Localizing signs in positional vertigo due to lateral canal cupulolithiasis

Background: Different types of benign positional vertigo (BPV) have been recognized. The variant with permanent apogeotropic direction-changing lateral 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 five patients without any evidence of neurologic disease and with this variant of positional vertigo, the behavior of nystagmus as a function of head position in space was studied. Results: In the supine position, a null point for lateral nystagmus was identified, beyond which the nystagmus changed direction. This null point was evident when the head was turned 10 to 20 degrees to the side. In this position, the ipsilateral cupula of the lateral semicircular canal is aligned with the gravity vector. In two of the five patients, a null point was identified in pitch, beyond which the nystagmus reversed direction. This null point corresponds to the head position where the lateral canals are in an earth horizontal plane. Conclusion: From the behavior of lateral nystagmus in different head positions, the lateral canal system of the inner ear is shown to be gravity-sensitive and the side on which the cupula is affected can be determined.