[Electric response audiometry in retrocochlear hearing disorders]

Abstract

The intensive clinical use of Electric Response Audiometry (ERA) has recently improved the diagnosis of sensorineural hearing loss. The Auditory Evoked Potentials (AEP) allow a precise topodiagnosis of the whole auditory system. However, specific limitations of the different potentials (cochlear, neural, brainstem, thalamic, cortical) have to be considered to avoid an overestimation of ERA. Based on the author's own material of more than 3500 recordings, most cases of a retrocochlear hearing loss are located at the auditory nerve and the brainstem. Therefore, brainstem electric response audiometry (BERA) and electrocochleography (ECoG) are the two most important methods of ERA. Typical Brainstem AEP (BAEP) features of retrocochlear disorders are shown. They are of different diagnostic value. A prolongation of the central conduction time or a break down of the later BAEP, both unequivocal signs of a retrocochlear lesion, were found in only 62 per cent of 100 cases of acoustic neuroma. However, diagnostic sensitivity can be improved to 96 per cent by additional use of ECoG and electrical promontory testing of the excitability of the auditory nerve. This value cannot be attained by any other audiometric test. These results show that BERA and ECoG are essential screening methods of high sensitivity in the differential diagnosis of sensorineural hearing loss. Additional values for diagnostic specificity and efficacy are also given in comparison with a group of patients initially suspected of having a cerebellopontine angle tumour. These values exceed those of conventional audiometric tests by far. Therefore, BERA and ECoG are reliable predictors of a retrocochlear lesion and can be judged as the best indicators for a CT scan or magnetic resonance imaging at present.