Changes in slow phase eye velocity and time constant of positional nystagmus at transform from cupulolithiasis to canalolithiasis.

Changes in slow phase eye velocity (SPEV) and time constant (TC) of benign paroxysmal positional nystagmus in horizontal canal type were examined at transitional period from cupulolithiasis (apogeotropic nystagmus) into canalolithiasis (geotropic nystagmus) in two patients. SPEV and TC of positional nystagmus were tri-dimensionally analyzed. The first patient showed an apogeotropic nystagmus. Head rotation to the left in supine position induced a right-beating nystagmus with an initial SPEVof 15.3 degrees/s and a TC of 133 s. The nystagmus then gradually declined with a TC of 31.3 s after reaching a maximum SPEV of 28.8 degrees/s. After the nystagmus disappeared, he showed a geotropic nystagmus. The second patient showed a left-beating nystagmus with an initial SPEV of 2.5 degrees/s and a TC of 141 s when his head was rotated to the right in supine position. The nystagmus then gradually declined with a TC of 8.05 s after reaching a maximum SPEV of 16.7 degrees/s. After the nystagmus disappeared, he showed a geotropic nystagmus. The present findings suggested that in both patients, at the period of an increase of SPEV of the positional nystagmus with the shortening of its TC, cupulolithiasis transformed into canalolithiasis.