[Ipsilateral masking of auditory evoked potentials: a method of studying multiple sclerosis]

20 normal subjects and 39 patients with multiple sclerosis were the control and the test groups. Auditory brainstem potentials to 60 dB nHL, 11/s clicks, were recorded under ipsilateral broad-band noise masking at S/N ratio of + 60 dB (unmasked condition), + 20 dB, + 10 dB and 0 dB. In the control group the ABP were absent only in 1 subject at S/N = 0 dB. In the group of 16/39 patients with definite multiple sclerosis, 11 had no ABP at S/N = 0 dB, 6 at S/N = + 10 dB and 5 at S/N = + 20 dB. The ABP waveform per se, in the same subjects, was abnormal in 7 and doubtful in 5. These results are discussed in terms of sensitivity, specificity and efficiency of the test to be applied. The best predictive value is achieved by combining a strict morphological criterion with the results of the ipsilateral masking. Moreover, the ipsilateral masking test positive findings are equally distributed in the group of patients with and without signs of neurological involvement of the brainstem.