Eye examination in the cat

Abstract

Practical relevance:

The ability to perform a complete eye examination in the cat is critically important for patients with an ocular disorder or a systemic disease, as well as for the geriatric cat.

Clinical challenges:

Cats may need short breaks between portions of the eye exam in order to minimize stress. For the clinician, use of ophthalmic equipment and interpretation of normal vs abnormal findings may take some practice in order to develop proficiency.

Audience:

This review is aimed at veterinary practitioners and outlines all the steps and equipment necessary to perform a complete ophthalmic examination in the cat.

Equipment:

Although some specialized equipment is required, a complete eye exam can be performed with a modest investment in equipment and supplies.

Evidence base:

This article draws on published references for normal parameters measured in the feline eye exam, as well as the experience of the authors.

Proficiency in performing an eye examination in the cat is critical for the patient presented with ocular disease, as well as for the patient that may have a systemic disorder. Although veterinary ophthalmologists use specialized equipment, a thorough eye exam can be performed with relatively simple equipment (Figure 1). Practice leads to proficiency, especially when performing fundic examinations.

Figure 1

Basic equipment needed for an ophthalmic exam (clockwise from top left): Optivisor loupe (x 3.5 magnification at 4 inches), 20 diopter indirect lens, direct ophthalmoscope and Finoff transilluminator

Vision and neuro-ophthalmic tests

Assessing vision in the cat can sometimes be more difficult than in the dog. Cats tend not to move around a strange room as a dog might, making assessments of visual behavior more tenuous.

• Menace response The menace response involves a blink with hand motion toward the eye. This response involves the retina, optic nerve, chiasm and optic tract (afferent) as well as facial nerve (efferent). Care must be taken not to touch the whiskers or vibrissae, nor create an air current that could stimulate the cornea. Some normal cats will have a very weak or even absent menace response; in others the response will be brisk. Each side should be tested independently by covering one eye with the examiner’s hand. A positive menace response allows the examiner to determine that at least some vision is present, but cannot assess quality or completeness of vision. Kittens do not develop a menace response until approximately 12 weeks of age.

• Dazzle reflex A dazzle reflex involves blinking in response to a bright light source such as a Finoff transilluminator being quickly shone on the eye. This reflex is subcortical and is thus not a test of vision. Instead its presence allows the examiner to know the pathways that include retina, optic nerve, chiasm, optic tract, rostral colliculus, and facial nucleus and nerve are intact. A dazzle reflex can be elicited at a much younger age than a menace response, essentially as soon as the eyelids open.

• Palpebral reflex If either the menace response or dazzle reflex are absent, it is important to test the palpebral reflex by gently tapping the eyelids at the medial and lateral canthi and observing a blink. The palpebral reflex tests the ophthalmic (medial canthus) and maxillary (lateral canthus) branches of the trigeminal nerve (afferent) as well as the facial nerve (efferent).

• Pupil size and shape Symmetry of pupil size and shape is evaluated with a light source held at arm’s distance and the light directed toward the cat’s forehead so that both pupils are equally illuminated by the tapetal reflection (retroillumination). This should be performed in normal room light, followed by dimming of the room lights, and is the best way to recognize subtle anisocoria (difference in pupil size).

• Pupillary light reflex The next step is to check the pupillary light reflex (PLR) of each eye. Using a Finoff transilluminator hold the light off the eye, then quickly move it onto one eye and observe the PLR of both eyes. Wait a few seconds for the pupils to return to resting size and repeat on the opposite eye. The PLR results in constriction of the pupil in response to stimulation by light (direct reflex), accompanied by a weaker constriction of the contralateral pupil (indirect reflex). The PLR is not a test of vision, but does test the pathway that includes retina, optic nerve, chiasm, optic tract, pretectal nucleus in the midbrain, oculomotor nerve, ciliary ganglion and iris sphincter muscle. The PLR will be present in kittens at the time the eyelids open. Some older cats may have mydriatic pupils with a weak or absent PLR due to iris sphincter muscle atrophy.

• Other tests of vision In addition to the menace response, vision testing can include the cotton ball test, whereby a cotton ball is dropped near the cat’s face and behavior indicating whether the cat sees the object is observed. A laser pointer light directed on the table top can also be used, but care must be taken not to point it toward the eye. Each eye can be tested independently. Visual placing is also useful in cats. Holding the cat, slowly advance it toward the exam table and watch for placement of a forelimb prior to contact with the table. Note that some normal cats will fail to perform visual placing, repeatedly allowing the limb to contact the table prior to reaching with the paw.

