Dynamics of canal response to head-shaking test in benign paroxysmal positional vertigo.

CONCLUSIONS Time constant and maximum slow phase velocity (SPV) of head-shaking nystagmus (HSN) demonstrated a differential canal response to head shaking in 24% of patients with posterior canal benign paroxysmal positional vertigo (BPPV). We suggest that vestibular lithiasis has a limited contribution to the mechanism that generates HSN. OBJECTIVE To determine the canal response to head shaking in BPPV. PATIENTS AND METHODS This was a case-control study including 104 individuals with BPPV. The diagnosis was based on the presence of vertigo and nystagmus during the positional test. Subjects were examined by the horizontal and vertical head-shaking test. Eye movements were recorded on a video camera to analyze the nystagmus. The head was shaken passively in the horizontal and sagittal planes, respectively, for horizontal and vertical HSN at a frequency of 2 Hz. HSN was considered when six consecutive beats of nystagmus with an SPV of at least 2 degrees/s were detected. Main outcome measures were the presence of horizontal and vertical HSN, maximum SPV of HSN, time constant of HSN, and canal paresis. RESULTS Maximum SPV of vertical HSN was higher in BPPV patients with posterior canal BPPV (n = 10) than in controls (p = 0.04). Moreover, the time constant of vertical HSN was significantly lower for posterior canal BPPV when compared with controls (p < 0.02).