The Acute Vestibular Syndrome with Skew Deviation, Gaze-evoked Nystagmus, and Bilaterally Abnormal Head Impulse Testing Due to AICA Stroke

This is a 60-year-old man with the acute onset of prolonged vertigo and nystagmus, consistent with the acute vestibular syndrome (AVS). HINTS (Head Impulse, Nystagmus, Test of Skew) exam demonstrated a central pattern: 1) Head impulse test (HIT) was abnormal to the right and to the left. An abnormal HIT, indicating vestibular hypofunction on the side that the head is turned (with a compensatory saccade in the opposite direction back to the target) generally suggests a peripheral and usually benign etiology for the AVS. Stroke in the distribution of the AICA is an exception and since the abnormal HIT in this situation often reflects labyrinthine ischemia, it is not a benign cause of the AVS. A bilaterally abnormal HIT (when acute in onset) may be seen in Wernicke's encephalopathy, trauma or ototoxicity (e.g., gentamicin) for instance. Since the cerebellar flocculus has a role in modulation of the high frequency vestibulo-ocular reflex, an acute unilateral infarction of the flocculus is another possible cause of bilaterally (usually contralesional more than ipsilesional) abnormal head impulse testing. 2) Nystagmus demonstrated spontaneous contralesional (right-beating) nystagmus in primary gaze and was left-beating in left gaze, also known as gaze-evoked nystagmus, which suggests a central etiology. In the AVS due to the most common cause, vestibular neuritis, the nystagmus is horizontal-torsional with the fast phase away from the involved ear and is unidirectional whether in primary position, left or right gaze or in vertical gaze. In contrast, central causes of acute vertigo commonly produce direction-changing (gaze-evoked) nystagmus, vertical or vertical-torsional nystagmus. However, small unilateral posterior fossa lesions can produce unidirectional nystagmus that is indistinguishable from "peripheral" nystagmus. 3) Test of skew - evaluation of alignment demonstrated a right hypertropia, which was relatively comitant and consistent with a skew deviation (as opposed to a right 4th nerve palsy). Since this was a left sided lesion and the left eye was hypotropic, damage to the utricle-ocular motor pathways would have occurred caudal to their decussation at the pontomedullary junction. ; The internal auditory artery (IAA) is normally a branch of the anterior inferior cerebellar artery (AICA), and usually arises from the basilar artery. The IAA supplies the labyrinth, and a labyrinthine stroke can be indistinguishable from a unilateral peripheral vestibulopathy. However, labyrinthine ischemia will almost always also cause ipsilateral loss of hearing due to cochlear ischemia. Therefore, the HINTS "plus" test is used, which adds a bedside evaluation of hearing to finger rub. It is important to note that, when compared to the standard reference pure-tone audiogram, bedside tests of hearing have been shown to have poor sensitivity, relatively good specificity, and variable positive predictive value1. However, when acute hearing loss is present in a patient with the AVS, a dangerous (ischemic) etiology should be suspected, especially in a patient with vascular risk factors. In his case, ischemia involving the left labyrinth could certainly cause spontaneous right-beating nystagmus and an abnormal HIT to the left (and potentially a skew deviation that is "peripheral", although skew deviations related to peripheral utricle injury are usually smaller in amplitude and short-lived). The left hemi-ataxia and direction changing nystagmus clearly indicate that this AVS localizes to the brainstem +/- cerebellum with or without additional labyrinth ischemia. Finally, there is a lower motor neuron (LMN) left facial palsy, which in his case was related to ischemia involving the root entry zone of cranial nerve 7 and 8. This is another potential explanation for an abnormal HIT to the left, but again would not explain an abnormal HIT contralesionally. Because the left flocculus is supplied by the left AICA, the abnormal HIT to the right would seem to be best explained by ischemia involving this structure +/- an abnormal HIT to the left from left labyrinthine ischemia and/or ischemia in the region of the left root entry zone of cranial nerves 7 and 8. ; Unfortunately additional clinical details (whether or not left sided hearing loss was present) and neuroimaging could not be recovered. ; 1.; Boatman DF, et al. How accurate are bedside hearing tests? Neurology 2007;68:1311-1314. [[Number of Videos and legend for each: 1, patient with the acute vestibular syndrome due to AICA stroke]]