Examination and diagnostic skills
An ocular exam is most likely to identify more subtle abnormalities when performed systematically on each occasion. Vision
and comfort are best assessed in the undisturbed patient in its familiar environment or under the conditions (bright light
or darkness) where the complaint arises. Indicators of discomfort include reduced palpebral fissure size or asymmetry (compare
eyelash and vibrissae angle), ocular discharge, increased blink rate, prominence of the nictitans, and the horse's avoidance
of contact with either globe. Palpebral fissure asymmetry is noteworthy and can reflect abnormalities with the orbit, globes
or eyelids. Vision testing should include the menace response from multiple angles, and a navigation (maze) test before sedation.
Visually oriented responses, such as avoiding objects and stepping over kerbs indicate vision, is present: partial visual
impairment is more challenging to identify.
Eyelid innervation and motor function is verified by the palpebral response (prior to blockade), and direct and consensual
pupillary light reflexes (PLR) are assessed with a bright light. Both rate of constriction and degree of completeness are
assessed because partial denervation is relevant. Apparent lack of menace can be caused by eyelid paralysis, but nictitans
excursion often confirms an intact visual pathway. Anisocoria is assessed in dim lighting by observing the tapetal reflection
outlining the pupils through the direct ophthalmoscope from 5-8 feet in front of the head. Buphthalmos or phthisis bulbi should
be differentiated from globe displacement (enophthalmos or exophthalmos) and relative nictitans prominence. Asymmetry might
be due to ocular pain (globe retraction), reduced orbital contents (enophthalmos), physical displacement by a mass, phthisis
bulbi/globe rupture, autonomic denervation (Horner's syndrome) or eyelid paralysis, and less commonly fracture or other damage
to the orbit.
The corneal reflection should be bright and sharply focused, indicating tear film quality and quantity. Dry eye is rare in
horses but might be associated with skull trauma, plant intoxication and infrequently with pharmaceutical intoxication or
eosinophilic keratitis. Corneal clarity and discoloration are important findings and are more easily identified in dim surroundings.
A generalized haze might reflect focal or geographic corneal edema, ulcerative keratitis, fibrosis or aberrant metabolic products
such as lipid or mineral. Opaque focal white-tan opacities within the cornea are particularly significant (see stromal abscess
in previous edition).
Fluorescein stain retention is best identified with cobalt-blue light. More subtle tear film and epithelial abnormalities
are seen with Rose bengal stain and bright white light [Akorn Inc., (800) 535-7155]. Lissamine Green stain is less useful.
Corneal staining should be performed before iatrogenically drying the tear film and causing false positives. Magnification
and focused light beams allow closer scrutiny. Fundic abnormalities can occur in 10 percent of horses, so indirect screening
funduscopy should be performed routinely, ideally after dilation, but few lesions are solely in the periphery. The fundus
includes the ONH, retina and highly vascular choroid, which dorsally contain the reflective tapetum fibrosum. Direct ophthalmoscopy
permits close examination of the optic nerve head (ONH), but the 7.9x magnification and reduced field of view make a complete
exam laborious. Indirect ophthalmoscopy can be performed more rapidly with a bright light source and indirect lens (20D magnification
0.79x, or 14D magnification 1.2x). Apparent lesions should be interpreted with other ipsilateral changes and contrasted with
the fellow (or other normal) eye.
Many normal variations are encountered, especially among subalbinotic horses. The entire length of retinal vessels (1-2 ONH
diameters) might not be seen unless the room is dark. The remainder of the retina is perfused perpendicularly by choroidal
capillaries, the "stars of Winslow". If the RPE is unpigmented, dramatic views of the choroidal vasculature are apparent and
should be differentiated from hemorrhage. Typically well-organized parallel vessels are completely normal unless the overlying
retina has been depigmented by some pathologic process.
Many aging changes pose little threat to vision but should be considered together with the clinical complaint and demonstrated
ability to navigate and perform visually oriented tasks.
The PanOptic® (Welch Allyn) is a new device which provides intermediate field-of-view and magnification while still being
operated with one hand. It is monocular and lacks stereopsis, but it permits more rapid screening of the fundus. The more
powerful lithium battery is recommended for adequate illumination because of the internal lenses. An extended rubber eyepiece
reduces ambient illumination, but many horses resist its contact with the eyelids. This device provides a single instrument
alternative to screening with indirect ophthalmoscopy, and scrutiny of potential abnormalities by direct funduscopy.