In outlining the clinical approach to the cat with ocular disease it is relevant to remember that 'clinical' is defined as concerned with, or based on, observation--and the feline eye, in health and disease, is particularly rewarding to observe. Knowledge of ocular disease--disease pertaining to the eye--is based on a thorough understanding of what is normal and some variations from normality are so characteristic that specific diagnosis of the ocular disease is possible from the history alone. In many cases, however, clinical diagnosis of ocular disease requires physical and neurological examination as well as examination of both eyes and so the clinical approach adopted in the presentation will concentrate on the ocular findings, supplemented by accompanying physical and neurological findings as appropriate. Additional investigative procedures, such as laboratory tests, that may on occasions be needed to confirm the clinical diagnosis will not be discussed, as they are not an integral part of the clinical part of the diagnostic process. Examples of feline ocular disease will be used to demonstrate the necessity for a meticulous clinical approach.
It is crucial that the clinical approach to ocular disease is underpinned by a comprehensive history and accurate recording of the clinical findings. Clinical records for ophthalmic cases should include illustrations as well as written details and sequential illustrations are an excellent means of recording changes over time. Photographs are a useful adjunct, but they should complement, rather than replace, simple drawings.
The history should record the age, breed, sex and vaccination status of the case, information about the present problem and its duration, as well as details of any previous health problems. It is important to ascertain the nature of any treatment, both for the presenting problem and any previous problems. Other relevant enquiries include the management and lifestyle of the cat and, for example, whether it is kept isolated from other cats, or is part of a multicat household. Details of any other animals with which the cat may have come into contact should also be obtained. Some assessment of the severity of the problem and the degree of pain should also be made, based on the client's own assessment of the cat and the clinician's observations.
Physical examination is a logical procedure based on a standard protocol. Physical examination should be carried out in a quiet room that can be darkened completely, to facilitate the subsequent ophthalmic and neuro-ophthalmological examination which forms part of the process.
Physical examination involves the whole body and usually consists of initial observation and inspection followed by checking the vital signs including respiratory rate, pulse (rate, regularity and pressure) and temperature. Inspection of body systems as part of routine physical examination includes palpation, auscultation and percussion. In adult cats it may also be sensible to measure the blood pressure, particularly if there are any clinical features that suggest systemic hypertensive disease. Usually it is the mean systolic blood pressure that is measured.
Ophthalmic examination is part of the physical examination and forms the main focus of the presentation. The cranial nerve tests which are routinely included as part of ophthalmic examination are tracking response, vestibulo-ocular reflex, dazzle reflex, menace response, palpebral and corneal reflexes and direct and consensual pupillary light reflex. Ophthalmic examination is performed in two parts; the first part in daylight or artificial light and the second part in the dark.
Initially the cat is observed from a distance in order to assess the nature and severity of the ocular problem. If appropriate, obstacles can be placed on the floor and the cat should be allowed to move freely about the consulting room, as a very crude way of assessing vision (the lighting intensity should be varied). Visual tracking can also be checked at this stage using cotton wool balls, or a bright spot of light, but the cat needs to be interested!
For complete examination of the lens, vitreous and fundus, instillation of a mydriatic is required, but this is not regarded as a routine feature of feline ocular examination. The pupil of normal cats responds briskly and more completely to bright light than that of dogs and narrows to a vertical slit, resulting in a very limited field of view. Tropicamide 1% is the drug of choice, but cats resent its bitter taste and it is often more rewarding to simply keep the light intensity low for basic examination, especially as many cats resent illumination with bright light. This sympathetic approach is also less likely to produce third eyelid protrusion (there is a voluntary element in the cat). If a mydriatic is used it must be applied after other tests such as the Schirmer tear test, have been performed.
The general appearance of the eyes and adnexa is observed in daylight or artificial light and each side compared to ensure that they are symmetrical. The position of the globe in relation to the orbit should be assessed from in front of the patient and from above. The incomplete bony orbital rim should also be inspected both visually and manually. The vestibulo-ocular, dazzle reflex, palpebral and corneal reflexes and menace response can also be tested at this point. The direct and consensual pupillary light reflex can also be checked and should be repeated under conditions of darkness.
The lacrimal apparatus is not evaluated in any detail at this stage, although the possibility of abnormalities of production, distribution and drainage may be suspected according to the clinical presentation. Once the basic ophthalmic examination is completed tear production can be assessed, usually with a Schirmer I tear test. The presence and position of the upper and lower lacrimal puncta should be confirmed and for this magnification is useful.
The margins, outer and inner surfaces of the upper and lower eyelids should be examined. There is close apposition of the upper and lower eyelids to the globe, so inspection of their inner surface is not always easy. The position of the third eyelid should be observed and its outer surface inspected once the eyelid has been protruded by pressure on the globe through the upper eyelid. The inner surface of the third eyelid is not examined routinely.
