A case of Arnold-Chiari malformation showing bow-tie nystagmus

―Three dimensional analysis of bow-tie nystagmususing a 240Hz high speed video oculography system―

Abstract

　Bow-tie nystagmus is a rare phenomenon, which is reportedly associated with cerebellar infarction, brain stem anomalies, and so on. We analyzed herein bow-tie nystagmus in a patient with an Arnold-Chiari malformation using a three-dimensional 240 Hz high speed video oculography (VOG) system. The patient's nystagmus consisted of two aspects―slow phases directed upward and quick phases altering the direction obliquely right to left downward in turn (square wave jerks). The dominant frequency of the vertical component was 3.1Hz, which was exactly twice as fast as that of the horizontal component at 1.55Hz. Appearance of slow phases of horizontal components suppressed the square wave jerks. Square wave jerks were not only synchronized with quick phases of downbeat nystagmus but also with quick phases of upbeat or torsional nystagmus. Therefore their frequency was not regular but depended on the frequencies of the vertical or torsional component. These data suggest that bow-tie nystagmus is not generated by a common bow-tie nystagmus generator. We propose that bow-tie nystagmus is composed of quick phases of a horizontal component corresponding to the slow phases of the vertical or torsional component which alters its direction left to right in turn in order to avoid lateralization of the position of the eyes.