Examination of the eye includes

 History

 General examination

 Inspection

 Control of vision

 Examination of outer structures

 Schirmer tear test

 Fluorescein staining

 Tonometry

 Examination of deeper structures

 Electrophysiology

 Imaging

Vision should be assessed before pupil dilation through menace response, pupillary light reflexes, falling cotton ball, and obstacle course under different light conditions.

This lecture is restricted to examination of intraocular structures.

The anterior chamber and iris

The anterior chamber should only contain clear fluid, and the iris should be clearly visible. Uveitis with increased cell content of the aqueous humour may cause opacity of the fluid. In mild cases the aqueous look dusty (Tyndall effect). Larger exudates of leucocytes and fibrin may form sterile aggregates (hypopyon) with or without red blood cells present. Especially in the cat, immune complexes may sometimes be seen ventrally on the inside of the cornea as small brownish spots. Haemorrhage in the anterior chamber may be seen in connection with tumours and inflammations, in bleeding disorders, and also in connection with the inherited disease Collie Eye Anomaly.

Pigmentation of the iris varies, often dependent on coat color. Iris in the two eyes may be of different colors, heterochromia. The iris contains small irregular folds. The pupil is round in dogs and large felines, biconvex in the cat. The most common congenital malformation is persistent pupillary membranes, more rarely iris colobomas. Especially in older dogs of small breeds, the iris may be more or less atrophied. This is seen as holes in the iris, or major parts of the iris may be lacking. The pupil will stay apparently dilated, which may cause some discomfort in bright weather.

Normally the iris rests on the anterior lens capsule. Subluxation or luxation of the lens causes a deepening of the anterior chamber and also a characteristic shivering of the lens when the eye moves (iridodonesis).

Iris cysts may be present in the anterior chamber. The cysts can be differentiated from tumours by their round and often transparent appearance.

Gonioscopy

Many dog breeds are predisposed to glaucoma. Inspection of the iridocorneal angle may be indicated as part of a breeding program in certain breeds. In cats the angle is easily visible, in dogs vision is obscured by the sclera. An extra curved lens, a gonioscopy lens, can be placed on the cornea after topical anaesthesia. The most commonly used gonioscopy lenses are the Barkan lo-vac and the Koeppe lens. The whole iridocorneal angle should be examined, and the width of the angle as well as the appearance of the pectinate ligament is noted. Gonioscopy findings of the two eyes are similar in almost all normotensive dogs.

Gonioscopy is also indicated when an intraocular tumor is present and as a means of differentiation between primary and secondary glaucoma in breeds predisposed to primary glaucomas.

EXAMINATION OF THE POSTERIOR SEGMENTS

Pupil dilatation

The most common medication for diagnostic use is Tropicamide. One drop is applied in the eye and in a normal eye maximum dilation is achieved after approximately 20 minutes. Repeated application may be indicated in some dogs. If uveitis is present, the effect of Tropicamide may be poor in the affected eye.

Lens

Initial examination is performed by directing light into the eye to allow the reflex from the fundus (retroillumination). Opacities may be seen as darker areas through the pupil. Directing the light from an angle allows localization of the opacities. The optimal instrument for examination of the lens, however, is a slit-lamp biomicroscope. This instrument consists of two parts; the microscope providing 10-20 x enlargements enables visualisation of small structures, and the light source, which can be directed into the eye at different angles and with a slit-formed light beam. Binocular vision also facilitates localisation of opacities within the lens.

Normal easily distinguished variations include the suture lines of the lens, nuclear rings and small remnants of the hyaloid artery. On the posterior lens capsule occasional small dots representing remnants of the tunica vasculosa lentis may be seen, and on the anterior lens capsule small, pigmented dots representing remnants of the pupillary membrane are not infrequently observed.

Ophthalmoscopy

There are two principles for ophthalmoscopy, direct and indirect. In direct ophthalmoscopy the effective field of vision is determined by the illuminating system. Most direct ophthalmoscopes project a beam of light about one disc diameter. The visual field is restricted, and using a direct ophthalmoscope may be compared to peeping through a keyhole: You get a detailed view within a small field of vision. Thus, the direct ophthalmoscope is a good tool for close examination of details, for instance in examining the optic disc, but orientation is more difficult than with the indirect one, and it is difficult to examine the peripheral retina. The panoptic direct ophthalmoscope may represent a better tool than the traditional one.

In indirect ophthalmoscopy the field of view is determined by the ratio of lens diameter and focal length. Given lenses of equal power, a larger lens provides a wider field of view. Given equal lens diameters, a stronger lens provides a wider field of view. To prevent image distortion, most indirect ophthalmoscopy lenses are aspheric with two different curved surfaces; the surface with the steeper curvature should face the examiner. Options for indirect ophthalmoscopy include the binocular indirect ophthalmoscope, where the light source sits on a headband, monocular indirect ophthalmoscope as a hand-held indirect ophthalmoscope, or a modified indirect ophthalmoscope with the use of as focal light source and a lens.

