EQUINE VETERINARY JOURNAL 17 The Ocular Fundus of the Horse K. c. BARNETT M.A., PH.D., B.SC., M.R.C.V.S. Department of Veterinary Clinical Studies. School of Veterinary Medicine, University of Cambridge THE ocular fundus of the horse (fig. 1) and of the other domestic animals, with the exception of the pig which lacks a tapetum lucidum, consists of an optic disc or optic papilla, retinal blood vessels, tapetum lucidum or tapetal fundus, and tapetum nigrum or non-tapetal fundus. The ocular fundus of the donkey (fig. 2) and horse are similar in every way. There are many variations in colour of the equine fundus which are seen in normal animals with apparently normal vision. Some of these variations are associated with the colour of the eye (iris) and the coat of the animal. All these colour variations must be recognised if a correct assessment is to be made of the horse s vision in an examination for soundness, and for the diagnosis of a retinopathy or other ocular defect. The eyes of the horse are large and as they are placed laterally on either side of the head near objects cannot be seen binocularly. To see a near object the horse has to turn its head and use monocular vision. The horse is one of the few animals with a sloping ramp retina, the ray family of fishes have a similar device. In this system the lens has no power of accommodation and focussing is obtained on different areas of the retina which are at different distances from the lens. The eye is therefore asymmetrical in vertical section. Distant objects are focussed on the inferior retina which is closer to the lens than the superior retina, on which near objects are focussed. Histologically the layers of cells of the retina are arranged in a mannercommon to all the domestic animals. The rods and cones are one of the outer layers and lie adjacent to the pigment epithelium. In the latter layer the cells contain pigment in the non-tapetal region (fig. 22) which gives the tapetum nigrum its dark colour. In the tapetal region the bright colours of the tapetum lucidum, which in the horse and all ungulates is a fibrous structure (tapetum fibrosum) unlike the cellular structure (tapetum cellulosum) of the carnivores, can be seen ophthalmoscopically through the unpigmented pigment epithelial cells, the tapetum being part of the underlying choroid. THE OPTIC DISC The optic disc of the horse varies from oval to almost circular in shape and is situated inside the tapetum nigrum, just below its junction with the lower horizontal border of the tapetum lucidum. Paper presented at the annual congress of the British Equine Veterinary Association, Cambridge 1970. The colour varies from pale pink (fig. 3) to brick red (fig. 4). The surface often appears uneven due to various intensities of colour. Frequently the margin is narrow and pale. The disc often, but not invariably, has a well defined edge and is clearly demarcated from the surrounding tapetum nigrum. The edge may be uneven (fig. 4) which gives the optic disc an irregularly oval shape, or has a notch in the lower border (fig. 3). Sometimes the edge may appear double in places (fig. 5) and not infrequently the region surrounding the disc is darkly Some of the Rfsults of Imperfect Vision (Mayhew s Horse Doctor 1896). pigmented, particularly along the lower border (fig. 4). Medullated or opaque nerve fibres associated with the optic disc are occasionally seen in the horse. They appear as tufts or whitish streaks which cross the border of the optic disc, enter the surrounding tapetum nigrum, and extend for approximately a half to one disc diameter on to the fundus (fig. 6). They may be very faint, which gives the border of the optic disc a hazy appearance, or much more dense which makes the disc appear larger. They are usually situated at the lower and outer segment of the edge, are commonly bilateral, but may not be symmetrical in both eyes. THE RETINAL BLOOD VESSELS About fifty retinal blood vessels are just visible ophthalmoscopically in the horse. Both arteries and veins are present but they cannot be differentiated. The blood vessels arise all around and immediately inside the border of the optic disc and radiate outwards in approximately straight lines onto the fundus. This type of retinal vascularisation is called pauran-
18 giotic i.e. small vessels extending only a short distance from the disc. All the retinal blood vessels are very fine (fig. 8), but some appear narrower than others. A few are branched and some more tortuous than others (fig. 3). Tortuous and prominent vessels are occasionally also present on the surface of the optic disc (fig. 4). THE TAPETUM LUCIDUM The equine tapetum lucidum extends over most of the upper two thirds of the ocular fundus and has a straight horizontal base just above the optic disc (fig. 23). The colour is usually green, but of many shades varying from yellow-green to blue-green. The surface is covered with a regular pattern of numerous fine dark dots i.e. the Stellulae of Winslow (fig. 1). The colour of the tapetum is either evenly distributed throughout the area or contains darker blotches varying from small spots (fig. 7) to larger areas (fig. 8). These are often a more intense blue than the rest of the tapetum. In