The best assessment of a cat’s vision comes from the results of multiple tests.

Adnexal and anterior segment examination

The adnexa consists of the eyelids, third eyelid and conjunctiva. The anterior segment includes the cornea, anterior chamber, iris, ciliary body (which cannot be visualized) and lens. All of these structures (other than the ciliary body) can be adequately examined with a transilluminator, an Optivisor loupe (x 3.5 magnification at 4 inches) and a direct ophthalmoscope set on the slit beam. A slit lamp biomicroscope is the ideal instrument for anterior segment examination but is typically only utilized by ophthalmologists.

• Eyelids and conjunctiva Using the Optivisor and transilluminator, first examine the eyelids for abnormal contour or thickness, entropion, hair contacting the cornea (trichiasis), ocular discharge or masses. Entropion in cats most often occurs in juveniles as a congenital defect or in geriatric cats secondarily to loss of orbital fat and resultant enophthalmos. Next examine the conjunctival surfaces for hyperemia, chemosis, discharge, follicles or masses. Evert the eyelids to look at the palpebral conjunctiva. The anterior face of the third eyelid can be examined by gently retropulsing the globe. The posterior face can be examined by instilling a topical anesthetic (see page 706) and then using a small forcep to reflect the third eyelid.

• Cornea The cornea should be transparent, free of any opacity or blood vessels, and have a moist glistening appearance. Examine the cornea from different angles of view and by moving the light into different positions. Pay particular attention to the innermost layer (endothelium) of the ventral cornea, as this is where keratic precipitates will be deposited in a cat with uveitis. These collections of cells are often quite small and easy to miss. Next examine the anterior chamber for the presence of abnormal components of the aqueous humor. Hyphema (blood) or hypopyon (neutrophils) will often settle to the ventral-most aspect of the anterior chamber and may be hard to see if the third eyelid is elevated. Fibrin may stick to the iris or lens, or settle ventrally.

• Anterior chamber Aqueous flare is seen as a cloudiness to the normally clear aqueous humor and is caused by leakage of protein from the uveal vasculature. The slit beam on the direct ophthalmoscope can be used to look for flare, holding the beam so that it is shining on the lower half of the cornea only and sharply focused on the cornea (Figure 2). This requires that the direct ophthalmoscope is held close to the cornea. A second light beam will be visible on the iris or lens depending on where it is directed. With the room lights out, observe the top of the light beam on the cornea from the side of the eye. In a normal eye the light is not visible in the anterior chamber. In a cat with aqueous flare the light beam will be visible. The intensity of the light beam in the anterior chamber approximates the severity of the breakdown of the blood–aqueous barrier and, therefore, the uveitis. The presence of aqueous flare is diagnostic for anterior uveitis, although cats may have uveitis without the presence of flare.

  The anterior chamber depth can be evaluated with the slit beam and one eye compared with the other. A shallow anterior chamber with a normal cornea and iris may indicate lens instability or aqueous misdirection syndrome. In the latter, aqueous humor is abnormally diverted into the vitreous body, resulting in vitreous expansion which causes the lens and iris to be displaced anteriorly. This condition often leads to glaucoma. By contrast, a shallow anterior chamber with a corneal wound may indicate a perforated, leaking eye.

Figure 2

Examination of the anterior chamber for aqueous flare using the slit beam of the direct ophthalmoscope and an Optivisor for magnification

Figure 3

The major arterial circle (arrow), located in the iris, is a large vessel varying in prominence from cat to cat

• Iris Fine capillary growth across the iris surface (rubeosis iridis) is abnormal and a sign of chronic uveitis (Figure 4). Another sign of chronic uveitis is the presence of grey, slightly raised iris nodules, which are aggregates of lymphocytes and plasma cells. Cats may have pigmented areas within the iris. These range from copper colored to dark brown. If the lesions are flat they most likely represent areas of iris melanosis and not neoplasia (Figure 5). Melanosis can transform to neoplasia over time, however, so continued monitoring is advised. Areas that become raised are more likely to represent iris melanoma, a tumor that is potentially metastatic in cats and may be life-threatening (Figure 6).^1,2 Iris cysts are uncommon in cats compared with dogs, and may potentially be mistaken for melanoma. Cysts may be noted at the pupillary margin or behind the iris when it is dilated. They are heavily pigmented, smooth, round to ovoid structures.

