Background: The pathogenesis of hearing loss caused by cerebellopontine angle tumors such as acoustic neuromas is unknown. The lack of loudness recruitment is thought to be one of the features of retrocochlear hearing impairment. In contrast to conventional suprathreshold tests, the categorial loudness scaling using the "Würzburger Hörfeld" is a valuable tool to describe the individual perception of sound. The aim of the present study was to analyze the loudness growth rate in patients with acoustic neuroma.
Patients and method: Pure tone and speech audiometry as well as auditory brainstem response and bilateral categorial loudness scaling were performed preoperatively in 54 patients with acoustic neuroma. Loudness scaling was done in free field switching off the contralateral ear by using an ear-plug.
Results: An abnormal rapid loudness growth function was found in 38 of the 54 patients (70.4%) at least at one frequency on the tumor side. The contralateral side was effected only in 57.4% of the patients. The incidence of a recruitment depended on the frequency with a maximum at 4 kHz. The slope of the loudness function showed a tendency to increase with increasing hearing loss.
Conclusions: Loudness recruitment is not a rare phenomenon in patients with acoustic neuroma. The underlying cause (a preexisting hair cell damage, hair cell changes resulting from an obstruction of the cochlear blood supply or a disruption of the cochlear efferents) still remains unclear.