0749-0739/92 $0.00
OPHTHALMOLOGY
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CONGENITAL OCULAR ANOMALIES Steven M. Roberts, DVM, MS
Ocular disorders of neonatal horses, although reported less commonly than corresponding disorders in dogs or cats, encompass a wide variety of conditions and may be inherited, congenital, or acquired. Acquired conditions that occur at or within a few weeks of birth may be misinterpreted as congenital in origin if the examiner is unfamiliar with the normal neonate eye, ocular development, or how other existing conditions may affect the eye. In addition, fetal remnants sometimes remain within the equine eye at birth, but may go unnoticed until some time later. Occasionally, it is difficult to be certain an abnormality is congenital, especially if the diagnosis is made when the foal is several weeks or months of age (e.g., cataract). Early examination of the equine eye is possible because the eyelids are open and ocular development is complete at the time of birth. One individual or group of individuals examining neonates will rarely encounter the full spectrum of congenital equine ocular defects because the overall prevalence is low (0.5%).48 Case reports of various congenital ocular defects in diverse domestic animal species abound in the literature. Liepold and others28 provide an excellent review of many congenital defects, including ocular disorders in horses. A recent textbook, edited by Gelatt, 9 contains informative sections on ocular embryology and equine ocular disease. It is not the purpose of this article to provide an exhaustive treatise, but to inform the busy veterinary practitioner and veterinary medical student of common ocular disorders documented in foals. Because congenital ocular defects occur sporadically, our awareness of these disorders tends to diminish. From the Department of Clinical Sciences, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado
VETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 8· NUMBER 3· DECEMBER 1992
459
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Hopefully, this article will maintain awareness of congenital conditions and provide a basis for answering questions, improving advice to clients, treating selected conditions, and offering a prognosis on ocular and animal function. Teratogenesis, a term often used instead of congenital, is defined as the in utero production of defects in offspring. Such defects can result from impaired cell proliferation, disrupted cell migration, and failure of cell differentiation, which cause disordered cell formation and development. A diverse group of problems, including genetic defects, extraneous teratogenic substances, infections, or trauma likely interact. 8 Extraneous teratogenic substances consist of drugs, toxins, vitamin deficiencies or excesses, and ionizing radiation. Idiopathic defects also occur and represent the largest etiologic group. The most common defects appear to be cataracts (::; 35.3%) and microphthalmia (::; 14.7%).8,21,25,37 Information from records of patients seen at the Colorado State University-Veterinary Teaching Hospital (CSU-VTH) between 1972 and 1991 indicated a similar prevalence, with cataracts representing 33.6% (38/113) and microphthalmia 7.1 % (8/113) of neonatal ocular diagnoses. Other common neonatal problems included retinal detachment and uveitis, 18.6% (21/113); nasolacrimal atresia, 8% (9/113); entropion, 7.1 % (8/113); persistent pupillary membrane, 7.1 % (8/113); microphthalmia, 7.1 % (8/113); congenital stationary night blindness, 3.5% (4/113); and retinal dysplasia, 3.5% (4/113) (Roberts SM: Unpublished data). Table 1 lists the congenital ocular conditions noted in the CSU-VTH records. Surprisingly, 5.3% of the diagnoses in horses with ocular problems were neonatal in nature. Table 2 presents a list of reported ocular defects of foals by associated breeds, proposed causes, essential features, and management methods. This article discusses neonatal ocular abnormalities, beginning with those affecting the external ocular structures and progressing deeper into the globe. Ocular colobomas are an exception, because they may involve several anatomic areas of the eye structures. Microphthalmia is discussed last because it impacts multiple ocular tissues.
DISORDERS Eyelid Positional Problems
Inversion of the eyelid margin, termed entropion, typically involves the lower eyelid. This problem is infrequent in horses, but occurs in neonates 9 , 21, 25 as either a congenital defect or early-acquired problem. The problem, possibly inherited in Thoroughbreds, 35 may involve multiple factors such as weakness of the tarsal plate, position of the globe, and spasm of the orbicularis oculi muscle. 36 Painful conditions of the conjunctiva and cornea frequently lead to or exacerbate entropion due to the pain-associated blepharospasm. Enophthalmia resulting from debilitation and weight loss or dehydration can produce entropion.
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Table 1. CONGENITAL OCULAR DIAGNOSES* Prevalence Disorder Diagnosis Amaurosis Amblyopia Aniridia Cataract-Y suture Cataract-nuclear Cataract-hyaloid remnant Cataract-mature Coloboma-eyelid Coloboma-lens Coloboma-optic nerve Congenital stationary night blindness Entropion Dermoid Nasolacrimal atresia-nasal puncta Nasolacrimal atresia-palpebral puncta Microphthalmia Microphakia Persistent pupillary membranes Optic nerve hypoplasia Retinal detachment Retinal dysplasia Uveitis Total
Male
Female
Total
0 1 1 1
0 1 1 2 6 1 8 1 0 1 3
3 1 1 8 15 3 12 1 1 2
4
4
1
1
2 2 4
4
1 3 0
0 5 1 3 2 5 54
8 2 6 3 8 1 8 1 7 4 14 113
3 0 0 6 9 2
4
4 2 9 59
1
4
4
0/0
2.6 0.9 0.9 7.1 13.3 2.6 10.6 0.9 0.9 1.8 3.5 7.1 1.8 5.3 2.6 7.1 0.9 7.1 0.9 6.2 3.5 12.4
*Based on medical records at the Colorado State University-Veterinary Teaching Hospital, a total of 1183 of 22,352 (5.3%) equine cases were documented with at least one ocular abnormality, representing 2137 diagnoses of ocular disease. Of all diagnoses, 113 (5.3%) represented congenital anomalies.
Of the CSU-VTH cases, six instances of entropion occurred in debilitated foals. Three suffered from failure of passive transfer, two had ocular pain associated with septicemia and uveitis, and one had an intestinal impaction. Typically, clinical signs develop when facial hair contacts the cornea and conjunctiva, thereby stimulating the trigeminal nerve. The result is increased lacrimation, blepharospasm, conjunctivitis, and keratitis. Ulcerative keratitis, if present, directly threatens vision. Severe, vision-threatening corneal ulceration occurred in one of the CSU-VTH cases. Treatment, which varies with both severity and chronicity, includes manual repositioning of the lids several times daily, subcutaneous injection of procaine penicillin G, placement of temporary repositioning sutures in a mattress or Lembert pattern,21, 27, 44, 45 placement of temporary wound closure staples in the skin, or excision of skin and wound closure adjacent to the lid margin to redirect the eyelid margin's position. 9, 46 Mild degrees of entropion often correct spontaneously within a few weeks, especially if topical ointments are used. Topical antibiotic ointments used with the procedures just mentioned help prevent tissue infection and increase animal comfort. If treatm.ent involves injection of material into the entropic lid for repositioning,
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N
Table 2. NEONATAL OCULAR DISORDERS IN FOALS Disorder
Breeds
Microphthalmia May be associated with other ocular defects, e.g., multiple ocular defect syndrome cataracts, retinal detachments.
All breeds. Thoroughbreds have higher incidence.
Strabismus
Appaloosa.
Etiology Not established; most cases are idiopathic, some due to toxic, infectious, or nutritional episodes.
Essential Features and Management Small eyes, prominent third eyelid. No treatment; lid surgery for secondary entropion.
Rare. Strabismus usually convergent; some patients have compensatory head deviation. Surgery to straighten eyes has been described.
Eyelid coloboma May be associated with multiple occular anomalies.
Defects in eyelid margin. Surgery, depending on extent of other problems.
Ankyloblepharon congenita Partial fusion of upper and lower lids. Surgery. Entropion Primary. Secondary.
Nasolacrimal Atresia of nasolacrimal meatus.
Some patients have oversized palpebral fissures. Prematurity or illness.
Inturned eyelids, epiphora, corneal ulceration. Manual eversion of the eyelids, palpebral injections or horizontal mattress sutures or surgery to turn out the eyelids. Treatment of underlying cause, then surgery. Absence of nasal opening. Watery eye; some patients have mucopurulent discharge. Dacrocystorhinography may aid in diagnosis. Surgery to create new opening.
Misplaced punctum.
Surgery to enlarge punctum.
Atresia of punctum.
Surgery to create new punctum.
Cornea Microcornea. May be associated with microphthalmia syndromes.
Small cornea. No treatment.
Corneal melanosis.
Superficial axial nonprogressive pigment. Superficial keratectomy.
Dermoid. Probably most common congenital anomaly. Aniridia May be associated with cataracts.
Belgian draft horse.
Not known to be inherited.
Lateral or ventral corneal conjunctival masses containing hair and sebaceous glands. Surgical removal.
Autosomal recessive.
Complete absence of iris tissue.
Uveal Cysts May be associated with heterochromia iridis. Heterochromia Iridis Cataracts Some associated with microphthalmia syndromes.
Persistent Pupillary Membranes (PPM) May be associated with other ocular anomalies. ~
0"\
Pigmented cysts in anterior chamber or pupillary edge. Can be transilluminated. Aspiration if large and vision obstructed. Many color-dilute breeds.
Pigment failure in one iris or part of one iris.
Tapatum may be absent, and iris can be hypoplastic.
Arabian; Belgian draft; Morgan; Thoroughbred; Quarter Horse.
May be inherited.
Spontaneous resorption can offer useful vision in stud animals. Consider surgery if the cataracts interfere with vision.
Other cataracts can be due to trauma (prenatal and foaling) or poor nutrition, or metabolic or toxic causes.
Note: Need to differentiate from prominent posterior Y sutures, which are present in the majority of foals.
Strands of iris tissue that can run to iris, cornea (opacity), or lens (cataracts). PPM may continue to atrophy during the first year of life.
W
Table continued on following page
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0'1
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Table 2. NEONATAL OCULAR DISORDERS IN FOALS Continued Disorder
Breeds
Etiology
Vitreous Remnants of hyaloid tissue.
Remnants of hyaloid tissues visible behind the lens. These are present in most foals and will atrophy by 6-9 months. Uncommon. Fibrovascular membrane behind the lens.
Persistent hyperplastic primary vitreous. Retina Retinal hemorrhages.
Retinal dysplasia; may be associated with other defects.
Mostly incidental. Difficult birth. Thoroughbred may have higher incidence.
Bilateral retinal disorganization; may also have retinal detachment.
Retinal detachment; may associate with dysplasia.
Complete retinal detachment.
Coloboma of the fundus.
Chorioretinal defect.
Retinal aplasia. Stationary night blindness.
Appaloosa and other breeds.
Possibly autosomal recessive.
Absence of the retina.
Autosomal recessive defect in neuroretinal transmission.
No lesion visible on ophthalmoscopic examination. Some patients have microphthalmia or dorsal strabismus/ nystagmus.
Optic Nerve Optic nerve hypoplasia (congenital optic atrophy); may be associated with other defects.
Papilledema.
Essential Features and Management
Small optic nerve heads either unilateral or bilateral. Slow or absent pupillary light reflex. Searching nystagmus. Maladjustment.
