Equine examination (Proceedings)
Equine ocular examination is very similar to small animal examination, with special considerations for the size and use of the animal being examined.
The first part of the examination would be a general examination, to include the general appearance and condition of the horse and its ability to maneuver on flat and uneven surfaces. A careful history should be elicited from the owner, and general observations should be made.The order of examination is from the outside inward, ending up with retinal and neuro-ophthalmic examination.
When approaching the horse head-on, the symmetry of the eyes should be noted, and in particular, the position of the eyelids and lashes. Closer inspection of the periocular tissue would include the degree of condition and the amount of orbital fat, which has an effect on the position of the globe. The eyelids are then noted, to include the eyelid margins for any prior lacerations or breaks in the eyelid margin.
The conjunctivae of the third eyelid and the palpebrae are evaluated for hyperemia and chemosis. The puncta of the nasolacrimal system should be observed in the dorsal and ventronasal lid area. The corneae and the conjunctivae are then evaluated next, and the corneae should be evaluated for clarity, any coloration, and change in the smoothness of the surface; this requires bright focal illumination and is preferably performed in a darkened stall with magnification (a slit-lamp biomicroscope is the best instrument for this purpose). Next, we proceed into the eye to examine the anterior chamber for clarity and the absence of flare and cells in the aqueous humor. The iris surface is inspected for normal architecture, and the corpora nigra (granula iridica) is then evaluated, both on the dorsal and lower pupillary margin; atrophy of this may indicate prior inflammation, and cystic formation should be noted. The lens is best examined through a dilated pupil in dark surroundings. Short-acting mydriatic agents act for three to four hours, and the horse should not be ridden for that period of time following instillation of short-acting products. Atropine, as a warning, may last for up to two to three weeks in a horse and should only be used as a therapeutic agent. The lens is evaluated for clarity and any coloration changes, adhesions of iris-to-lens capsule, and pigment migration on the surface of the lens capsule. Intraocular pressures can then be evaluated by using one of the newer tonometry systems; both rebound tonometry or applanation tonometry are useful (Schiotz tonometry is impractical in a horse). Next, the posterior segment, to include the vitreous and the fundus, is examined; this will require either a direct ophthalmoscope or indirect ophthalmoscopy, which can be performed by a number of different techniques. The vitreous may be liquified or have clouding in older horses, or in horses with inflammation. Evaluation of the fundus should be systematic, and I typically recommend starting with the optic nerve and then evaluating each quadrant in a systematic fashion. The optic disc is typically pink-to-salmon-colored and has anywhere from 40-60 small arterioles and venules that emanate a short distance around the optic disc. The disc always lies in the nontapetal fundus dorsally. The tapetal fundus can be evaluated; it has a granular appearance and may have radiating, star-like color changes that have to do with capillary vessels within the tapetal and retinal area. The neuro-ophthalmic examination would include eyelid reflexes, menace response, and positional changes with movement of the head. Consensual pupillary light reflexes can be evaluated by one examiner shining a bright light in one eye and looking for the associated response in the other.