Head-shaking nystagmus (HSN) is a unidirectional horizontal jerk nystagmus that occurs following about 20 cycles of head oscillation in yaw at a frequency of about 2 Hz and an excursion of about 30 degrees off midline on each side. This finding generally reflects vestibular weakness on the side opposite the fast phase of nystagmus (i.e., the nystagmus “beats away from the lesion”), though the mechanism mediating this is uncertain.