A 'Canal Jam' During Head Impulse Testing in a Patient With Horizontal Canal BPPV

A 70-year-old man reported brief episodes of positional vertigo. Ten years prior, he had undergone gamma knife radiosurgery for a vestibular schwannoma at the left cerebellopontine angle. Video head impulse testing (vHIT) showed reduced gains and corrective saccades in the planes of the left horizontal and anterior canals, consistent with his known left vestibular loss. On review of vHIT video playback, robust right-beating nystagmus was seen after around 35 seconds of repeated horizontal head impulses. The technician stopped performing head impulses once the vertigo and spontaneous nystagmus began, and both resolved without intervention over the next 10 seconds. Further video-oculography (VOG) testing showed left-beating nystagmus with bow position, and geotropic nystagmus with supine roll testing (more left-beating nystagmus in left roll than right-beating nystagmus in right roll) - suggestive of left geotropic horizontal canal BPPV. In view of the left horizontal canal BPPV, the right-beating nystagmus triggered during video head-impulse testing was felt to reflect a transient horizontal canal occlusion. A ‘canal jam' is characterized by direction-fixed horizontal nystagmus irrespective of head position, and is thought to occur due to blockage of the horizontal canal lumen by otoconia moving from a larger to narrower portion of the horizontal semicircular canal.1 In this case, presumably otoconia in the left horizontal canal created a transient canal jam, which can occur spontaneously, or triggered by head movement or repositioning maneuvers.1 This case illustrates that horizontal canal jam can also be a consequence of head impulse testing.