Downbeat Nystagmus with Active Horizontal Head Shaking

This is a 70-year-old man who presented with one single complaint for 10 years - if he moved his head too quickly (even one single horizontal head movement to the right or the left), he would experience the abrupt loss of balance and dizziness. His typical episodes were reproducible, and interestingly, his symptoms could not be brought on by passive head-shaking by the examiner, while imbalance, dizziness and downbeat nystagmus (DBN) were brought on robustly by active head-shaking (HS) by the patient. Otherwise, general neurologic examination was normal including cerebellar and gait assessment, and the only ocular motor abnormality was very mild DBN that could only be appreciated in down and lateral gaze. With video Frenzel examination and removal of fixation, there was very mild spontaneous DBN (<1 degree/second peak slow phase velocity, SPV), very mild 1 d/s SPV DBN with passive horizontal head-shaking (asymptomatic), and mild 2 d/s SPV DBN with hyperventilation. While the constellation of findings was compatible with cerebellar (mainly flocculus/paraflocculus) impairment, these signs were quite mild with the exception of robust DBN and abrupt imbalance when moving his own head back and forth. He was seated while asked to reproduce his symptoms in clinic (for this recording), otherwise he would typically fall if standing during these episodes. Several theories exist for this so-called ‘perverted' head-shaking nystagmus or ‘cross-coupling' response (i.e., horizontal HS causes DBN): 1) If the cerebellar flocculus/paraflocculus is involved, there is typically a disinhibition of the (upward) anterior canal pathways. Therefore, horizontal HS could create an upward bias since all semicircular canals are stimulated by this movement (although more horizontal than vertical canal stimulation). This upward bias could create the slow phase, followed by a downward fast phase (i.e., DBN). Of note, 6 canal video head impulse test was performed, which demonstrated normal gains and no corrective saccades in the planes of horizontal, anterior and posterior semicircular canals. There was also no significant asymmetry between anterior and posterior canals - i.e., it was very unlikely that his HS-induced DBN was due to peripheral vertical canal asymmetry. 2) Or, the cross-coupled HS response could relate to nodulus/uvula dysfunction and a disinhibited velocity-storage mechanism. Other subtle findings suggestive of a cerebellar disorder in this patient included: 1) hyperventilation-induced DBN, which is thought to relate to the sensitivity of cerebellar calcium channels, and 2) very mild DBN in lateral gaze. Investigations for reversible causes of cerebellar dysfunction including contrast-enhanced MRI and blood work were unrevealing, and he remained otherwise stable for years.