Localizing signs in positional vertigo due to lateral canal cupulolithiasis

Background: Different types of benign positional vertigo (BPV) have been re cognized. The variant with permanent apogeotropic direction-changing latera l nystagmus in the supine position is particularly difficult to distinguish from central etiologies. Objective: To identify clinical features of this variant of BPV, helping to establish its peripheral etiology. Methods: In f ive patients without any evidence of neurologic disease and with this varia nt of positional vertigo, the behavior of nystagmus as a function of head p osition in space was studied. Results: In the supine position, a null point for lateral nystagmus was identified, beyond which the nystagmus changed d irection. This null point was evident when the head was turned 10 to 20 deg rees to the side. In this position, the ipsilateral cupula of the lateral s emicircular canal is aligned with the gravity vector. In two of the five pa tients, a null point was identified in pitch, beyond which the nystagmus re versed direction. This null point corresponds to the head position where th e lateral canals are in an earth horizontal plane. Conclusion: From the beh avior of lateral nystagmus in different head positions, the lateral canal s ystem of the inner ear is shown to be gravity-sensitive and the side on whi ch the cupula is affected can be determined.