[Changes in the compound action potential in patients with acoustic neuroma]

Thirty-eight patients with proven acoustic neuroma were divided into three groups: 13 patients with normal threshold on pure tone audiogram, 15 patients with a hearing loss of 40 dB-80 dB (HL) in the low and middle frequencies, and 10 totally or almost deaf patients. Results of the first two groups are reported, using intra-tympanic electrodes with alternating clicks of 90 dB (nHL) intensity and a stimulus rate of 20/s. A total of 500 sweeps were averaged. Widening of the CAP complex by a mean of 1.6 ms, compared to 1.1 ms in normal hearing subjects, was the most striking finding. Summation potentials (ratio of SP amplitude to CAP amplitude) were enlarged to 0.6, compared to 0.2 in healthy persons and 0.9 in patients suffering from Ménière's disease. CAP amplitudes as well as CM amplitudes were reduced to almost one-third of normal values. The most noticeable result was the loss of amplitude using very short interstimulus intervals. With a stimulus rate of 97/s amplitudes of acoustic neuroma patients were reduced by 49.9% - reduction in normal and Ménière patients was 41.6% and 41.3%. Acoustic neuroma patients show pathological inner ear potentials. Increased summation potentials are often found with Ménière patients, which are said to be caused by endolymphatic hydrops. For tumours of the acoustic nerve the hydrops theory does not seem likely; the loss of amplitude due to ++adaptation++ within the first neuron seems to point to mechanical reasons due to the pressure of the tumour itself.