■ OBJECTIVE: To review the location of acoustic neuroma in the internal auditory canal, especially focusing on cerebrospinal fluid (CSF) space in the fundus, and hearing outcomes and to analyze the origin of acoustic neuroma in magnetic resonance imaging (MRI).

■ METHODS: Retrospective review of 82 acoustic neuroma patients who underwent hearing examination and MRI. To investigate the origin of acoustic neuroma, we measured where the tumor localized from the fundus to the cerebellopontine angle in 24 patients whose acoustic tumor had not touched fundus, cerebellum or pons.Results: 44 patients (54%) showed CSF space in the fundus of the internal auditory canal (Group 1), whereas 38 patients (46%) did not show CSF space (Group 2) on MRI. The initial symptoms in Group 1 occurred more suddenly than those in Group 2. The mean pure-tone average using 0.5, 1 and 2 kHz frequencies was 52.2 dB in Group 1 and 65.8 dB in Group 2. The mean speech discrimination score was 75.8% and 67.7%, respectively. The abnormality rate for auditory brainstem response (ABR) was 92.7% and 97.3%, respectively. The mean distance from the fundus to the brainstem was 22.6 +/- 2.9 mm. The mean distance from the  fundus to the porus was 12.8 +/- 1.4 mm. The center of the tumor was localized at a mean distance of 8.2 +/- 2.4 mm from the fundus.

■ CONCLUSION: Comparison of the patients with/without CSF space in the fundus of the internal auditory canal demonstrated a significant difference in the onset pattern of symptoms and puretone hearing level. Speech discrimination score of patients with CSF space was better, but there was no significant difference between two groups. Auditory brainstem response showed a high rate abnormality in both groups. In this MRI study, the acoustic tumor tended to originate from the one-third peripheral portion of the porus. Acoustic tumor may occur from a more peripheral portion considering the structure of the internal auditory canal and cerebellopontine angle.