Geotropic direction-changing positional nystagmus is considered to be caused by canalolithiasis of the horizontal semicircular canal. However, horizontal nystagmus, which persists without decreasing over time, can not be explained by canalolithiasis. The pathophysiology of the persistent type of geotropic direction-changing positional nystagmus is considered to be a result of light debris cupulolithiasis of the horizontal semicircular canal. In the supine position, permanent horizontal nystagmus toward the intact side is observed. In neutral position 1, with the head turned about 30 degrees to the affected side, the nystagmus decreases and eventually stops. With greater headturn, nystagmus toward the earth lasts for more than one minute. These phenomena are considered to indicate that the cupula is deflected by the buoyancy of attached light debris which is hypodense compared with the endolymph. In the nose-down position, persistent horizontal nystagmus to the affected side is detected. In neutral position 2, with the head turned about 30 degrees to the affected side, the nystagmus stops. To evaluate Ewald's first law, we quantitatively compared horizontal nystagmus in the supine position and that in the nose-down position with the use of video-oculography. The patients were fifteen women and four men with persistent horizontal positional nystagmus toward the ground, ranging in age from 23 to 83 years (average, 59.2 years). They had no evidence of neurologic disease. In fifteen patients (78.9%), the maximum slow phase velocity in the nose-down position was greater than that in the supine position. Therefore, an ampullopetal deviation of the horizontal semicircular canal cupula was considered to bring about stronger nystagmus than an ampullofugal displacement of the cupula.