• Lens The lens can only be fully examined after pharmacologic dilation of the pupil with tropicamide (see page 708). Retroillumination (as described for examining the pupils) will best demonstrate nuclear sclerosis, a normal aging change to the lens noted in cats older than about 8 years. The central lens will refract light differently due to compacted lens fibers, and appear as a circular area of mild cloudiness. Cataracts appear as dark regions within the tapetal glow. Retroillumination also aids in assessing the position of an opacity within the lens. An opacity that moves in the direction of eye movement would be in the anterior portion of the lens, while an opacity that moves in the opposite direction to eye movement would be in the posterior portion.

  The edge or equator of the lens is not visible in a normal eye. If the edge of the lens is visible, along with an aphakic crescent, the lens is subluxated. A fully luxated lens will tend to fall either into the anterior chamber, where it is easy to visualize, or posteriorly into the vitreous cavity. If the lens appears not to be present, it is likely in the ventral-most portion of the vitreous cavity.

Figure 4

Rubeosis iridis, or neovascularization on the iris surface, in a cat with chronic uveitis. Courtesy of Michigan State University Ophthalmology Service

Figure 5

Iris melanosis. Note the individual areas of hyperpigmentation that are flat with the iris surface

Figure 6

Iris melanoma, a potentially life-threatening neoplasm. Note the extensive hyperpigmentation with areas raised from the iris surface

Schirmer tear test

The Schirmer tear test (STT) involves placing the test strip in the lower conjunctival sac for 60 seconds and measuring the amount of wetting in millimeters during this time. This test assesses only the aqueous portion of tears, and not the lipid or mucin component. Normal STT values for cats range from 7–34 mm/min.^3,4

Keratoconjunctivitis sicca (KCS) is uncommon in cats compared with dogs. In fact, excess tearing is a more frequent clinical finding and thus the STT can often be omitted from the eye exam in a cat without losing vital information.

If the STT is performed, it must be prior to placing any fluid on the ocular surface. It has previously been speculated that stress in cats can increase sympathetic tone, causing a temporary decrease in tear production.^3,5 However, a more recent study of normal cats did not substantiate this. ⁴ Instead, this study suggested that an STT <9 mm/min is consistent with KCS, and no normal cat was found to have an STT <7 mm/min. It was also found that cats that were stressed actually seemed to wet the STT paper more rapidly. ⁴

Due to variability among cats, a low STT value must be interpreted carefully. Low STT values must be found in conjunction with clinical signs such as corneal ulceration, lusterless cornea and mucoid discharge before making a diagnosis of KCS.

Tonometry

Tonometry is a non-invasive diagnostic tool commonly used to approximate the intraocular pressure (IOP). Accurate estimations of IOP play an essential role in diagnosing glaucoma (high IOP) as well as helping to confirm acute uveitis (low IOP). The normal range of IOP in cats is 12–19 mmHg. There is a normal circadian rhythm to IOP, with the highest values occurring at night. ⁷

There are two types of tonometers in general use in practice today – applanation and rebound tonometers; the most common examples being the Tono-Pen and the TonoVet, respectively (Figures 7 and 8). These instruments differ in the way they obtain an estimation of IOP. Applanation tonometry measures the force needed to flatten an area of the cornea with the instrument’s footplate. In comparison, rebound tonometry measures the probe deceleration as it is propelled from the instrument, contacts the cornea and returns.^8,9 One study in cats comparing these two indirect measuring devices with the gold standard of manometry, reported that the readings of the TonoVet were closer to the true IOP than those of the Tono-Pen, especially at IOPs above 20 mmHg. ¹⁰

Figure 7

The Tono-Pen applanation tonometer works by flattening the cornea with the instrument’s footplate

Figure 8

The TonoVet rebound tonometer works by measuring the speed with which the probe returns to the instrument following contact with the cornea

Regardless of the method used to assess IOP, it is important to ensure that pressure is not being placed around the cat’s neck or on the globes as the eyelids are held open. Restraint under the chin and behind the head will help to prevent movement of the cat without affecting the IOP. A topical anesthetic must be used when utilizing applanation tonometry, but is not necessary for rebound tonometry due to the short contact time between the probe and the cornea.

