The examiner should begin by placing him- or herself at eye level with the cat and observing from a distance while avoiding excessive manipulation of the face. This allows for detection of nonocular abnormalities that may be related to the ocular disease, such as facial asymmetry, blepharospasm, oral or nasal discharge, or the presence of a head tilt. The examiner should then assess for the presence of anisocoria using retroillumination, by using the transilluminator held at arm’s length from the cat’s face (Fig. 32.1). It is especially important to first direct the light into both eyes equally while observing both pupils simultaneously. This reduces the potential of missing a subtle anisocoria. The examiner should then perform the neuro-ophthalmic examination (Table 32.1). The neuro-ophthalmic assessment for cats can yield different results than for the dog. For example, the menace response tends to be inconsistently elicited in cats, with many normal cats failing to blink in response to a menacing gesture (Fig. 32.2). Likewise, stressed cats with higher sympathetic tone often have resting mydriasis and diminished pupillary light reflexes (PLRs) unless a very bright light source is used. The PLRs should be checked by directing the light into each eye separately. The direct PLR describes pupillary constriction in the illuminated eye. To document the indirect, or consensual, PLR, the light is then quickly directed into the contralateral eye where the pupil should already be constricted to almost the extent of the directly illuminated eye. It is important to observe the pupil in the contralateral eye immediately after the light is moved to avoid inadvertently stimulating a direct PLR that can be mistaken for the indirect reflex. Once this has been completed, the light stimulus should be removed for 20–30 seconds before checking direct and indirect PLRs in the other eye. The indirect PLR is one of the most important prognostic tests in veterinary ophthalmology. While there are many reasons a direct PLR may be absent in a diseased eye, the absence of an indirect PLR from the affected to the normal contralateral eye (i.e., lack of constriction in the contralateral eye when shining the light into the diseased eye) suggests retinal or optic nerve dysfunction in the affected eye, either of which greatly diminish prognosis for return of vision in that eye. The dazzle reflex, which evaluates cranial nerves II and VII and the subcortical visual pathways, can be checked at the same time as the PLR. A positive dazzle reflex refers to closure of the eyelids of one or both eyes when light is directed into one eye. Conversely, absence of eyelid closure when light is directed into an eye is referred to as a negative dazzle reflex. A positive dazzle reflex should not be used to infer a visual eye; shining light into a blind eye can often elicit a dazzle reflex. However, a negative dazzle reflex is often confirmatory for a blind eye. A negative dazzle reflex is therefore more clinically informative than a positive dazzle reflex.

The STT should be completed before any eye drops are applied to the eye. It is performed in the same manner as in the dog (Fig. 32.3). Because normal STT measurements vary widely in cats, ^1–4 and cats with significant keratoconjunctival disease may have STT results within the normal range,⁵ this test was previously considered to have little value by practitioners and was therefore underutilized. The belief that stress results in falsely low STT readings was another reason the STT was not in routine use for the feline ophthalmic examination. However, the STT is apparently unaffected by sympathetic stimulation and seems to be a reliable test for diagnosing keratoconjunctival disease in cats.⁴ It should therefore routinely be included in the ophthalmic examination. Based on data from a comprehensive study of normal cats, the median STT reading is 18 (range, 9 to 34) mm/min.³ The same study revealed no healthy cat with a STT value <7 mm/min, and suggested that STT values <9 mm/min, combined with clinical signs of ocular surface disease, would support a diagnosis of tear film deficiency. In a retrospective study of cats with low STT, the majority of eyes with STT <9 mm/min had ocular surface disease, and the mean STT of those with corneal and/or conjunctival disease was 2.4 ± 3.1 mm/min.⁶ This emphasizes the importance of interpreting the STT in conjunction with findings from the complete ophthalmic examination and comparing STT values between the affected and unaffected eyes in cats with unilateral or asymmetric ocular disease.

