Veterinary Ophthalmology (2014) 17, Supplement 1, 140–148
DOI:10.1111/vop.12180
Spectral-domain optical coherence tomography evaluation of the cornea, retina, and optic nerve in normal horses Nelson I. Pinto and Brian C. Gilger Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606, USA
Address communications to: B. C. Gilger Tel.: (919) 513-1273 Fax: (919) 513-6711 e-mail: [email protected]
Abstract Purpose To determine the feasibility of using a handheld spectral-domain optical coherence tomography (SD-OCT) instrument to characterize normal corneal, retinal, and optic nerve head anatomy in vivo in standing horses. Methods Clinically normal horses under sedation, palpebral nerve blockage, and pharmacologically induced mydriasis were imaged with a SD-OCT instrument (Envisu SD-OCT, Bioptigen, Inc., Morrisville, NC). Radial volumes from the cornea (axial, superior, inferior, nasal, and temporal), and rectangular volumes from the retina (dorsal, ventral, nasal, and temporal) and optic nerve head were acquired. Manual measurements of the corneal layers within the five regions, retinal and nerve fiber layer thickness in the four different regions adjacent to the ONH, and vertical and horizontal axis of the optic nerve head (ONH) and optic cup (OC) were obtained using the same device. Results Total corneal thickness (mean SD) measurements were 800 50, 937 61, 956 61, 912 65, and 884 68 lm for the axial, superior, inferior, nasal, and temporal regions, respectively. The highest total retinal and nerve fiber layer thickness (mean SD), at the level of the ONH, was found nasally 459 115 and 377 116 lm, respectively, followed by the temporal, dorsal, and ventral quadrants. The dimensions of the ONH and OC (mean SD) were 3.682 0.276 and 2.175 0.502 mm for the horizontal, and 3.012 0.278 and 2.035 0.488 mm for the vertical axis. Conclusions The SD-OCT instrument employed in this study may be used on sedated horses and allows the acquisition of high-resolution images, and thickness measurements involving the cornea, retina, and optic nerve. Key Words: SD-OCT, equine, cornea, retina, pachymetry
INTRODUCTION
Detailed microscopic examination of the cornea and retina was, until the middle 1980s, only assessable through routine histopathology after surgical keratectomy and/or enucleation. As a result of those limitations, noninvasive, detailed, high-resolution, diagnostic imaging of ocular tissues has been a major need in comparative ophthalmology. Confocal microscopy of the cornea in vivo was first reported in 1985, although Minsky first designed it on 1955.1,2 In 1991, the use of optical coherence tomography (OCT) on the retina was published.3 An OCT obtains high-resolution cross-sectional images (axial resolution 1–15 lm) of tissue architecture in vivo by interferometry. Interferometry correlates the
light backscattered from the tissue and the light that has traveled a known distance (reference arm length) to measure the magnitude and echo time delay of backscattered light.4 Since 1991, OCT technology has progressed and, although it was initially designed to evaluate retinal tissues, is now used to image nearly all ocular structures.5–8 The use of OCT in human medicine has reached diverse specialties, including dermatology, gastroenterology, dentistry, orthopedics, surgery, among others.9 The advantage of OCT imaging is that the high resolution and magnification of ocular tissues in vivo approaches the histopathological level (i.e., resolution of 5–10 lm). However, OCT is limited by the need for clear media to facilitate penetration of the light beam, as well as © 2014 American College of Veterinary Ophthalmologists
ocular sd-oct in normal horses 141
