Eosinophilic keratitis is associated with infiltration of eosinophils into the feline cornea. It may be an unusual immune response to latent feline herpesvirus. Treatment with topical steroids usually is sufficient, but some cases do not respond. Oral megestrol acetate (0.5 mg/kg/day until a response is noted, then 1.25 mg, PO, 2–3 times weekly as required) helps to improve or resolve the corneal inflammation via an unknown mechanism. However, its use is associated with adverse effects such as diabetes mellitus, adrenocortical suppression, and uterine hyperplasia, and this drug should be used with extreme caution. Megestrol acetate can also be dangerous to women handling the pills.
Older cats can present with sudden blindness due to serous retinal detachments secondary to systemic hypertension, or less commonly hyperthyroidism. Treatment is with the calcium channel blocker amlodipine (0.625 mg/cat once to twice daily; some cats may require 1.25 mg/cat, bid) and systemic corticosteroids (prednisone, 0.5–1 mg/kg, PO) to help control the posterior inflammation. Retinas can reattach once blood pressure returns to the normal range. At least 50% of cats will regain some clinical vision if treated early. Cats should be rechecked 1 wk after starting therapy, and blood pressure regularly monitored. Adverse effects of amlodipine are uncommon but include azotemia, lethargy, hypokalemia, and tachycardia.