Diagnostic Ophthalmology Ophtalmologie diagnostique Lynne S. Sandmeyer, Bianca S. Bauer, Bruce H. Grahn
A
History and clinical signs
3-year-old, castrated male American Paint horse was examined at the ophthalmology service at the Western College of Veterinary Medicine for evaluation of a mass within the right iris. The menace responses, palpebral, and occulocephalic reflexes were present bilaterally. The direct and consensual pupillary light reflexes were present bilaterally. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were 19 mm/min and 15 mm/min in the right and left eye, respectively. The intraocular pressures were estimated with a rebound tonometer (Tonvet; Tiolat, Helsinki, Finland) and were 25 mmHg and 23 mmHg in the right and left eye, respectively. Results from fluorescein staining (Fluorets; Bausch & Lomb Canada, Markham, Ontario) were negative bilaterally. Distant examination showed that the right iris was blue, while the left iris was brown. On direct examination, the dorsal aspect of the right iris appeared to bulge forward and had a corrugated appearance. Biomicroscopic examination (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) revealed the dorsal stroma of the iris to be very thin and transillumination through this area was possible. The right and left irides dilated fully following application of 0.5% tropicamide (Mydriacyl; Alcon, Canada) and the protruding area became less prominent. Indirect ophthalmoscopic examination (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) following dilation was completed and no abnormalities were noted. A photograph of the right eye prior to dilation is provided for your assessment (Figure 1).
What are your clinical diagnosis, differential diagnoses, therapeutic plan, and prognosis? Discussion Our clinical diagnosis was iris hypoplasia of the right eye. The protruding iris region represents an area of reduced development of the iris stroma. This is a non-progressive, congenital condition which is most commonly noted in the dorsal area of a blue iris (1). It may be unilateral or bilateral and there is no known age, gender, or breed predilection for this condition (1,2). Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4. Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office ([email protected]) for additional copies or permission to use this material elsewhere. 514
Figure 1. Photograph of the right eye of a 3-year-old American Paint horse.
Structurally, the normal iris has a superficial anterior border layer and stroma, derived from neural crest mesenchyme, and a bilayered posterior epithelium derived from neuroectoderm of the optic cup; the anterior epithelial layer includes the dilator muscle, and the posterior pigmented epithelium lines the deep surface. The smooth iris sphincter muscle within the stroma is also derived from neuroectoderm (3). Histologic findings in iris hypoplasia include a thin iris stroma with a poorly developed dilator muscle, and somewhat less developed constrictor muscle (2). It is likely that there is embryonic failure of induction of differentiation of the iris stroma and this may be related to defective growth of the neuroepithelium at the rim of the optic cup (4). The hypoplasia seen in this case was relatively mild; however, extreme examples of iris hypoplasia would include coloboma, or anirida (4). The clinical manifestations of iris hypoplasia include a darkened area of iris stroma that bulges forward. The area bulges forward due to the relative lack of resistance of this portion of the iris to aqueous humor flow from the posterior to anterior chamber. This resistance is increased when the pupil is constricted due to contact of the iris with the anterior lens. The thin, hypoplastic area of iris is, thus, pushed forward by the aqueous humor (2). The abnormality becomes less noticeable when the pupil is dilated as there is peripheral compression of the iris and less contact of the iris with the anterior lens leading to facilitation of aqueous humor flow (1). The main differential diagnoses for a darkened, bulging portion of iris is an iris cyst or uveal neoplasia, with iris melanoma being the most common primary intraocular neoplasm (4,5). Iris hypoplasia is occasionally and historically misinterpreted as a uveal cyst (6). CVJ / VOL 54 / MAY 2013
CVJ / VOL 54 / MAY 2013
References 1. Hollingsworth SR. Diseases of the anterior uvea. In: Gilger BC, ed. Equine Ophthalmology. St. Louis, Missouri: Elsevier, 2005:259–260. 2. Buyukmihci NC, MacMillan A, Scagliotti RH. Evaluation of zones of iris hypoplasia in horses and ponies. J Am Vet Med Assoc 1992;200:940–942. 3. Samuelson DA. Ophthalmic anatomy. In: Gelatt KN, ed. Veterinary Ophthalmology. 4th ed. Ames, Iowa: Blackwell Publishing, 2007:63–70. 4. Crispin SM. Developmental anomalies and abnormalities of the equine iris. Vet Ophthalmol 2000;3:93–98. 5. Barnett KC, Platt H. Intraocular melanomata in the horse. Equine Vet J Suppl 1990;10:76–82. 6. Rubin LF. Cysts of the equine iris. J Am Vet Med Assoc 1966;149: 151–154.
515
D I AG N O S T I C O P H T H A LM O LO GY
Diagnosis of iris hypoplasia requires complete ophthalmic examination prior to and following dilation. Due to stromal thinning, it is possible to retroilluminate through the stroma with a bright light. Mydriasis will result in reduction of the bulging appearance. Ocular ultrasound can be used when needed to rule out a space-occupying mass such as iris neoplasia or uveal cyst. Iris hypoplasia is a benign condition that does not cause ocular discomfort or vision problems; therefore, no treatment is necessary and the prognosis is excellent. It is important that this condition be recognized such that it is not confused with more serious ocular neoplasia.
