Objective: Only a small percentage of auditory brainstem implant (ABI) recipients treated for neurofibromatosis type 2 (NF2) have proved capable of identifying words using only the sound from the ABI. Recently, the ABI was applied to a series of patients with no cochlear nerve or with cochlear disorders that could not benefit from a cochlear implant (i.e., cochlear nerve aplasia or posttraumatic avulsion) or whose benefit was or would be severely compromised. A significant number of these patients have proven capable of understanding speech, including effortless telephone use. In the present study, a series of psychophysical tests were administered to determine the cause of the difference in performance between tumor (T) and nontumor (NT) ABI patients.
Study design: Retrospective case review.
Setting: Tertiary referral center.
Patients: Twenty patients with ABIs participated in the investigation. Ten were NF2 patients and 10 NT subjects. Patient ages ranged from 24 to 61 years. Eleven were males and nine females.
Intervention: Auditory rehabilitation in auditory disconnection caused by cochlea or cochlear nerve disorders.
Results: There was a significant correlation between modulation detection thresholds and speech understanding and a significant difference in modulation detection between T and NT patients.
Conclusions: The difference in modulation detection between the two groups suggests a difference in the survival of specific cells in the cochlear nucleus that support modulation. The pattern of results indicates a separate pathway of auditory processing that is specialized for modulated sounds, and that pathway is critical for speech understanding. In NF2 patients, the tumor and surgery may selectively damage this pathway, resulting in poor speech recognition with prosthetic stimulation.