necessary patient cooperation (voluntary gaze fixation) to minimize motion artifacts. In the veterinary ophthalmic literature, the use of OCT was first reported in 2007 in a study evaluating retinal structure in the cat and in 2008 to describe the retinopathy of the Coton de Tulear dogs.10,11 Recently, several studies have used OCT to characterize the normal cornea, iridocorneal angle, and retina in several species, as well as corneal abnormalities in dogs and cats.12–21 In the majority of these studies, the animals were under general anesthesia or deep sedation to avoid eye movement during imaging.10,14,16–21 In equine ophthalmology, several studies have measured corneal thickness using confocal microscopy (in vivo), SD-OCT (in vivo), specular microscopy (postmortem), or ultrasonic pachymetry (in vivo and postmortem).21–26 The equine optic nerve head morphology has been reviewed, and postmortem characterization of the equine retina has been described using histopathology.27,28 In the author’s knowledge, in vivo characterization of the equine retina has not been reported and SD-OCT imaging in the horse has only been used on the axial cornea.21 The purpose of this study was to determine the feasibility of using a handheld spectral-domain optical coherence tomography (SD-OCT) device to characterize normal corneal, retinal, and optic nerve anatomy in vivo in standing horses. METHODS
Client-owned horses presented to the North Carolina State University Veterinary Health Complex (NCSUVHC) with normal ophthalmic examinations and without history of ocular disease were included in this study. Use of animals was approved and monitored by the North Carolina State University Institutional Animal Care and Use Committee (NCSU-IACUC) and was in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. All horses had a complete ophthalmic examination performed by a boardcertified veterinary ophthalmologist or a veterinary ophthalmology resident, consisting of slit-lamp biomicroscopy (Kowa SL-15; Kowa Company Ltd, Tokyo, Japan) and indirect ophthalmoscopy (Keeler Vantage Plus; Keeler Ltd, Windsor, Berkshire, UK) following dilation (Tropicamide Ophthalmic Solution 1%; Falcon Pharmaceuticals, Forth Worth, TX, USA) prior to SD-OCT imaging. Topical fluorescein dye (Ful Glo; Akorn Pharmaceuticals, Lake Forest, IL, USA) was applied and applanation tonometry (Tono-Pen Vet; Reichert Inc, Depew, NY, USA) was performed following instillation of topical anesthetic (proparacaine hydrochloride ophthalmic solution 0.5%; Akorn Pharmaceuticals, Lake Forest, IL, USA) upon completion of SD-OCT imaging.
For imaging purposes, the horses were placed in stocks and sedated with detomidine hydrochloride (Dormosedan; Orion Corp, Espoo, Finland) administered at a dose range between 10 and 15 mcg/kg IV. Palpebral nerve blocks using lidocaine HCl 2% (VEDCO Inc, St. Joseph, MO, USA) were performed to facilitate the manipulation of the upper eyelid and to minimize pressure against the globe. The horses’ heads were maintained at the level of the withers by use of a padded table. During imaging, the cornea was irrigated with sterile irrigation solution (BSS; Alcon Laboratories Inc, Forth Worth, TX, USA). Only one eye was imaged for each horse. A SD-OCT instrument (Envisu SD-OCT, Bioptigen, Inc., Morrisville, NC), which contains a super-luminescent light-emitting diode (SLED) that delivers light at a wavelength of 840 nm, was used to image the cornea, retina, and optic nerve. The SD-OCT instrument has an axial pixel resolution in tissue of 3 mm) in the nasal and ventral quadrants were not imaged in all horses, This could be the result of variation in the probe position within horses and individual difference in the curvature of the posterior aspect of the globe that requires adjustments on the reference arm length. The general retina lens used on the SD-OCT is developed for the geometry of the human eye; the different size and curvature of the equine globe requires adjustments of the arm reference length for different areas of the retina. Due to this limitation, retinal image areas of 36 mm2 were selected for the present study. More extensive coverage of the retina with the SD-OCT was only possible by adding additional 6 mm rectangular volumes throughout the retina, but without the presence of retinal vasculature for orientation in the peripheral retina, accurate positioning of the rectangular volume was not feasible.
