Cerebellar Rebound Nystagmus Explained as Gaze-Evoked Nystagmus Relative to an Eccentric Set Point: Implications for the Clinical Examination.

A brain stem/cerebellar neural integrator enables stable eccentric gaze. Cerebellar loss-of-function can cause an inability to maintain gaze eccentrically (gaze-evoked nystagmus). Moreover, after returning gaze to straight ahead, the eyes may drift toward the prior eye position (rebound nystagmus). Typically, gaze-evoked nystagmus decays during continuously held eccentric gaze. We hypothesized this adaptive behavior to be prerequisite for rebound nystagmus and thus predicted a correlation between the velocity decay of gaze-evoked nystagmus and the initial velocity of rebound nystagmus. Using video-oculography, eye position was measured in 11 patients with cerebellar degeneration at nine horizontal gaze angles (15° nasal to 25° temporal) before (baseline), during, and after attempted eccentric gaze at ± 30° for 20 s. We determined the decrease of slow-phase velocity at eccentric gaze and the slow-phase velocity of the subsequent rebound nystagmus relative to the baseline. During sustained eccentric gaze, eye drift velocity of gaze-evoked nystagmus decreased by 2.40 ± 1.47°/s. Thereafter, a uniform change of initial eye drift velocity relative to the baseline (2.40 ± 1.35°/s) occurred at all gaze eccentricities. The velocity decrease during eccentric gaze and the subsequent uniform change of eye drift were highly correlated (R2 = 0.80, p < 0.001, slope = 1.09). Rebound nystagmus can be explained as gaze-evoked nystagmus relative to a set point (position with least eye drift) away from straight-ahead eye position. To improve detection at the bedside, we suggest testing rebound nystagmus not at straight-ahead eye position but at an eccentric position opposite of prior eccentric gaze (e.g., 10°), ideally using quantitative video-oculography to facilitate diagnosis of cerebellar loss-of-function.