A
History and clinical signs
3-year-old, castrated male American Paint horse was examined at the ophthalmology service at the Western College of Veterinary Medicine for evaluation of a mass within the right iris. The menace responses, palpebral, and occulocephalic reflexes were present bilaterally. The direct and consensual pupillary light reflexes were present bilaterally. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were 19 mm/min and 15 mm/min in the right and left eye, respectively. The intraocular pressures were estimated with a rebound tonometer (Tonvet; Tiolat, Helsinki, Finland) and were 25 mmHg and 23 mmHg in the right and left eye, respectively. Results from fluorescein staining (Fluorets; Bausch & Lomb Canada, Markham, Ontario) were negative bilaterally. Distant examination showed that the right iris was blue, while the left iris was brown. On direct examination, the dorsal aspect of the right iris appeared to bulge forward and had a corrugated appearance. Biomicroscopic examination (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) revealed the dorsal stroma of the iris to be very thin and transillumination through this area was possible. The right and left irides dilated fully following application of 0.5% tropicamide (Mydriacyl; Alcon, Canada) and the protruding area became less prominent. Indirect ophthalmoscopic examination (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) following dilation was completed and no abnormalities were noted. A photograph of the right eye prior to dilation is provided for your assessment (Figure 1).
What are your clinical diagnosis, differential diagnoses, therapeutic plan, and prognosis? Discussion Our clinical diagnosis was iris hypoplasia of the right eye. The protruding iris region represents an area of reduced development of the iris stroma. This is a non-progressive, congenital condition which is most commonly noted in the dorsal area of a blue iris (1). It may be unilateral or bilateral and there is no known age, gender, or breed predilection for this condition (1,2). Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4. Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office ([email protected]) for additional copies or permission to use this material elsewhere. 514
Figure 1. Photograph of the right eye of a 3-year-old American Paint horse.
Structurally, the normal iris has a superficial anterior border layer and stroma, derived from neural crest mesenchyme, and a bilayered posterior epithelium derived from neuroectoderm of the optic cup; the anterior epithelial layer includes the dilator muscle, and the posterior pigmented epithelium lines the deep surface. The smooth iris sphincter muscle within the stroma is also derived from neuroectoderm (3). Histologic findings in iris hypoplasia include a thin iris stroma with a poorly developed dilator muscle, and somewhat less developed constrictor muscle (2). It is likely that there is embryonic failure of induction of differentiation of the iris stroma and this may be related to defective growth of the neuroepithelium at the rim of the optic cup (4). The hypoplasia seen in this case was relatively mild; however, extreme examples of iris hypoplasia would include coloboma, or anirida (4). The clinical manifestations of iris hypoplasia include a darkened area of iris stroma that bulges forward. The area bulges forward due to the relative lack of resistance of this portion of the iris to aqueous humor flow from the posterior to anterior chamber. This resistance is increased when the pupil is constricted due to contact of the iris with the anterior lens. The thin, hypoplastic area of iris is, thus, pushed forward by the aqueous humor (2). The abnormality becomes less noticeable when the pupil is dilated as there is peripheral compression of the iris and less contact of the iris with the anterior lens leading to facilitation of aqueous humor flow (1). The main differential diagnoses for a darkened, bulging portion of iris is an iris cyst or uveal neoplasia, with iris melanoma being the most common primary intraocular neoplasm (4,5). Iris hypoplasia is occasionally and historically misinterpreted as a uveal cyst (6). CVJ / VOL 54 / MAY 2013
CVJ / VOL 54 / MAY 2013
References 1. Hollingsworth SR. Diseases of the anterior uvea. In: Gilger BC, ed. Equine Ophthalmology. St. Louis, Missouri: Elsevier, 2005:259–260. 2. Buyukmihci NC, MacMillan A, Scagliotti RH. Evaluation of zones of iris hypoplasia in horses and ponies. J Am Vet Med Assoc 1992;200:940–942. 3. Samuelson DA. Ophthalmic anatomy. In: Gelatt KN, ed. Veterinary Ophthalmology. 4th ed. Ames, Iowa: Blackwell Publishing, 2007:63–70. 4. Crispin SM. Developmental anomalies and abnormalities of the equine iris. Vet Ophthalmol 2000;3:93–98. 5. Barnett KC, Platt H. Intraocular melanomata in the horse. Equine Vet J Suppl 1990;10:76–82. 6. Rubin LF. Cysts of the equine iris. J Am Vet Med Assoc 1966;149: 151–154.
515
D I AG N O S T I C O P H T H A LM O LO GY
Diagnosis of iris hypoplasia requires complete ophthalmic examination prior to and following dilation. Due to stromal thinning, it is possible to retroilluminate through the stroma with a bright light. Mydriasis will result in reduction of the bulging appearance. Ocular ultrasound can be used when needed to rule out a space-occupying mass such as iris neoplasia or uveal cyst. Iris hypoplasia is a benign condition that does not cause ocular discomfort or vision problems; therefore, no treatment is necessary and the prognosis is excellent. It is important that this condition be recognized such that it is not confused with more serious ocular neoplasia.