Following the STT, IOP should be assessed. While retropulsion is of value in detecting space-occupying lesions of the orbit, it is not considered an acceptable technique for estimation of IOP. Both the Schiotz tonometer and the TonoPen (Reichert Inc., Depew, NY) require application of a topical anesthetic agent prior to obtaining an IOP assessment, while topical anesthetic should not be applied prior to using the TonoVet (Reichert Inc., Depew, NY; Fig. 32.4). The TonoVet and, to a lesser extent, the TonoPen can be used judiciously in the presence of corneal injuries, but the Schiotz should be avoided in corneas that have ruptured or are at risk of rupture. While there is some variability, normal feline IOP tends to be 10 to 25 mm Hg.⁷ However, differences in measurements among instruments should be kept in mind. For example, the TonoVet tends to overestimate IOP while the TonoPen tends to underestimate IOP.^8–10 Two studies suggest the TonoVet is more accurate than the TonoPen in cats.^9,10 Regardless, it is important that some means of assessing IOP is always included in the feline eye examination, and that it is done consistently using the same type of tonometer in individual cats over time. If the IOP is within normal limits, a single drop of 0.5% or 1% tropicamide should be applied to each eye to achieve pupillary dilation, which is the only way to ensure examination of all ocular structures. Intraocular structures should be examined before and after pharmacologic dilation to maximize the potential for visualizing all lesions. For example, the anterior surface of the iris is a frequent site of pathology in cats and is only visible in its entirety when the pupil is miotic. It therefore must be evaluated prior to dilation. By contrast, thorough assessment of the lens, vitreous, and fundus can only be performed after full pupillary dilation. In cats, tropicamide-induced mydriasis occurs within 15 minutes and persists for 8–9 hours.¹¹ Pharmacologic mydriasis is not recommended if the IOP is above normal as dilation may further increase it.

Sequential examination of all ocular structures moving from peripheral to axial and from superficial to deep ensures a complete and orderly examination and may be performed during and after pupillary dilation. The techniques used in cats differ little from those used in other species. Magnification with a head loupe ( e-Fig. 32.1) should be employed throughout the entire ophthalmic examination except for the fundic examination. The examiner should employ focal illumination (Fig. 32.5) from various angles when assessing the ocular surface. Surface contours, lesion depth, and three-dimensional spatial relationships are best assessed using the slit beam of the direct ophthalmoscope. Detection of aqueous flare (plasma protein in the anterior chamber) is pathognomonic for anterior uveitis; therefore, evaluation for aqueous flare forms a critical part of the ophthalmic examination in all cats. Aqueous flare is best detected when the room is completely darkened, and the smallest, most focal white light beam on the direct ophthalmoscope head is held close to the cat’s eye. The examiner should check the clarity of the anterior chamber by examining it from an angle while the light beam is directed across the anterior chamber. Aqueous flare occurs when the beam is visible as it traverses the anterior chamber. Finding aqueous flare should prompt investigation for other signs of anterior uveitis that might better define causes or sequelae of the uveal inflammation. Aqueous flare may be more easily detected after full mydriasis is achieved, as the black background of the pupil provides contrast for viewing the smoky grey beam of light crossing the anterior chamber. Once complete mydriasis is achieved, the examiner should repeat retroillumination (Fig. 32.1) to identify opacities in the clear ocular media (tear film, cornea, aqueous humor, lens, and vitreous humor) that may not have been visible prior to dilation.

Direct and indirect ophthalmoscopy are the two traditional methods for examining the fundus. Examples of the normal feline fundus and variations are found in Fig. 32.6, e-Fig. 32.2, e-Fig. 32.3 and e-Fig. 32.4. Direct ophthalmoscopy is relatively easy to learn and provides the examiner with an upright image of the retina but is not a good method for examination of the fundus. The main disadvantage of direct ophthalmoscopy is the limited field of view, a result of extreme magnification. This makes complete retinal examination difficult, and peripheral regions and smaller lesions are often missed. Indirect ophthalmoscopy is considered the best method for viewing the fundus, despite a steeper learning curve. The image produced is an inverted representation of the fundus. When a binocular headset ( e-Fig. 32.5) is used, indirect ophthalmoscopy provides superior depth perception. The larger field of view makes complete fundic examination easier and allows the practitioner to view a larger portion of the fundus in less time, which is extremely valuable for uncooperative patients, and permits comparison of features among regions of the fundus. The direct ophthalmoscope can then be used for more detailed examination of any focal lesions identified. The Panoptic (Welch Allyn; Chicago, IL) ophthalmoscope offers a field of view and level of magnification intermediate between the two traditional ophthalmoscopes (Fig. 32.7). Although it is unable to provide much depth perception, its upright image, ease of use, and moderate field of view make it a reasonable compromise between direct and indirect ophthalmoscopy.

