Bilateral riMLF Syndrome Causing Vertical Saccadic Palsy and Loss of Ipsitorsional Fast Phases

This is a 60-year-old man who developed fatigue and diabetes insipidus about 12 months prior to this video, and MRI demonstrated hypothalamic enhancement at that time. Nine months prior to this video, he gradually noticed that he was unable to look down. Work-up for ischemic, infectious, inflammatory (especially neurosarcoidosis), neoplastic (lymphoma), and paraneoplastic (panel + anti-MaTa) etiologies was unrevealing, although a mass was discovered on head/neck PET scan, which led to the diagnosis of squamous cell nasopharyngeal carcinoma. Subsequent contrast-enhanced MRIs showed persistent hypothalamic enhancement with addition bilateral 3rd nerve enhancement (normal midbrain). There was no change in ocular motor symptoms/exam with pulse dose steroids, chemotherapy, or radiation therapy. ; ; At the time of this video (9 months from onset of downgaze complaints), he described that he could only look up or down by tracking his finger. Exam showed absent downward saccades, severe slowing of upward saccades, and no vertical fast phases with an optokinetic stimulus directed upward or downward. However, vestibulo-ocular reflex (VOR), smooth pursuit and range of eye movements were normal both vertically and horizontally, and horizontal saccades were normal. The selective vertical saccadic palsy with preserved vertical pursuit and VOR indicated a supranuclear abnormality and localized to the rostral interstitial medial longitudinal fasciculus (riMLF) - the structure responsible for the generation of vertical saccades and torsional fast phases. Generation of downward saccades is through unilateral riMLF projections, while generation of upward saccades is through bilateral riMLF projections (an analogous example is a facial palsy - i.e., upper face is bilaterally innervated, explaining upper face sparing with an upper motor neuron lesion similar to bilateral innervation for upward saccades, explaining upward saccade sparing); therefore, a unilateral riMLF can cause a selective downward saccade disorder with relative sparing of upward saccades. In this case, bilateral riMLF lesions were suspected for two reasons: 1) there was severe impairment of both downward and upward saccades, and 2) when the head was rolled or tilted to the right and to the left, this produced a slow torsional phase (an ocular counterroll, seen in the video), but no ipsitorsional fast phase to the right or to the left (e.g., tilt the head right, and top poles of both eyes counter-roll toward the left ear, which should be followed by torsional fast phases back toward the right ear to keep the top poles of the eyes aligned with vertical). Because the riMLF is responsible for these torsional fast phases, they were absent bilaterally in this case. Additional work-up is ongoing.