Eyeball Position & Movement (III, IV, VI, VIII)
While examining the head, the clinician should assess whether or not each eye is in a central position within the orbit, which requires normal function of the peripheral and central components of the vestibular system; cranial nerves III (oculomotor), IV (trochlear), and VI (abducent); and the extraocular muscles that they innervate. Ventrolateral strabismus is associated with oculomotor nerve dysfunction, a medial strabismus with abducent nerve dysfunction, and eyeball extorsion with trochlear nerve dysfunction. Trochlear dysfunction in cats causes a lateral rotation of the dorsal aspect of the pupil, which cannot be recognized on external ocular examination in dogs because their pupils are round. In dogs, fundic examination is required and would show lateral deviation of the retinal vein.