一側性聴神経鞘腫症例における健側耳蝸電図所見

抄録

Electrocochleography (EcochG) was performed in contralateral ears in 27 cases of unilateral acoustic neurinoma, and the relationships between the EcochG findings, tumor size and pure tone thresholds were investigated. As to EcochG findings, negative summating potential to compound action potential ratio (-SP/AP ratio) was examined. The tumor size was assessed by magnetic resonance imaging (MRI). Pure tone audiometry was performed just before EcochG recording. Abnormal-SP/AP ratio exceeding 0.40 of upper limit of normal range was found in 7 (25.9%) of 27 ears. Pure tone thresholds elevated at the low and high frequencies. However, these pure tone threshold shifts had no correlation to abnormal-SP/APratio. Incidence of abnormal-SP/APratiowas 30.0%in cases with a large tumor (n=10), 33.3% in cases with a medium-sized tumor (n=6) and 18.2% in cases with a small tumor (n=11). There was no correlation between incidence of abnormal-SP/AP ratio and tumor size. Averages of-SP/AP ratios were 0.242±0.138, 0.327±0.124 and 0.30±0.130 in cases with large, medium-sized and small tumors respectively. The average of -SP/AP ratio has no statistical difference among the three groups. Moreover, there was no correlation between-SP/AP ratio and pure tone threshold at the EcochG recording. These results suggest that any size of acoustic neurinoma may affect on the contralateral electrocochleographic findings.
One of possible causes may be dysfunction of olivocochlear efferent system. The other may be delayed endolymphatic hydrops at present, however, the causes of-SP/AP ratio increase are still unknown. Further investigation on this matter will be needed.