頭位性めまいを主訴とした脊髄小脳失調症6型 (SCA6) 例の ENG 検討

Abstract

A 57-year-old woman presented at our outpatient clinic with the recent and frequent experience of paroxysmal positional vertical vertigo similar to the sensation of a somersault, especially changing position from sitting to lying supine, accompanied with spells of nausea and blackout. On examination, we observed prominent downbeat nystagmus in the primary eye position and lateral gaze. MRI disclosed prominent cerebellar cortical atrophy, especially in the vermal regions. The patient had a family history of spinocerebellar ataxia type 6 (SCA6) and had herself received a definitive diagnosis of the same phenotype disease from genetic studies. The points of the ENG findings were as follows: (1) downbeat nystagmus was prominent in the primary eye position and was combined with gaze nystagmus in the lateral gaze; (2) both horizontal & vertical rebound nystagmus were noted; and (3) impairment of both horizontal & vertical smooth pursuit were seen. In horizontal and upward pursuit, catch-up saccades (lower gain) were intermingled. In contrast, backup-saccades (higher gain) were intermingled in downward pursuit. Furthermore, (4) Horizontal saccades were hypometric, though vertical saccades were hypermetric (overshoot); (5) In both horizontal and vertical OKN, the slow phase of OKN was severely impaired. Finally, (6) vestibular caloric nystagmus was well induced bilaterally. Both sides of visual suppression of caloric nystagmus were severely impaired. Most of present results are well explainable with the pure cerebellar cortical atrophy as the typical pathological finding of SCA6. However, two important findings should be mainly discussed and stressed here. One, the gain of the horizontal smooth pursuit was lower, while the gain of the downward pursuit was higher. It seemed to be inconsistent that a single part of the cerebellum would simultaneously modulate decreasing gain of the horizontal and increasing gain of vertical smooth pursuit systems. Therefore, this finding suggested that several parts, not a single part, of the cerebellum would be simultaneously engaged in the gain modulation of the smooth pursuit system. Second, the severe impairments of both horizontal and vertical OKN implied disturbances of both the pursuit and the velocity storage systems. In Cohen & Raphan's model, the slow phase of OKN is composed of the “direct pathway” and the “indirect pathway.” The “direct pathway” utilizes the ocular pursuit system in the flocculus/paraflocculus, while the “indirect pathway” is mediated through the velocity storage system in the medial vestibular nuclei/prepositus hypoglossi nuclei or the interstitial nuclei of Cajal in the brainstem. Consequently, the present results of OKN strongly suggested that the neural circuits related to the velocity storage integrator in the brainstem would also be perturbed, together with the disturbance of the ocular pursuit system in the cerebellum.