Posterior semicircular canal nystagmus is conjugate and its axis is parallel to that of the canal

A patient with a postoperative fistula of the left posterior semicircular c anal is presented. Negative pressure in the external ear canal produced upb eat-torsional nystagmus, which was recorded in three dimensions using binoc ular scleral search coils. The nystagmus was conjugate, without skew deviat ion, and its trajectory corresponded to the anatomic axis of the left poste rior canal. The current study helps validate Ewald's first law in humans: t he axis of nystagmus should match the anatomic axis of the semicircular can al that generated it. This law is clinically useful in diagnosing pathology of the vestibular end-organ, such as benign paroxysmal positional vertigo or the superior semicircular canal dehiscence syndrome.