Twelve patients with direction-changing horizontal positional nystagmus in the lateral position (7 geotropic and 5 apogeotropic) were examined. Their positional nystagmus showed some characteristic features : prolonged duration, mild vertigo and no fatiguability on repeated testing. The findings suggested a peripheral origin, and neurotological examinations and clinical courses showed no evidence of a central origin. Provocation of positional nystagmus occurred only with head positions and only slightly with head rotation. Positional nystagmus was also observed with every head position in the sagittal plane, which had a common pathogenesis with positional nystagmus in lateral positions. Positional nystagmus disappeared in the sitting position and in the head hanging position ; the direction of horizontal nystagmus was reversed in the sitting position. The slow phase velocity of the positional nystagmus varied like a sine curve with different head positions. These results suggest that positional nystagmus might be caused not by lateral canal dysfunction (especially due to specific gravity alteration) but by utricle dysfunction.