Swelling of optic nerve associated with cerebral edema.
From Slatter D: Fundamentals of Veterinary Ophthalmology. Philadelphia, WB Saunders, pp 554,555, 1990; with permission.
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do not inject potentially irritating materials such as mineral oil or paraffin. 7 To do so risks a severe foreign body reaction that could result in serious lid conformation abnormalities and corneal irritation. If questions arise relating to the usefulness of a horse that has or has had entropion, it should be considered a blemish, not an unsoundness, unless keratitis has caused loss of vision. Visual loss often requires a finding of unsoundness. The CSU-VTH records indicated three cases of ectropion, eversion of the eyelid margin. Two were due to trauma and one to overzealous surgical correction of entropion. Temporary procedures are preferred for entropion correction in neonates. When treatment involves surgical excision of skin, plan carefully to avoid overcorrection. Cases of ectropion, regardless of their cause, require blepharoplastic procedures to correct the problem. Ectropion surgical correction results in slight deformity of the eyelid. Refer to the article by Moore for additional information. Nasolacrimal Atresia
Congenital anomalies of the nasolacrimal excretory system reported in the horse include atresia of the eyelid punctum, atresia of the nasal meatus punctum, and ectopic eyelid puncta. The most common anomaly is atresia of the nasal meatus puncta. 9, 18, 23, 24, 27, 29, 45 Such was the case in six of nine CSU-VTH cases. Atresia involved the proximal bony portion in two horses and one had atresia of the bony portion and canaliculi. To date, there is no evidence to support an inherited basis in the horse. In humans, congenital atresia of the nasolacrimal duct is common and potentially familial. 10 The lacrimal passages are formed in humans by epithelial cords beneath the cleft between the nasal and maxillary processes. Canalization begins by the end of the first trimester of gestation. Communication exists between the eye and nose by the end of the second trimester. Patency of the canaliculi lumina develops during midgestation. The eyelid puncta open onto the eyelid margins early during the third trimester of gestation. A mucosal membrane often covers the nasal meatus puncta at birth. 34 This developmental process likely is quite similar in the horse. Clinical signs of atresia of the nasolacrimal system consist of epiphora and mucopurulent ocular discharge, the latter secondary to dacryocystitis. 23, 30 The onset of symptomatic epiphora sometimes is delayed until horses are several months of age. Explanations of this phenomenon include a lower tear volume in neonates, distention of the existing nasolacrimal duct portion,27 and transmucosal absorption of tears across the duct wall. 23 Differentiation of nasolacrimal obstructions from chronic conjunctivitis hinges on the quantity of ocular discharge. Profuse discharge occurs with nasolacrimal obstructions, especially when the palpebral puncta are flushed (Fig. 1).
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Figure 1. Atresia of the nasal punctum has resulted in chronic accumulation of secretions in the nasolacrimal system. Epiphora and spillage of some mucoid discharge onto the face had been present since 1 month of age. Upon flushing saline into the upper palpebral punctum, mucopurulent material exited from the lower palpebral punctum.
Clinical diagnosis is accomplished by (1) irrigation of the proximal nasolacrimal duct with saline solution through the palpebral puncta, (2) examination and palpation of the floor of the nasal vestibule for a distended structure, and (3) radiographic contrast studies. 22- 24, 35 Irrigation procedures result in abnormal material refluxing from a punctum, a distended structure on the floor of the nasal vestibule that changes in size with irrigation, and a reduced amount or lack of irrigant exiting the nasal meatus punctum. Collection of reflux material for bacterial culture and antibiotic sensitivity testing is desirable because of the secondary dacryocystitis usually present. Treatment requires surgical placement of a retention tube for 3 to 8 weeks to allow epithelization of the opening and resolution of the dacryocystitis. 18, 23, 24, 27, 29 A flexible catheter such as a male urinary catheter is passed through the nasolacrimal duct from the upper palpebral punctum. The tip of the catheter is usually palpable slightly proximal to the expected location of the nasal punctum. The nasal mucosa is cut with a scalpel blade, the catheter tip grasped with a hemostat, and the catheter pulled through the new punctal opening and sutured to skin within the nostril or, after passing the catheter through the lateral false nostril wall to the skin surface, secured with sutures. Hemorrhage from the extensive vascular supply of the ventral nasal concha may require control by direct pressure, local application of a-adrenergic agents, or local application of cocaine. Following retention tube placement, the nasolacrimal system is flushed with saline or 1:20 povidone-iodine solution in saline. After surgery, topical antibiotic or antibiotic-corticosteroid preparations and systemic antibiotics should
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be used for several weeks. Surgery is successful in most instances. Refer to the article by Moore for additional information. Although rare in comparison to nasal meatus punctal problems, atresia or ectopic placement of the eyelid puncta also occurs. If epiphora results from either condition, surgical intervention to open, slightly enlarge, or move the punctum is possible. If atresia or agenesis cannot be corrected, creation of an alternative nasolacrimal outflow pathway or dacryoconjunctivorhinostomy is possible. All nasolacrimal anomalies represent an unsoundness until corrected surgically. Dermoid
The term dermoid means resembling skin. Dermoids are choristomas and are defined as normal tissue elements located at an abnormal site. They represent a congenital tumor of skin and related appendage tissues. External ocular involvement may include the cornea, conjunctiva, nictitating membrane, or eyelids. Most commonly, the temporal limbus and neighboring cornea and sclera show superficial involvement. 27, 32, 35, 43, 50 Deeper tissue involvement is possible and epibulbar dermoids in humans reportedly can involve the full corneal thickness and intraocular structures. lO No genetic association exists in horses. Some dermoids have hair on their surface, causing mechanical irritation of the cornea. The degree of visual impairment depends on the extent of corneal involvement and the occurrence or absence of disease secondary to corneal problems. Large dermoids may extensively involve the globe and adnexa, causing corneal opacity, nictitans deformity, eyelid disfigurement, and nasolacrimal punctal occlusion. Removal requires a superficial keratectomy, although small lesions may go untreated. 27 In most instances, complete removal without perforation of the globe is possible. The corneal stroma beneath the lesion typically is transparent (Fig. 2). Treatment of superficial keratectomy defects is identical to conventional ulcer management (see the article in this issue on ulcerative keratitis by Nassise). All wounds require topical treatment and some require corneal mechanical support, with conjunctival flap grafts, nictitans flaps, collagen shields, or tarsorrhaphies. Some degree of mild corneal opacity remains following surgical removal. Uveal Defects
Aniridia means absence of the iris. Iris hypoplasia, in which a rudimentary iris bud is present, is included within the scope of the definition. True aniridia, the total absence of iris, is rare in all species. The anomaly arises from a third wave mesenchyme failure of ingrowth and disorderly differentiation. The first and second mesenchyme waves, which form the corneal endothelium and stroma, are rarely involved in congenital defects. The major flaw appears to be a maldevelopment
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Figure 2. A large corneal dermoid involving 60% of the corneal surface has been removed and the conjunctiva sutured to the limbus. Notice the transparent nature of the corneal stroma at the site of excision. Approximately two thirds of the cornea was removed to achieve complete lesion removal. This defect was managed with a subpalpebral medication tube and was negative to fluorescein in 5 days.
of capsulopupillary vessels of the tunica vasculosa lentis, thus preventing growth of the iris. lO A single family of Belgian draft horses with aniridia that had an autosomal dominant pattern of inheritance has been reported. 12 More recent reports document sporadic aniridia in a Quarter Horse stallion20 and a Welsh-Thoroughbred filly. 19 Aniridia is usually bilateral and frequently is confused with an abnormally large pupil. In distinction from simple mydriasis, aniridia allows visualization of the lens equator and ciliary processes. Associated clinical signs are common and include blepharospasm, cataract formation, 12, 19 ocular discharge, perilimbal keratitis,20 photophobia, and reduced vision. There are no specific treatments for aniridia except symptomatic supportive measures. Protective masks may help reduce ambient illumination and increase patient comfort. Vision is degraded and affected horses are unsound. Iris cysts involving the posterior pigmented epithelium and protruding through a thinned iris stroma occur in horses with blue irides. 9 One report41 mentioned a similar abnormality in older horses that was effectively treated by surgical removal of the anterior cyst wall in two individuals. Treatment is not necessary if there are no clinical signs and the cyst does not increase in size. Iris cysts transilluminate, appearing hollow upon examination. If confusion between an iris cyst and melanoma occurs, histopathology will determine the exact lesion nature. Persistent pupillary membrane problems occasionally are reported. 2, 25, 27, 50 Figure 3 illustrates several manifestations of this anomaly. Strands that radiate from iris to iris are most common and cause no apparent problems. More serious are strands that radiate from iris to lens or iris to cornea. Persistent pupillary membranes that touch the lens leave pigment deposits on the anterior lens capsule or cause anterior capsular cataract. If strands touch the corneal endothelium,
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Figure 3. This yearling was examined because of the presence of corneal leukoma since birth. Several persistent pupillary membranes are present ventrally and extend to the cornea, making contact with the endothelial cell. This form of leukoma is referred to as adherent leukoma. Notice the two foci of pigment on the anterior lens capsule, indicating persistent pupillary membrane contact with the lens capsule (arrows). Subtle persistent pupillary membrane strands are present near the nasal edge of the pupil.
leukoma from endothelial cell disturbance occurs. Such a leukoma will be fluorescein negative because the epithelial surface is not involved. Surgical removal of persistent pupillary membranes is usually not recommended. There is no known inherited tendency in the horse and the anomaly should be considered a blemish if strands radiate from iris to iris. If strands radiate from iris to lens or cornea, the horse should be considered unsound.
Coloboma
This term means mutilation and was first coined in 1821 10 to describe a missing part of an ocular structure, resulting from a failure of the optic fissure to close. Colobomas are either "typical" (i.e., positioned along the line of closure of the optic fissure) or "atypical" (i.e., positioned outside the line of closure). Ocular structures involved include the eyelids,22, 27 iris,22, 35 ciliary body, lens,2, 22, 27, 35 retinachoroid,45 optic disc,27 and optic nerve. Figure 4 depicts an optic nerve coloboma and retinal detachment in a Quarter Horse foal. All forms of coloboma are rare in the horse, but when they occur, they frequently are associated with concomitant ocular disorders. Although the cause of colobomas usually are unknown, typical optic nerve and scleral colobomas result from abnormal eversion of the neurosensory retina through the embryonic fissure. lO This theory, supported by work in the Collie dog, suggests that defective neurosensory retinal differentiation is responsible for optic disc colobomal formation. 26 Eyelid coloboma or agenesis, although reported in the horse, has not been described in detail. 37 Eyelid colobomas do not result from
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Figure 4. A large coloboma of the optic nerve is shown in a Quarter Horse filly. The superior portion of the retina was detached, and several retinal folds have resulted (arrow).
embryonic fissure abnormalities. Experimental work suggests that ischemia to a rapidly developing embryonic eyelid may cause infarction of the lid farthest from the blood supply, 34 thus resulting in a focal defect. Congenital, atypical colobomas have been reported, involving the retina and choroid of the horse. 42 Repairs of eyelid colobomas require blepharoplastic surgery. In some situations, the repair process requires several stages. If the defect is minor, complex repair is unnecessary and simple cryoepilation of eyelashes or facial hair contributing to corneal irritation provides symptomatic relief. Surgery must correct exposure keratopathy and other associated problems, if present. Figure 5 illustrates atresia that involved
Figure 5. Atresia of the eyelids frequently causes corneal irritation or ulceration. This case had mild atresia of the nasal aspect of the upper lid (arrow), yet suffered severe corneal ulceration. The atresia also involved the nasal canthus causing malfunction of the palpebral puncta. Blepharoplastic repair recreated a mucocutaneous junction by splitting the eyelid into cutaneous and conjunctival flaps. The conjunctival portion was advanced and sutured to the cutaneous flap. In addition, the palpebral puncta were cannulated and reconstructed. Following surgery, recovery was uneventful and no further problems were noted for 1.5 years; then, the horse was lost to follow-up.
