The relation between head size and auditory brain-stem response interpeak latency maturation.

In developmental populations, duration of auditory brain-stem response (ABR) I-III, III-V, and I-V vary substantially across individuals, particularly among preterm infants. Adult ABR interpeak latency has a strong correlation with brain-stem size and weaker correlation with head size. To determine if head size might contribute to this increased interpeak latency variability among infants, ABR data were normalized based on head circumference. Normalization by head circumference did not reduce interpeak variability. Further analyses revealed a negative correlation between interpeak latency and head circumference that varied as a function of age. Before 42 weeks conceptional age (CA), a significant relation exists between increased head circumference and decreased duration of the III-V and I-V intervals, but not the I-III interval. For infants older than 42 weeks CA, there was a significant relation between increased head circumference and decreased duration for the I-III intervals but not the III-V and I-V intervals. An age-dependent correlation between decreasing interpeak latency and increasing head circumference suggests that improved neural transmission through the auditory nerve and brain-stem pathway offset or even overcompensate for developmental lengthening of the sensory pathway. Also, developmental time constants obtained from nonlinear curve fit analyses were shorter for normalized than non-normalized data, particularly for the I-V interval. Therefore, correction of ABR data for the length of the sensory pathway may be important to estimate accurately maturation rate for developmental populations.