Application of fluorescein dye should be performed only after all other parts of the examination are complete as it will affect results of microbial testing, the STT, and the appearance of many ocular structures. It is used to assess for corneal or conjunctival ulceration, corneal perforation, tear film stability, and patency of the nasolacrimal drainage system. The Jones test uses fluorescein dye to assess nasolacrimal duct patency (Fig. 32.8). Unlike most mesaticephalic and dolichocephalic dogs, fluorescein within the nasolacrimal apparatus in cats almost always enters the nose far enough caudally that it drips backward over the soft palate and into the oral cavity rather than out the nares. Therefore, the nasolacrimal apparatus of each eye should be tested independently with fluorescein dye first placed into the conjunctival sac of the eye in which obstruction is suspected, then the mouth (as well as the ipsilateral naris) is examined for evidence of fluorescein. The time necessary for appearance of fluorescein at the nares may be as short as a few seconds, but can be greater than 30 minutes.¹² Although presence of fluorescein in the mouth or nostril (a positive Jones test) confirms patency of the nasolacrimal duct, absence of fluorescein (a negative Jones test) can occur in normal cats and so should be followed with nasolacrimal flushing. This may require sedation in the cat but otherwise is performed as for the dog.

The tear film break-up time (TFBUT) uses fluorescein to evaluate stability of the precorneal tear film. It is the time between eyelid opening and the first spot of evaporation within the precorneal tear film. The examiner first places fluorescein dye into the conjunctival sac, then closes the eyelids. Timing begins when the eyelids are opened. Using magnification and a blue light source, the examiner observes one area of the cornea, usually the dorsolateral aspect. Timing ends when the first black spot, signifying evaporation, appears within the fluorescing tear film. The normal TFBUT in a cat is >10 seconds.3,13 A more rapid TFBUT suggests tear film instability due typically to mucin or lipid dysfunction rather than the more common aqueous-deficient dry eye syndrome seen in dogs.

Techniques for ancillary diagnostic tests such as corneal scrapings (Fig. 32.9), conjunctival swabs, and conjunctival biopsies (Fig. 32.10) are identical to those for the dog. However, the duration of effect of topical anesthetics required to obtain these samples is shorter in cats than in dogs. While a single drop of 0.5% proparacaine provides up to 45 minutes of corneal anesthesia in the dog, the same dose achieves only 25 minutes of corneal anesthesia in the cat, with maximal effect occurring 5 minutes following application.^14,15

Advanced imaging is required when opaque ocular media prevent complete examination of the globe, or when orbital or neurologic disease is suspected. Ocular ultrasound is useful for characterizing lesions within the globe, but may not allow full evaluation of orbital lesions (Fig. 32.11).¹⁶ Skull radiography and computed tomography (CT) also may not clearly show the borders of a lesion, but will detect bony changes.¹⁶ Magnetic resonance imaging (MRI) is of some value for characterization of bony lesions and offers excellent resolution of soft tissues, including the globe, orbital contents, and optic nerves.¹⁷

Orbital cellulitis and/or abscess formation in the cat is diagnosed and treated in a similar manner as for dogs. As in dogs, orbital extension of dental disease is responsible for many of these cases, but foreign body migration, fungal infections, and iatrogenic trauma (mainly during dental procedures) are also documented causes.20,21 However, these conditions occur much less frequently in cats than in dogs, and neoplasia should always be considered when a cat presents with orbital disease.²¹ In cats, the majority of orbital neoplasia is secondary, with direct extension from adjacent structures accounting for 71% of cases.¹⁸ Squamous cell carcinoma (SCC) is the most common orbital neoplastic process in cats,¹⁸ but many other neoplasms may occur.^{17,18,22–27} Regardless of type, 90% of orbital neoplasms are malignant, with mean postdiagnosis survival times of <2 months.^18,25

