Test Your Knowledge - Central and Peripheral Vestibular and Ocular Motor Signs Due to a Large Vestibular Schwannoma

Which of the following is least likely to be the correct localization or etiology given the findings seen in the video? 1) Acute right 8th cranial neuropathy 2) Right-sided vestibular schwannoma 3) Right vestibular nucleus infarction 4) Right anterior inferior cerebellar artery distribution stroke Answers: 1) Incorrect. With a right-sided peripheral 8th cranial neuropathy (as in vestibular neuritis), there should be 1) an abnormal head impulse test (HIT) to the right, 2) absence of a skew deviation, and 3) left-beating unidirectional nystagmus, increasing in left gaze (in accordance with Alexander's law. In this case, there was bidirectional (direction-changing) or gaze-evoked nystagmus in lateral gaze, suggestive of a central disorder. 2) Correct. This patient has a right sided vestibular schwannoma causing a right unilateral vestibular loss (right 8th cranial nerve compression), and given its cerebello-pontine angle location and its size, there was also right-sided compression of the brainstem/cerebellum causing right-beating gaze-evoked nystagmus (GEN) in right and up gaze (upbeating in upgaze). There is left-beating nystagmus in left gaze that is most likely due to a combination of gaze-evoked nystagmus (unilateral posterior fossa lesions can cause bilateral gaze-evoked nystagmus, usually greater ipsilesional than contralesional) AND right unilateral vestibular loss. The combination of gaze-evoked (larger amplitude, lower frequency) nystagmus in one direction and vestibular (smaller amplitude, higher frequency) nystagmus in the other is also referred to as Bruns nystagmus and is commonly related to cerebello-pontine angle tumors. In this case, there is not a clear distinction between the two types of nystagmus, presumably related to the large size of the tumor and the fact that gaze-evoked nystagmus is present in all directions of gaze. However, when looking to the left, a slight torsional nystagmus (top poles beating toward left ear) can be appreciated in the video - mixed horizontal-torsional nystagmus is seen in unilateral vestibular loss (seen to the left), as opposed to the pure horizontal gaze-evoked nystagmus (seen to the right). For a more typical example of Bruns nystagmus, see https://collections.lib.utah.edu/details?id=1248764; 3) Correct. Since the horizontal canal afferents synapse in the medial vestibular nucleus (MVN), it is possible to have an abnormal HIT that is "central". Since the MVN-nucleus prepositus hypoglossi (NPH) complex is responsible for horizontal gaze-holding, unilateral or bilateral injury can also cause gaze-evoked nystagmus. Since the MVN-NPH complex may be preferentially affected by Wernicke's encephalopathy, this provides the explanation for gaze-evoked nystagmus and loss of the horizontal vestibulo-ocular reflex (with relative sparing of vertical VOR) in this condition. 4) Correct. A stroke in the distribution of the right anterior inferior cerebellar artery (AICA) can cause right labyrinthine ischemia (causing right unilateral vestibular loss, although generally accompanied by ipsilesional hearing loss due to cochlear ischemia), and simultaneous brainstem/cerebellar ischemia which can result in gaze-evoked nystagmus (e.g., right flocculus).