Characteristics and mechanism of perverted head-shaking nystagmus in central lesions: Video-oculography analysis.

OBJECTIVE: Perverted downbeat head-shaking nystagmus (pdHSN) has been considered a sign of central pathology. Two hypotheses have been advanced as the mechanisms of pdHSN without proper validation. This study aimed to elucidate the mechanism of pdHSN.

METHODS: Eighteen patients with pdHSN due to central lesions were subjected to analyses of their oculographic characteristics. The peak velocity, temporal features including the duration and time constant (TC), rotational axis of pdHSN were analyzed. To determine the most relevant mechanism of pdHSN, we compared the TCs of pdHSN with those of downbeat nystagmus after vertical head-shaking in four subjects, and with the TCs of horizontal head-shaking nystagmus (HSN) in three subjects who had both horizontal and downbeat nystagmus after head-shaking.

RESULTS: The duration of pdHSN ranged from 14 to 25s and the estimated TC was from 3.9 to 7.8s. When the data were pooled after the intensities of the nystagmus in each patient were normalized, the TC was 5.2 (95% CI=5.0-5.4) seconds from nonlinear regression test. The rotational vectors of pdHSN (n=8) were mostly aligned between the anterior semicircular canals. The estimated TC of downbeat nystagmus after vertical head-shaking (5.8s) was similar to that of pdHSN (6.0s) in four subjects tested. In contrast, the TC of horizontal HSN (10.9s) was significantly larger than that of pdHSN (4.9s) in three subjects.

CONCLUSIONS: The characteristics of TCs of HSN in our patients with central lesions suggest that pdHSN is due to enhanced activities of the central anterior canal pathway.

SIGNIFICANCE: pdHSN is a sign indicative of central pathology causing cerebellar dysfunction, especially when it associated with other neuro-otological signs.