Ocular proptosis is seen less commonly in cats than in dogs, likely due to their deep orbits and relatively tightly fitting adnexa. For these reasons, the amount of force required to displace the globe from the feline orbit is typically large, and cats with ocular proptosis usually present with severe intraocular trauma, as well as concurrent cranial or systemic injuries that require more immediate attention. Ocular proptosis in cats often occurs in conjunction with skull fractures and trauma to the globe.28 As with dogs, enucleation or emergency replacement of the globe followed by temporary tarsorrhaphy is required. However, the prognosis following traumatic ocular proptosis in cats is worse than in dogs. In one retrospective review, all proptosed feline eyes were permanently blind, and 12 of 18 required enucleation.²⁸

Although not an ophthalmic condition, enucleation warrants discussion because it is commonly performed by practitioners and because cats are at higher risk of intraoperative harm than dogs. Because the feline optic nerve is relatively short compared to the canine optic nerve, and because the feline globe fits more tightly into the orbit, there is a higher likelihood of inadvertently applying excessive force to the optic nerve during routine surgical manipulations.29 This may result in injury to the optic chiasm as well as the optic nerve of the contralateral eye. Optic nerve atrophy and retinal degeneration result in irreversible blindness of the contralateral eye on anesthetic recovery. Absent menace responses, reduced or absent PLRs, and absent dazzle reflexes may be noted.^29,30 Funduscopic examination will initially reveal lesions in the peripapillary retina.²⁹ With time, atrophy of the optic nerve and surrounding retina will become visible.²⁹ To reduce the potential for complications, care must be taken to handle orbital tissues and the globe as gently as possible. In addition, optic nerve ligation should be avoided since this has been documented as a specific cause of residual optic nerve damage in cats. Adequate hemostasis can be achieved by applying pressure to the orbital tissues.³¹ Utilizing the subconjunctival enucleation technique may also reduce tissue disturbance, and using smaller instruments lessens the amount of traction needed to place an instrument behind the globe.³²

Feline restrictive orbital myofibroblastic sarcoma (FROMS) was thought to be an inflammatory disease and was therefore originally referred to as feline orbital pseudotumor (FOP). However, it has been reclassified as a low-grade but highly invasive spindle cell neoplasm.33 The disease manifests itself reasonably consistently, affecting middle-aged to older cats^34,35 ultimately bilaterally, even if only one eye is affected at initial presentation. Onset of clinical signs is insidious and is characterized by progressive exophthalmos, lagophthalmos, exposure keratitis, and restriction of ocular and eyelid movements.^34–36 The presence of keratitis can lead to a misdiagnosis of ­primary ocular surface disease; however, careful ophthalmic examination will reveal decreased retropulsion and globe movement, as well as thickened, immobile ­eyelids.^33,36 There may also be concurrent inflammation of the oral and/or nasal tissues.^33,36 Adnexal SCC may produce similar clinical changes and should be included as a differential when these changes are seen.³⁷

No specific changes are seen on bloodwork, and fine-needle aspirates tend to be nondiagnostic. Ultrasound may show increased echogenicity of orbital tissues.³⁵ Advanced imaging, such as CT and MRI, appears to be more useful, often revealing physical compression of the globe, thickening of orbital and periorbital tissues, and sometimes destruction of bony structures. Cross-sectional imaging techniques (CT or MRI) are very helpful for assessing disease extent and refining the diagnosis. In the one report documenting MRI findings in a cat affected by FROMS, precontrast images showed abnormalities only when fat suppression techniques were used, emphasizing the importance of including these techniques when planning MRI.³⁶ Ultimately, histopathology of orbital tissues (often obtained during enucleation/exenteration) is required for definitive diagnosis. In contrast to the standard recommendation for submission of globes for histologic examination, the globe from a cat with suspected FROMS should be submitted with eyelids and orbital tissue still attached. Histopathology will reveal neoplastic spindle cells spreading along fascial planes, entrapment and atrophy of normal orbital tissues, and lymphoplasmacytic inflammation of orbital tissues.³³