The optic nerve is formed mainly by the axons of retinal ganglion cells (RGCs), which comprise the nerve fiber layer (NFL) within the retina and form the neuroretinal rim within the optic nerve head. The OC/ONH ratios in this study were 0.594 and 0.693 for the horizontal and vertical axis, respectively. The only previous report of this ratio, described in a review article, was 0.61.27 The purpose of this study was not to standardize this lens for its use throughout the entire equine retina, but to prove that the equine retina could be imaged with a SD-OCT device. There are many variables that may interfere with accurate measurement of the retinal and nerve fiber layer thickness and optic nerve head axes using OCT. Ideally, the retinal lens used in the SDOCT device needs to be adjusted according to the refractive error of the patient being evaluated. However, horses imaged in this study were not subjected to preOCT retinoscopy or axial globe length measurement, but instead an average equine axial length was used.34,35 In pediatric human ophthalmology, the refractive error and the globe length are important parameters to minimize the magnification artifact, particularly when the ONH is being assessed.36,37 Although not routinely performed prior to adult human OCT, the effect that variations of axial length and refractive state have on OCT values in the adult equine eye is not known and should be the subject of further study. The data in the present study cannot be used, due to the small numbers, to evaluate age-, gender-, and/or breed-related differences. Subsequent studies should evaluate those variables. It has been reported that RGC density varies by breed.45 Breed differences in RGC density may have been a source of variation for TNFLT and therefore the TRT in this study. The handheld SD-OCT device used in this study allowed for the acquisition of high-quality images of the cornea, retina, and optic nerve in standing horses. Further studies are warranted to evaluate the diagnostic capabilities of SD-OCT for the equine eye. ACKNOWLEDGMENTS
The authors would like to thank Drs. Alison Clode and Keith Montgomery for their comments on this manuscript. Special thanks to the technicians (ophthalmology and large animal services) who helped patiently with the horse restraint. REFERENCES 1. Lemp MA, Dilly PM, Boyde A. Tandem scanning confocal microscopy of the full thickness cornea. Cornea 1985; 4: 205–209. 2. Minsky M. Memoir on inventing the confocal scanning microscope. Scanning 1987; 10: 128–138. 3. Huang D, Swanson EA, Lin EP et al. Optical coherence tomography. Science 1991; 254: 1178–1181.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 140–148
ocular sd-oct in normal horses 147 4. Fujimoto JG, Drexler W. Introduction to optical coherence tomography. In: Optical Coherence Tomography, 1st edn. (eds Fujimoto JG, Drexler W) Springer-Verlag, Berlin, 2008; 1–46. 5. Sarunic MV, Asrani S, Izatt JA. Imaging the ocular anterior segment with real-time, full-range Fourier-domain optical coherence tomography. Archives on Ophthalmology 2008; 126: 537–542. 6. Bizheva K, Hutchings N, Sorbara L et al. In vivo volumetric imaging of the human corneo-scleral limbus with spectral domain OCT. Biomedical Optics Express 2011; 2: 1794–1802. 7. Jeong HW, Lee SW, Kim BM. Spectral-domain OCT with dual illumination and interlaced detection for simultaneous anterior segment and retina imaging. Optics Express 2012; 20: 19148– 19159. 8. Tao A, Shao Y, Zhong J et al. Versatile optical coherence tomography for imaging the human eye. Biomedical Optics Express 2013; 4: 1031–1044. 9. Fujimoto JG. Optical coherence tomography: introduction. In: Handbook of Optical Coherence Tomography, 1st edn. (eds Bouma BE, Tearney GJ) Marcel Dekker, New York, NY, 2002; 1–40. 10. Gekeler F, Gmeniner H, V€ olker M et al. Assessment of the posterior segment of the cat eye by optical coherence tomography (OCT). Veterinary Ophthalmology 2007; 10: 173–178. 11. Grahn BH, Sandmeyer LL, Breaux C. Retinopathy of Coton de Tulear dogs clinical manifestations, electroretinographic, ultrasonographic, fluorescein and indocyanine green angiographic, and optical coherence tomographic findings. Veterinary Ophthalmology 2008; 11: 242–249. 