Normal variability of the brain-stem auditory evoked response in young and old adult subjects.

The brain-stem auditory evoked response was studied under three combinations of intensity and rate of click stimulation in 25 young and 25 old adult subjects. The responses show a great intra and inter subject variability. Waves 1, 3 and 5 are constant and reproducible markers of the response while waves 2, 4, 6, and 7 are variable and frequently asymmetrical or absent. Wave peak latencies increase with an increase in stimulation rate, a decrease in stimulus intensity and an increase in age. Interpeak conduction times, except those involving wave 2, are unaffected by a change in stimulus intensity. Wave 1-3 time increases with an increase in stimulus rate and an increase in age while wave 3-5 time is not affected by any change in stimulus parameters or age. Wave amplitude alone is not a reliable measure of normality. Normal values for peak latencies, interpeak conduction times and interear symmetry are presented. One tailed 95 and 99% confidence intervals for normal interpeak conduction times are presented with appropriate corrections for stimulus rate used and patient age. No corrections need to be made for changes in stimulus intensity. This suggests that no correction is necessary when studying patients with conductive type hearing deficits. Laboratories using the same stimulation technique appear to produce equivalent normative conduction time values.