Equine ocular examination: Review of normal anatomy, diagnostics, procedures, differentials, drugs that aid in diagnostic examination and therapeutics (Proceedings)
The equine ocular exam is a routine ophthalmic exam with special consideration given to the size, temperament and use of the animal being examined.
Prior to the ocular exam, general observations should include the appearance and condition of the horse and its ability to maneuver on both a flat and uneven surface. A careful history should be elicited from the owner and general observations recorded.The order of the ocular exam always starts on the outside and moves inward in a routine and consistent fashion to avoid missing any shuttle lesions.
Start with the head and periocular tissues, and work your way to the retina. When viewing the horse head-on, note symmetry of the eyes, with particular interest on the position of the eyelids and eyelashes. Closer inspection of the periocular tissue will include the condition and amount of the orbital fat with their effect on the position of the globe. The eyelids are noted to include the lid margins for any evidence of prior lacerations. The conjunctival surfaces of the eyelid, third eyelid and are evaluated for hyperemia and chemosis. The nasolacrimal puncta are observed, both at the dorsonasal and ventronasal lid margins and at the nares.
The cornea and bulbar conjunctiva are evaluated next. The cornea should be evaluated for clarity, coloration and any breaks or changes in the surfaces. This requires bright focal illumination and is preferably performed in a dark stall with 2-4x magnification; the biomicroscope is the best instrument for this purpose. The conjunctiva should be noted for coloration or lack of coloration, especially at the lateral limbus.
Next, proceed to examination of the anterior chamber, to include the iris and the aqueous. The aqueous fluid should be clear with an absence of flare or cells. The iris surface is inspected for normal architecture, including any changes in coloration and size and appearance of the granula iridica. Atrophy of the granula iridica may indicate prior inflammation, and any cystic formation should be noted.
The lens is best examined through a dilated pupil in a dark stall. Short-acting mydriatic agent (tropicamide) will act for three to four hours, and the horse should not be ridden during this period of time. Warning: atropine, although useful for therapeutic purpose in dilating the pupil, may last upto three to four weeks and should only be used as a therapeutic agent.
The lens is evaluated for clarity and any adhesions of the iris to the lens capsule, or pigment migration on the surface of the lens capsule.
Intraocular pressures can be evaluated by using one of the newer tonometry systems. The rebound and applanation tonometers have both been used. Schiotz tonometry is impractical.
The posterior segment, including the vitreous and fundus, is examined next and requires either a direct ophthalmoscope or indirect ophthalmoscopy system. There are a number of techniques for performing indirect ophthalmoscopy, and these will be discussed. The vitreous will be evaluated for any infiltrates, floaters and liquefaction. A cloudy, liquefied vitreous is not uncommon in older horses and especially those that have prior episodes of uveitis. Evaluation of the fundus should be systematic. We recommend starting at the optic nerve and then evaluating each quadrant in a systematic fashion. The optic nerve is typically faint salmon-colored and has anywhere from 40-60 small arterioles and venules that emanate a short distance around the optic disc, and the disc always lies in the nontapetal fundus. The tapetal fundus typically has a very granular appearance, and end-on vessels, called "stars of Winslow," are often seen and are normal. A wide variety of normal colorations are noted in the tapetal fundus, and these are often related to coat color.
The neuro-ophthalmic examination should include eyelid reflexes, menace response and positional changes of the globe with movement of the head. Consensual pupillary reflexes should be evaluated by one examiner shining a bright light in one eye and looking for the associated response in the other.