Examination of the Eye of the Horse

Take Home Message  

To be able to perform a proper ophthalmic examination it is necessary to have a bright focal light  source such as a transilluminator or a direct ophthalmoscope. The head is examined for symmetry,  globe size, movement and position of the globe, ocular discharge, and blepharospasm. The general  appearance of the eyes and adnexa is noted. It can be useful to examine the angle of the eyelashes on  the upper lid to the cornea of the two eyes, as droopiness of the lashes of the upper lid may well  indicate blepharospasm, ptosis, enophthalmos, or exophthalmos. Normally the eyelashes are almost  perpendicular to the corneal surface. The first sign of a painful eye often is the eyelashes pointing  downwards.  

Examination Techniques  

Reflex Testing  

Making a quick, threatening motion toward the eye to cause a blink response and/or a movement of  the head tests the menace response. This is a crude test of vision. Care is taken not to create air  currents toward the eye when performing this test. Horses have a very sensitive menace response.  

The horse should also quickly squint or “dazzle” when a bright light is abruptly shown close to the  eye.  

The palpebral reflex is tested by gently touching the eyelids and observing the blink response.  

Vision could be further assessed with maze testing with blinkers alternatively covering each eye.  The maze tests should be done under dim and light conditions.  

The pupillary light reflex (PLR; direct and indirect) (Fig. 1) evaluates the integrity of the retina,  optic nerve, midbrain, oculomotor nerve, and iris sphincter muscle. The normal equine pupil  responds somewhat sluggishly and incompletely unless the stimulating light is particularly bright.  Stimulation of one eye results in the constriction of both pupils. The PLR is valuable in testing  potential retinal function in eyes with severe corneal opacity.  

Diagnostic Testing  

It is important to approach each eye problem in the horse in an ordered and systematic manner. The majority of cases can be diagnosed by using standard ophthalmic clinical examination techniques.  

Intravenous sedation, a nose twitch, and supraorbital sensory and auriculopalpebral motor nerve  blocks may be necessary to facilitate the examination (Fig. 2).  

The auriculopalpebral nerve (motor nerve to the orbicularis oculi muscle) can be palpated under  the skin and blocked with 2-3 ml of lidocaine or mepivicaine (Carbocaine) just lateral to the highest  point of the zygomatic arch.  

The frontal or supraorbital nerve (sensory to the medial two thirds of the upper lid) can be  blocked at the supraorbital foramen. This foramen can be palpated medially at the superior orbital  rim where the supraorbital process begins to widen. Line blocks can be used near the orbital rim to  desensitize other regions.
 

Schirmer tear testing is a method to measure reflex tearing and should be used for chronic ulcers  and eyes in which the cornea appears dry. The Schirmer tear test must be done prior to instillation of  any medications into the eye. The test strip is folded at the notch and the notched end inserted over  the temporal lower lid margin. The strip is removed after one minute and the length of the moist end  measured. Strips are frequently saturated in horses after one-minute with values ranging from 14-34-mm wetting/minute considered normal. Values less than 10-mm wetting/minute are diagnostic for a  tear deficiency state.  

Corneal cultures using microbiologic culture swabs should be obtained prior to placing any topical  medications in the eye. The swabs should be gently touched to the corneal ulcer.  [...]