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the nasal aspect of the upper eyelid. Corneal ulceration secondary to mechanical irritation resulted in corneal scarring and vascularization as sequelae. Defects located within the globe are not amenable to surgical correction. A coloboma that directly or indirectly causes visual loss renders the horse unsound, unless surgical correction is possible.
Cataract
The normal lens is biconvex and suspended behind the pupil. It is transparent, although it appears slightly bluish-grey when viewed along an examining light beam pathway. The lens is unique in that it represents surface ectoderm sequestered within the globe and enclosed by a basement membrane. Lens epithelial cells and fibers thus retained throughout life are subject to insult. They react to insult by cell death, abnormal cell proliferation, and loss of osmotic homeostasis. The result is vacuolation and loss of transparency, termed cataract formation. A cataract may partially or totally involve the lens cortex, nucleus, or capsule and formation typically is not uniform throughout the lens. One part of the lens therefore may show partial opacity, whereas another part is totally opaque. Complete congenital cataracts are totally opaque and are termed mature cataracts. The term immature cataract describes partially opaque lenses. Depending on the cataract's severity or degree of maturity, slight vision impairment or total blindness results. Regardless of the degree of involvement, a cataract represents an unsoundness. Congenital cataracts are the most common neonatal ocular abnormality and may be the most common cause of blindness or vision impairment in foals. Owners and handlers frequently complain that the affected foal displays visual deficits, clumsiness, and tendency toward repeated traumatic injuries. Aniridia, microphthalmia, and other defects frequently accompany congenital cataracts. 11 - 13, 19-21, 33 In most instances, the cause of congenital cataracts remains unknown. Potential causes include faulty in utero nutrition, genetic factors, ocular inflammation occurring in utero, radiation, and prenatal trauma. 38 Heredity rarely has been documented in the horse. 3 , 12,49 A congenital cataract, by definition, is present at birth, having developed during fetal life. Most are bilateral. Four basic types of congenital cataracts described in horses include (1) completely mature cataract, (2) nuclear cataract, (3) Y suture-type cataract, and (4) cataract associated with persistent hyaloid vasculature. 9, 27 The CSU-VTH medical record database documented 38 diagnoses of congenital cataract in horses (Roberts SM: Unpublished data). The distribution by type included 15 nuclear, 12 mature, 8 Y suture, and 3 hyaloid-related cataracts. Breeds included American Saddlebred (n = 3), Arabian (n = 9), Quarter Horse (n = 24), Appaloosa (n = 1), and Thoroughbred (n = 1). In these cases, two of three cataracts secondary to a persistent hyaloid artery and all Y suture-type cataracts did not seriously impair vision. Complete cortical or nuclear congenital cataracts were more
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serious, with a potential for visual loss. Some nuclear cataracts also had extensive cortical involvement. Figure 6 demonstrates a nuclear cataract involving the fetal nucleus. In seven cases, nuclear and Y suture cataract changes were noted in the lenses. In addition to the data just summarized, reports of heritable, congenital cataracts exist for the Belgian,12 Morgan, 3 and Thoroughbred45 breeds. Cataracts in the Morgan horse typically are bilateral, nonprogressive, and nuclear, causing minimal visual impairment. Noninherited nuclear cataracts also are reported in Morgan horses. 4 The genesis of congenital cataracts is not simply a matter of genetic or environmental factors, but involves complex developmental issues. Traditionally, descriptions of lens development include four stages: (1) formation of the lens plate, (2) formation of the lens vesicle, (3) formation of the primary lens fibers, and (4) formation of the secondary lens fibers. 34 During early fetal development, the primordial neural retina induces lens formation. The resulting lens placode undergoes cellular division to form the lens cup, with an initial hollow lumen, which becomes sealed off and surrounded by the epithelial basement membrane. This arrangement prevents other cell types from mixing with the lens cells and limits permeability of the lens. Extrinsic factors transforming the lens vesicle into a mature lens include the proximity of a healthy, developing neural retina 34 and proper conformation between the developing lens and optic cup rim. Proper lens development involves elongation of posterior epithelial cells to form primary lens fibers that obliterate the lens vesicle lumen. Division, equatorial migration, and elongation of the anterior lens epithelial cells produce the secondary lens fibers. The result is crowding of primary lens fibers centrally to form the fetal nucleus. The unique lens organization of cells surrounded and sequestered by the epithelial basement membrane
Figure 6. A dense cataract involving the fetal portion of the nucleus of this Quarter Horse weanling. This cataract is small enough that no serious vision impairment resulted. As the eye and lens grew, the cataract remained the same absolute size, resulting in a relative decrease in cataract size as normal lens material was formed around the lesion.
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prevents exfoliation of secondary lens cells. Thus, unlike the surface ectoderm of skin, the lens retains all cellular material throughout the animal's life. It is little wonder that cataracts frequently form and is surprising they are not more common. The congenital nuclear cataracts noted often in horses are a lamellar type, involving that portion of the lens immediately external to the fetal nucleus, which represents the inner part of the adult nucleus (Fig. 7). Such a circumscribed zone of opacity occurs as a cataractogenic influence acts on newly formed lens fibers during a precise time. The stage of lens formation during the cataractogenic influence determines the location and extent of the opacity. Lens fibers developing before or after the cataractogenic influences are transparent. Some horses exhibiting moderate to severe visual impairment soon after birth therefore undergo clinical visual improvement as normal lens fibers form around the cataractous fibers. This translates into a decrease in the percentage of cataract involvement as lens growth continues. Occasionally, however, adjacent normal lens fibers opacify over time, leading to cataract progression and further visual impairment. Currently, surgical removal is the only treatment for visual loss due to congenital cataract. The article by McLaughlin in this issue deals specifically with cataract surgery in horses. Before scheduling cataract surgery in a foal, the possibility of deprivation amblyopia requires evaluation. Deprivation amblyopia results from inadequate visual stimulation to the central nervous system, producing irreversible functional and structural abnormalities in the lateral geniculate nuclei and visual cortex. 45 Foals with a searching, wandering-type nystagmus are suspect of having deprivation amblyopia. In this situation, successful cataract removal may offer little improvement in actual visual function. Congenital Stationary Night Blindness
Congenital stationary night blindness, an incompletely understood condition reported in Appaloosa horses of the United States, 39, 51-53 occasionally affects other breeds. 51 Visual impairment ranges from poor vision during reduced light conditions to poor vision during the day
Figure 7. A lamellar cataract involving the junction between the fetal and adult nucleus. The posterior surface of the nucleus was involved and the entire nucleus displaces slightly posterior, a condition referred to as lenticonus interna. This horse had clinically normal vision from the affected eye.
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and blindness at night. Proposed inheritance modes include autosomal recessive 51 or sex-linked recessive, with the defect X chromosome. 52 The CSU-VTH data do not support a sex-linked recessive mode of inheritance, given that three of four affected Appaloosa horses were female (see Table 1). A history of poor vision, behavior associated with poor vision, and a normal ocular fundus allows presumptive diagnoSiS. 39,51-53 Other clinical signs include subtle microphthalmia, holding the head in a "stargazing" position when visualizing objects, seeking lighted conditions, and displaying a dorsomedial strabismus. Definitive diagnosis by electroretinography shows a characteristic large negative waveform. The disorder may result from abnormal neural transmission between the photoreceptor and inner nuclear layers of the retina. No treatment is available and the condition is nonprogressive. Affected horses are unsound and unsuitable for breeding. Equestrian use of even mildly affected horses requires the riders' clear understanding of the degree of visual impairment. Medico-legal issues should be addressed by issuing a written statement, signed by the involved parties, explaining the degree of visual impairment and any recommended restrictions in the use of the horse.
Microphthalmia
The normal globe fills the orbit, thereby providing a curved surface for the eyelids to move across and creating a normal palpebral fissure shape. Microphthalmia or nanophthalmia represents one of the more common congenital ocular anomalies and ranges from minimal to severe reductions in globe size. Severe microphthalmia should not be confused with anophthalmos or total absence of a globe. True anophthalmia is very rare and most suspected cases are, in reality, microphthalmia. Microphthalmia may be unilateral or bilateral and affected foals display a small palpebral fissure and protruded nictitating membrane. The associated abnormal anatomy of the globe, orbit, and eyelid results in poor nasolacrimal drainage and subsequent ocular discharge. Secondary conjunctivitis from environmental irritation and infection frequently is seen. Orbital and periocular asymmetry may become more pronounced as the foal grows because the orbit fails to grow properly. The affected globe is usually blind and corneal opacity due to leukoma or pigmentation is common. Cataracts are common and were present in five of eight CSU-VTH microphthalmia cases. A corneal dermoid occurred in one of the CSU-VTH microphthalmia cases. Microphthalmia is due to defective organogenesis. Complex and closely interrelated factors determine the final globe size. Embryologically, the primary optic vesicle buds from the forebrain and differentiates into the eye. Retarded growth of the optic vesicle and associated degeneration or atrophy lead to microphthalmia. Three classifications of microphthalmia include (1) pure microphthalmia (nanophthalmos), with a small but otherwise normal eye; (2) colobomatous microphthal-
CONGENITAL OCULAR ANOMALIES
475
mia, with a defect resulting from failure of the optic vesicle to involute or the embryonic fissure to close; and (3) complicated microphthalmia, in which anomalous development occurs independent of fissure closure. 10 The severity of microphthalmia depends on the gestational stage during which the insult occurs: It is most severe when insult occurs early in gestation, at the time of optic vesicle and lens formation. 1 If the optic vesicle tip contacts surface ectoderm over less than the normal area, a perfectly formed microphthalmic eye results. 6 In humans, defective closure of the embryonic fissure appears to cause most microphthalmia. 34 Reports of microphthalmia in the horse 2, 8, 11, 13, 21, 33, 37, 43, 46, 50 suggest optic vesicle degeneration is a common cause, because affected globes contain intraocular tissue remnants showing wide morphologic variation, faulty formation, and disorganization. Colobomatous microphthalmia perhaps best describes this condition. Although the orbit forms, volume reduction is evident and abnormal globe formation leads to hypoplastic or atrophic optic nerves. Most equine cases are sporadic, with an unknown cause. There is no genetic association in horses. A case of equine microphthalmia associated with sulfadimethoxine administration to the mare during week six of gestation 11 raises the question of a teratogenic effect similar to that seen in humans. Although microphthalmia is not correctable, secondary problems may be amenable to treatment. For unilateral cases, enucleation is helpful in preventing secondary problems associated with abnormal ocular discharge. Entropion, developing from poor eyelid conformation or painful ocular conditions, requires surgical correction. Because the u~derlying problem remains, however, entropion frequently recurs. Most foals with unilateral involvement adapt to their monocular state with maturity. Bilateral involvement, causing blindness or near blindness, may necessitate destruction of the animal. Microphthalmia represents a serious handicap to horses. Some may be functional depending on their intended use, but microphthalmia represents an unsoundness in all cases.
