Test Your Knowledge - Vertical-Torsional Nystagmus

Question #1: Watch the first portion of the video until you are told to stop. Is this vestibular nystagmus more likely to be peripheral or central? A. Peripheral B. Central Answer for #1: A. Incorrect. While the patient has upbeat-torsional (top poles beating toward the right ear) nystagmus which is typical of right posterior canal (PC, canalithiasis) benign paroxysmal positional vertigo (BPPV), and while the patient's head position is not shown, the relatively mild nystagmus seen in the video persists without a crescendo-decrescendo appearance as would be expected in BPPV, and typical PC-BPPV usually lasts <15-30 seconds. Although a PC cupulolithiasis (otoconia adhere to the cupula as opposed to free-floating within the canal itself) could cause a mild and more prolonged upbeat (UB)-torsional nystagmus, this patient had nystagmus due to an acute left medial longitudinal fasciculus (MLF) stroke. B. Correct. This patient had a right MLF stroke. PC-BPPV creates UB-torsional nystagmus due to excitation/hyperactivity of the right PC due to flow of otoconia. Spontaneous (not provoked) vertical-torsional spontaneous nystagmus is highly suspicious of a central etiology. Since the posterior and anterior canal (AC) afferents travel through the MLFs, lesions that cause asymmetry in these pathways can create spontaneous nystagmus with vertical and torsional components. In this case, the stroke involves the right MLF, and so injures those AC and PC pathways that originated in the left labyrinth. The AC afferents can be thought of as the upward or anti-gravity pathways (e.g., if you pitch your head forward, you stimulate the AC and your eyes move up), while the PC afferents can be thought of as the downward or gravity pathways (e.g., if you pitch your head backward, you stimulate the PC and your eyes move down). So if the AC/upward pathways are preferentially infarcted or hypoactive, there will be a predominance of PC/downward tone which creates a downward slow phase followed by an upward (UB) fast phase. Torsion and other patterns of spontaneous nystagmus seen with MLF injury are discussed below. Question #2: Which of the following patterns of spontaneous nystagmus would be unexpected with an acute unilateral MLF lesion? A. Ipsiversive conjugate torsional nystagmus B. Upbeat nystagmus, more in the contralesional eye C. Downbeat nystagmus, more in the ipsilesional eye D. Hemi- or jerky see-saw nystagmus E. Left-beating horizontal-torsional nystagmus. Answers for #2 (please read reference 1). A. Correct. When vertical-torsional nystagmus present and related to unilateral injury, it will always be ipsiversive. In the case of right MLF syndrome, the AC and PC afferents that originated in the (contralateral) left labyrinth are involved. However, those AC and PC afferents that originated in the (ipsilateral) right labyrinth and traverse the left MLF are unaffected. Therefore, the torsional nystagmus results from unopposed torsional slow phases (top poles moving toward the left ear) generated by the normally functioning right anterior and posterior semicircular canal pathways, which is followed by a quick phase towards the right ear, and an ipsiversive (top poles toward the right ear) spontaneous nystagmus. B. Correct. To explain this pattern, consider the AC or upward/anti-gravity pathways. The excitatory fibers from the left AC decussate at the pontomedullary junction and ascend in the right MLF (in addition to the brachium conjunctivum and ventral tegmental tract) to innervate the left superior rectus (SR) and right inferior oblique (IO). Injury to the right MLF that mainly involves AC pathways would therefore cause more PC or downward tone/slow phase, and UB nystagmus. Because the primary action of left SR is supraduction, there should be more UB in the left (contralesional) eye. Because the primary action of right IO is excycloduction, there should be more torsion in the right (ipsilesional) eye. C. Correct. To explain this pattern, consider the PC or downward/gravity pathways. The excitatory fibers from the left PC decussate at the pontomedullary junction and ascend in the right MLF to innervate the left superior oblique (SO) and right inferior rectus (IR). Injury to the right MLF that mainly involves PC pathways would therefore cause more AC or upward tone/slow phase, and downbeat (DB) nystagmus. Because the primary action of left SO is incycloduction, there should be more torsion in the left (contralesional) eye. Because the primary action of right IR is infraduction, there should be more DB in the right (ipsilesional) eye. D. Correct. In hemi- or jerky see-saw nystagmus, the vertical components are in opposite directions - e.g., DB in the ipsilesional eye and UB in the contralesional eye, but with a conjugate ipsiversive fast phase. Damage to utricle and/or vertical semicircular canal pathways are thought to be responsible for this nystagmus. The utricle fibers originating in the left labyrinth will decussate and ascend in the right MLF to innervate ipsilateral SR and contralateral IR. Since a skew deviation is a common consequence of an acute MLF lesion as in this case (right MLF lesion will result in the right eye being too high and the left eye being too low) the upward movement OD and downward movement OS due to the skew can be thought of as the slow phases, so that the fast phases are DB OD (ipsilesional) and UB OS (contralesional). E. Incorrect. Left-beating horizontal-torsional nystagmus would usually be seen with an acute peripheral vestibulopathy (vestibular neuritis) on the right, although a central lesion can sometimes mimic this pattern, for instance a stroke involving the right vestibular nucleus. Summary: This 55 year-old woman with hypertension presented to the emergency department for acute onset dizziness. She had adduction paresis OD and abducting nystagmus OS, consistent with a right internuclear ophthalmoplegia (INO). She also had a skew deviation with right hypertropia, related to utricle-ocular motor pathway involvement, in addition to spontaneous UB-torsional (top poles toward right ear) nystagmus, with more UB in the left eye, and more torsion in the right eye. Of note, this patient's nystagmus also increased with fixation removed (see the first part of the video, where nystagmus was recorded with infrared VOG in the dark), which is typically thought of as a ‘peripheral' feature of vestibular nystagmus, but which is well recognized to be present in some ‘central' cases as well2. By 48 hours, the nystagmus had resolved completely, which is typical of the spontaneous nystagmus seen acutely due to an MLF lesion. Although MRI in this case did not show an acute stroke, small posterior fossa strokes may be missed on MRI, especially in the first 24-48 hours, and especially with brainstem ischemia. Clinically, the lesion would have to involve the right MLF given the patient's constellation of highly localizing signs.