The most important thing when checking patients over time is to utilize the same measuring device so that trends in IOP are more easily assessed. ¹¹

Collection of cytology sample

See box on page 707.

Fluorescein staining

Fluorescein is a water-soluble dye and is thus absorbed by hydrophilic tissue (corneal and conjunctival stroma) but not by lipophilic tissue (conjunctival and corneal epithelium and Descemet’s membrane).

Placing a drop of topical anesthetic on the eye and waiting 30 s before fluorescein staining will make the cat more tolerant of rinsing. Because the fluorescein strips are highly concentrated, only a small drop of eye wash is needed to partially wet the strip. While retracting the upper lid, touch the wetted fluorescein strip to the bulbar conjunctiva rather than to the cornea itself. Allow the patient to blink, which distributes the dye across the surface of the cornea. Rinse excess stain from the cornea with eye wash.

Any exposed corneal stroma (indicating loss of corneal epithelium) will absorb fluorescein. If a deep ulcer with no stain uptake at its base is present, this is most likely a descemetocele. If aqueous humor mixed with fluorescein is seen running from the surface of a cornea with an ulcer this would be considered a positive Seidel test and indicates a corneal perforation. The cobalt blue filter on the direct ophthalmoscope along with a source of magnification should be used for enhanced visualization of small areas of fluorescein uptake.

Fluorescein can also be utilized to assess patency of the nasolacrimal system. For this assessment, a large amount of stain is placed on the ocular surface, but not rinsed, and then the nares are observed for appearance of the dye. This test is known as the fluorescein passage or Jones test. In cats, the time it takes for fluorescein to reach the nares is quite variable, typically ranging from a few seconds to a few minutes. One study reported a range of 4–1800 s. ¹³ Some cats may also have a nasolacrimal duct opening into the oral cavity. Therefore, it is important to observe the oropharynx for dye if fluorescein is not observed at the cat’s nares.

Figure 9

Microbrushes are ideal for collecting ocular surface samples for cytology

Figure 10

The Microbrush should be rolled firmly across the conjunctiva or cornea several times. The brush is then rolled on a glass slide to transfer the cells

Figure 11

Cytologic preparation from a kitten with conjunctivitis. Purulent inflammation is present as well as a cytoplasmic inclusion body within an epithelial cell (arrow), consistent with Chlamydia felis. Wright’s stain, x 100

Figure 12

Cytologic preparation from a cat with a raised pinkish-white plaque and corneal vascularization. Purulent inflammation with eosinophils (arrow) and a mast cell (arrowhead) is present, confirming eosinophilic keratitis. Wright’s stain

Figure 13

Cytologic preparation from a cat with conjunctivitis. Purulent inflammation is present, a finding consistent with, but not diagnostic for, herpesvirus infection. Wright’s stain

Fundic examination

Fundic examination is critically important in geriatric cats or any cat with systemic illness. The fundus can be examined by direct or indirect ophthalmoscopy. While a standard direct ophthalmoscope produces an image that is right side up, it is magnified approximately x 15 and provides only a 5° angle view, making visualization of the entire fundus difficult. Conversely, indirect ophthalmoscopy produces an image that is upside down and inverted but with a magnification of only about x 3, making visualization of large portions of the fundus possible. Intermediate between the two is the Welch Allyn Panoptic direct ophthalmoscope which provides a 25° upright image view.

Several technical aspects of indirect ophthalmoscopy are important to appreciate in order to obtain the best view of the fundus. Indirect lenses are biconvex although the curvature of each side is different. The flatter side (usually marked by a silver ring on the rim) should always be positioned towards the patient. A transilluminator should be held next to the observer’s eye. To start, rest the hand holding the lens gently on the cat’s head with the lens pointed away from the eye. Attain the best tapetal reflection possible, then drop the lens into place to observe the fundus (Figure 14). The observer’s eye, the light beam, the lens and the cat’s eye should all be in a straight line. The indirect lens should be held with the observer’s arm fully extended (do not bend the elbow and lean forward).

Figure 14

Indirect ophthalmoscopy using a Finoff transilluminator and a 20 diopter indirect lens

The lens needs to be at the correct distance from the cornea, so that the image of the fundus entirely fills the lens. If eyelids or iris are seen within the lens, then the lens is too close (usually) or too far from the cornea. An assistant looking from the side can help the examiner learn correct placement by indicating when the light spot on the cornea is very small and focused (correct focal point).