Rare Conditions
In addition to the conditions already discussed, a wide range of rare conditions include glaucoma,14, 22 lens luxation,13, 31, 50 lenticonus,27, 50 lentiglobus,27, 50 microphakia,27, 50 optic nerve hypoplasia (Fig. 8) and atrophy, 17, 50 retinal cysts,41 retinal dysplasia, 14, 27,35,50 retinal detachment (Fig. 9),27, 35, 40 and strabismus. 16, 27 Although not strictly viewed as a congenital problem, distichiasis has been observed at the C5U-VTH in young horses. In some situations, congenital anomalies within the eyelid tarsal glands ultimately result in aberrant hair growth and some instances of distichiasis represent ectopic locations of hair follicles at the lid margins. Thus, some cases may be congenital anomalies. Retinal cysts, mild retinal dysplasia, and strabismus do not interfere seriously with vision. Such minor conditions represent a blemish rather
476
ROBERTS
Figure 8. This weanling Quarter Horse filly was examined for unilateral blindness. Examination detected a hypoplastic optic nerve and peripapillary pigment dispersion of the retinal pigment epithelium. Notice the reduced size of the optic disc. The disc was recessed below the level of the sclera. Histologically, the optic nerve and ganglion cell layer were hypoplastic and the peripapillary retina dysplastic. The cause of the changes was unknown.
than an actual unsoundness. Conditions directly threatening vision represent unsoundness. Four cases of retinal dysplasia in the CSUVTH data displayed severe bilateral involvement, with either poor vision or no vision. Two had cataracts and one had retinal detachments and lens luxations. SUMMARY
This discussion provides an idea of the diversity and relative prevalence of certain congenital ocular conditions of horses. Many are not difficult to diagnose, yet curative treatment may be impossible. When dealing with owners of horses affected with unusual anomalies,
Figure 9. A complete retinal detachment is shown with the retina still attached to the optic disc. This foal had been blind since birth. No cause for the congenital detachment could be found.
CONGENITAL OCULAR ANOMALIES
477
responsible client service requires veterinarians to provide accurate information and to know where answers to unusual questions can be found. Again, most veterinarians never encounter all of the diverse congenital defects. As a result, the horse owner frequently receives misinformation. Hopefully, this brief coverage of congenital ocular anomalies will provide useful information and assist in appropriate communication to concerned parties.
References 1. Apple OJ, Naumann GOH: Malformations and anomalies of the eye. In Naumann GOH, Apple OJ (eds): Pathology of the Eye. New York, Springer-Verlag, 1986, p 63 2. Barnett KC: The eye of the newborn foal. J Reprod Fertil (suppl) 23:701, 1975 3. Beech J, Aquirre G, Gross S: Congenital nuclear cataracts in the Morgan horse. J Am Vet Med Assoc 184:1363, 1984 4. Beech J, Irby N: Inherited nuclear cataracts in the Morgan horse. J Hered 76:371, 1985 5. Bistner SI, Rubin LF, Saunders LZ: Ocular lesions of bovine viral diarrhea-mucosal disease. Pathologica Veterinaria 7:275, 1970 6. Coulombre AJ: Experimental embryology of the vertebrate eye. Invest Ophthalmol Vis Sci 4:411, 1965 7. Craven JR: Significance of lesions of the cornea and lens in the examination of horses for soundness. Equine Vet J 3:141, 1971 8. Crowe MW, Swerczek TW: Equine congenital defects. Am J Vet Res 46:353, 1985 9. Davidson MG: Equine ophthalmology. In Gelatt KN (ed): Veterinary Ophthalmology, ed 2. Philadelphia, Lea & Febiger, 1991, p 576 10. Duke-Elder S: Normal and abnormal development. In Duke-Elder System of Ophthalmology, vol III, part 2. St Louis, CV Mosby, 1963, pp 456, 458-459, 571 11. Dziezyc J, Kern TJ, Wolf ED: Microphthalmia in a foal. Equine Vet J 15:15, 1983 12. Ericksson K: Heriditary aniridia with secondary cataract in horses. Nord Vet Med 7:773, 1955 13. Garner A, Griffiths P: Bilateral congenital ocular defects in a foal. Br J Ophthalmol 53:513, 1969 14. Gelatt KN: Ophthalmoscopic studies in the normal and diseased ocular fundi of horses. J Am Anim Hosp Assoc 7:158, 1971 15. Gelatt KN: Glaucoma and lens luxation in a foal. Vet Med Small Anim Clin 68:261, 1973 16. Gelatt KN, McClure JR: Congenital strabismus and its correction in two Appaloosa horses. J Equine Med Surg 1(13):240, 1977 17. Gelatt KN, Leipold HW, Coffman JR: Bilateral optic nerve hypoplasia in a colt. J Am Vet Med Assoc 155:627, 1969 18. Hjorth P: Atresia of the nasolacrimal duct in a horse. Nord Vet Med 23:260, 1971 19. Irby NL, Aquirre GO: Congenital aniridia in a pony. J Am Vet Med Assoc 186:281, 1985 20. Joyce JR: Aniridia in a Quarter Horse. Equine Vet J 15:21, 1983 21. Koch SA, Cowles RR Jr, Schmidt GR, et al: Ocular disease in the newborn horse: A preliminary report. J Equine Surg 2(4):167, 1978 22. Latimer CA, Wyman M: Atresia of the nasolacrimal duct in three horses. J Am Vet Med Assoc 184:989, 1984 23. Latimer CA, Wyman M: Neonatal ophthalmology. Vet Clin North Am Equine Pract 1(1):235, 1985 24. Latimer CA, Wyman M, Diesem CD, et al: Radiographic and gross anatomy of the nasolacrimal duct of the horse. Am J Vet Res 45:451, 1984 25. Latimer CA, Wyman M, Hamilton J: An ophthalmic survey of the neonatal horse. Equine Vet J (suppl 2) 2:9, 1983 26. Latshaw WK, Wyman M, Venzke WG: Embryonic development of an anomaly of the ocular fundus in Collies. Am J Vet Res 30:211, 1969 27. Lavach JD: Large Animal Ophthalmology. St Louis, CV Mosby, 1990
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28. Leipold HW, Saperstein G, Woollen NE: Congenital defects in foals. In Smith BP (ed): Large Animal Internal Medicine. St Louis, CV Mosby, 1990, P 1576 29. Lundvall RL, Carter JD: Atresia of the nasolacrimal meatus in the horse. J Am Vet Med Assoc 159:289, 1971 30. Mason TA: Atresia of the nasolacrimal orifice in two Thoroughbreds. Equine Vet J 11:19, 1979 31. Mathews AG, Handscombe MC: Bilateral cataract formation and subluxation of the lenses in a foal: A case report. Equine Vet J (suppl 2) 2:23, 1983 32. McLaughlin SA, Brightman AH: Bilateral ocular dermoids in a colt. Equine Pract 5:10, 1983 33. Mosier DA, Engleman RW, Confer AW, et al: Bilateral multiple congenital ocular defects in Quarter Horse foals. Equine Vet J 15:18, 1983 34. Mullaney J: Normal development and developmental anomalies of the eye. In Gamer A, Klintworth GK (eds): Pathobiology of ocular disease. New York, Marcel Dekker, 1982 35. Munroe GA, Barnett KC: Congenital ocular disease in the foal. Vet Clin North Am Large Anim Pract 6(3):519, 1984 36. Peiffer RL Jr, Williams R, Schenk M: Correction of congenital entropion in a foal. Vet Med Small Anim Clin 72:1219, 1977 37. Priester WA: Congenital ocular defects in cattle, horses, cats, and dogs. J Am Vet Med Assoc 160:1504, 1972 . 38. Rathbun WB: Biochemistry of the lens and cataractogenesis: Current concepts. Vet Clin North Am Small Anim Pract 10(2):377, 1980 39. Rebhun WC: Equine retinal lesions and retinal detachments. Equine Vet J (suppl 2) 2:86, 1983 40. Rebhun WC, Loew ER, Riis RC, et al: Clinical manifestations of night blindness in the Appaloosa horse. Compend Contin Educ Pract Vet 6(2):S103, 1984 41. Rubin LF: Cysts of the equine iris. J Am Vet Med Assoc 149:151, 1966 42. Rubin LF: Atlas of Veterinary Ophthalmoscopy. Philadelphia, Lea & Febiger, 1974, p 296 43. Saunders LZ, Rubin LF: Ophthalmic Pathology of Animals. New York, S Karger, 1975, pp 180, 184 44. Severin GA: Veterinary Ophthalmology Notes, ed 2. Ft Collins, Colorado State University, 1976, p 94 45. Slatter DH: Fundamentals of Veterinary Ophthalmology. Philadelphia, WB Saunders, 1990, pp 164, 241, 376, 379, 468 46. Trapp CW: Congenital maldevelopment of the eyes of a colt. Cornell Vet 47:467, 1957 47. Vestre WA, Brightman AH: Correction of cicatricial entropion and trichiasis in the horse. Equine Pract 2:13, 1980 48. Walde I: Some observations on congenital cataracts in the horse. Equine Vet J 15:27, 1983 49. Weber W: Hereditary cataract, a recessive character in the horse. Schweiz Arch Teirheilk 89:397, 1947 50. Wilcock BP: Ocular anomalies. In Peiffer RL (ed): Comparative Ophthalmic Pathology. Springfield, IL, CC Thomas, 1983, p 3 51. Witzel DA, Joyce JR, Smith EL: Electroretinography of congenital night blindness in an Appaloosa filly. J Equine Med Surg 1:226, 1977 52. Witzel DA, Riis RC, Rebhun WC, et al: Night blindness in the Appaloosa: sibling occurrence. J Equine Med Surg 1:383, 1977 53. Witzel DA, Smith RD, Wilson RD, et al: Congenital stationary night blindness: an animal model. Invest Ophthalmol Vis Sci 17:788, 1978
Address reprint requests to Steven M. Roberts, DVM, MS Veterinary Teaching Hospital Colorado State University College of Veterinary Medicine and Biomedical Sciences 300 West Drake Fort Collins, CO 80523
OPHTHALMOLOGY
+ .20
CONGENITAL OCULAR ANOMALIES Steven M. Roberts, DVM, MS
Ocular disorders of neonatal horses, although reported less commonly than corresponding disorders in dogs or cats, encompass a wide variety of conditions and may be inherited, congenital, or acquired. Acquired conditions that occur at or within a few weeks of birth may be misinterpreted as congenital in origin if the examiner is unfamiliar with the normal neonate eye, ocular development, or how other existing conditions may affect the eye. In addition, fetal remnants sometimes remain within the equine eye at birth, but may go unnoticed until some time later. Occasionally, it is difficult to be certain an abnormality is congenital, especially if the diagnosis is made when the foal is several weeks or months of age (e.g., cataract). Early examination of the equine eye is possible because the eyelids are open and ocular development is complete at the time of birth. One individual or group of individuals examining neonates will rarely encounter the full spectrum of congenital equine ocular defects because the overall prevalence is low (0.5%).48 Case reports of various congenital ocular defects in diverse domestic animal species abound in the literature. Liepold and others28 provide an excellent review of many congenital defects, including ocular disorders in horses. A recent textbook, edited by Gelatt, 9 contains informative sections on ocular embryology and equine ocular disease. It is not the purpose of this article to provide an exhaustive treatise, but to inform the busy veterinary practitioner and veterinary medical student of common ocular disorders documented in foals. Because congenital ocular defects occur sporadically, our awareness of these disorders tends to diminish. From the Department of Clinical Sciences, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado
VETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 8· NUMBER 3· DECEMBER 1992
459
460
ROBERTS
Hopefully, this article will maintain awareness of congenital conditions and provide a basis for answering questions, improving advice to clients, treating selected conditions, and offering a prognosis on ocular and animal function. Teratogenesis, a term often used instead of congenital, is defined as the in utero production of defects in offspring. Such defects can result from impaired cell proliferation, disrupted cell migration, and failure of cell differentiation, which cause disordered cell formation and development. A diverse group of problems, including genetic defects, extraneous teratogenic substances, infections, or trauma likely interact. 8 Extraneous teratogenic substances consist of drugs, toxins, vitamin deficiencies or excesses, and ionizing radiation. Idiopathic defects also occur and represent the largest etiologic group. The most common defects appear to be cataracts (::; 35.3%) and microphthalmia (::; 14.7%).8,21,25,37 Information from records of patients seen at the Colorado State University-Veterinary Teaching Hospital (CSU-VTH) between 1972 and 1991 indicated a similar prevalence, with cataracts representing 33.6% (38/113) and microphthalmia 7.1 % (8/113) of neonatal ocular diagnoses. Other common neonatal problems included retinal detachment and uveitis, 18.6% (21/113); nasolacrimal atresia, 8% (9/113); entropion, 7.1 % (8/113); persistent pupillary membrane, 7.1 % (8/113); microphthalmia, 7.1 % (8/113); congenital stationary night blindness, 3.5% (4/113); and retinal dysplasia, 3.5% (4/113) (Roberts SM: Unpublished data). Table 1 lists the congenital ocular conditions noted in the CSU-VTH records. Surprisingly, 5.3% of the diagnoses in horses with ocular problems were neonatal in nature. Table 2 presents a list of reported ocular defects of foals by associated breeds, proposed causes, essential features, and management methods. This article discusses neonatal ocular abnormalities, beginning with those affecting the external ocular structures and progressing deeper into the globe. Ocular colobomas are an exception, because they may involve several anatomic areas of the eye structures. Microphthalmia is discussed last because it impacts multiple ocular tissues.
DISORDERS Eyelid Positional Problems
Inversion of the eyelid margin, termed entropion, typically involves the lower eyelid. This problem is infrequent in horses, but occurs in neonates 9 , 21, 25 as either a congenital defect or early-acquired problem. The problem, possibly inherited in Thoroughbreds, 35 may involve multiple factors such as weakness of the tarsal plate, position of the globe, and spasm of the orbicularis oculi muscle. 36 Painful conditions of the conjunctiva and cornea frequently lead to or exacerbate entropion due to the pain-associated blepharospasm. Enophthalmia resulting from debilitation and weight loss or dehydration can produce entropion.
CONGENITAL OCULAR ANOMALIES
461
Table 1. CONGENITAL OCULAR DIAGNOSES* Prevalence Disorder Diagnosis Amaurosis Amblyopia Aniridia Cataract-Y suture Cataract-nuclear Cataract-hyaloid remnant Cataract-mature Coloboma-eyelid Coloboma-lens Coloboma-optic nerve Congenital stationary night blindness Entropion Dermoid Nasolacrimal atresia-nasal puncta Nasolacrimal atresia-palpebral puncta Microphthalmia Microphakia Persistent pupillary membranes Optic nerve hypoplasia Retinal detachment Retinal dysplasia Uveitis Total
Male
Female
Total
0 1 1 1
0 1 1 2 6 1 8 1 0 1 3
3 1 1 8 15 3 12 1 1 2
4
4
1
1
2 2 4
4
1 3 0
0 5 1 3 2 5 54
8 2 6 3 8 1 8 1 7 4 14 113
3 0 0 6 9 2
4
4 2 9 59
1
4
4
0/0
2.6 0.9 0.9 7.1 13.3 2.6 10.6 0.9 0.9 1.8 3.5 7.1 1.8 5.3 2.6 7.1 0.9 7.1 0.9 6.2 3.5 12.4
*Based on medical records at the Colorado State University-Veterinary Teaching Hospital, a total of 1183 of 22,352 (5.3%) equine cases were documented with at least one ocular abnormality, representing 2137 diagnoses of ocular disease. Of all diagnoses, 113 (5.3%) represented congenital anomalies.
Of the CSU-VTH cases, six instances of entropion occurred in debilitated foals. Three suffered from failure of passive transfer, two had ocular pain associated with septicemia and uveitis, and one had an intestinal impaction. Typically, clinical signs develop when facial hair contacts the cornea and conjunctiva, thereby stimulating the trigeminal nerve. The result is increased lacrimation, blepharospasm, conjunctivitis, and keratitis. Ulcerative keratitis, if present, directly threatens vision. Severe, vision-threatening corneal ulceration occurred in one of the CSU-VTH cases. Treatment, which varies with both severity and chronicity, includes manual repositioning of the lids several times daily, subcutaneous injection of procaine penicillin G, placement of temporary repositioning sutures in a mattress or Lembert pattern,21, 27, 44, 45 placement of temporary wound closure staples in the skin, or excision of skin and wound closure adjacent to the lid margin to redirect the eyelid margin's position. 9, 46 Mild degrees of entropion often correct spontaneously within a few weeks, especially if topical ointments are used. Topical antibiotic ointments used with the procedures just mentioned help prevent tissue infection and increase animal comfort. If treatm.ent involves injection of material into the entropic lid for repositioning,
t+:-
0"\
N
Table 2. NEONATAL OCULAR DISORDERS IN FOALS Disorder
Breeds
Microphthalmia May be associated with other ocular defects, e.g., multiple ocular defect syndrome cataracts, retinal detachments.
All breeds. Thoroughbreds have higher incidence.
Strabismus
Appaloosa.
Etiology Not established; most cases are idiopathic, some due to toxic, infectious, or nutritional episodes.
Essential Features and Management Small eyes, prominent third eyelid. No treatment; lid surgery for secondary entropion.
Rare. Strabismus usually convergent; some patients have compensatory head deviation. Surgery to straighten eyes has been described.
Eyelid coloboma May be associated with multiple occular anomalies.
Defects in eyelid margin. Surgery, depending on extent of other problems.
Ankyloblepharon congenita Partial fusion of upper and lower lids. Surgery. Entropion Primary. Secondary.
Nasolacrimal Atresia of nasolacrimal meatus.
Some patients have oversized palpebral fissures. Prematurity or illness.
Inturned eyelids, epiphora, corneal ulceration. Manual eversion of the eyelids, palpebral injections or horizontal mattress sutures or surgery to turn out the eyelids. Treatment of underlying cause, then surgery. Absence of nasal opening. Watery eye; some patients have mucopurulent discharge. Dacrocystorhinography may aid in diagnosis. Surgery to create new opening.
Misplaced punctum.
Surgery to enlarge punctum.
Atresia of punctum.
Surgery to create new punctum.
Cornea Microcornea. May be associated with microphthalmia syndromes.
Small cornea. No treatment.
Corneal melanosis.
Superficial axial nonprogressive pigment. Superficial keratectomy.
Dermoid. Probably most common congenital anomaly. Aniridia May be associated with cataracts.
Belgian draft horse.
Not known to be inherited.
Lateral or ventral corneal conjunctival masses containing hair and sebaceous glands. Surgical removal.
Autosomal recessive.
Complete absence of iris tissue.
Uveal Cysts May be associated with heterochromia iridis. Heterochromia Iridis Cataracts Some associated with microphthalmia syndromes.
Persistent Pupillary Membranes (PPM) May be associated with other ocular anomalies. ~
0"\
Pigmented cysts in anterior chamber or pupillary edge. Can be transilluminated. Aspiration if large and vision obstructed. Many color-dilute breeds.
Pigment failure in one iris or part of one iris.
Tapatum may be absent, and iris can be hypoplastic.
Arabian; Belgian draft; Morgan; Thoroughbred; Quarter Horse.
May be inherited.
Spontaneous resorption can offer useful vision in stud animals. Consider surgery if the cataracts interfere with vision.
Other cataracts can be due to trauma (prenatal and foaling) or poor nutrition, or metabolic or toxic causes.
Note: Need to differentiate from prominent posterior Y sutures, which are present in the majority of foals.
Strands of iris tissue that can run to iris, cornea (opacity), or lens (cataracts). PPM may continue to atrophy during the first year of life.
W
Table continued on following page
~
0'1
~
Table 2. NEONATAL OCULAR DISORDERS IN FOALS Continued Disorder
Breeds
Etiology
Vitreous Remnants of hyaloid tissue.
Remnants of hyaloid tissues visible behind the lens. These are present in most foals and will atrophy by 6-9 months. Uncommon. Fibrovascular membrane behind the lens.
Persistent hyperplastic primary vitreous. Retina Retinal hemorrhages.
Retinal dysplasia; may be associated with other defects.
Mostly incidental. Difficult birth. Thoroughbred may have higher incidence.
Bilateral retinal disorganization; may also have retinal detachment.
Retinal detachment; may associate with dysplasia.
Complete retinal detachment.
Coloboma of the fundus.
Chorioretinal defect.
Retinal aplasia. Stationary night blindness.
Appaloosa and other breeds.
Possibly autosomal recessive.
Absence of the retina.
Autosomal recessive defect in neuroretinal transmission.
No lesion visible on ophthalmoscopic examination. Some patients have microphthalmia or dorsal strabismus/ nystagmus.
Optic Nerve Optic nerve hypoplasia (congenital optic atrophy); may be associated with other defects.
Papilledema.
Essential Features and Management
Small optic nerve heads either unilateral or bilateral. Slow or absent pupillary light reflex. Searching nystagmus. Maladjustment.
