Perverted Head Shaking Nystagmus due to Cerebellar Lesions: Possible Mechanisms

ABSTRACT

BACKGROUND AND OBJECTIVES: Perverted head-shaking nystagmus (HSN) refers to nystagmus that develops in the plane other than that being stimulated by head oscillation, i.e., downbeat or upbeat HSN after head oscillation in the yaw plane. However, the mechanism of perverted HSN is unknown. To elucidate the mechanism of perverted HSN by measuring tilt suppression of the vestibule-ocular reflex (VOR). MATERIALS AND METHOD: Twenty two consecutive patients with perverted HSN from cerebellar lesions (13 circumscribed and 9 diffuse) and 50 normal volunteers underwent recording of spontaneous, head-shaking and positional nystagmus, smooth pursuit, saccades, and the VOR. Tilt suppression method was to pitch patients' head forward further at the end of the step rotation stimuli. RESULTS: HSN was pure downbeat in nine and mixed horizontal-downbeat in 13 patients. The mean VOR time constants did not differ between normal controls and patients. However, tilt suppression of the post-rotatory nystagmus was impaired in the patients (p<0.01). Downbeating (n=12) and central positional nystagmus (n=12) were frequently accompanied findings. CONCLUSION: Uvulonodular dysfunction was frequently observed in patients with perverted HSN from cerebellar lesions. Uvulonodulus may be involved in the spatial organization of the velocity storage mechanism and uvulonodular dysfunction may be responsible for developing perverted HSN in cerebellar lesions.