Electrocochleographic and pure-tone audiometric findings in contralateral ear of unilateral acoustic neurinoma.

OBJECTIVE

Auditory disorders of the contralateral ear in patients with unilateral acoustic neurinoma have previously been reported using the auditory brainstem response. From our survey of the literature, there is no reported result about electrocochleography (ECoG) in the contralateral ear of acoustic neurinoma cases. In the present study, we reported ECoG and pure-tone audiometric results of the contralateral ears in cases of unilateral acoustic neurinoma and the relationship between ECoG findings and tumor size was discussed.

METHODS

ECoG and pure-tone audiometry were performed for the contralateral ears in 27 cases of unilateral acoustic neurinoma, and the relationships between tumor size and contralateral ECoG findings and between pure-tone threshold and ECoG findings were investigated. As for ECoG findings, the negative summating potential to the compound action potential ratio (-SP/AP ratio) was examined. The tumor size was assessed by magnetic resonance imaging.

RESULTS

Abnormal -SP/AP ratio exceeding 0.40 of the upper limit of the normal range was found in 7 (25.9%) of 27 ears. Incidences of abnormal -SP/AP ratio were 30.0% in large tumors (n = 10), 33.3% in mid-size tumors (n = 6) and 18.2% in small tumors (n = 11). There was no correlation between the incidences of abnormal -SP/AP ratio and tumor size. Moreover, there was no correlation between the -SP/AP ratio and pure-tone threshold at the ECoG recording.

CONCLUSION

These results suggest that any size of acoustic neurinoma may affect contralateral electrocochleographic findings. One possible cause may be dysfunction of the olivocochlear efferent system and another may be endolymphatic hydrops (e.g. delayed endolymphatic hydrops). Currently, however, the causes of -SP/AP ratio increase are still unknown and further investigations are required.