Swelling of optic nerve associated with cerebral edema.
From Slatter D: Fundamentals of Veterinary Ophthalmology. Philadelphia, WB Saunders, pp 554,555, 1990; with permission.
CONGENITAL OCULAR ANOMALIES
465
do not inject potentially irritating materials such as mineral oil or paraffin. 7 To do so risks a severe foreign body reaction that could result in serious lid conformation abnormalities and corneal irritation. If questions arise relating to the usefulness of a horse that has or has had entropion, it should be considered a blemish, not an unsoundness, unless keratitis has caused loss of vision. Visual loss often requires a finding of unsoundness. The CSU-VTH records indicated three cases of ectropion, eversion of the eyelid margin. Two were due to trauma and one to overzealous surgical correction of entropion. Temporary procedures are preferred for entropion correction in neonates. When treatment involves surgical excision of skin, plan carefully to avoid overcorrection. Cases of ectropion, regardless of their cause, require blepharoplastic procedures to correct the problem. Ectropion surgical correction results in slight deformity of the eyelid. Refer to the article by Moore for additional information. Nasolacrimal Atresia
Congenital anomalies of the nasolacrimal excretory system reported in the horse include atresia of the eyelid punctum, atresia of the nasal meatus punctum, and ectopic eyelid puncta. The most common anomaly is atresia of the nasal meatus puncta. 9, 18, 23, 24, 27, 29, 45 Such was the case in six of nine CSU-VTH cases. Atresia involved the proximal bony portion in two horses and one had atresia of the bony portion and canaliculi. To date, there is no evidence to support an inherited basis in the horse. In humans, congenital atresia of the nasolacrimal duct is common and potentially familial. 10 The lacrimal passages are formed in humans by epithelial cords beneath the cleft between the nasal and maxillary processes. Canalization begins by the end of the first trimester of gestation. Communication exists between the eye and nose by the end of the second trimester. Patency of the canaliculi lumina develops during midgestation. The eyelid puncta open onto the eyelid margins early during the third trimester of gestation. A mucosal membrane often covers the nasal meatus puncta at birth. 34 This developmental process likely is quite similar in the horse. Clinical signs of atresia of the nasolacrimal system consist of epiphora and mucopurulent ocular discharge, the latter secondary to dacryocystitis. 23, 30 The onset of symptomatic epiphora sometimes is delayed until horses are several months of age. Explanations of this phenomenon include a lower tear volume in neonates, distention of the existing nasolacrimal duct portion,27 and transmucosal absorption of tears across the duct wall. 23 Differentiation of nasolacrimal obstructions from chronic conjunctivitis hinges on the quantity of ocular discharge. Profuse discharge occurs with nasolacrimal obstructions, especially when the palpebral puncta are flushed (Fig. 1).
466
ROBERTS
Figure 1. Atresia of the nasal punctum has resulted in chronic accumulation of secretions in the nasolacrimal system. Epiphora and spillage of some mucoid discharge onto the face had been present since 1 month of age. Upon flushing saline into the upper palpebral punctum, mucopurulent material exited from the lower palpebral punctum.
Clinical diagnosis is accomplished by (1) irrigation of the proximal nasolacrimal duct with saline solution through the palpebral puncta, (2) examination and palpation of the floor of the nasal vestibule for a distended structure, and (3) radiographic contrast studies. 22- 24, 35 Irrigation procedures result in abnormal material refluxing from a punctum, a distended structure on the floor of the nasal vestibule that changes in size with irrigation, and a reduced amount or lack of irrigant exiting the nasal meatus punctum. Collection of reflux material for bacterial culture and antibiotic sensitivity testing is desirable because of the secondary dacryocystitis usually present. Treatment requires surgical placement of a retention tube for 3 to 8 weeks to allow epithelization of the opening and resolution of the dacryocystitis. 18, 23, 24, 27, 29 A flexible catheter such as a male urinary catheter is passed through the nasolacrimal duct from the upper palpebral punctum. The tip of the catheter is usually palpable slightly proximal to the expected location of the nasal punctum. The nasal mucosa is cut with a scalpel blade, the catheter tip grasped with a hemostat, and the catheter pulled through the new punctal opening and sutured to skin within the nostril or, after passing the catheter through the lateral false nostril wall to the skin surface, secured with sutures. Hemorrhage from the extensive vascular supply of the ventral nasal concha may require control by direct pressure, local application of a-adrenergic agents, or local application of cocaine. Following retention tube placement, the nasolacrimal system is flushed with saline or 1:20 povidone-iodine solution in saline. After surgery, topical antibiotic or antibiotic-corticosteroid preparations and systemic antibiotics should
CONGENITAL OCULAR ANOMALIES
467
be used for several weeks. Surgery is successful in most instances. Refer to the article by Moore for additional information. Although rare in comparison to nasal meatus punctal problems, atresia or ectopic placement of the eyelid puncta also occurs. If epiphora results from either condition, surgical intervention to open, slightly enlarge, or move the punctum is possible. If atresia or agenesis cannot be corrected, creation of an alternative nasolacrimal outflow pathway or dacryoconjunctivorhinostomy is possible. All nasolacrimal anomalies represent an unsoundness until corrected surgically. Dermoid
The term dermoid means resembling skin. Dermoids are choristomas and are defined as normal tissue elements located at an abnormal site. They represent a congenital tumor of skin and related appendage tissues. External ocular involvement may include the cornea, conjunctiva, nictitating membrane, or eyelids. Most commonly, the temporal limbus and neighboring cornea and sclera show superficial involvement. 27, 32, 35, 43, 50 Deeper tissue involvement is possible and epibulbar dermoids in humans reportedly can involve the full corneal thickness and intraocular structures. lO No genetic association exists in horses. Some dermoids have hair on their surface, causing mechanical irritation of the cornea. The degree of visual impairment depends on the extent of corneal involvement and the occurrence or absence of disease secondary to corneal problems. Large dermoids may extensively involve the globe and adnexa, causing corneal opacity, nictitans deformity, eyelid disfigurement, and nasolacrimal punctal occlusion. Removal requires a superficial keratectomy, although small lesions may go untreated. 27 In most instances, complete removal without perforation of the globe is possible. The corneal stroma beneath the lesion typically is transparent (Fig. 2). Treatment of superficial keratectomy defects is identical to conventional ulcer management (see the article in this issue on ulcerative keratitis by Nassise). All wounds require topical treatment and some require corneal mechanical support, with conjunctival flap grafts, nictitans flaps, collagen shields, or tarsorrhaphies. Some degree of mild corneal opacity remains following surgical removal. Uveal Defects
Aniridia means absence of the iris. Iris hypoplasia, in which a rudimentary iris bud is present, is included within the scope of the definition. True aniridia, the total absence of iris, is rare in all species. The anomaly arises from a third wave mesenchyme failure of ingrowth and disorderly differentiation. The first and second mesenchyme waves, which form the corneal endothelium and stroma, are rarely involved in congenital defects. The major flaw appears to be a maldevelopment
468
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Figure 2. A large corneal dermoid involving 60% of the corneal surface has been removed and the conjunctiva sutured to the limbus. Notice the transparent nature of the corneal stroma at the site of excision. Approximately two thirds of the cornea was removed to achieve complete lesion removal. This defect was managed with a subpalpebral medication tube and was negative to fluorescein in 5 days.
of capsulopupillary vessels of the tunica vasculosa lentis, thus preventing growth of the iris. lO A single family of Belgian draft horses with aniridia that had an autosomal dominant pattern of inheritance has been reported. 12 More recent reports document sporadic aniridia in a Quarter Horse stallion20 and a Welsh-Thoroughbred filly. 19 Aniridia is usually bilateral and frequently is confused with an abnormally large pupil. In distinction from simple mydriasis, aniridia allows visualization of the lens equator and ciliary processes. Associated clinical signs are common and include blepharospasm, cataract formation, 12, 19 ocular discharge, perilimbal keratitis,20 photophobia, and reduced vision. There are no specific treatments for aniridia except symptomatic supportive measures. Protective masks may help reduce ambient illumination and increase patient comfort. Vision is degraded and affected horses are unsound. Iris cysts involving the posterior pigmented epithelium and protruding through a thinned iris stroma occur in horses with blue irides. 9 One report41 mentioned a similar abnormality in older horses that was effectively treated by surgical removal of the anterior cyst wall in two individuals. Treatment is not necessary if there are no clinical signs and the cyst does not increase in size. Iris cysts transilluminate, appearing hollow upon examination. If confusion between an iris cyst and melanoma occurs, histopathology will determine the exact lesion nature. Persistent pupillary membrane problems occasionally are reported. 2, 25, 27, 50 Figure 3 illustrates several manifestations of this anomaly. Strands that radiate from iris to iris are most common and cause no apparent problems. More serious are strands that radiate from iris to lens or iris to cornea. Persistent pupillary membranes that touch the lens leave pigment deposits on the anterior lens capsule or cause anterior capsular cataract. If strands touch the corneal endothelium,
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Figure 3. This yearling was examined because of the presence of corneal leukoma since birth. Several persistent pupillary membranes are present ventrally and extend to the cornea, making contact with the endothelial cell. This form of leukoma is referred to as adherent leukoma. Notice the two foci of pigment on the anterior lens capsule, indicating persistent pupillary membrane contact with the lens capsule (arrows). Subtle persistent pupillary membrane strands are present near the nasal edge of the pupil.
leukoma from endothelial cell disturbance occurs. Such a leukoma will be fluorescein negative because the epithelial surface is not involved. Surgical removal of persistent pupillary membranes is usually not recommended. There is no known inherited tendency in the horse and the anomaly should be considered a blemish if strands radiate from iris to iris. If strands radiate from iris to lens or cornea, the horse should be considered unsound.
Coloboma
This term means mutilation and was first coined in 1821 10 to describe a missing part of an ocular structure, resulting from a failure of the optic fissure to close. Colobomas are either "typical" (i.e., positioned along the line of closure of the optic fissure) or "atypical" (i.e., positioned outside the line of closure). Ocular structures involved include the eyelids,22, 27 iris,22, 35 ciliary body, lens,2, 22, 27, 35 retinachoroid,45 optic disc,27 and optic nerve. Figure 4 depicts an optic nerve coloboma and retinal detachment in a Quarter Horse foal. All forms of coloboma are rare in the horse, but when they occur, they frequently are associated with concomitant ocular disorders. Although the cause of colobomas usually are unknown, typical optic nerve and scleral colobomas result from abnormal eversion of the neurosensory retina through the embryonic fissure. lO This theory, supported by work in the Collie dog, suggests that defective neurosensory retinal differentiation is responsible for optic disc colobomal formation. 26 Eyelid coloboma or agenesis, although reported in the horse, has not been described in detail. 37 Eyelid colobomas do not result from
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Figure 4. A large coloboma of the optic nerve is shown in a Quarter Horse filly. The superior portion of the retina was detached, and several retinal folds have resulted (arrow).
embryonic fissure abnormalities. Experimental work suggests that ischemia to a rapidly developing embryonic eyelid may cause infarction of the lid farthest from the blood supply, 34 thus resulting in a focal defect. Congenital, atypical colobomas have been reported, involving the retina and choroid of the horse. 42 Repairs of eyelid colobomas require blepharoplastic surgery. In some situations, the repair process requires several stages. If the defect is minor, complex repair is unnecessary and simple cryoepilation of eyelashes or facial hair contributing to corneal irritation provides symptomatic relief. Surgery must correct exposure keratopathy and other associated problems, if present. Figure 5 illustrates atresia that involved
Figure 5. Atresia of the eyelids frequently causes corneal irritation or ulceration. This case had mild atresia of the nasal aspect of the upper lid (arrow), yet suffered severe corneal ulceration. The atresia also involved the nasal canthus causing malfunction of the palpebral puncta. Blepharoplastic repair recreated a mucocutaneous junction by splitting the eyelid into cutaneous and conjunctival flaps. The conjunctival portion was advanced and sutured to the cutaneous flap. In addition, the palpebral puncta were cannulated and reconstructed. Following surgery, recovery was uneventful and no further problems were noted for 1.5 years; then, the horse was lost to follow-up.
