Assessing Utricle Pathway Function and the Effects of Convergence on Nystagmus in Acute Vestibular Neuritis

A 35-year-old woman presented a few days after the onset of room-spinning vertigo. She denied diplopia, dysarthria, dysphagia, dysphonia, incoordination, numbness, and weakness. On examination, she had subtle spontaneous right-beat nystagmus (RBN). This nystagmus increased in amplitude and frequency when looking to the right (in the direction of the fast phase - i.e., Alexander's law) with a torsional (top poles of both eyes beating toward the right ear) component, but also increased with convergence. There was an abnormal bedside and video head impulse test in the planes of the left horizontal and anterior semicircular canals (an indication of superior vestibular nerve involvement), and skew deviation was absent with alternate cover testing. Otoscopy was normal and there was no hearing loss. With fixation-removed video-oculography, there was spontaneous mixed horizontal-torsional RBN. On provocative maneuvers, including head-shaking and vibration, there was an increase in RBN. The bucket test (a comparison between the patient's perception of vertical and true earth vertical) showed a leftward 15 degree tilt in the subjective visual vertigo (SVV). An SVV tilt can be thought of as a perceptual consequence of the ocular tilt reaction (OTR), which is suggestive of utriculo-ocular motor pathway involvement (tilting toward the left assuming a left peripheral vestibulopathy). There was also an ocular counter-roll (OCR, top poles of each eye rotated towards the left ear) apparent with fundus photographs, another indication of left utriculo-ocular motor pathway involvement. Therefore, her HINTS exam (head impulse, nystagmus, and test of skew) [Kattah et al. 2009] was consistent with a diagnosis of a left vestibular neuritis. The maneuvers above, including head impulse test, head-shaking and vibration, were used to uncover the presence of an underlying semicircular canal pathway imbalance, while the SVV tilt and OCR test and fundus photography were used to identify a static utricle pathway imbalance. Not surprisingly, there was also a 1 prism diopter (PD) right hyperphoria with Maddox rod test suggestive of a skew deviation, which is another component of the OTR (head tilt was lacking). Small 1-2 PD skew deviations that do not cause diplopia and cannot be seen with alternate cover (‘test of skew') are common acutely and resolve over days to weeks. The significant augmentation of her RBN with convergence is a curious finding. Few studies exist examining the effects of convergence on acute vestibular nystagmus, however it was recently shown that convergence damped nystagmus in six patients with acute unilateral vestibulopathies (Ujjainwala et al. 2022) - i.e., the opposite of what we observed in our patient. Nystagmus secondary to a peripheral vestibulopathy is known to damp with fixation and augment while fixation is removed, during VOG for instance. It is thus more likely that her nystagmus was damped while fixating at a distance rather than truly augmented while converging. If that is so, her translational vestibulo-ocular reflex (tVOR, modulated by otolithic, or utricular, inputs and the distance from the orbits to the visual target) gain may not have adequately adapted by the time of her examination. This likely has much to do with the extent of utricle pathway involvement, which was clearly present in this patient based on her skew deviation, SVV tilt and OCR. Abnormal tVOR gain could therefore lead to failure to suppress nystagmus at near, but not at distance. The fact that convergence can have opposite effects on nystagmus in different patients with acute vestibulopathies (i.e., damping vs provoking) deserves further investigation, although the severity of utriculo-ocular motor pathway involvement may be a key factor. Seen at the beginning of the video are the traces from the video head impulse test (vHIT), which measures and quantifies the eye movement response to head movements (i.e., the vestibulo-ocular reflex). The vHIT was performed in the planes of all 6 semicircular canals, and demonstrates a vestibular weakness of the left side that can be seen in vestibular neuritis. The low gains and both overt and covert corrective saccades are seen in the planes of the left anterior and lateral (horizontal) canals, which follows a left superior vestibular nerve distribution.