The ocular surface (defined as the continuous epithelium which begins at the lid margin, extends onto the back of the upper and lower eyelids, and both surfaces of the third eyelid, into the fornices and onto the globe) is examined next. Naked eye examination should indicate whether the appearance of the ocular surface is normal. A penlight can be used to ensure that the corneal reflex is normal (in this situation the corneal 'reflex' is the light from the penlight reflected in miniature on the corneal surface without disruption). It may also be appropriate to check corneal sensitivity at this stage, particularly in those situations in which corneal anaesthesia may be part of the clinical presentation (e.g., herpetic keratitis). This can be done in an empirical fashion by touching the cornea with a fine wisp of cotton wool, which should elicit a brisk blink in the normal cat. A more elegant and accurate method utilises an aesthesiometer. Additional investigations such as swabs and scrapes for culture and the application of ophthalmic stains may be performed once the basic examination has been completed.
Darkness minimises distracting reflections and is the next--and essential--part of ophthalmic examination.
The anterior segment (the internal structures of the globe up to and including the lens) is examined with a light source and magnification, or a slit lamp biomicroscope. The pupillary light response in darkness can be evaluated using focal illumination, before the eye itself is examined.
The limbus and cornea are examined first. Most of the limbus is invisible in the normal cat except, sometimes, laterally. The limbal zone is usually clearly defined because of a rim of pigment on the corneal side.
The anterior chamber should be optically clear. A slit beam, rather than a diffuse beam, is used to detect subtle opacities within the aqueous. The depth of the anterior chamber is most easily assessed by use of a slit beam, or by shining a beam of light across the eye from lateral to medial. The anterior chamber is deep and the pectinate ligament of the iridocorneal angle can be observed directly, if somewhat imperfectly, without a gonioscopy lens.
The iris of most cats is lightly pigmented and the distinction between the pupillary zone (usually darker) and ciliary zone (usually lighter) at the collarette is not always present, so that the iris is of uniform colour. Colour variations may be present between irides and within different sectors of the same iris. Variations of pigmentation produce a range of colours. In the least pigmented, almost albinotic iris, which is very pale in colour, the iris is often so thin that it can be transilluminated.
The adult pupil is round when dilated and narrows to a vertical slit on constriction. It is important to observe the size and shape of the pupil, paying particular attention to the pupillary margin, as deviations from normal may indicate posterior synechiae or neurological abnormalities.
The whole lens can only be examined in detail when a mydriatic has been used. The light source is used to demonstrate the anterior and posterior lens surfaces by observing the catoptric images which are visualised on the anterior lens capsule (erect) and the posterior lens capsule (inverted). It is easier to establish these boundaries by noting the relative movement of the images in relation to the light source (parallax).
The posterior segment (the internal structures of the globe beyond the lens) is examined next using some or all of a light source, slit lamp biomicroscope, indirect ophthalmoscope and direct ophthalmoscope.
The anterior vitreous is most easily examined with a penlight or slit lamp and should be free of obvious opacities.
Both indirect ophthalmoscopy and direct ophthalmoscopy are used to examine the ocular fundus and, to some extent, the posterior vitreous. Indirect ophthalmoscopy provides low power examination of a wide area and is particularly useful when the ocular media lack optical clarity. Direct ophthalmoscopy provides a magnified view of a relatively small area.
With either type of ophthalmoscopy the optic nerve head (optic disc or papilla) which is situated within the tapetal fundus when a tapetum is present, is located first and its size, shape and colour should be noted. In cats the optic nerve head is usually unmyelinated so that it is round in shape and slightly recessed, the optic nerve becoming myelinated posterior to the lamina cribrosa. The retinal vasculature is examined next, paying particular attention to the number and distribution of the retinal vessels, both arterioles and venules, as they hook over the rim of the optic nerve head. The terminal choroidal vessels appear as dark dots where they are viewed end on. In poorly pigmented eyes, larger choroidal vessels will also be visible, and they too should be examined. Finally, all four quadrants (dorso-medial, dorso-lateral, ventro-lateral, vento-medial) of the ocular fundus are checked.
It is important to note that because the feline eye is well adapted to light collection and vision under dim lighting conditions (large eye, large cornea, large lens, rod-rich retina, tapetum cellulosum) the examiner must be prepared to examine the ocular fundus without the use of excessively bright light. A non-compliant patient with the ability to move the third eyelid voluntarily, together with constriction of the pupil to a narrow vertical slit, will make the process of ophthalmic examination both difficult and incomplete. In addition, the feline tapetum is such an effective mirror that it is easy to miss subtle lesions if the light intensity is too high. However, the feline fundus exhibits fewer normal variants than the canine fundus, so it is relatively easy to identify pathology provided that this simple rule is followed.