Abstract
The horizontal canal type of benign paroxysmal positional vertigo (H-BPPV) is the most common cause of direction-changing positional nystagmus. H-BPPV with geotropic positional nystagmus is caused by canalolithiasis in the horizontal semicircular canal (HSCC). When a patient with HSCC canalolithiasis makes a lateral head turn toward the affected ear, the free-floating debris creates an ampullopetal endolymph flow. As a result, a geotropic nystagmus (fast phase toward the ground) is caused. When the patient turns away from the affected side, the free-floating debris creates an ampullofugal flow. The flow ends when the debris reaches its lowest position in the canal and the geotropic nystagmus disappears rapidly. On the other hand, some cases show geotropic positional nystagmus, which is not transient but persistent. Persistent geotropic positional nystagmus is induced by a light cupula. In this study, to classify these types of geotropic positional nystagmus as transient (transient geotropic positional nystagmus, TGPN) or persistent (persistent geotropic positional nystagmus, PGPN), the time constant (TC) of nystagmus was calculated. Geotropic positional nystagmus was recorded by video-oculography and analyzed three-dimensionally. The slow phase eye velocity of nystagmus against time was approximated exponentially and TC was calculated as the reciprocal of the coefficient of time. Instances of geotropic positional nystagmus in patients with H-BPPV were classified as TGPN with a TC of ≤35 sec or PGPN with a TC of >35 sec. In conclusion, we showed that the TC of positional nystagmus is an objective index of TGPN or PGPN and proposed that a TC of ≤35 sec indicates a TGPN while a TC of >35 sec indicates a PGPN.
