of the ocular fundus in dogs

N ormal variation

of the ocular fundus in dogs G.H.R.R. Janssens (1)

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The fundus is the part of the posterior eye segment that can be seen when using an ophthalmoscope. The different visible structures, from the tapetal and non-tapetal fundus, to the optic disc and the retinal blood vessels, will be discussed. The (sub)albinotic fundus will also be discussed in short.

INTRODUCTION

This paper originally appeared in: Vlaams Diergeneeskundig Tijdschrift * 2000, 69, p. 207-210

The normal appearances of the ocular fundus in dogs vary a lot. Therefore, it is important to know what is normal, to be able to diagnose abnormalities, both hereditary and postinflammatory. There is an apparent connection between the colour of the iris and the outlook of the fundus, especially where the colour of the non-tapetal fundus is concerned. Besides this, there is also variation depending of the colour of the coat, length of the hair and size of the dog. Individual differences are also possible. A good fundic examination can only be done after the eye has been sufficiently dilated. 0.5% tropicamide is used for this (Tropicol ® Bournonville Pharma). The effect is visible after about 15 to 20 minutes. We can see the tapetal fundus (formerly known as tapetum lucidum), the non-tapetal fundus (tapetum nigrum), the optic disc (papilla) and the retinal blood vessels. The position and form of the disc, the colour and the form of the tapetal and non-tapetal fundus are usually very similar in both eyes. The retinal blood vessels are usually each other’s mirror-image. With heterochromia iridis and subalbinotic eyes, there are more differences between both eyes.

the tapetal and non-tapetal fundus are not differentiated yet. There is a small disc, of which the anterior part does not bulge out of the retinal surface and of which the blood vessels are relatively wide.

At the age of three weeks, the area that will develop into the tapetal fundus will become a little bit paler, the area that will become the non-tapetal area will get darker. When the pup is four weeks old, the tapetal fundus will seem to have a purple colour. Over the following weeks, it will become more granular and more blue (Fig. 1). After this, usually at four months but sometimes even only after six months, the fundus will get its permanent colour.

TAPETAL FUNDUS The tapetal fundus is situated in the upper half of the posterior eye segment. The tapetal area is usually triangular (with the sharpest angle pointing towards the nose) or semicircular, with a horizontal line as base and completely surrounded by the non-tapetal fundus. The tapetal fundus is best developed in gazehounds. In smaller breeds, like the Yorkshire Terrier and Chihuahua, the tapetal fundus is often poorly developed, only barely half the size of that of other breeds. It is situated mainly to the upper lateral side of the disc.

DEVELOPMENT OF THE FUNDUS

Histologically, the central area of the tapetum (which is part of the choroid) is made up of a maximum of 15 layers of rectangular-shaped cells. Towards the periphery, the number

When a puppy is two weeks old, and no sooner, the fundus can be checked. It appears dull and uniform grey-brown, and

(1) UNO-Iaan 141 B-2620 Hemiksem, or Algemene Dierenkliniek Ranstad Frans Beirenslaan 155 B-2150 Borsbeek. E-mail: [email protected] * Presented by SAVAB (Belgium). The Author acknowledges the help a guidance given in Dr. Barnetts Compendium paper mentioned in Further reading.

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of cell layers decreases. In the central area, there is no pigmentation in the retinal pigmented epithelium cells, which cover the tapetum. Because of this, the tapetum can be seen with an ophthalmoscope. Sometimes the tapetum is absent (Fig. 2) or poorly developed (Fig. 3, 4). This is common in merle breeds, e.g. in Collies (rough- and shorthaired), Shelties, Border Collies, Cardigan Welsh Corgies and Harlequin Great Danes.

also can be situated in the tapetal fundus, and are called a tigroid fundus. Sometimes, there can be small islands of tapetal tissue in the non-tapetal fundus.

ALBINOTIC AND SUBALBINOTIC FUNDUS

When you examine the fundus and there is no tapetum and the pigmentation in the retinal pigmented epithelium is absent, then you see a red colour (red fundus, ruby eye, Fig. 5). This variation can be seen in the tricolour Beagle with a pale-yellow iris.

A merle coloured coat is often associated with pale blue irises or with heterochromia iridis. In such eyes, albinotic or subalbinotic areas are possible, randomly spread in the tapetal and non-tapetal fundus. These areas are characterised by the absence of pigmentation in the retinal pigmented epithelium and in the choroid, causing the white sclera to become visible between the choroidal blood vessels (Fig. 13, 14).

