with otitis media in large animals or with petrosal bone fractures. Inadvertent perijugular injection of drugs is a relatively frequent cause of Horner syndrome when the compound spreads to the adjacent cervical vagosympathetic trunk. The effect with local anesthetic compounds including α‐2 drugs is usually temporary. But depending on the degree of tissue inflammation caused by other, more irritant compounds, any resulting Horner syndrome can last for hours to months and may be permanent. In horses, the sympathetic fibers innervating the eye are more often damaged in and around the guttural pouch in the region of the cranial cervical ganglion. Finally, many systemic toxins, such as those mediated by atropine‐like alkaloids and the common antimuscarinic colic drug butylscopolamine, cause degrees of mydriasis (Figure 10.1), and those acting with anticholinesterase activity can result in miosis.
References
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11 Strabismus
Strabismus refers to abnormal deviation of the central axis of the eyeball. Loss of function of the extraocular muscles innervated by the oculomotor nerve should result in a lateral and slightly ventral strabismus. A medial strabismus should result with abducent nerve lesions. Finally, rotation of the globe such that the medial aspect of the pupil moves dorsomedial should result with trochlear nerve involvement. In each case, the eyeball cannot be moved out of the deviated position by movement and manipulations of the head and thus can be described as fixed strabismus (Figure 11.1). These are rarely seen alone as acquired syndromes in large animals. Both bilateral medial (convergent) strabismus, or esotropia, with exophthalmos (Figure 11.2), and lateral (divergent) strabismus, or exotropia, occur in cattle and are believed to be due to genetic brainstem nuclear defects.1–9 An eyeball deviation, particularly when it stays in a ventral or dorsal position but can be made to move from that abnormal position with head movements, is seen frequently. Usually, this is due to lesions involving the vestibular system and is referred to as vestibular strabismus (Figures 11.3 and 11.4).10–12 This may not be very evident with the head in a normal posture but usually is exaggerated when the muzzle is raised. Also, performing a fundic examination to focus on the position of the optic disc can assist in defining asymmetric positioning of the pair of eyeballs.
Figure 11.1 Although rarely encountered alone in large animals,