Physiologic nystagmus (oculocephalic or oculovestibular responses) is evaluated by moving the head in a horizontal plane from side to side. This stimulates cranial nerve VIII to relay impulses to the brainstem, on to the vestibular nuclei and the medial longitudinal fasciculus, and finally to abducent and oculomotor neurons for abduction and adduction of the eyeball, respectively.
Strabismus and nystagmus commonly are associated with dysfunction of the vestibular system. A positional, ventrolateral strabismus commonly is associated with vestibular disease. Nystagmus is an involuntary oscillation of the eyeball. Resting or spontaneous nystagmus is continual and observed when the head is in any position, whereas positional nystagmus is seen only when the head is held in certain positions. The latter often is seen in patients that have accommodated for lesions in the vestibular system. Occasionally, positional nystagmus can be elicited in compensated patients placed on their backs with their head and neck extended.