The colour of the tapetum can vary a lot. Possible colours include yellow, green, blue or orange (Fig. 6, 7, 8), and combinations of these colours are often seen too. Very frequent is a yellow tapetum with a green border, e.g. in Retrievers and Spaniels (Fig. 9) or a green tapetum with a blue border, e.g. in miniature Schnauzers. The tapetal cells do not contain any pigmentation, but resorb and reflect different wavelengths of light, resulting in a particular colour.

If there is no pigmentation at all in the retinal pigmented epithelium and in the choroid, the tapetum is completely absent. When the pigmentation is only partially absent, like in a number of merle dogs, the tapetum is smaller that usual.

The junction between the tapetal and non-tapetal area can be gradual (Fig. 10), more often seen in longhaired breeds like the Golden Retriever, or can be sharply demarcated (Fig. 11), which is more the case in shorthaired breeds like the Whippet. Forms between both are also possible.

OPTIC DISC With an ophthalmoscope, you can see that the disc is situated close to the junction of tapetal to non-tapetal fundus. It is often situated just inside the tapetal area (Fig. 6, 9, 11) or a little bit further in. The latter is especially the case in large breeds. In some dogs, especially smaller ones, the disc is situated entirely in the non-tapetal fundus (Fig. 12). These differences in situation are not due to a different position of the optic nerve in the eyeball, but to a variation in extent of the tapetal fundus.

Sometimes, small islands of normally pigmented tapetum nigrum can be seen in the tapetal area (Fig. 12). These can be sharply demarcated or can gradually transcend into the surrounding area.

NON-TAPETAL FUNDUS

The disc also varies a lot in size (Fig. 3, 10). Due to the presence of myelinated nerve fibres on the anterior side of the lamina cribrosa, the disc may appear a lot larger (Fig. 7) and it does not have a sharply demarcated rim. The myelin causes the disc to bulge out into the vitreous body (Fig. 8). This is called pseudopapilloedema and is especially common in Golden Retrievers and German Shepherds. In other breeds, like Poodles, Cocker Spaniels and Shelties, it is common for the disc to be situated somewhat deeper than the surface of the retina.

The non-tapetal fundus can be seen around the tapetal fundus and can be judged best in the area under the disc. The non-tapetal area covers three quarters or more of the entire fundus and is non-reflecting. There are variations in colour, depending on the colour of the iris. The non-tapetal fundus is usually dark brown, grey or black (Fig. 6, 7). The colour depends on the amount of pigmentation in the retinal pigmented epithelium and in the underlying choroid. Sometimes, the area closest to the tapetal fundus is a lighter brown than the lower part (Fig. 10, 12). The non-tapetal fundus can be less pigmented and therefore appear lighter brown, red brown or more red in dogs with a chocolate or liver coloured coat and light brown, yellow irises. This is seen in e.g. liver-and-white Pointers, English Springer Spaniels, brown miniature Poodles and chocolate Labradors (Fig. 11). In the less heavily pigmented non-tapetal fundus the area, in which choroid blood vessels are visible, have a tigroid or striped pattern (Fig. 2). This is due to the radial course of these blood vessels, from the area of the disc. These choroidal blood vessels also have a lighter colour than the retinal blood vessels. These areas can be small or large and

The term micropapilla is used for a small, but functional papilla. This is sometimes hard to distinguish from an optic nerve hypoplasia, in which case the eye is blind, with a dilated pupil and no direct pupillary reflex. There is also variation in the form and colour of the disc. It is often round (Fig. 10), but sometimes it may also be oval, triangular or irregularly shaped. The colours also vary, ranging from white to pink (Fig. 3, 11). This depends on the amount of blood in the retinal blood vessels. When the disc is entirely situated in a dark nontapetal fundus, it also appears to be paler than when it is surrounded by the tapetal fundus (Fig. 12).

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Fig. 1: Border Collie puppy, seven weeks old, with a purple-blue tapetal fundus.

Fig. 2: Sheltie. The tapetum is absent, but there is a “tigroid” or striped fundus (some choroidal blood vessels are visible). The rest of the fundus contains pigmentation in the retinal pigmented epithelium.

Fig. 3: Australian Shepherd Dog (blue merle). Some tapetal islands are visible (left and top). Note the large, white disc.