12. Wang X, Wu Q. Normal corneal thickness measurements in pigmented rabbits using spectral-domain anterior segment optical coherence tomography. Veterinary Ophthalmology 2013; 16: 130– 134. 13. Alario AF, Pirie CG. A spectral-domain optical coherence tomography device provides reliable corneal pachymetry measurements in canine eyes. Veterinary Record 2013; 172: 23. doi:10.1136/vr.101530. 14. Alario AF, Pirie CG. Central corneal thickness measurements in normal dogs: a comparison between ultrasound pachymetry and optical coherence tomography. Veterinary Ophthalmology 2013; doi:10.1111/vop.12074. 15. Alario AF, Pirie CG. Intra and inter-user reliability of central corneal thickness measurements obtained in healthy feline eyes using a portable spectral-domain optical coherence tomography device. Veterinary Ophthalmology 2013; 16: 446–450. 16. Famose F. Assessment of the use of spectral domain optical coherence tomography (SD-OCT) for evaluation of the healthy and pathological cornea in dogs and cats. Veterinary Ophthalmology 2013; doi:10.1111/vop.12028. 17. Almazan A, Tsai S, Miller PF et al. Iridocorneal angle measurements in mammalian species normative data by optical coherence tomography. Veterinary Ophthalmology 2013; 16: 163–166. 18. Rosolen SG, Riviere MK, Lavillegrand S et al. Use of a combined slit-lamp SD-OCT to obtain anterior and posterior segment images in selected animal species. Veterinary Ophthalmology 2012; 15(Suppl 2): 105–115. 19. Hernandez-Merino E, Kecova H, Jacobson SJ et al. Spectral domain optical coherence tomography (SD-OCT) assessment of the healthy female canine retina and optic nerve. Veterinary Ophthalmology 2011; 14: 400–405. 20. Huang Y, Cideciyan AV, Papastergiou GI et al. Relation of optical coherence tomography to microanatomy in normal and rd chickens. Investigative Ophthalmology and Visual Science 1998; 39: 2405–2416.
21. Pirie CG, Alario AF, Barysauskas CM et al. Manual corneal thickness measurements of healthy equine eyes using a portable spectral-domain optical coherence tomography device. Equine Veterinary Journal 2013; doi:10.1111/evj.12198. 22. van der Woerdt A, Gilger BC, Wilkie DA et al. Effect of auriculopalpebral nerve block and IV administration of xylazine on IOP and corneal thickness in horses. American Journal of Veterinary Research 1995; 56: 155–158. 23. Andrew SE, Ramsey DT, Hauptman JG et al. Density of corneal endothelial cells and corneal thickness in eyes of euthanatized horses. American Journal of Veterinary Research 2001; 62: 479–482. 24. Plummer CE, Ramsey DT, Hauptman JG. Assessment of corneal thickness, intraocular pressure, optical corneal diameter, and axial globe dimensions in Miniature Horses. American Journal of Veterinary Research 2003; 64: 661–665. 25. Ledbetter EC, Scarlett JM. In vivo confocal microscopy of the normal equine cornea and limbus. Veterinary Ophthalmology 2009; 12(Suppl 1): 57–64. 26. Ramsey DT, Hauptman JG, Petersen-Jones SM. Corneal thickness, intraocular pressure, and optical corneal diameter in Rocky Mountain Horses with cornea globosa or clinically normal corneas. American Journal of Veterinary Research 1999; 60: 1317– 1321. 27. Brooks DE, Komaromy AM, K€allberg ME. Comparative retinal ganglion cell and optic nerve morphology. Veterinary Ophthalmology 1999; 2: 3–11. 28. Ehrenhofer MC, Deeg CA, Reese S et al. Normal structure and age-related changes of the equine retina. Veterinary Ophthalmology 2002; 5: 39–47. 29. Hirano K, Ito Y, Suzuki T et al. Optical coherence tomography for the noninvasive evaluation of the cornea and epithelium using optical coherence tomography. Cornea 2001; 20: 281–289. 30. Wirbelauer C, Winkler J, Bastian GO et al. Histopathologcal correlation of corneal diseases with optical coherence tomography. Graefes Archive for Clinical and Experimental Ophthalmology 2002; 240: 727–734. 31. Hildebrand GD, Fielder AR. Anatomy and physiology of the retina. In: Pediatric Retina. (eds Reynolds JD, Olitsky SE) Springer-Verlag, Berlin, 2011; 39–65. 32. Williams R, Fink BA, King-Smith PE et al. Clinical applications of optical coherence tomography in the posterior pole: the 2011Jose Manuel Espino lecture – part 1. Clinical Ophthalmology 2013; 7: 2165–2179. 33. Harman AM, Moore S, Hoskins R et al. Horse vision and explanation for the visual behavior originally explained by the ‘ramp retina’. Equine Veterinary Journal 1999; 31: 384–390. 