A 70-year-old woman with trigeminal cystic schwannoma developed imbalance and dizziness over a year and a half. Neurologic examination revealed mild abduction limitation of the right eye and positive head impulse test toward the rightward and right peripheral facial palsy.
Videooculography showed a large-amplitude right-beating nystagmus with the rightward gaze and a small-amplitude left-beating nystagmus with the leftward gaze (Supplementary Video 1). Brain magnetic resonance imaging with enhancement demonstrated a well-defined huge mass (about 7×4 cm) in the right cerebellopontine angle compressing the middle cerebellar peduncle and pons (Fig. 1).
Brun’s nystagmus is a combination of ipsilesional gaze-evoked nystagmus with decreasing velocity waveform and contralesional gaze-induced vestibular nystagmus with linear velocity waveform [1]. The ipsilesional large-amplitude slow nystagmus is the result of impairment of gaze holding due to compression or ischemia of the horizontal neural integrator [2]. The contralesional small-amplitude fast nystagmus is caused by the impairment of the ipsilateral vestibular system. Since this rare directional changing nystagmus indicates localization and lateralization of a lesion, we must know the clinical meaning of this.
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