Figure 15

Note the dorsal tapetal and ventral melanotic portions in the normal feline fundus. The optic disc is non-myelinated and has three main arteriole–venule pairs extending from it, along with a few other small vessels

Figure 16

Normal fundus in a blueeyed, color-dilute cat. No tapetum or pigment is present, allowing Visualization of choroidal vasculature and sclera in addition to retinal blood vessels

A cat will tend to retract its globe when a very bright light is shone into the eye. This leads to elevation of the third eyelid and loss of the fundic image. The examiner should stop, let the cat relax and try again. This is mitigated to some extent by using light of just sufficient brightness to provide a good image and avoiding excessive illumination. Another helpful maneuver in some cats is to have an assistant hold the cat toward the edge of the table, as often this prompts it to lift its eyes such that the third eyelid retracts.

• Normal fundus The appearance of a normal feline fundus is described on page 708.

• Hypertensive retinopathy Systemic hypertension is common in older cats and hypertensive retinopathy is one of the most prevalent fundic abnormalities in this species. By routinely performing a fundic examination in geriatric cats, hypertensive retinopathy may be identifiable, and thus treated, before a cat becomes blind. The early signs include small retinal hemorrhages (Figure 17) and/or focal bullous retinal detachments (appearing as grey bubble-like elevations; Figure 18). More advanced lesions include large areas of hemorrhage, and detachment of all or a significant portion of the retina, resulting in vision loss (Figures 19 and 20). Some cats may regain vision if the retina reattaches with medical management of the hypertension. Cats with significant hemorrhage usually have a poor prognosis for return of vision.

• Retinal degeneration Retinal degeneration can be recognized by tapetal hyperreflectivity and attenuation or loss of retinal blood vessels (Figure 21). Some breeds, such as the Abyssinian, Somali and Siamese, may have an inherited form of retinal degeneration. Administration of enrofloxacin, particularly to cats with suboptimal renal function, may lead to acute and irreversible retinal degeneration.^14–16

• Chorioretinitis Chorioretinitis is an inflammatory condition that may be caused by some infectious diseases such as feline infectious peritonitis (Figure 22), systemic mycoses (cryptococcosis, histoplasmosis, blastomycosis or coccidioidomycosis) (Figure 23), toxoplasmosis or feline leukemia virus infection. Chorioretinitis is characterized by cellular infiltrates in the choroid, which lies posterior to the retina. Additionally, the retina itself may have inflammatory infiltrates and varying degrees of retinal detachment may occur. Lymphoma can also cause chorioretinitis that is indistinguishable in appearance from an infectious cause.

Figure 17

Multifocal small retinal hemorrhages in a cat with systemic hypertension and hypertensive retinopathy

Figure 18

Multifocal areas of intra- and subretinal fluid accumulation (arrows) in a cat with systemic hypertension and hypertensive retinopathy

Figure 19

Large areas of hemorrhage within the retina and in the vitreous in a cat with systemic hypertension and hypertensive retinopathy

Figure 20

Complete serous retinal detachment in a cat with systemic hypertension and hypertensive retinopathy. Note that no hemorrhages are present in this cat

Figure 21

Advanced retinal degeneration in a cat. Note the tapetal hyperreflectivity, loss of retinal blood vessels and optic nerve degeneration

Figure 22

Retinal vasculitis in a cat with feline infectious peritonitis. Note the perivascular cuffing of inflammatory cells

Figure 23

Focal chorioretinitis (arrow) in a cat with cryptococcosis

Any patient in which chorioretinitis is identified should undergo a thorough systemic work-up in an attempt to determine an underlying cause.

Key points

• A complete eye exam is critical for the cat presented for ocular disease or a systemic illness.

• Many normal cats will have a weak menace response and poor visual placing. The cotton ball test often elicits the best visual response.

• When performing tonometry it is important that no pressure is placed on the cat’s neck or on the globes when holding open the eyelids.

• Hypertensive retinopathy is the most common cause of vision loss in older cats.

• Fundic examination should be performed on every geriatric cat, regardless of the reason for the veterinary visit. Early signs of hypertensive retinopathy may be identified before the cat goes blind.

• It takes practice to master the technique of indirect fundic examination, as well as to distinguish normal and abnormal findings.