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the nasal aspect of the upper eyelid. Corneal ulceration secondary to mechanical irritation resulted in corneal scarring and vascularization as sequelae. Defects located within the globe are not amenable to surgical correction. A coloboma that directly or indirectly causes visual loss renders the horse unsound, unless surgical correction is possible.
Cataract
The normal lens is biconvex and suspended behind the pupil. It is transparent, although it appears slightly bluish-grey when viewed along an examining light beam pathway. The lens is unique in that it represents surface ectoderm sequestered within the globe and enclosed by a basement membrane. Lens epithelial cells and fibers thus retained throughout life are subject to insult. They react to insult by cell death, abnormal cell proliferation, and loss of osmotic homeostasis. The result is vacuolation and loss of transparency, termed cataract formation. A cataract may partially or totally involve the lens cortex, nucleus, or capsule and formation typically is not uniform throughout the lens. One part of the lens therefore may show partial opacity, whereas another part is totally opaque. Complete congenital cataracts are totally opaque and are termed mature cataracts. The term immature cataract describes partially opaque lenses. Depending on the cataract's severity or degree of maturity, slight vision impairment or total blindness results. Regardless of the degree of involvement, a cataract represents an unsoundness. Congenital cataracts are the most common neonatal ocular abnormality and may be the most common cause of blindness or vision impairment in foals. Owners and handlers frequently complain that the affected foal displays visual deficits, clumsiness, and tendency toward repeated traumatic injuries. Aniridia, microphthalmia, and other defects frequently accompany congenital cataracts. 11 - 13, 19-21, 33 In most instances, the cause of congenital cataracts remains unknown. Potential causes include faulty in utero nutrition, genetic factors, ocular inflammation occurring in utero, radiation, and prenatal trauma. 38 Heredity rarely has been documented in the horse. 3 , 12,49 A congenital cataract, by definition, is present at birth, having developed during fetal life. Most are bilateral. Four basic types of congenital cataracts described in horses include (1) completely mature cataract, (2) nuclear cataract, (3) Y suture-type cataract, and (4) cataract associated with persistent hyaloid vasculature. 9, 27 The CSU-VTH medical record database documented 38 diagnoses of congenital cataract in horses (Roberts SM: Unpublished data). The distribution by type included 15 nuclear, 12 mature, 8 Y suture, and 3 hyaloid-related cataracts. Breeds included American Saddlebred (n = 3), Arabian (n = 9), Quarter Horse (n = 24), Appaloosa (n = 1), and Thoroughbred (n = 1). In these cases, two of three cataracts secondary to a persistent hyaloid artery and all Y suture-type cataracts did not seriously impair vision. Complete cortical or nuclear congenital cataracts were more
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serious, with a potential for visual loss. Some nuclear cataracts also had extensive cortical involvement. Figure 6 demonstrates a nuclear cataract involving the fetal nucleus. In seven cases, nuclear and Y suture cataract changes were noted in the lenses. In addition to the data just summarized, reports of heritable, congenital cataracts exist for the Belgian,12 Morgan, 3 and Thoroughbred45 breeds. Cataracts in the Morgan horse typically are bilateral, nonprogressive, and nuclear, causing minimal visual impairment. Noninherited nuclear cataracts also are reported in Morgan horses. 4 The genesis of congenital cataracts is not simply a matter of genetic or environmental factors, but involves complex developmental issues. Traditionally, descriptions of lens development include four stages: (1) formation of the lens plate, (2) formation of the lens vesicle, (3) formation of the primary lens fibers, and (4) formation of the secondary lens fibers. 34 During early fetal development, the primordial neural retina induces lens formation. The resulting lens placode undergoes cellular division to form the lens cup, with an initial hollow lumen, which becomes sealed off and surrounded by the epithelial basement membrane. This arrangement prevents other cell types from mixing with the lens cells and limits permeability of the lens. Extrinsic factors transforming the lens vesicle into a mature lens include the proximity of a healthy, developing neural retina 34 and proper conformation between the developing lens and optic cup rim. Proper lens development involves elongation of posterior epithelial cells to form primary lens fibers that obliterate the lens vesicle lumen. Division, equatorial migration, and elongation of the anterior lens epithelial cells produce the secondary lens fibers. The result is crowding of primary lens fibers centrally to form the fetal nucleus. The unique lens organization of cells surrounded and sequestered by the epithelial basement membrane
Figure 6. A dense cataract involving the fetal portion of the nucleus of this Quarter Horse weanling. This cataract is small enough that no serious vision impairment resulted. As the eye and lens grew, the cataract remained the same absolute size, resulting in a relative decrease in cataract size as normal lens material was formed around the lesion.
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prevents exfoliation of secondary lens cells. Thus, unlike the surface ectoderm of skin, the lens retains all cellular material throughout the animal's life. It is little wonder that cataracts frequently form and is surprising they are not more common. The congenital nuclear cataracts noted often in horses are a lamellar type, involving that portion of the lens immediately external to the fetal nucleus, which represents the inner part of the adult nucleus (Fig. 7). Such a circumscribed zone of opacity occurs as a cataractogenic influence acts on newly formed lens fibers during a precise time. The stage of lens formation during the cataractogenic influence determines the location and extent of the opacity. Lens fibers developing before or after the cataractogenic influences are transparent. Some horses exhibiting moderate to severe visual impairment soon after birth therefore undergo clinical visual improvement as normal lens fibers form around the cataractous fibers. This translates into a decrease in the percentage of cataract involvement as lens growth continues. Occasionally, however, adjacent normal lens fibers opacify over time, leading to cataract progression and further visual impairment. Currently, surgical removal is the only treatment for visual loss due to congenital cataract. The article by McLaughlin in this issue deals specifically with cataract surgery in horses. Before scheduling cataract surgery in a foal, the possibility of deprivation amblyopia requires evaluation. Deprivation amblyopia results from inadequate visual stimulation to the central nervous system, producing irreversible functional and structural abnormalities in the lateral geniculate nuclei and visual cortex. 45 Foals with a searching, wandering-type nystagmus are suspect of having deprivation amblyopia. In this situation, successful cataract removal may offer little improvement in actual visual function. Congenital Stationary Night Blindness
Congenital stationary night blindness, an incompletely understood condition reported in Appaloosa horses of the United States, 39, 51-53 occasionally affects other breeds. 51 Visual impairment ranges from poor vision during reduced light conditions to poor vision during the day
Figure 7. A lamellar cataract involving the junction between the fetal and adult nucleus. The posterior surface of the nucleus was involved and the entire nucleus displaces slightly posterior, a condition referred to as lenticonus interna. This horse had clinically normal vision from the affected eye.
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and blindness at night. Proposed inheritance modes include autosomal recessive 51 or sex-linked recessive, with the defect X chromosome. 52 The CSU-VTH data do not support a sex-linked recessive mode of inheritance, given that three of four affected Appaloosa horses were female (see Table 1). A history of poor vision, behavior associated with poor vision, and a normal ocular fundus allows presumptive diagnoSiS. 39,51-53 Other clinical signs include subtle microphthalmia, holding the head in a "stargazing" position when visualizing objects, seeking lighted conditions, and displaying a dorsomedial strabismus. Definitive diagnosis by electroretinography shows a characteristic large negative waveform. The disorder may result from abnormal neural transmission between the photoreceptor and inner nuclear layers of the retina. No treatment is available and the condition is nonprogressive. Affected horses are unsound and unsuitable for breeding. Equestrian use of even mildly affected horses requires the riders' clear understanding of the degree of visual impairment. Medico-legal issues should be addressed by issuing a written statement, signed by the involved parties, explaining the degree of visual impairment and any recommended restrictions in the use of the horse.
Microphthalmia
The normal globe fills the orbit, thereby providing a curved surface for the eyelids to move across and creating a normal palpebral fissure shape. Microphthalmia or nanophthalmia represents one of the more common congenital ocular anomalies and ranges from minimal to severe reductions in globe size. Severe microphthalmia should not be confused with anophthalmos or total absence of a globe. True anophthalmia is very rare and most suspected cases are, in reality, microphthalmia. Microphthalmia may be unilateral or bilateral and affected foals display a small palpebral fissure and protruded nictitating membrane. The associated abnormal anatomy of the globe, orbit, and eyelid results in poor nasolacrimal drainage and subsequent ocular discharge. Secondary conjunctivitis from environmental irritation and infection frequently is seen. Orbital and periocular asymmetry may become more pronounced as the foal grows because the orbit fails to grow properly. The affected globe is usually blind and corneal opacity due to leukoma or pigmentation is common. Cataracts are common and were present in five of eight CSU-VTH microphthalmia cases. A corneal dermoid occurred in one of the CSU-VTH microphthalmia cases. Microphthalmia is due to defective organogenesis. Complex and closely interrelated factors determine the final globe size. Embryologically, the primary optic vesicle buds from the forebrain and differentiates into the eye. Retarded growth of the optic vesicle and associated degeneration or atrophy lead to microphthalmia. Three classifications of microphthalmia include (1) pure microphthalmia (nanophthalmos), with a small but otherwise normal eye; (2) colobomatous microphthal-
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mia, with a defect resulting from failure of the optic vesicle to involute or the embryonic fissure to close; and (3) complicated microphthalmia, in which anomalous development occurs independent of fissure closure. 10 The severity of microphthalmia depends on the gestational stage during which the insult occurs: It is most severe when insult occurs early in gestation, at the time of optic vesicle and lens formation. 1 If the optic vesicle tip contacts surface ectoderm over less than the normal area, a perfectly formed microphthalmic eye results. 6 In humans, defective closure of the embryonic fissure appears to cause most microphthalmia. 34 Reports of microphthalmia in the horse 2, 8, 11, 13, 21, 33, 37, 43, 46, 50 suggest optic vesicle degeneration is a common cause, because affected globes contain intraocular tissue remnants showing wide morphologic variation, faulty formation, and disorganization. Colobomatous microphthalmia perhaps best describes this condition. Although the orbit forms, volume reduction is evident and abnormal globe formation leads to hypoplastic or atrophic optic nerves. Most equine cases are sporadic, with an unknown cause. There is no genetic association in horses. A case of equine microphthalmia associated with sulfadimethoxine administration to the mare during week six of gestation 11 raises the question of a teratogenic effect similar to that seen in humans. Although microphthalmia is not correctable, secondary problems may be amenable to treatment. For unilateral cases, enucleation is helpful in preventing secondary problems associated with abnormal ocular discharge. Entropion, developing from poor eyelid conformation or painful ocular conditions, requires surgical correction. Because the u~derlying problem remains, however, entropion frequently recurs. Most foals with unilateral involvement adapt to their monocular state with maturity. Bilateral involvement, causing blindness or near blindness, may necessitate destruction of the animal. Microphthalmia represents a serious handicap to horses. Some may be functional depending on their intended use, but microphthalmia represents an unsoundness in all cases.
