Positional Elliptical Pendular Nystagmus in Inferior Olivary Pseudohypertrophy

Dear Editor,Secondary oculopalatal tremor (OPT) typically develops months after lesions occur within the Guillain-Mollaret triangle and is characterized by 1-3 Hz pendular nystagmus with rhythmic palatal contractions. 1,2Although the nystagmus of OPT has traditionally been assessed in the upright position, its modulation by vestibular stimuli remains largely unexplored.A 66-year-old female presented with a 2-year history of motion-induced dizziness.A physical examination revealed no spontaneous nystagmus or saccadic intrusions during fixation.Transient direction-changing gaze-evoked nystagmus followed by low-amplitude square-wave jerks were elicited during horizontal gaze but not during vertical gaze.Smooth pursuit was fragmented, but saccades were preserved.Fixation removal elicited spontaneous horizontal square-wave jerks with a magnitude of 1-2.Head shaking did not evoke additional oscillations, but moving to the supine head-hanging (SHH) position elicited 3-4 Hz paroxysmal pendular nystagmus.This comprised combined leftward/upward and rightward/downward slow-phase drifts, resulting in an elliptical oscillation that lasted 13 seconds (Supplementary Video 1 in the online-only Data Supplement).Similar but attenuated nystagmus was observed during Dix-Hallpike maneuvers, whereas the supine roll test did not induce a significant response.A laryngopharyngeal examination revealed rhythmic contractions of the soft palate (Supplementary Video 2 in the online-only Data Supplement), but other neurological examinations revealed no abnormalities.Brain MRI performed 2 years previously had revealed a 1-cm right pontomesencephalic tegmental lesion, which was suspicious of capillary telangiectasia based on characteristic imaging features (Fig. 1A andB).PET and follow-up MRI revealed benign and stable lesion characteristics, which supported the diagnosis.However, follow-up MRI demonstrated a new T2-weighted hyperintensity in the inferior olivary nuclei (ION) that was more prominent on the right side (Fig. 1C andD).Although capillary telangiectasia is typically asymptomatic, lesions exceeding 1 cm may exert a mass effect or compromise local perfusion. 3We therefore assumed that this lesion was disrupting the descending central tegmental tract, leading to ION pseudohypertrophy and secondary OPT.OPT is explained by a dual-mechanism model. 4First, the loss of GABAergic inhibition leads to the formation of abnormal soma-soma gap junctions between ION neurons, resulting in the clustering of adjacent neurons within the ION.These clusters are characterized by increased electrotonic coupling and synchronized oscillatory activity, 5 and so they could function as intrinsic 2-Hz oscillators that generate rhythmic climbing-fiber inputs to cerebellar Purkinje cells. 6Second, a cerebellar learning process shapes the output.Repetitive climbing-fiber inputs result in Purkinje cells-which normally inhibit the vestibular/deep cerebellar nuclei-learning to pause at the expected timing of the rhythmic signals. 7Variations in the delays of individual Purkinje cells result in their collective pauses being temporally dispersed, leading to the gradual release of the vestibular/deep cerebellar nuclei from Purkinje