[Electrocochleographic detection of the summation potential in patients with inner ear deafness]

Pre-operative information about cochlear mechanical function is a pre-requisite for intra-cochlear implants. In more than 300 pre-operative examinations we demonstrated cochlear microphonics in many patients suffering from complete inner ear deafness; we interpret this phenomenon as evidence of mechanical function of the cochlea. A summation potential in patients with sensorineural deafness is more certain proof. We have now performed thorough electrocochleography on 43 deaf patients whose acoustic nerve was intact, i.e. whose promontory test was positive. After stimulation firstly with rarefaction tonebursts and later with condensation tonebursts and consecutive, off-line addition and subtraction we found small amplitude microphonics in 32 patients. Of these patients, 16 showed a negative potential that we interpreted as a summation potential. If these findings are verified by further investigations it will be possible to examine patients with inner ear deafness electrophysiologically. Furthermore this investigation is a very sensitive indicator of residual mechanical function of the cochlear basilar membrane.