Rare Conditions
In addition to the conditions already discussed, a wide range of rare conditions include glaucoma,14, 22 lens luxation,13, 31, 50 lenticonus,27, 50 lentiglobus,27, 50 microphakia,27, 50 optic nerve hypoplasia (Fig. 8) and atrophy, 17, 50 retinal cysts,41 retinal dysplasia, 14, 27,35,50 retinal detachment (Fig. 9),27, 35, 40 and strabismus. 16, 27 Although not strictly viewed as a congenital problem, distichiasis has been observed at the C5U-VTH in young horses. In some situations, congenital anomalies within the eyelid tarsal glands ultimately result in aberrant hair growth and some instances of distichiasis represent ectopic locations of hair follicles at the lid margins. Thus, some cases may be congenital anomalies. Retinal cysts, mild retinal dysplasia, and strabismus do not interfere seriously with vision. Such minor conditions represent a blemish rather
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Figure 8. This weanling Quarter Horse filly was examined for unilateral blindness. Examination detected a hypoplastic optic nerve and peripapillary pigment dispersion of the retinal pigment epithelium. Notice the reduced size of the optic disc. The disc was recessed below the level of the sclera. Histologically, the optic nerve and ganglion cell layer were hypoplastic and the peripapillary retina dysplastic. The cause of the changes was unknown.
than an actual unsoundness. Conditions directly threatening vision represent unsoundness. Four cases of retinal dysplasia in the CSUVTH data displayed severe bilateral involvement, with either poor vision or no vision. Two had cataracts and one had retinal detachments and lens luxations. SUMMARY
This discussion provides an idea of the diversity and relative prevalence of certain congenital ocular conditions of horses. Many are not difficult to diagnose, yet curative treatment may be impossible. When dealing with owners of horses affected with unusual anomalies,
Figure 9. A complete retinal detachment is shown with the retina still attached to the optic disc. This foal had been blind since birth. No cause for the congenital detachment could be found.
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responsible client service requires veterinarians to provide accurate information and to know where answers to unusual questions can be found. Again, most veterinarians never encounter all of the diverse congenital defects. As a result, the horse owner frequently receives misinformation. Hopefully, this brief coverage of congenital ocular anomalies will provide useful information and assist in appropriate communication to concerned parties.
References 1. Apple OJ, Naumann GOH: Malformations and anomalies of the eye. In Naumann GOH, Apple OJ (eds): Pathology of the Eye. New York, Springer-Verlag, 1986, p 63 2. Barnett KC: The eye of the newborn foal. J Reprod Fertil (suppl) 23:701, 1975 3. Beech J, Aquirre G, Gross S: Congenital nuclear cataracts in the Morgan horse. J Am Vet Med Assoc 184:1363, 1984 4. Beech J, Irby N: Inherited nuclear cataracts in the Morgan horse. J Hered 76:371, 1985 5. Bistner SI, Rubin LF, Saunders LZ: Ocular lesions of bovine viral diarrhea-mucosal disease. Pathologica Veterinaria 7:275, 1970 6. Coulombre AJ: Experimental embryology of the vertebrate eye. Invest Ophthalmol Vis Sci 4:411, 1965 7. Craven JR: Significance of lesions of the cornea and lens in the examination of horses for soundness. Equine Vet J 3:141, 1971 8. Crowe MW, Swerczek TW: Equine congenital defects. Am J Vet Res 46:353, 1985 9. Davidson MG: Equine ophthalmology. In Gelatt KN (ed): Veterinary Ophthalmology, ed 2. Philadelphia, Lea & Febiger, 1991, p 576 10. Duke-Elder S: Normal and abnormal development. In Duke-Elder System of Ophthalmology, vol III, part 2. St Louis, CV Mosby, 1963, pp 456, 458-459, 571 11. Dziezyc J, Kern TJ, Wolf ED: Microphthalmia in a foal. Equine Vet J 15:15, 1983 12. Ericksson K: Heriditary aniridia with secondary cataract in horses. Nord Vet Med 7:773, 1955 13. Garner A, Griffiths P: Bilateral congenital ocular defects in a foal. Br J Ophthalmol 53:513, 1969 14. Gelatt KN: Ophthalmoscopic studies in the normal and diseased ocular fundi of horses. J Am Anim Hosp Assoc 7:158, 1971 15. Gelatt KN: Glaucoma and lens luxation in a foal. Vet Med Small Anim Clin 68:261, 1973 16. Gelatt KN, McClure JR: Congenital strabismus and its correction in two Appaloosa horses. J Equine Med Surg 1(13):240, 1977 17. Gelatt KN, Leipold HW, Coffman JR: Bilateral optic nerve hypoplasia in a colt. J Am Vet Med Assoc 155:627, 1969 18. Hjorth P: Atresia of the nasolacrimal duct in a horse. Nord Vet Med 23:260, 1971 19. Irby NL, Aquirre GO: Congenital aniridia in a pony. J Am Vet Med Assoc 186:281, 1985 20. Joyce JR: Aniridia in a Quarter Horse. Equine Vet J 15:21, 1983 21. Koch SA, Cowles RR Jr, Schmidt GR, et al: Ocular disease in the newborn horse: A preliminary report. J Equine Surg 2(4):167, 1978 22. Latimer CA, Wyman M: Atresia of the nasolacrimal duct in three horses. J Am Vet Med Assoc 184:989, 1984 23. Latimer CA, Wyman M: Neonatal ophthalmology. Vet Clin North Am Equine Pract 1(1):235, 1985 24. Latimer CA, Wyman M, Diesem CD, et al: Radiographic and gross anatomy of the nasolacrimal duct of the horse. Am J Vet Res 45:451, 1984 25. Latimer CA, Wyman M, Hamilton J: An ophthalmic survey of the neonatal horse. Equine Vet J (suppl 2) 2:9, 1983 26. Latshaw WK, Wyman M, Venzke WG: Embryonic development of an anomaly of the ocular fundus in Collies. Am J Vet Res 30:211, 1969 27. Lavach JD: Large Animal Ophthalmology. St Louis, CV Mosby, 1990
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28. Leipold HW, Saperstein G, Woollen NE: Congenital defects in foals. In Smith BP (ed): Large Animal Internal Medicine. St Louis, CV Mosby, 1990, P 1576 29. Lundvall RL, Carter JD: Atresia of the nasolacrimal meatus in the horse. J Am Vet Med Assoc 159:289, 1971 30. Mason TA: Atresia of the nasolacrimal orifice in two Thoroughbreds. Equine Vet J 11:19, 1979 31. Mathews AG, Handscombe MC: Bilateral cataract formation and subluxation of the lenses in a foal: A case report. Equine Vet J (suppl 2) 2:23, 1983 32. McLaughlin SA, Brightman AH: Bilateral ocular dermoids in a colt. Equine Pract 5:10, 1983 33. Mosier DA, Engleman RW, Confer AW, et al: Bilateral multiple congenital ocular defects in Quarter Horse foals. Equine Vet J 15:18, 1983 34. Mullaney J: Normal development and developmental anomalies of the eye. In Gamer A, Klintworth GK (eds): Pathobiology of ocular disease. New York, Marcel Dekker, 1982 35. Munroe GA, Barnett KC: Congenital ocular disease in the foal. Vet Clin North Am Large Anim Pract 6(3):519, 1984 36. Peiffer RL Jr, Williams R, Schenk M: Correction of congenital entropion in a foal. Vet Med Small Anim Clin 72:1219, 1977 37. Priester WA: Congenital ocular defects in cattle, horses, cats, and dogs. J Am Vet Med Assoc 160:1504, 1972 . 38. Rathbun WB: Biochemistry of the lens and cataractogenesis: Current concepts. Vet Clin North Am Small Anim Pract 10(2):377, 1980 39. Rebhun WC: Equine retinal lesions and retinal detachments. Equine Vet J (suppl 2) 2:86, 1983 40. Rebhun WC, Loew ER, Riis RC, et al: Clinical manifestations of night blindness in the Appaloosa horse. Compend Contin Educ Pract Vet 6(2):S103, 1984 41. Rubin LF: Cysts of the equine iris. J Am Vet Med Assoc 149:151, 1966 42. Rubin LF: Atlas of Veterinary Ophthalmoscopy. Philadelphia, Lea & Febiger, 1974, p 296 43. Saunders LZ, Rubin LF: Ophthalmic Pathology of Animals. New York, S Karger, 1975, pp 180, 184 44. Severin GA: Veterinary Ophthalmology Notes, ed 2. Ft Collins, Colorado State University, 1976, p 94 45. Slatter DH: Fundamentals of Veterinary Ophthalmology. Philadelphia, WB Saunders, 1990, pp 164, 241, 376, 379, 468 46. Trapp CW: Congenital maldevelopment of the eyes of a colt. Cornell Vet 47:467, 1957 47. Vestre WA, Brightman AH: Correction of cicatricial entropion and trichiasis in the horse. Equine Pract 2:13, 1980 48. Walde I: Some observations on congenital cataracts in the horse. Equine Vet J 15:27, 1983 49. Weber W: Hereditary cataract, a recessive character in the horse. Schweiz Arch Teirheilk 89:397, 1947 50. Wilcock BP: Ocular anomalies. In Peiffer RL (ed): Comparative Ophthalmic Pathology. Springfield, IL, CC Thomas, 1983, p 3 51. Witzel DA, Joyce JR, Smith EL: Electroretinography of congenital night blindness in an Appaloosa filly. J Equine Med Surg 1:226, 1977 52. Witzel DA, Riis RC, Rebhun WC, et al: Night blindness in the Appaloosa: sibling occurrence. J Equine Med Surg 1:383, 1977 53. Witzel DA, Smith RD, Wilson RD, et al: Congenital stationary night blindness: an animal model. Invest Ophthalmol Vis Sci 17:788, 1978
Address reprint requests to Steven M. Roberts, DVM, MS Veterinary Teaching Hospital Colorado State University College of Veterinary Medicine and Biomedical Sciences 300 West Drake Fort Collins, CO 80523