Examination of the retina includes description of the optic disc, retinal vessels, tapetum and non-tapetum.

There are substantial normal variations in fundus color, size of tapetum, distribution of blood vessels and myelination of the optic disc. The optic disc is situated at the horizontal midline in the lower part of the tapetum. The degree of myelination around the nerve fibres decides the size and shape of the optic disc. A central cupping of the disc is often present, but must be distinguished from pathologic changes (colobomas). Cat discs are poorly myelinated and appear smaller than in the dog.

Normally, 3-5 retinal veins from the periphery of the retina merge to form a vessel ring on the optic disc. About 20 small arterioles extend radially from around the disc.

The tapetum is a triangular reflective layer situated in the dorsal half of the fundus. In puppies less than 8-12 weeks old the differentiation of the retinal layers are not yet complete, and the whole fundus appears blue with no visible tapetum. The color of the tapetum in dogs older than 12 weeks varies from yellow to bluish-green. In miniature breeds the tapetum can be limited to a minor area above the optic disc, or may even be totally absent. Only small reflective islets of tapetal tissue may be seen in some dogs. Except from some white animals, the tapetum is present in the cat.

Non-tapetal fundus is mostly darkly pigmented, the degree of pigmentation varying according to coat and iris color. If the retina is poorly pigmented, brighter vessels from the underlying choroid can be seen extending radially towards the periphery. This is termed tigroid fundus.

Albinotic fundus is tigroid fundus also lacking choroidal pigmentation. This is often seen in eyes with a blue iris ("wall eye" or "china eye").

Retinoscopy

Retinoscopy is performed to evaluate the animal's refractive status, that is, if the animal is near-sighted or far-sighted. Familiar myopia has been described in some dog breeds, and retinoscopy may be indicated in some behavioural problems. A streak retinoscope represents a practical, although not very sophisticated tool for veterinary ophthalmology. The retinoscope is held with one hand at a fixed distance from the animal's eye, and the examiner tries to neutralise the movements of light across the animal's fundus by adding lenses of varying strength in front of the animal's eye.

Electroretinography

Electroretinography (ERG) registers the response from the retina to light stimuli. After pupil dilation a contact electrode is placed on the retina and a reference electrode as well as a ground electrode on the head. The animal should be anesthetised for the procedure. The retina is stimulated with light of different wavelengths before, during and after dark-adaptation, and after light adaptation. There are many types of ERG equipment available. The important factor is to standardise examination methods, both regarding anesthesia, dark adaptation and recording. ERG can be used to establish an early diagnosis of photoreceptor disease, to distinguish blindness caused by photoreceptor disease from blindness from other causes and also to evaluate the function of the retina where it cannot be visualised, as in cataracts.

Ultrasound and other imaging techniques

Ultrasound is indicated to record space-occupying lesions and retinal detachment, aw well as other intraocular conditions, especially in cases where the fundus cannot be visualized. Radiography, MRI (magnetic resonance imaging) and CT (computer tomography) are also routinely used, but may be more indicated in central neuro-ophthalmic disease affecting vision, or in changes in periocular structures.

For recording or diagnosis of retinal disease, fundus photography as well as fluorescein angiography represent useful tools. Fluorescein angiography is a dynamic method to explore the fundus vasculature and tissues. Serial photographs are taken the first 20 seconds after intravenous injection of a 10% fluorescein sodium solution, then less frequently during the following 3-5 minutes. The following stages are recorded: The choroidal phase, the arterial phase, the arteriovenous phase, the capillary phase and the venous phase. Changes can present as either hypofluorescence, representing either a filling defect or a masking effect, or hyperfluorescence representing leakage of dye from vessels, neovascularisation or a "window effect" where the choroidal vasculature is seen through the retinal pigment epithelium.

Histology

For histology the eye should be taken out as soon as possible after death. It should be carefully dissected, removing as much extraocular tissue as possible before fixation. Preference for fixation fluid may vary, one should discuss with the pathologist who performs the examination of the eye what to use. One common mistake is to use too small amount of fixation fluid; the amount should be about 10 x the size of the eye.

References

1.  Davidson M. Clinical Retinoscopy for the Veterinary Ophthalmologist. Vet Comp Ophthalmol 1997; 7: 128-137.

2.  Narfstrom K, Ekesten B, Rosolen SG, Spiess BM, Percicot CL, Ofri R; Committee for a Harmonized ERG Protocol, European College of Veterinary Ophthalmology. Guidelines for clinical electroretinography in the dog. Doc Ophthalmol. 2002; 105: 83-92.

3.  Lescure, F. In: Pratique Medicale et Chirurgicale de l'Animal de Compagnie ed. Fluorescein angiographic atlas of the small animal ocular fundus. Paris: CNVSPA, 1998.