Published papers

Pure-tone audiometry and transiently evoked otoacoustic emission (TEOAE) measurements were made before and after glycerol administration in the ears of sensorineural hearing loss with three different clinical entities, i.e. Meniere's disease, cochlear Meniere's disease and unilateral sensorineural hearing loss of unknown cause without hearing fluctuation (SNHLWF). TEOAE was evoked after glycerol administration in several ears of Meniere's disease and cochlear Meniere's disease, in which it was not evoked before glycerol administration. Especially in patients with cochlear Meniere's disease, TEOAE was altered without an accompanying significant change in mean hearing level. The change in TEOAE after glycerol administration indicates that the active motile property of outer hair cells may be affected by endolymphatic hydrops.

It has been postulated that impairment of cochlear blood flow (CoBF) is one of the most important causes of hearing loss occurring during acoustic neuroma (AN) surgery. However, it remains unclear how the degree of cochlear ischemia influences the evoked responses in electrocochleography (ECochG) which has been used for monitoring cochlear functions. In the present study, we investigated alterations in ECochG during cochlear ischemia of varying degree in the guinea pig. In order to induce cochlear ischemia, the anterior inferior cerebellar artery (AICA) was mechanically compressed via the transclival approach. The compression of AICA resulted in a reduction of CoBF in 55 out of 70 guinea pigs. A constant reduction of CoBF was maintained during the compression of AICA in 44 (63%) guinea pigs. CoBF abruptly decreased upon compressing AICA, and promptly recovered after releasing the compression. N1 and N2 in ECochG were also altered by compression. During 3-min ischemia, N1 and N2 disappeared in 36% and 41% of the cases, respectively. The residual CoBF in cases whose N1 and N2 disappeared was significantly lower that that in other cases whose N1 and N2 were sustained during 3-min ischemia. In addition, there was a tendency that the lower the residual CoBF was, the shorter the survival time of N1 and N2. In cases whose NI and N2 did not disappear, the prolongation of N1 and N2 latencies after 3-min ischemia was positively correlated to the residual CoBF. On the basis of these results, we discuss the mechanisms underlying the changes in CoBF and ECochG during cochlear ischemia, and conclude that the degree of cochlear ischemia during AN surgery can be estimated with ECochG.

Hearing preservation is one of the ultimate goals of acoustic neuroma (AN) surgery. In this paper, we present the surgical results of hearing preservation surgery in our department, as well as our recent basic research on hearing preservation in AN surgery. Subjects consisted of 95 AN patients who underwent hearing preservation surgery via the extended middle cranial fossa approach and the middle cranial fossa approach. The overall hearing preservation rate was 51%. From the analysis of surgical results, we discussed the indications for hearing preservation surgery and the evaluation of pre- and post-operative hearing. In addition, we proposed a new classification system for pre- and post-operative hearing evaluation. We also present our basic research on "the relationship between cochlear blood flow and electrocochleographic findings during cochlear ischemia", and "the morphological studied of the VIII cranial nerve in AN patients". The changes in N1 and N2 in electrocochleography depended on the cochlear blood flow. Thus, we can estimate the degree of cochlear ischemia using the electrocochleography. Furthermore, the impairment of cochlear blood flow and cochlear nerve damage could be distinguished by the electrocochleographic findings. The morphological findings of the VIII cranial nerves of AN patients demonstrated tumor invasion in approximately 67% of the cochlear nerves, suggesting that total tumor removal together with hearing preservation could not be performed in these patients. In addition, gliosis in the transitional region of the cochlear nerve in AN patients was observed, which might lead to a change in capillary distribution. Angiogenesis was also found in tumor tissues from the cochlear nerve. These morphological findings indicated that the impairment of cochlear blood flow may be easily induced by manipulation during tumor removal, especially those around the transitional region.