STRATEGY FOR THE DIAGNOSIS OF SMALL ACOUSTIC NEUROMAS

Twenty small (extra-meatal size < 15 mm) acoustic neuromas have been diagnosed since high-resolution (HR) magnetic resonance imaging (MRI) became available in our clinic. Among these tumors, 18 had sensorineural hearing loss and 16 enlarged internal auditory meatus in X-ray photo, but only 8 tumors out of 18 tested showed diminished caloric response. Auditory brainstem responses (ABR) has been believed as the most reliable test for the diagnosis of acoustic neuroma despite several reports of false-negatives. In our series, 4 tumors out of 18 tested had normal ABR. The false-negative rate was 22%, which is much higher than expected. In CT, only 11 tumors were recognized. Although the total number is not large, present results clearly suggest the limits of these examinations. At present, HR-MRI is the most reliable diagnostic method for acoustic neuromas with no false-negative reported; ordinary MRI may have false-negatives. For the effective use of MRI, the results of audiometry, X-ray photo, ABR and other examinations must be evaluated properly.