同時に三種類の rebound nystagmus を認めた Arnold-Chiari I 型奇形例

抄録

　A 67-year-old woman began to experience sudden paroxysmal dizziness (or a fainting sensation) without any auditory symptoms upon turning her head or extending her neck to look upwards. The dizziness usually resolved within a moment even if the provoking position was maintained. Nystagmus was not induced by positioning or positional change tests. An MRI examination revealed a downward shift in the cerebellar tonsil beneath the level of the foramen magnum, and the patient was diagnosed as having an Arnold-Chiari malformation (type I). The characteristic ENG findings were as follows: (1) horizontal rebound nystagmus (RN) in the dark, (2) downbeat nystagmus mainly during shifting eyes leftward or downward in the dark, (3) vertical (upward) pursuit intermingled with back-up (overshoot) saccades, (4) slightly reduced peak slow phase velocity of OKN with preserved frequencies, and (5) OKAN were within normal limits. Of special note, three different kinds of RN in the dark were concurrently recognized in a single ENG study: (i) rightward (leftward) nystagmus during rightward (leftward) gazing followed by leftward (rightward) nystagmus upon returning to a straight-ahead eye position, (ii) rightward (leftward) nystagmus followed by reversed nystagmus in the opposite direction during rightward (leftward) gazing and continued reversed leftward (rightward) nystagmus after returning to a straight-ahead eye position, and (iii) very few nystagmus events during leftward (rightward) gazing followed by a remarkable rightward (leftward) nystagmus upon returning to a straight-ahead eye position. Although the definite pathophysiological mechanisms of RN remain unclear, the compression of the cerebellar tonsil (nodulus) or flocculus/paraflocculus caused by downward herniation in the present case might be correlated with the induction of such RN.