"Onion bulb" formation associated with a solitary neoplasm of the eighth nerve sheath

Audiometric patterns associated with retrocochlear disorders include abnormal adaptation, delay of auditory evoked potential latencies, and characteristic abnormalities of speech intelligibility functions. The anatomic substrate of these psychoacoustic and electrophysiologic abnormalities is unknown. This paper describes the incidental observation of "onion bulbs" in a solitary neoplasm involving the eighth nerve sheath and in the cochlear nerve lateral to the neoplasm. Such hypertrophic neuropathy in the eighth nerve has not been described previously. This occurrence raises the question of sequential demyelination and remyelination in a neoplasm-bearing nerve as a possible histopathologic correlate to retrocochlear dysfunction.