Medial Longitudinal Fasciculus Syndrome with Prominent Spontaneous Nystagmus

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: This is a 60-year-old man who experienced the abrupt onset of diplopia and imbalance. He had typical features of a left medial longitudinal fasciculus (MLF) syndrome including left internuclear ophthalmoplegia (INO) and left hypertropia from skew deviation, but he also had very prominent upbeat-torsional (top poles beating towards left ear) nystagmus. MRI demonstrated a left MLF stroke. Spontaneous nystagmus with acute MLF injury is common, but is usually subtle and short-lived. This is a case of prominent nystagmus due to disruption of the semicircular canal and/or utricule-ocular motor pathways, both of which originated in the right ear, decussated and ascended via the left MLF. The torsional component of MLF-related spontaneous nystagmus is almost always ipsiversive - e.g., with a left MLF stroke, the top poles will beat towards the left ear, and this is because of damage to central fibers that originated in the 1) right posterior semicircular canal (SCC), 2) right anterior SCC or 3) right posterior and anterior SCCs. Because of unopposed left anterior and posterior SCC afferents (when normally stimulated, the left anterior and posterior SCCs cause the top poles to move towards the right ear), the slow torsional phase will be towards the right ear, and the fast phase will cause the top poles to beat towards the left ear. Additionally, there are often dissociated vertical components, again mainly due to central vertical (anterior and posterior) SCC injury. The most common pattern is upbeat-torsional (ipsiversive) nystagmus, where there is more upbeat in the contralesional eye than in the ipsilesional eye. This is the pattern seen in the video. Because the right anterior SCC, when normally stimulated, causes excitation of the right superior rectus and left inferior oblique, a lesion involving the right anterior SCC pathway (at the level of the left MLF in this case) will lead to less excitation of the elevators and relative hyperactivity of the antagonist depressors. Because the right inferior rectus is a strong depressor, there is a more of a downward slow phase OD compared to OS and therefore, a more marked upbeat component OD compared to OS. The other patterns of spontaneous nystagmus are: downbeat-torsional (ipsiversive) nystagmus, where there is more downbeat in the ipsilesional eye than in the contralesional eye (related to posterior SCC pathway damage, or the opposite of the situation described above); hemi-see saw or jerky see saw nystagmus where the torsional component is ipsiversive, there is upbeat OD and downbeat OS, related to injury of the posterior and anterior SCC pathways and/or utricle-ocular motor pathways (that are also responsible for skew deviation - i.e., slow phase up in the hypertropic left eye (fast phase down, or downbeat OS), and slow phase down in the hypotropic right eye (fast phase up, or upbeat OD). 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: This patient suffered a left MLF stroke, and had a skew deviation causing a left hypertropia (due to utriculo-ocular motor pathway involvement in the MLF), in addition to a left internuclear ophthalmoplegia causing an exotropia and adduction paresis OS (due to involvement of the interneurons connecting right 6th nucleus to left medial rectus subnucleus by the left MLF). There was particularly prominent spontaneous upbeat-torsional nystagmus in this case as well (due to involvement of the central vertical semicircular canal pathways in the MLF). The torsional component of MLF-related spontaneous nystagmus is almost always ipsiversive - e.g., with a left MLF stroke, the top poles will beat towards the left ear, and this is because of damage to central fibers that originated in the 1) right posterior semicircular canal (SCC), 2) right anterior SCC or 3) right posterior and anterior SCCs. Because of unopposed left anterior and posterior SCC afferents (when normally stimulated, the left anterior and posterior SCCs cause the top poles to move towards the right ear), the slow torsional phase will be towards the right ear, and the fast phase will cause the top poles to beat towards the left ear. Additionally, there are often dissociated vertical components, again mainly due to central vertical (anterior and posterior) SCC injury. The most common pattern is upbeat-torsional (ipsiversive) nystagmus, where there is more upbeat in the contralesional eye than in the ipsilesional eye. This is the pattern seen in the video. Because the right anterior SCC, when normally stimulated, causes excitation of the right superior rectus and left inferior oblique, a lesion involving the right anterior SCC pathway (at the level of the left MLF in this case) will lead to less excitation of the elevators and relative hyperactivity of the antagonist depressors. Because the right inferior rectus is a strong depressor, there is a more of a downward slow phase OD compared to OS and therefore, a more marked upbeat component OD compared to OS. The other patterns of spontaneous nystagmus are: downbeat-torsional (ipsiversive) nystagmus, where there is more downbeat in the ipsilesional eye than in the contralesional eye (related to posterior SCC pathway damage, or the opposite of the situation described above); hemi-see saw or jerky see saw nystagmus where the torsional component is ipsiversive, there is upbeat OD and downbeat OS, related to injury of the posterior and anterior SCC pathways and/or utricle-ocular motor pathways (that are also responsible for skew deviation - i.e., slow phase up in the hypertropic left eye (fast phase down, or downbeat OS), and slow phase down in the hypotropic right eye (fast phase up, or upbeat OD). (Video and legend created with the assistance of Dr. Roksolyana Tourkevich) https:// collections.lib.utah.edu/ark:/87278/s6rz39rq