Lateral Pontine Stroke Involving the Superior Vestibular Nucleus Causing Spontaneous Upbeat-torsional Nystagmus

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: A 65-year-old man presented to the emergency department with spontaneous vertigo and unsteadiness, and had was noted to have spontaneous upbeat nystagmus (UBN), also with a torsional component (top poles beating toward the right ear) that was most noticeable in right and up gaze. General neurologic exam was non-focal although there was severe imbalance. Brain MRI demonstrated a small left dorsolateral pontine stroke (see end of video for diffusion-weighted imaging on the left and FLAIR on the right), that involved the left superior vestibular nucleus. While the posterior semicircular canal (SCC) afferents synapse in the vestibular nucleus and then ascend the medial longitudinal fasciculus (MLF) only, the anterior SCC afferents synapse in the vestibular nucleus and ascend via one of three pathways: 1) MLF, 2) ventral tegmental tract, and 3) brachium conjunctivum (BC, aka, superior cerebellar peduncle). Anterior SCC afferents synapse in the rostral SVN on their way to join the BC, and in this patient's case, it was thought that ischemia in this region was responsible for his UB-torsional nystagmus. Anterior SCC afferents destined for the BC would originate in the left labyrinth and synapse in the left SVN prior to their decussation. If left anterior SCC afferents were injured - normally responsible for torsional slow phase with top poles rotating toward the right ear and an upward slow phase (sometimes referred to as the "anti-gravity" pathway) - there would be a relative predominance of posterior SCC tone (causing downward slow phase) and right sided anterior and posterior SCC tone (causing torsional slow phase with top poles toward the left ear). These slow phases in turn generate fast phases upward (UBN) and torsional toward the right ear, which is what is seen in this patient. While UBN is typically thought to localize to the midline medulla or to paramedian pontomesencephalic regions (MLF, BC), this patient demonstrates that a lateral pontine insult can also cause this finding. 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: A 65-year-old man presented to the emergency department with spontaneous vertigo and unsteadiness, and was noted to have spontaneous upbeat nystagmus (UBN), also with a torsional component (top poles beating toward the right ear) that was most noticeable in right and up gaze. General neurologic exam was non-focal although there was severe imbalance. Brain MRI demonstrated a small left dorsolateral pontine stroke (see end of video for diffusion-weighted imaging on the left and FLAIR on the right), that involved the left superior vestibular nucleus. While the posterior semicircular canal (SCC) afferents synapse in the vestibular nucleus and then ascend the medial longitudinal fasciculus (MLF) only, the anterior SCC afferents synapse in the vestibular nucleus and ascend via one of three pathways: 1) MLF, 2) ventral tegmental tract, and 3) brachium conjunctivum (BC, aka, superior cerebellar peduncle). Anterior SCC afferents synapse in the rostral SVN on their way to join the BC, and in this patient's case, it was thought that ischemia in this region was responsible for his UB-torsional nystagmus. Anterior SCC afferents destined for the BC would originate in the left labyrinth and synapse in the left SVN prior to their decussation. If left anterior SCC afferents were injured - normally responsible for torsional slow phase with top poles rotating toward the right ear and an upward slow phase (sometimes referred to as the "anti-gravity" pathway) - there would be a relative predominance of posterior SCC tone (causing downward slow phase) and right sided anterior and posterior SCC tone (causing torsional slow phase with top poles toward the left ear). These slow phases in turn generate fast phases upward (UBN) and torsional toward the right ear, which is what is seen in this patient. While UBN is typically thought to localize to the midline medulla or to paramedian pontomesencephalic regions (MLF, BC), this patient demonstrates that a lateral pontine insult can also cause this finding. (Video courtesy of Dr. Tzu-Pu Chang) https://collections.lib.utah.edu/ark:/87278/s6kx02v4