Fig. 4: Border Collie (merle). Presence of many tapetal islands. Choroidal blood vessels and some sclera are visible in the nontapetal area (subalbinotic).

Fig. 5: Australian Shepherd Dog (black tricolour). The tapetum is absent and so is the pigmentation in the retinal pigment epithelium. This image is comparable to a red fundus.

Fig. 6: Eurasier with yellow tapetum and dark nontapetal fundus. The disc is situated in the tapetal area and is partly surrounded by a pigmented ring. Note the “pit” or depression in the disc.

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In the middle of the disc, there is a small depression (pit or cup), which is sometimes seen as a small grey spot in the centre (Fig. 6, 13). This is the place where the hyaloid artery used to be.

the arterioles. The veins form a venous circle in the central part of the disc. This circle can be fully closed (Fig. 15) or not (Fig. 9, 14) and be located on the surface of the disc or partially deeper in it. Sometimes, some kind of “pulsation” is seen in that venous ring, which can disappear during the examination. It is suspected that this phenomenon is in relation to the pressure put on the thin sclera by contraction of the extraocular muscles.

When the disc is entirely surrounded by tapetum, there can be a pigmented ring (partial or complete) present around the disc (Fig. 6). In that area, the tapetum is absent and the choroidal pigmentation is visible. A hyper-reflecting ring around the disc, called conus or halo, may also occur. There, the peripapillar retinal layers are thinner, causing an increased reflectivity.

There are usually fifteen to twenty arterioles. They start from inside the edge of the disc, radiating outward over the fundus. These arterioles are finer, lighter in colour and are sometimes more tortuous than the veins (Fig. 16). With an ophthalmoscope, you cannot see a lot of difference between these arterioles and smaller veins.

RETINAL BLOOD VESSELS

The retinal blood vessels are usually spread over the entire fundus. An exception is the area centralis. This is the area with the largest concentration of cones. It is situated laterally and somewhat dorsally from the disc, about two diameters of a disc away. There are no large retinal blood vessels in the area centralis itself, but there is a fine capillary network. The large retinal blood vessels surround this area.

Dogs have a holangiotic fundus. There are three (Fig. 15) or four and sometimes even five, primary retinal veins (Fig. 12). They are either equally spread around the disc or there is one dorsal vein, with the others situated along the ventral side of the disc (which is more common). The primary veins are wider, darker (Fig. 15), less in number and less tortuous than

Fig. 7: Siberian Husky with bluish tapetal area, brownish non-tapetal area and large, white disc.

Fig. 8: Chow Chow. There are many yellow-orange tapetal areas present. Note how the myelinated disc bulges out into the vitreous body.

Fig. 9: Labrador. The yellow tapetum has a green rim. The disc is situated in the tapetal area and contains an incompletely closed venous circle and a central depression (“pit”).

Fig. 10: Lhasa Apso. The transition of tapetal to nontapetal fundus is gradual.

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Fig. 11: Labrador (chocolate). There is a sharp transition of the tapetal fundus to the red-brown non-tapetal area. The colour of the disc is more pink than white.

Fig. 12: Sheltie. Some pigmented islands are visible in the tapetal area. Myelinated nerve fibres are situated in the non-tapetal area. Note the four primary retinal veins.

Fig. 13: Australian Shepherd Dog (blue merle) without tapetum. The fundus contains a subalbinotic area with visible sclera between the choroidal blood vessels. Note the central depression in the disc (“pit”).

Fig. 14: Australian Shepherd Dog (blue merle) without tapetum. In the subalbinotic fundus, you can see the sclera between the choroidal blood vessels. The disc contains an incompletely closed venous circle.

Fig. 15: Pointer. In the red-brown non-tapetal area, some choroidal blood vessels are visible. The three primary retinal veins are wider and darker than the arterioles and come together in a closed venous circle on the disc.

Fig. 16: Australian Shepherd Dog (black tricolour). The arterioles are more tortuous than the veins. The disc has an incompletely closed venous circle and a central depression (“pit”).

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FURTHER READING

Slatter (D.) The normal fundus. In: Fundamentals of Veterinary Ophthalmology, Second edition, W.B. Saunders Company, Philadelphia, 1990: 98-101.

Barnett (K.C.) The canine ocular fundus: normal variations. The Compendium,1994 March, volume3, 348-356. Gelatt (K.N.) The normal ocular fundus. In: Veterinary Ophthalmology, Third edition, Lippincott Williams & Wilkins, Philadelphia, 873-877.

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