Central HINTS (With an Abnormal Head Impulse Sign) in the Acute Vestibular Syndrome Due to Lateral Pontine/Middle Cerebellar Peduncle Demyelination

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: This is a 30-year-old man presenting with vertigo, diplopia and mild left facial weakness (not seen in the video). On exam, there was right-beating nystagmus (RBN) in primary gaze that increased in right gaze (in accordance with Alexander's law), and the RBN stayed unidirectional, but lessened, in left gaze. This is a pattern of nystagmus that is usually peripheral, especially when nystagmus increases when fixation is removed. However, this can also be central, especially when there is no change in nystagmus with removal of fixation. There was a positive or abnormal head impulse test (HIT) with leftward impulses of the head, which is also usually a sign of peripheral pathology. With alternate cover testing, there was a right hyperdeviation, which was comitant and associated with a left head tilt and leftward ocular counterroll (top poles of the eyes towards the left ear). Taken together, this was thought to be related to utricle pathway pathology causing an ocular tilt reaction, with the skew deviation responsible for his diplopia. Using HINTS (Head Impulse, Nystagmus, Test of Skew), the presence of a skew deviation should lead the examiner to assume a "central" etiology until proven otherwise, despite the "peripheral" appearance of the nystagmus and HIT. ; ; It is important to note that while unidirectional nystagmus and an abnormal HIT are usually suggestive of a peripheral etiology, either one can be seen with a central etiology. A skew deviation may result from peripheral utricle pathology (at the level of labyrinth or 8th cranial nerve - e.g., post acoustic neuroma resection), although generally these "peripheral" skews tend to be small in magnitude and short-lived. Regardless, since peripheral skews are rare, when present, a central etiology must first be ruled out. This patient was found to have a demyelinating lesion (leading to the diagnosis of MS) in the region of the middle cerebellar peduncle (MCP)/root entry zone of cranial nerves 7 and 8 (see anatomic diagram of the pons, https://collections.lib.utah.edu/details?id=1256243). MCP lesions are common in MS, and patients typically present with vertigo and nystagmus. The ocular tilt reactions seen in demyelinating and ischemic lesions involving the MCP are almost always ipsiversive as it was in this case. Finally, there was a mild left lower motor neuron facial palsy, which along with the abnormal HIT is suggestive of a root entry zone localization if due to a central etiology. Or, an inflammatory, neoplastic or infectious process (e.g., herpes zoster/Ramsay Hunt syndrome) causing multiple cranial neuropathies should be considered when peripheral. Our patient's RBN and abnormal HIT to the left could have been explained by a left 8th cranial neuropathy, although the prominent skew deviation would have been atypical. In this patient's case, possible localizations for the abnormal HIT to the left include (in order of most to least likely): root entry zone of CN 7/8; intra-axial 8th nerve fascicle; vestibular nucleus complex. Had this patient suffered a stroke, left labyrinthine ischemia (due to AICA-territory ischemia) would be another explanation for abnormal HIT on the left and unidirectional RBN, although ipsilesional hearing loss is generally present due to cochlear ischemia. 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: This patient presented with vertigo, diplopia and mild left facial weakness (not seen in the video). On exam, there was right-beat-ing nystagmus (RBN) in primary gaze that increased in right gaze (in accordance with Alexander's law), and the RBN stayed unidirectional, but lessened, in left gaze. This is a pattern of nystagmus that is usually peripheral, especially when nystagmus increases when fixation is removed. However, this can also be central, especially when there is no change in nystagmus with removal of fixation. There was a positive or abnormal head impulse test (HIT) with leftward impulses of the head, which is also usually a sign of peripheral pathology. With alternate cover testing, there was a right hyperdeviation, which was comitant and associated with a left head tilt and leftward ocular counterroll (top poles of the eyes toward the left ear). Taken together, this was thought to be related to utricle pathway pathology causing an ocular tilt reaction, with the skew deviation responsible for his diplopia. Using HINTS (Head Impulse, Nystagmus, Test of Skew), the presence of a skew deviation should lead the examiner to assume a "central" etiology until proven otherwise, despite the "peripheral" appearance of the nystagmus and HIT. It is important to note that while unidirectional nystagmus and an abnormal HIT can suggest a peripheral etiology, either one can be seen with a central etiology. A skew deviation may result from peripheral utricle pathology (at the level of labyrinth or eighth cranial nerve-e.g., post acoustic neuroma resection), although generally these "peripheral" skews tend to be small in magnitude and shortlived. Regardless, since peripheral skews are rare, when present, a central etiology must first be ruled out. This patient was found to have a demyelinating lesion (leading to the diagnosis of MS) in the region of the MCP/root entry zone of cranial nerves 7 and 8. MCP lesions are common in MS, and patients typically present with vertigo and nystagmus. The ocular tilt reactions seen in demyelinating and ischemic lesions involving the MCP are almost always ipsiversive as it was in this case. Finally, there was a mild left lower motor neuron facial palsy, which along with the abnormal HIT is suggestive of a root entry zone localization if due to a central etiology. Or, an inflammatory, neoplastic or infectious process (e.g., herpes zoster/Ramsay Hunt syndrome) causing multiple cranial neuropathies should be considered when peripheral. Our patient's RBN and abnormal HIT to the left could have been explained by a left eighth cranial neuropathy, although the prominent skew deviation would have been atypical. In this patient's case, possible localizations for the abnormal HIT to the left include (in order of most to least likely): root entry zone of CN 7/8; intraaxial eighth nerve fascicle; vestibular nucleus complex. Had this patient suffered a stroke, left labyrinthine ischemia (due to AICA-territory ischemia) would be another explanation for abnormal HIT on the left and unidirectional RBN, although ipsilesional hearing loss is generally present due to cochlear ischemia https://collections.lib.utah.edu/details?id=1291717. (Video courtesy of Dr. Tzu-Pu Chang)