SURGICAL APPROACH TO THE INTERNAL AUDITORY MEATUS IN ACOUSTIC NEUROMASURGERY - SIGNIFICANCE OF PREOPERATIVE HIGH-RESOLUTION COMPUTED-TOMOGRAPHY

OBJECTIVE AND METHOD: To establish a surgical approach that provides b etter access to the internal auditory meatus (IAM) in acoustic tumor r emoval via a suboccipital transmeatal route, the anatomic location of the labyrinth with regard to the sigmoid-fundus line and the extent of bone removal from the IAM were evaluated by pre- and postoperative hi gh-resolution computed tomography for 47 patients. RESULTS: The labyri nth was located lateral to the sigmoid-fundus line in 22 cases (latera l type, 46.8%), on the line in 9 (on-the-line type, 19.1%), and medial to the line in 16 (medial type, 34.0%). Postoperative high-resolution computed tomography showed that the mean length of the posterior wall from the fundus was shortened to 5.1 +/- 1.1 mm (n = 27) in the parti ally resected group and to 2.0 +/- 0.6 mm (n = 20) in the widely opene d IAM group. The labyrinth was damaged in four cases from the widely o pened IAM group; the preoperative status of the labyrinth was medial t o the sigmoid-fundus line in three and on the line in one. The cochlea r nerve was preserved in seven (31%) partially resected and in eight ( 50%) widely opened IAM cases. The overall success rate of hearing pres ervation was 4 of 22 (18.2%) partially resected and 5 of 16 (31.2%) wi dely opened cases. CONCLUSION: Preoperative evaluation of the anatomic relation between the IAM and the labyrinth by high-resolution compute d tomography provides an efficient surgical approach to the IAM and co ntributes to anatomic and functional preservation of the cochlear nerv e.