HOW-TO SESSION: OPHTHALMOLOGY


slightly higher than small-animal patients and have been found to generally range between 15 and 30 mm Hg.5,6 If a tonometer is not available, digital tonometry can be performed by placing both index fingers over the eyelid when covering the eye, and gentle, alternating digital pressure can be used to palpate for normal mild globe indentability. Alter- natively, a soft, blunt device (eg, a cotton-tipped applicator) can be used to attempt to gently indent the cornea after application of topical anesthesia. These methods give an approximation of eye pres- sure through subjective indentation and may help to identify soft or firm eyes consistent with uveitis or glaucoma, respectively, if a tonometer is not available. Though tear film deficiency is not commonly re-


ported in horses, a Schirmer tear test may be per- formed in animals suspected of having keratoconjunctivitis sicca, such as those with a dull corneal surface, unexplained corneal pathology, or those with facial nerve dysfunction. The tear test strip is folded at the notch while still in its plastic packaging and is then gently placed over the lower eyelid margin so that the folded tip sits within the conjunctival fornix. After 1 minute, the strip is removed and the reading is immediately recorded where moisture has traversed the test strip. Nor- mal tear values in horses are highly variable, with results in one study never below 10 mm/min and sometimes exceeding 35 mm/min.7 When beginning an eye exam general facial and


ocular symmetry should first be assessed from a distance in addition to looking for any signs of bleph- arospasm or ocular discharge. Cranial nerve eval- uation can then be performed with a palpebral reflex (evaluates trigeminal and facial nerves), menace re- sponse (evaluates optic and facial nerves), and pupil light reflex testing (evaluates optic and oculomotor nerves), although pupil light reflexes are commonly slow and incomplete in horses. Animals that do not menace because of impediments in the ocular media (eg, profound corneal edema, hyphema, cataract, etc) can have a dazzle reflex assessed by rapidly shining a very bright light at the eye and observing for a blink response or head jerk to indicate retinal and optic nerve functioning. Ocular motility can also be consciously evaluated because both eyes should be able to move in all directions and to do so concurrently (evaluates oculomotor, trochlear, and abducens nerves). Periocular palpation can be used to assess around the orbital rim. Globe retro- pulsion allows for further orbital examination by attempting to caudally displace the globe through digital pressure on a partially closed upper eyelid. Retropulsion also allows easier evaluation of the third eyelid because of its passive elevation with posterior globe movement. Retroillumination is a technique in which light is


shone toward the eyes at arm’s length (2–3 feet from the cornea) and the fundic reflection is visible through the pupil. It allows for easy assessment of


pupil size and symmetry and draws immediate at- tention to any impediments in the reflection (eg, focal corneal scar, cataract, etc). Detailed exami- nation of the ocular structures should proceed in a systemic manner with both direct and transillumi- nation, in which light is directed with the line of gaze, and at alternating perpendicular angles to the line of gaze so that the eye is truly assessed in all three dimensions. The individual anatomical structures that should


be consciously examined during a complete ocular evaluation include the eyelids, third eyelid, conjunc- tiva, sclera, cornea, anterior chamber, iris, lens, vit- reous, and fundus. Anterior segment examination requires only a light source and, ideally, a means for magnification. Inexpensive head loops or the oto- scope head on a Welch Allyn examination set serve as good magnification tool options. Complete lens and posterior segment examination should be per- formed after pharmacologic dilation with 1% tropic- amide (acts within 20–30 minutes), though adequate cursory evaluation is commonly possible in a darkened environment with the rheostat on the light source dimmed down slightly to minimize the pupil light reflex. In addition to fundus evaluation, the direct ophthalmoscope can also be used to assess for aqueous flare, an indicator of anterior segment inflammation, by selecting the smallest focal circu- lar beam of light and holding the instrument 5 to 10 mm in front of the cornea while viewing from the side (45–90° angle) in a very dark exam setting. Normal eyes will show the light beam hitting the cornea, a void in the anterior chamber, then light hitting the anterior lens capsule and coursing through the lens to end at the posterior lens capsule. An eye for which the light beam continues through the anterior chamber (like a head light beam in the fog) to connect the cornea and lens has aqueous flare, which is common with intraocular inflamma- tion or uveitis. Fundic examination can be performed with the


direct ophthalmoscope focused at 0 to 2 diopters (red numbers) to give a highly magnified direct up- right image. This method is good for detailed optic disc or fundic lesion assessment but is difficult for general examination, given the very limited field of view (2% of the fundus). The Welch Allyn Pan- Optic attachment is also a direct ophthalmoscope that gives a 5-times-larger field of view with less magnification. Indirect ophthalmoscopy with the use of a light source (eg, Finoff transilluminator) and handheld condensing lens (eg, 20 diopter) gives the greatest field of view to more easily assess the entirety of the fundus and is the author’s preferred initial assessment tool. The view with this tech- nique is inverted and reversed; therefore observed anatomy is effectively 180° off. Fluorescein stain should be performed on every


horse with an ocular complaint to evaluate corneal health and nasolacrimal duct patency. Stain can be applied to the ocular surface either by direct


AAEP PROCEEDINGS  Vol. 59  2013 147


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