Brainstem auditory evoked response at five years and prenatal and postnatal blood lead

Abstract

Previous work from this laboratory demonstrated an association between higher maternal blood lead level at 20 weeks of pregnancy and increased I–V and III–V interpeak intervals in the brainstem auditory evoked response (BAER) recorded in 1-month-old infants. We repeated the BAER measurements with a larger group of children ( n = 100–113) from the same study at 5–7 years. Maternal blood lead level at 20 weeks of pregnancy (geometric mean = 7.7 μg/dl; range 1–30.5 μg/dl) was the only prenatal blood lead level significantly associated with I–V and III–V interpeak interval in a multiple regression model controlling for head circumference and age at time of testing and sex. In contrast to the findings at 1 month of age, interpeak intervals decreased as a linear function of increasing 20-week maternal blood lead. A nonlinear, orthogonal, second-order polynomial model was a significantly better fit to the data than the linear model. The nonlinear model showed I–V and III–V interpeak intervals decreased as blood lead rose from 1 to 8 μg/dl, and then increased as blood lead rose from 8 to 30.5 μg/dl. We hypothesized that the negative linear term was related to lead effect on brainstem auditory pathway length, and that the positive quadratic term was related to neurotoxic lead effect on synaptic transmission or conduction velocity. We found support for the brainstem length interpretation in the data, showing that 6-year-old head circumference in these children significantly decreased with increased maternal 20-week blood lead level. Increasing postnatal blood lead at 12 and 48 months was related only to decreased BAER conduction intervals across the entire blood lead range, suggesting only pathway length effects. Alterations in BAER at this age may indicate that the effect of prenatal lead exposure on the auditory brainstem is permanent, as response latencies reach essentially adult values by 4 years of age.