Reduced variance in the latency and amplitude of the fifth wave of auditory brain stem response after normalization for head size.

The value of the Auditory Brain Stem Response (ABR) in the assessment and detection of neurological disorders could be considerably enhanced if the normative standards of ABR characteristic parameters take into account all other systematic sources of variance. The present study attempts to take into account the influence of head size on latency and amplitude of the ABR components. We examined amplitude and latency as a function of head size in 40 normal male subjects (age 20-40 years). Significant negative correlation was found between amplitude and head radius. The experimental data were fitted using a theoretical curve of the potential on the surface of a three-concentric sphere model representing the human head. The fitted curve of amplitude versus radius can be applied to normative data in order to substantially reduce dispersion and consequently increase the diagnostic value of this parameter. Moreover, a substantial effect of radius on wave V latency was detected. Normalization of the latencies with reference to the head radius, assuming that the latter is proportional to the length of brain stem, resulted in a significant reduction in the standard deviation of these data as compared to the original.