some horses the tapetum has a regular arrangement of spots and streaks radiating from a centre above the disc and well inside the tapetal region (fig. 9). These spots or streaks are blue or red in colour and represent areas where the tapetum covering the underlying blood vessels is either thin or absent. In some equines the tapetal region is clearly separated into large and small areas of two different coloursthe normal green of the tapetum lucidum and a pale to dark orange (fig. 10). This difference of colour is due to a variation in pigmentation of the choroid coupled with the effect of the underlying choroidal vasculature. This phenomenon is known as partial albinism of the choroid (or albinismus fundi partialis). It has also been referred to as a coloboma of the tapetum but this is not a correct description. Other authors have described it as tapetal islands, but this is inaccurate and this term should be used for another condition of the fundus, described under the section Tapetum Nigrum. In a cream horse examined (white mane and tail with pale blue irides) the tapetal region showed varying shades of pink (fig. 11). A darker red tapetal region (fig. 12) was present in a skewbald (brown and white with heterochromia of one iris) but this is not common to all skewbald horses e.g. (fig. 4). The albinotic fundus (fig. 13) of a white horse (with pale blue irides) showed complete absence of a tapetum. In this case the absence of pigment in the pigment epithelium of the retina, and in the choroid resulted in the choroidal blood vessels being clearly seen superimposed on the white sclera. The appearance was the same throughout the tapetal and non-tapetal regions. This horse was a cross between a Palomino and a true albino. (The Palomino has a brown iris and a normal fundus with green tapetum lucidum and pigmented dark tapetum nigrum). In the tapetal region patches of dark pigment are occasionally seen (fig. 14). These represent areas where the tapetum is absent and the pigment epithelium of the retina contains pigment, as in the tapetum nigrum region. Partial albinism of the fundus, involving both tapetum lucidum and tapetum nigrum regions, is commonly seen in Welsh ponies and their crosses. It is frequently associated with a heterochromic iris or wall eye (figs. 10 and 12). EQUINE VETERINARY JOURNAL THE TAPETUM NIGRUM The tapetum nigrum is usually a dark brownish-grey region (fig. 5), frequently paler, due to a reduction in pigment, in the narrow area immediately above the optic disc (fig. 1). Often choroidal blood vessels are visible in this area. An area of partial albinism in the tapetum nigrum appears reddish due to the circulation of the underlying choroid and a reduction of pigment in the pigment epithelium of the retina (figs. 4 and 8). Total albinism of this region was seen in both the cream pony (fig. 1 I ) and white pony (fig. 13) described above. Occasionally pale grey or silvery spots are seen in the non-tapetal fundus and also small areas of tapetum lucidum situated inside the tapetum nigrum-these are the true islands of tapetum referred to in the previous section. OPTIC ATROPHY A case of atrophy of the optic nerve, of unknown aetiology, has been seen as a unilateral condition. The optic disc was much paler when compared with the other eye and the retinal blood vessels were absent. The horse had no direct pupillary light reflex in the affected eye but a very good consensual light reflex. and unless examined in a dark box or by very careful ophthalmoscopic examination the total blindness of the one eye could easily be missed. The appearance of this horse s optic discs are shown in figs. 15 and 16. PERIPAPILLARY RETINOPATHY The ophthalmoscopic appearance of a peripapillary retinopathy is shown in fig. 17. It appears as pale silvery grey areas extending from the edge of the optic disc into the tapetum nigrum. The areas are focal, non-progressive, asymmetrical and in this case were only present in one eye. This horse showed no evidence of defective vision but histological examination of the lesion (fig. 24) revealed loss of rods and cones and their associated nuclear layer, indicating that a blind area must be present. Roberts (1962) described circumpapillary chorioretinitis, as a focal lesion of equine periodic ophthalmia, which showed many similarities to the above case. Andrasic et. al. (1963), however, do not consider that these areas of depigmentation are a sequel to periodic ophthalmia and in the case described here there was no history of any inflammatory eye disease or evidence of an iridocyclitis. PIGMENTARY R ETIN OPATHY Fig. 18 shows a case of pigmentary retinopathy. It i4 characterised by dark, irregular, pigmented areas scattered in the tapetum lucidum, reminiscent of the bone corpuscle type of pigmentation typical of retinitis pigmentosa in man. The horse had defective vision and the only abnormality detected on ophthalmoscopic examination was as described above. Unfortunately no histological examination of the retina was possible. Startup (1970) has described a similar ophthalmoscopic appearance, and as long ago as 1864 two horses which had defective vision due to retinitis pigmentosa were examined and recorded by Magnusson (1911).