34. Knill LM, Eagleton RD, Harver E. Physical optics of the equine eye. American Journal of Veterinary Research 1977; 38: 735–737. 35. Coile DC, O’Keefe LP. Schematic eyes for domestic animals. Ophthalmic and Physiological Optics 1988; 8: 215–220. 36. Huynh SC, Wang XY, Rochtchina E et al. Distribution of optic disc parameters measured by OCT: findings from a population-based study of 6-year-old Australian children. Investigative Ophthalmology and Visual Science 2006; 47: 3276–3285. 37. Leung CK, Cheng AC, Chong KK et al. Optic disc measurements in myopia with optical coherence tomography and confocal scanning laser ophthalmoscopy. Investigative Ophthalmology and Visual Science 2007; 48: 3178–3183. 38. Wirbelauer C, Thannha€ user CL, Pham DT. Influence of corneal curvature on central and paracentral pachymetry with optical coherence tomography. Cornea 2009; 28: 254–260.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 140–148
148 pinto and gilger 39. Ishibazawa A, Igarashi S, Hanada K et al. Central corneal thickness measurements with Fourier-Domain optical coherence tomography versus ultrasonic pachymetry and rotating Scheimpflug camera. Cornea 2011; 30: 615–619. 40. Ponce CM, Rocha KM, Smith SD et al. Central and peripheral corneal thickness measured with optical coherence tomography, Scheimpflug imaging, and ultrasound pachymetry in normal, keratoconus-suspect, and post-laser in situ keratomileusis eyes. Journal of Cataract & Refractive Surgery 2009; 35: 1055–1062. 41. Beutelspacher SC, Serbecic N, Scheuerle AF. Assessment of central corneal thickness using OCT, ultrasound, optical low coherence reflectometry and Scheimpflug pachymetry. European Journal of Ophthalmology 2011; 21: 132–137.
42. Williams R, Fink BA, King-Smith PE et al. Central corneal thickness measurements: using an ultrasonic instrument and 4 optical instruments. Cornea 2011; 30: 1238–1243. 43. Haque S, Jones L, Simpson T. Thickness mapping of the cornea and epithelium using optical coherence tomography. Optometry and Vision Science 2008; 85: 963–976. 44. Hebel R. Distribution of retinal ganglion cells in five mammalian species (pig, sheep, ox, horse, dog). Anatomy and Embryology 1976; 150: 45–51. 45. Evans KE, McGreevy PD. The distribution of ganglion cells in the equine retina and its relationship to skull morphology. Anatomia, Histologia, Embryologia 2007; 36: 151–156.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 140–148
DOI:10.1111/vop.12180
Spectral-domain optical coherence tomography evaluation of the cornea, retina, and optic nerve in normal horses Nelson I. Pinto and Brian C. Gilger Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606, USA
Address communications to: B. C. Gilger Tel.: (919) 513-1273 Fax: (919) 513-6711 e-mail: [email protected]
Abstract Purpose To determine the feasibility of using a handheld spectral-domain optical coherence tomography (SD-OCT) instrument to characterize normal corneal, retinal, and optic nerve head anatomy in vivo in standing horses. Methods Clinically normal horses under sedation, palpebral nerve blockage, and pharmacologically induced mydriasis were imaged with a SD-OCT instrument (Envisu SD-OCT, Bioptigen, Inc., Morrisville, NC). Radial volumes from the cornea (axial, superior, inferior, nasal, and temporal), and rectangular volumes from the retina (dorsal, ventral, nasal, and temporal) and optic nerve head were acquired. Manual measurements of the corneal layers within the five regions, retinal and nerve fiber layer thickness in the four different regions adjacent to the ONH, and vertical and horizontal axis of the optic nerve head (ONH) and optic cup (OC) were obtained using the same device. Results Total corneal thickness (mean SD) measurements were 800 50, 937 61, 956 61, 912 65, and 884 68 lm for the axial, superior, inferior, nasal, and temporal regions, respectively. The highest total retinal and nerve fiber layer thickness (mean SD), at the level of the ONH, was found nasally 459 115 and 377 116 lm, respectively, followed by the temporal, dorsal, and ventral quadrants. The dimensions of the ONH and OC (mean SD) were 3.682 0.276 and 2.175 0.502 mm for the horizontal, and 3.012 0.278 and 2.035 0.488 mm for the vertical axis. Conclusions The SD-OCT instrument employed in this study may be used on sedated horses and allows the acquisition of high-resolution images, and thickness measurements involving the cornea, retina, and optic nerve. Key Words: SD-OCT, equine, cornea, retina, pachymetry
