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.