Basic and clinical aspects of hearing preservation in acoustic neuroma surgery

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.