Medial acoustic neuromas. A new clinical entity.

The medial acoustic neuroma, a new clinical entity, is defined as an extrameatal tumor without tumor mass laterally in the internal acoustic meatus. During a 12-year period in Denmark, in a prospective analysis of 400 acoustic neuromas on which surgery was performed by the translabyrinthine approach, 48 tumors (12%) were medial tumors, corresponding to an incidence of 0.8 tumors per million inhabitants per year. Analysis of the symptoms and results showed that medial tumors are generally larger with more severe involvement of the cerebellum, the trigeminal nerve, and the brain stem, compared with the 352 nonmedial tumors. Because of its onset in the medial part of the vestibular nerve, the tumor may grow silently and to a considerable size without any widening of the internal auditory canal and with relatively good hearing. Meaningful hearing preservation is impossible in medial tumors, because the smallest medial tumor (subject's hearing, 40 dB or better) measured 3 cm, and the majority of tumors are giant tumors.