Prognosis for patients with FROMS is grave. Although early exenteration, with or without adjunctive therapy, is recommended, this is based on a small number of cases, and specific therapeutic guidelines do not exist. No published treatments, including immunosuppressive doses of systemic corticosteroids, oral antibiotics, oral antiviral medications, radiation therapy, and enucleation or exenteration, have been successful in halting disease progression.^34,35,38 In almost all published cases, cats were eventually euthanatized due to either recurrence of disease, appearance of the disease in the contralateral orbit, or invasion of nearby structures.^33,35

Eyelid agenesis, or eyelid coloboma, is the most common congenital eyelid disease of cats.39 It has been reported in Persian, Burmese, and domestic short hair cats, as well as snow leopards, and a Texas cougar.^40–44 Affected cats have bilateral incomplete development of the upper lateral eyelids ( e-Fig. 32.7 and Fig. 32.13). The extent of the defect ranges widely from a barely perceptible absence of the eyelid margin at the lateral extent of the eyelid to complete absence of the upper eyelid margin. The cause is unknown; viral etiologies, intrauterine events, and genetic mutations are proposed mechanisms although there is no clear evidence supporting any of these theories.⁴⁰

Most owners do not notice abnormalities until kittens are a few months of age, likely due to the small eye size of young kittens. On occasion, eyelid agenesis has been mistaken for upper eyelid entropion; however, close inspection reveals absence rather than inversion of the eyelid margin. Varying degrees of chronic keratitis due to trichiasis and lagophthalmos almost always accompany the eyelid defect ( e-Fig. 32.7). The conjunctival fornix at the site of the malformed eyelid also tends to be shallow or replaced by a thin band of conjunctiva directly connecting the globe to the eyelid. Other ocular abnormalities may accompany eyelid agenesis; most common are persistent pupillary membranes, which are remnants of dysplastic uveal tissue visible as thin strands or sheets of pigmented tissue commonly extending from the iris to the cornea. Colobomas of the iris, choroid or optic nerve, retinal dysplasia, and keratoconjunctivitis sicca may also accompany eyelid agenesis.^40,45 Like the eyelid defect itself, the associated developmental abnormalities range in severity, sometimes even among kittens from the same litter.⁴⁰

Treatment varies with the extent of the eyelid malformation. Very small defects may require cryoepilation or primary closure for resolution of trichiasis; however, most defects of clinical significance require more extensive blepharoplasty. Surgical reconstruction usually involves two surgeries; the first to correct the defect, and the second to correct residual trichiasis. The most common procedure is rotation of a flap of skin from the lower eyelid (Fig. 32.14). Subsequent cryoepilation or a Hotz–Celsus procedure (excision of a crescent-shaped section of eyelid with the underlying orbicularis oculi muscle) can be employed to address trichiasis from the skin flap.

Breed-related entropion in cats is often more subtle than the analogous condition in dogs. It occurs most often in brachycephalic cats; they tend to be mildly affected at the medial aspect of the lower eyelid ( e-Fig. 32.8). Clinical signs may be absent, or there may be associated epiphora and tear-staining due to wicking of the tears by trichiasis, kinking of the nasolacrimal canaliculi, functional obstruction of the lacrimal puncta, and frictional irritation of the cornea. Treatment may not be necessary if there are no associated signs, or if signs are mild. Breed-related entropion is suggested to be more clinically significant in young, intact male Maine Coon cats where it is associated with excessive skin around the face and can result in pronounced lower eyelid entropion.⁴⁶

Unlike breed-related entropion, acquired entropion in cats is often associated with clinically significant keratitis (Fig. 32.15). In general, acquired entropion occurs more often in older cats and in response to a primary pathologic process, typically blepharospasm, symblepharon formation due to keratoconjunctivitis, or abnormalities in globe position (e.g., enophthalmos) or size (e.g., microphthalmos or phthisis). Enophthalmos is more common in older cats or cachexic cats due to loss of orbital fat.

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