前庭核中心に plaque 病変を認めた多発性硬化症2例における視運動性眼振所見の継時的検討

Abstract

Electronystagmography (ENG) findings were chronologically studied through the clinical courses in two multiple sclerosis patients (1 male and 1 female, both 25 years of age). At the initial bout, they showed left facial palsy and horizontal gaze nystagmus. The MRI demonstrated a single plaque lesion around the left vestibular nuclei at the ponto-medullary junction in both cases. In ENG recordings of both cases, the slow phase velocities of optokinetic nystagmus (OKN) were severely attenuated bilaterally at onset. In contrast, the smooth pursuit was relatively preserved in both cases through their clinical courses. In several months activities of daily living had recovered to normal. In accordance with their recovery, the OKN finally improved to the normal level in both cases. The Present ENG studies showed a definite dissociation between eye movements of pursuit and optokinetic systems. In Cohen and Raphan's model, the slow phase eye velocity of OKN is comprised of two components: the “direct pathway” involving the flocculus, which is the mechanism responsible for the rapid rise in slow phase eye velocity and is also utilized in mediating ocular pursuit. The second component is the “indirect pathway” which is responsible for producing slow phase eye velocity during vestibular nystagmus, OKN, and optokinetic after-nystagmus (OKAN). A key element in the indirect pathway is velocity storage, which is closely related to the mechanism of the vestibular nuclei. Although the neural circuits that produce velocity storage remain unclear, our previous experimental studies demonstrated that microstimulation mainly in the central medial vestibular nuclei (MVN) of monkeys elicited horizontal nystagmus and after-nystagmus related to velocity storage. Furthermore, after microinjection of muscimol (a GABA-A agonist) into the stimulus effective sites in the MVN, the slow component of OKN, the dominant time constant of aVOR and OKAN transiently disappeared bilaterally. These findings suggested that the horizontal velocity storage is generated in the MVN. The results of our present ENG studies were quite coincident with those of our previous experimental studies. Consequently, in both cases in the present study, it was assumed that the “indirect pathway” was transiently disturbed by the plaque lesion around the VN. In addition, two important points were particularly emphasized. First, the neural networks related to the horizontal velocity storage should be situated at the restricted area of the MVN, mainly the central MVN. Second, a single unilateral vestibular lesion caused a deficit in the horizontal velocity storage in both directions, in a similar manner to the results of studies in the MVN on a single experimental lesion. In conclusion, the findings of the present study and previous studies suggest that the horizontal velocity storage is generated in similar neural circuits of the human vestibular nuclei.