INTRODUCTION
Detailed microscopic examination of the cornea and retina was, until the middle 1980s, only assessable through routine histopathology after surgical keratectomy and/or enucleation. As a result of those limitations, noninvasive, detailed, high-resolution, diagnostic imaging of ocular tissues has been a major need in comparative ophthalmology. Confocal microscopy of the cornea in vivo was first reported in 1985, although Minsky first designed it on 1955.1,2 In 1991, the use of optical coherence tomography (OCT) on the retina was published.3 An OCT obtains high-resolution cross-sectional images (axial resolution 1–15 lm) of tissue architecture in vivo by interferometry. Interferometry correlates the
light backscattered from the tissue and the light that has traveled a known distance (reference arm length) to measure the magnitude and echo time delay of backscattered light.4 Since 1991, OCT technology has progressed and, although it was initially designed to evaluate retinal tissues, is now used to image nearly all ocular structures.5–8 The use of OCT in human medicine has reached diverse specialties, including dermatology, gastroenterology, dentistry, orthopedics, surgery, among others.9 The advantage of OCT imaging is that the high resolution and magnification of ocular tissues in vivo approaches the histopathological level (i.e., resolution of 5–10 lm). However, OCT is limited by the need for clear media to facilitate penetration of the light beam, as well as © 2014 American College of Veterinary Ophthalmologists
ocular sd-oct in normal horses 141
necessary patient cooperation (voluntary gaze fixation) to minimize motion artifacts. In the veterinary ophthalmic literature, the use of OCT was first reported in 2007 in a study evaluating retinal structure in the cat and in 2008 to describe the retinopathy of the Coton de Tulear dogs.10,11 Recently, several studies have used OCT to characterize the normal cornea, iridocorneal angle, and retina in several species, as well as corneal abnormalities in dogs and cats.12–21 In the majority of these studies, the animals were under general anesthesia or deep sedation to avoid eye movement during imaging.10,14,16–21 In equine ophthalmology, several studies have measured corneal thickness using confocal microscopy (in vivo), SD-OCT (in vivo), specular microscopy (postmortem), or ultrasonic pachymetry (in vivo and postmortem).21–26 The equine optic nerve head morphology has been reviewed, and postmortem characterization of the equine retina has been described using histopathology.27,28 In the author’s knowledge, in vivo characterization of the equine retina has not been reported and SD-OCT imaging in the horse has only been used on the axial cornea.21 The purpose of this study was to determine the feasibility of using a handheld spectral-domain optical coherence tomography (SD-OCT) device to characterize normal corneal, retinal, and optic nerve anatomy in vivo in standing horses. METHODS
Client-owned horses presented to the North Carolina State University Veterinary Health Complex (NCSUVHC) with normal ophthalmic examinations and without history of ocular disease were included in this study. Use of animals was approved and monitored by the North Carolina State University Institutional Animal Care and Use Committee (NCSU-IACUC) and was in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. All horses had a complete ophthalmic examination performed by a boardcertified veterinary ophthalmologist or a veterinary ophthalmology resident, consisting of slit-lamp biomicroscopy (Kowa SL-15; Kowa Company Ltd, Tokyo, Japan) and indirect ophthalmoscopy (Keeler Vantage Plus; Keeler Ltd, Windsor, Berkshire, UK) following dilation (Tropicamide Ophthalmic Solution 1%; Falcon Pharmaceuticals, Forth Worth, TX, USA) prior to SD-OCT imaging. Topical fluorescein dye (Ful Glo; Akorn Pharmaceuticals, Lake Forest, IL, USA) was applied and applanation tonometry (Tono-Pen Vet; Reichert Inc, Depew, NY, USA) was performed following instillation of topical anesthetic (proparacaine hydrochloride ophthalmic solution 0.5%; Akorn Pharmaceuticals, Lake Forest, IL, USA) upon completion of SD-OCT imaging.