EQUINE VETERINARY JOURNAL 19 Fig. 23. Normal Ocular Fundus Showing Extent of Tapetum. u Fig. 21. Retina (Tapetum Lucidum). SENILE RETINOPATHY An aged Shetland pony (approximately 25 years old) had pigmentary changes in the fundus (fig. 19) which histologically showed loss of rods and cones and the normal layer organisation of the retina, together with changes in the pigment epithelium cells. Cystic degeneration of the inner layers of the peripheral retina was also present (figs. 25 and 26). RETINAL DETACHMENT Retinal detachment is not common in the horse, although it has been described in cases of periodic ophthalmia (Formston 1964). The case illustrated (fig. 20) was a 6 months old crossbred Arab filly, congenitally blind due to microphthalmos, cataract and various abnormalities of the fundus. Ophthalmoscopic examination also revealed the typical picture of retinal detachment i.e. grey translucent folds in front of a highly reflective tapetum lucidum and these can be seen to undulate slightly in the vitreous with each movement of the eye. SUMMARY The ocular fundus of the horse is described, including the structure of the tapetum lucidum, tapetum nigrum, and retina. Fig. 22. Retina (Tapetum Nigrum). Fig. 24. Periperpillary Retinopathy.
20 EQUINE VETERINARY JOURNAL Fig. 25. Senile Retinapathy. Fig. 26. Senile Retinopathy. The normal variations in the ophthalmoscopic appearance of the tapetal and non-tapetal fundus, the optic disc, and the retinal blood vessels are discussed. The colour relationships between the fundus and the coat and iris are shown. Certain pathological conditions of the ocular fundus are also described, including their ophthalmoscopic and histopathological examinations. RESUME Le fond de l oeil du cheval est dccrit: structures du tapis clair, du tapis sombre et de la rctine. Les variations normales de l aspect ophtalmoscopique des tapis clair et sombre, de la papille optique et des vaisseaux sanguins de la rctine sont exposkes et l association de certains aspects avec la robe et la couleur de I i ri s est prkci sce. Quelques aspects pathologiquqs du fond de I oeil sont indiqucs y compris les aspects ophtalmoscopiques et histopathologiques. estructuras del tapetum lucidum, tapetum nigum y la retina. Las variaciones normales observadas con el osftalmoscopio del fondo del tapetum y no-tapetum, del disco optic0 y 10s vasos de la retina son expuestos junto con la asociacion de este con el pelaje y el color del iris. Se describen algunas condiciones patologicas del fondo ocular, incluyendo 10s examenes oftalmoscbpicos e histopatologicos. ACKNOWLEDGEMENT The author thanks the Horserace Betting Levy Board and the British Equine Veterinary Association Trust for financial assistance to cover the cost of the colour plates. REFERENCES Ammann, K. and Muller, A. (1968). The Normal Picture of the Fundus Oculi in the Horse. Bed. Munch. Timxtl. Wschr. 81, 370. Andrasic, N.,,Marolt, J., Frank, A., Vukelic, E., Cormak, h. and Sankovic, F. (1963). A Contribution to the Study of Abnormal Colouring of the Fundus in Horses. Zhl Vet. Med. 10, 566. ZUSAMMENFASSUNG Barnett, K. C. (1965). Retinal Atrophy. Vet. Rec. 77, 1543. Der Augenhintergrund des Pferdes wird beschrieben, Formston, C. (1964). Some Aspects of Ocular Disease in the einschliesslich der Struktur des Tapetum lucidum, des Horse. Proc. B.E.V.A. 3, 3. Tapetum nigrum und der Retina. Die normalen Gelatt, K. N. and Finocchio, E. J. (1970). Variations in the Normal Equine Eye. Vet. Med. 65, 569. Variationen des ophthalmoskopischen Befundes im Fundus, an der Papilla und den Retinagefassen werden Kamer, 0. (1960). Colour Anomalies of the Fundus cif the Eye in Horses, Cattle, Dogs and Cats. Schweizer A lrcll. diskutiert und zum Teil mit der Haar- und Irisfarbe in Tierheilk. 102. 501. Zusam menhang gebracht. Komar, G. and Szutter, L. (1968). Veterinary Ophthalmology. Einige pathologische Zustande am Augenhintergrund verlag, Paul Parey, Berlin and Hamburg. werden beschrieben einschliesslich ophthalmoskopischer Magnusson, H. (191 1). Arch. Vevgh. Ophthal. 2, 147. und histopathologischer Befunde. Roberts, S. R. (1962). Fundus Lesions in Equine Periodic Ophthalmia. J. Am. Vet. Med. Ass. 141, 229. Startup, F. G. (1970). Personal communication. SUMARIO Szutter, W. (1962). The So-called Pigment Colobomas of El fondo ocular del caballo se describe incluyendo las the Fundus of the Eye in Horses. Acta Vet. Hung.