Internal auditory canal vascular loops: audiometric and vestibular system findings

Prominent loops of the anterior inferior cerebellar artery in the cerebellopontine angle are found frequently during anatomic studies of this region. These vascular loops are suspected of causing hearing loss, tinnitus, and vertigo, and surgery has been advocated to separate the vascular loop from the eighth cranial nerve. Previous reports have described pathologic anatomy, surgical approaches, and results of treatment. In this study, we report the results of a uniform battery of audiometric and vestibular system test results administered to fifteen patients with prominent vascular loops in the internal auditory canal diagnosed by pneumo-CT. All patients were tumor suspects before CT because of unilateral (or asymmetric) tinnitus or hearing loss. Hearing losses ranged from mild to profound, and most were of a cochlear type with excellent speech discrimination. Only one-third of the patients had abnormal caloric tests, but spontaneous nystagmus was detected in all but one of the patients by photoelectric nystagmography. The wide range of audiometric and vestibular system test results probably reflects the complex interaction between the vascular loop and eighth cranial nerve, in which the loop exerts pressure on the nerve, and the nerve compromises inner ear circulation. Eighth nerve tumors and vascular loops produce similar symptoms, but a cochlear type of hearing loss with good speech discrimination and normal caloric testing should raise suspicion of a vascular loop. Pneumo-CT is an effective means of diagnosing vascular loops and differentiating them from other lesions of the cerebellopontine angle.