突発性難聴後BPPV様眼振の1例から考える両側性内リンパ水腫の病態

抄録

BPPV-like nystagmus is a common occurrence in patients presenting with inner ear disorders such as sudden hearing loss. In this report, we describe a case of a patient with refractory BPPV who developed direction-changing ageotropic nystagmus following the development of sudden hearing loss. A 75-year-old man was diagnosed as having sudden right-sided hearing loss with vertigo and was treated with intraventricular Dexate injection on day 6 after symptom onset. Neurological examination revealed ageotropic nystagmus with a predominant leftward horizontal nystagmus in the supine right lower head position, the null position in the midline, and no nystagmus induced by the bow & lean test. The duration of the nystagmus was 1 minute, with a latency of 1 to 2 seconds. Based on these findings, we made the diagnosis of left lateral semicircular canal type BPPV cupula stone disease, and attempted otolith replacement by the Brandt-Daroff method and stone release by administering jumping exercises, but the patient failed to show improvement. We examined the mechanism by which ageotropic nystagmus, as seen in this case, occurs within BPPV-like nystagmus and determined that refractory nystagmus with inconsistent laterality developed due to the overlapping effects of two mechanisms: 1) endolymphatic hydrops with contralateral dominance, and 2) disinhibition of the otolith-ocular reflex. Meanwhile, contrast-enhanced MRI of the inner ear also revealed bilateral endolymphatic edema in this patient. Recently, the existence of osmotic receptors in the inner ear has been confirmed, and ADH secretion is now considered as a possible cause of endolymphatic edema formation. The patient was elderly, but he exercised habitually, including activities such as mountain climbing and gardening, had been showing signs of dehydration for the past three years, and was developing chronic elevation of the serum osmolality, which was thought to be the background predisposing to the bilateral endolymphatic edema formation.