When all one does is deal with the “eye” day in and day out, one may (or should) get a feel for certain things that can go wrong. You might say that such is my case. I have been able to observe some common pitfalls in equine ophthalmology that we all may fall into. I can try to map these out for you here, but if you’re as bad at reading maps as I am, you may still “fall in” occasionally. But, that’s what life’s all about...seeing trouble ahead, but still getting into it anyhow.
Let’s start easy. Take a simple equine corneal ulcer, for instance. The ulcer is small, shallow and puncture-like. You treat it correctly with topical antibiotics and atropine. The ulcer re-epithelializes very quickly (in fact, too quickly) and all is well. The ulcer site is fluorescein negative. To decrease scarring, (or really because the owner keeps bugging you about the white spot on the cornea) you immediately switch to a topical corticosteroid, such as dexamethasone. The eye becomes intensely inflamed, vessels start appearing in the cornea, a white spot (abscess) appears in the cornea and the eye goes to the netherworld. The second mistake now would be to continue topical steroids to decrease this inflammation.
Be wary of a few things when dealing with simple ulcers, especially small puncture types of ulcers. The epithelium can quickly grow over a small ulcer site by rapid mitosis and seal bacteria within the stroma. The topical antibiotic may not be given a chance to destroy bacteria in the wound. Corticosteroids on top of this decrease the normal inflammatory cell response, allowing bacteria to persist and infect the stroma. In essence, a corneal abscess is formed. The only chance for the eye now is to 1) somehow get antibiotics to the abscess and 2) allow the natural inflammatory response to clean up the abscess. To avoid all of this in the first place, be careful of early use of topical corticosteroids when dealing with equine corneal ulcers and monitor closely those small ulcers that re-epithelialize very rapidly, especially puncture type ulcers that are more likely to abscess.
Another area in which problems often arise is that of the fungal corneal ulcer. These ulcers can be difficult to diagnose and treat and can develop into a mess if not dealt with quickly and adequately. A fungal etiology should be suspected in any ulcer that is non-responsive to topical antibiotic therapy, in any ulcer that shows any degree of “melting," or in any ulcer that exhibits a white-plaque appearance. Definitive diagnosis is made by corneal cytology, culture, of histopathology. Unfortunately, fungi can be rather elusive at times and a negative cytology or culture does not necessarily rule out the disease. I have often initiated antimycotic therapy for ulcers I have suspected as being fungal, despite negative diagnostics. Keep in mind that therapy of fungal ulcers is very intensive and frequently requires conjunctival grafting for a cure.
Regarding equine uveitis, the first problem I’ve seen is in cases of severe corneal ulcers and secondary uveitis. Several mechanisms may be involved here. The first involves the trigeminal reflex between the cornea and anterior uvea. Corneal pain may cause uveitis simply by the trigeminal nerve reflex. A more severe uveitis may occur from exotoxins released from bacterial or fungal organisms within an ulcer or abscess or from inflammatory by-products. These may diffuse through the cornea and enter the anterior chamber and cause severe uveitis. Likewise, direct extension of infectious organisms from a corneal lesion into the anterior chamber may cause severe uveitis. When dealing with corneal disease, don’t miss the anterior uveitis that often results from it and likewise, don’t miss treating the uveitis as well.
Acute equine recurrent uveitis (ERU) is unmistakable by its clinical signs of blepharospasm, conjunctivitis, corneal edema, aqueous flare, miosis and hypopyon. The two most common problems I see are not treating the acute aggressively enough and not continuing therapy for an adequate period of time. Unfortunately, I can’t give you a hard and fast rule for this, as this varies with the case, but allow me to say that I have treated severe cases of uveitis as often as every one to two hours round the clock. Subpalpebral lavage systems are lifesavers in these cases. Treatment may need to be continued on a reduced basis for a period of several months.
Along this same line, be careful of the signs of ERU in its quiescent stages while doing prepurchase examinations. These signs may include a corneal opacity, pigment on the anterior lens capsule, anterior or posterior synechiae, cataracts, “butterfly lesions” around the optic disc, retinal detachment, phthisis bulbi, or loss of vision.
Finally, just a brief word about when to refer a case. Immediately is the answer in emergencies that you do not feel comfortable in handling yourself. This may include deep or melting corneal ulcers or corneal lacerations, acute anterior uveitis, or sudden blindness. Referral may be an option if an ocular condition does not respond as you would expect within a period of 1-2 weeks of treatment for less serious conditions (ex: conjunctivitis, masses, simple ulcers), or as soon as 1-2 days for more serious conditions (ex: deeper ulcers, uveitis). Finally, referral may be an option if the mode of diagnosis or treatment requires specialized instrumentation (ex; tonography, slit lamp biomicroscopy, cryothermy, ultrasonography, CO2 laser, ocular surgical instrumentation).
If you have any questions or concerns regarding Common Equine Problems, please call Eye Care for Animals.