For imaging purposes, the horses were placed in stocks and sedated with detomidine hydrochloride (Dormosedan; Orion Corp, Espoo, Finland) administered at a dose range between 10 and 15 mcg/kg IV. Palpebral nerve blocks using lidocaine HCl 2% (VEDCO Inc, St. Joseph, MO, USA) were performed to facilitate the manipulation of the upper eyelid and to minimize pressure against the globe. The horses’ heads were maintained at the level of the withers by use of a padded table. During imaging, the cornea was irrigated with sterile irrigation solution (BSS; Alcon Laboratories Inc, Forth Worth, TX, USA). Only one eye was imaged for each horse. A SD-OCT instrument (Envisu SD-OCT, Bioptigen, Inc., Morrisville, NC), which contains a super-luminescent light-emitting diode (SLED) that delivers light at a wavelength of 840 nm, was used to image the cornea, retina, and optic nerve. The SD-OCT instrument has an axial pixel resolution in tissue of 3 mm) in the nasal and ventral quadrants were not imaged in all horses, This could be the result of variation in the probe position within horses and individual difference in the curvature of the posterior aspect of the globe that requires adjustments on the reference arm length. The general retina lens used on the SD-OCT is developed for the geometry of the human eye; the different size and curvature of the equine globe requires adjustments of the arm reference length for different areas of the retina. Due to this limitation, retinal image areas of 36 mm2 were selected for the present study. More extensive coverage of the retina with the SD-OCT was only possible by adding additional 6 mm rectangular volumes throughout the retina, but without the presence of retinal vasculature for orientation in the peripheral retina, accurate positioning of the rectangular volume was not feasible.
The optic nerve is formed mainly by the axons of retinal ganglion cells (RGCs), which comprise the nerve fiber layer (NFL) within the retina and form the neuroretinal rim within the optic nerve head. The OC/ONH ratios in this study were 0.594 and 0.693 for the horizontal and vertical axis, respectively. The only previous report of this ratio, described in a review article, was 0.61.27 The purpose of this study was not to standardize this lens for its use throughout the entire equine retina, but to prove that the equine retina could be imaged with a SD-OCT device. There are many variables that may interfere with accurate measurement of the retinal and nerve fiber layer thickness and optic nerve head axes using OCT. Ideally, the retinal lens used in the SDOCT device needs to be adjusted according to the refractive error of the patient being evaluated. However, horses imaged in this study were not subjected to preOCT retinoscopy or axial globe length measurement, but instead an average equine axial length was used.34,35 In pediatric human ophthalmology, the refractive error and the globe length are important parameters to minimize the magnification artifact, particularly when the ONH is being assessed.36,37 Although not routinely performed prior to adult human OCT, the effect that variations of axial length and refractive state have on OCT values in the adult equine eye is not known and should be the subject of further study. The data in the present study cannot be used, due to the small numbers, to evaluate age-, gender-, and/or breed-related differences. Subsequent studies should evaluate those variables. It has been reported that RGC density varies by breed.45 Breed differences in RGC density may have been a source of variation for TNFLT and therefore the TRT in this study. The handheld SD-OCT device used in this study allowed for the acquisition of high-quality images of the cornea, retina, and optic nerve in standing horses. Further studies are warranted to evaluate the diagnostic capabilities of SD-OCT for the equine eye. ACKNOWLEDGMENTS
