PRESERVATION OF COCHLEAR NERVE FUNCTION IN ACOUSTIC NEURINOMA SURGERY

A total of 55 cases with unilateral acoustic neurinoma which were oper ated on by the lateral suboccipital approach was studied to elucidate factors which influence postoperative hearing acuity. We analyzed seve ral factors: preoperative hearing level, tumour size, tumour consisten cy (cystic or solid), and anatomical location of the cochlear nerve. T he size of the tumours ranged from 1.2 to 5.8 cm in diameter. Thirty o f 55 cases (55%) preoperatively had remaining cochlear function. The s maller the size of tumour, the higher was the preoperative hearing lev el excepting those tumours with a diameter of 5cm or greater, which ha d relatively good hearing and often contained large cysts. As to the c onsistency of the tumours, 41 were solid and 14 were cystic, where 19 (46%) and 11 (79%) cases had had preoperative hearing, respectively. A natomical continuity of the cochlear nerve was maintained at surgery i n 15 of 30 cases with preoperatively remaining hearing; cochelar funct ion was preserved after surgery in 9 of the 15 cases. It was located c ounter-clockwise (caudally) to the facial nerve at an angle of 50 degr ees on average when they were projected on the right side. The distanc e or interrelation between the two nerves had no bearing on postoperat ive hearing preservation. Postoperatively, hearing acuity was improved in 6 cases (20%) with a mean value of 5.6 dB, unchanged in 3 (10%), a nd deteriorated in 21 (70%) among the 30 cases with remaining preopera tive-hearing. When the tumour was less than 2cm or cystic, better hear ing preservation was expected. Hearing was preserved in 4 cases of the 19 solid tumours (21%) and in 5 of the 11 cystic tumours (45%). No ca ses with preoperative hearing deficit greater than 60 dB showed postop erative improvement to a useful hearing level.