Veterinary Articles | Why Cats Are Not Little Dogs
Veterinary Articles > Why Cats Are Not Little Dogs
Why Cats Are Not Little Dogs – Some Thoughts on Feline Ophthalmology
by Noelle La Croix, DVM, Dip. ACVO
Veterinarians face unique challenges when a cat with an "eye problem" arrives at their practice. The presentation, diagnosis, treatment, and underlying pathology of feline ophthalmic disorders often differ from their canine counterparts. This brief review will contrast feline and canine ophthalmology, and describe some novel features of feline ocular anatomy.
Eyelids – In dogs, meibomian gland tumors are fairly common and usually benign. They tend to distort the eyelids and can affect corneal health. In cats, the most common eyelid tumor is squamous cell carcinoma. This malignant tumor is often locally invasive, but it can also metastasize. Masses of the feline eyelid should therefore be biopsied and/or removed soon after initial examination. An exception is the benign apocrine hidrocystoma (presenting as multiple black nodules along the eyelid margin) of a Persian cat (Figure 1). Apocrine hidrocystomas (Moll’s cysts) derive from the sweat glands of the eyelid margin. These masses can be removed but recurrence is likely.
Conjunctiva – Any “red eye” should be subjected to typical diagnostics including Schirmer tear testing, fluorescein staining, and tonometry. In the dog, allergic conjunctivitis in common and will usually resolve with topical steroid therapy. In contrast, feline allergic conjunctivitis is very rare. The most common causes of feline conjunctivitis are infections of Herpesviridae, Chlamydia, Mycoplasma, or Caliciviridae. Topical steroid therapy can exacerbate these infections. Conjunctival redness and chemosis may initially subside, but subsequent steroidal immunosuppression will keep the infection from resolving. Therefore topical steroid therapy is not recommended for treating feline conjunctivitis.
Third Eyelid or Nictitating Membrane – “Cherry eye” (prolapse of the gland of the third eyelid) is common in dogs, but very uncommon in the cat. Only Burmese cats are known to regularly present with third eyelid gland prolapse. Third eyelid disorders in dogs can also result from an infiltration of inflammatory cells (via autoimmune disease). In contrast, cancerous cells are most often involved in feline infiltrative third eyelid disease.
Cornea – Corneal health relies upon adequate tear production. Low tear production in the dog is easy to diagnosis via Schirmer tear testing. Anything below 10-15 mm/min is defined as low tear production. Dogs will increase tear production in response to topically applied cyclosporine and/or tacrolimus. However, normal feline tear production can vary from as low as 5 mm/min to an average of 16 mm/min. The diagnosis of low feline tear production must be matched with other clinical signs. Most commonly, the decreased tear production is associated with a herpes viral infection. Both cyclosporine and tacrolimus act as T cell suppressors in the cat, and this immunosuppression can reactivate dormant Herpesviridae. Cyclosporine and/or tacrolimus are therefore not optimal for stimulating feline tear production. Artificial tear supplementation and herpesvirus remission will often resolve decreased tear production symptoms in the cat.
Dark opacities of the cornea also commonly differ in the dog and cat. In dogs the opacities are generally foreign bodies (e.g., seed hulls). These can often be simply removed along with the application of topical anaesthesia. In cats, the opacities are more often areas of corneal necrosis (corneal sequestra). Feline corneal sequestra require more complicated surgical removal (e.g., superficial keratectomy).
Glaucoma – A dog typically presents acute glaucoma with dramatic clinical signs including injected sclera, diffuse corneal edema, and a mid-range to dilated pupil. These glaucomatous eyes are very painful. Feline glaucoma typically presents with subtle clinical signs, or solely minor pupil dilation. Routine screening of the intraocular pressure is therefore recommended for all older cats. Evaluating the optic nerve for glaucoma-related damage is more difficult in cats than in dogs. A normal cat’s dark (non-myelinated) optic nerve can be mistaken for an indication of glaucoma. In a dog, dark optic nerves are a sign of glaucomatous damage.
Uvea - Uveal melanoma is found in both the cats and dogs. In the dog, uveal melanoma is usually nodular, locally aggressive, and rarely metastatic. But in the feline, uveal melanoma is typically diffused, infiltrative to the iris, and often metastatic. Early referral to an ophthalmologist is important in both cases, and diode laser ablation of the masses may save an eye.
Lens – Cataracts are common in dogs, but rarer in cats. Canine cataracts are associated with various genetic factors, and can also develop secondary to diabetes. In contrast, feline cataracts are rarely associated with diabetes, and are usually secondary to uveitis or trauma. Trauma to the feline lens can also lead to intraocular sarcoma, a malignant tumor unique to cats.
The primary cause of lens luxation in the dog is a genetically derived deterioration of the lens zonules. Lens luxation in the dog is an acute emergency. The canine anterior chamber volume (0.4 ml) is smaller then a lens (0. 5 ml) luxated within it, resulting in acute glaucoma and pain. The primary cause of lens luxation in the cat is an inflammatory breakdown of the lens zonules. All cats with a luxated lens should therefore be examined for signs of uveitis. Following lens luxation, the feline lens volume (0.3 ml) can be accommodated by the larger anterior chamber (0.6 ml). Acute pain is therefore not usually associated with feline lens luxation. The luxated feline lens will however need to be removed to prevent subsequent glaucoma.
Proptosis – Canine proptosis is generally secondary to trauma, and the prognosis for the return of vision is guarded. Favorable prognostic indicators for globe replacement include a positive direct or consensual pupillary light response, an undamaged posterior segment, and vision at initial presentation. Brachycephalic dogs are generally the best candidates for vision after globe replacement. In contrast, feline proptosis is generally secondary to massive trauma, and the prognosis for the return of vision is always grave (Figure 2). The protective feline orbit is simply much less susceptible to trauma than the canine orbit. All cats with an ocular proptosis should therefore be examined for other trauma (i.e., chest and skull radiographs) and the patient should stabilized as necessary.
This list of contrasting canine and feline ophthalmic phenomena is by no means exhaustive. My hope is that it will help guide you in your examination, diagnosis, and treatment of your next feline eye patient.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: Apocrine hidrocystoma in the right lower lid, upper lid and medial canthus of a Persian cat.
Figure 2: Proptosis of the left eye in a cat. The eye and orbit are infected.