The authors would like to thank Drs. Alison Clode and Keith Montgomery for their comments on this manuscript. Special thanks to the technicians (ophthalmology and large animal services) who helped patiently with the horse restraint. REFERENCES 1. Lemp MA, Dilly PM, Boyde A. Tandem scanning confocal microscopy of the full thickness cornea. Cornea 1985; 4: 205–209. 2. Minsky M. Memoir on inventing the confocal scanning microscope. Scanning 1987; 10: 128–138. 3. Huang D, Swanson EA, Lin EP et al. Optical coherence tomography. Science 1991; 254: 1178–1181.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 140–148
ocular sd-oct in normal horses 147 4. Fujimoto JG, Drexler W. Introduction to optical coherence tomography. In: Optical Coherence Tomography, 1st edn. (eds Fujimoto JG, Drexler W) Springer-Verlag, Berlin, 2008; 1–46. 5. Sarunic MV, Asrani S, Izatt JA. Imaging the ocular anterior segment with real-time, full-range Fourier-domain optical coherence tomography. Archives on Ophthalmology 2008; 126: 537–542. 6. Bizheva K, Hutchings N, Sorbara L et al. In vivo volumetric imaging of the human corneo-scleral limbus with spectral domain OCT. Biomedical Optics Express 2011; 2: 1794–1802. 7. Jeong HW, Lee SW, Kim BM. Spectral-domain OCT with dual illumination and interlaced detection for simultaneous anterior segment and retina imaging. Optics Express 2012; 20: 19148– 19159. 8. Tao A, Shao Y, Zhong J et al. Versatile optical coherence tomography for imaging the human eye. Biomedical Optics Express 2013; 4: 1031–1044. 9. Fujimoto JG. Optical coherence tomography: introduction. In: Handbook of Optical Coherence Tomography, 1st edn. (eds Bouma BE, Tearney GJ) Marcel Dekker, New York, NY, 2002; 1–40. 10. Gekeler F, Gmeniner H, V€ olker M et al. Assessment of the posterior segment of the cat eye by optical coherence tomography (OCT). Veterinary Ophthalmology 2007; 10: 173–178. 11. Grahn BH, Sandmeyer LL, Breaux C. Retinopathy of Coton de Tulear dogs clinical manifestations, electroretinographic, ultrasonographic, fluorescein and indocyanine green angiographic, and optical coherence tomographic findings. Veterinary Ophthalmology 2008; 11: 242–249. 12. Wang X, Wu Q. Normal corneal thickness measurements in pigmented rabbits using spectral-domain anterior segment optical coherence tomography. Veterinary Ophthalmology 2013; 16: 130– 134. 13. Alario AF, Pirie CG. A spectral-domain optical coherence tomography device provides reliable corneal pachymetry measurements in canine eyes. Veterinary Record 2013; 172: 23. doi:10.1136/vr.101530. 14. Alario AF, Pirie CG. Central corneal thickness measurements in normal dogs: a comparison between ultrasound pachymetry and optical coherence tomography. Veterinary Ophthalmology 2013; doi:10.1111/vop.12074. 15. Alario AF, Pirie CG. Intra and inter-user reliability of central corneal thickness measurements obtained in healthy feline eyes using a portable spectral-domain optical coherence tomography device. Veterinary Ophthalmology 2013; 16: 446–450. 16. Famose F. Assessment of the use of spectral domain optical coherence tomography (SD-OCT) for evaluation of the healthy and pathological cornea in dogs and cats. Veterinary Ophthalmology 2013; doi:10.1111/vop.12028. 17. Almazan A, Tsai S, Miller PF et al. Iridocorneal angle measurements in mammalian species normative data by optical coherence tomography. Veterinary Ophthalmology 2013; 16: 163–166. 18. Rosolen SG, Riviere MK, Lavillegrand S et al. Use of a combined slit-lamp SD-OCT to obtain anterior and posterior segment images in selected animal species. Veterinary Ophthalmology 2012; 15(Suppl 2): 105–115. 19. Hernandez-Merino E, Kecova H, Jacobson SJ et al. Spectral domain optical coherence tomography (SD-OCT) assessment of the healthy female canine retina and optic nerve. Veterinary Ophthalmology 2011; 14: 400–405. 20. Huang Y, Cideciyan AV, Papastergiou GI et al. Relation of optical coherence tomography to microanatomy in normal and rd chickens. Investigative Ophthalmology and Visual Science 1998; 39: 2405–2416.
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