Assessing windows of susceptibility to lead-induced cognitive deficits in Mexican children

Background: The identification of susceptible periods to Pb-induced decrements in childhood cognitive abilities remains elusive. Objective: To draw inferences about windows of susceptibility using the pattern of associations between serial childhood blood lead (BPb) concentrations and children's cognitive abilities at 4 years of age among 1035 mother child pairs enrolled in 4 prospective birth cohorts from Mexico City. Methods: Multiple longitudinally collected BPb measurements were obtained from children (1, 2, 3, and 4 years) between 1994 and 2007. Child cognitive abilities were assessed at 4 years using the general cognitive index (GCI) of the McCarthy Scales of Children's Abilities. We used multivariable linear regression to estimate the change in cognitive abilities at 4 years of age with a 10 mu g/dL increase in childhood BPb concentrations adjusting for maternal IQ education, marital status, child sex, breastfeeding duration, and cohort. Results: In separate models for each BPb measurement, 2 year BPb concentrations were most strongly associated with reduced GCI scores at 4 years after adjusting for confounders (beta: -3.8; 95% confidence interval CI: -6.3, -1.4). Mutual adjustment for other BPb concentrations in a single model resulted in larger, but less precise estimate between 2 year BPb concentrations and GCI scores at 4 years of age (beta: -7.1; 95% CI: -12, -2.0). The association between 2 year BPb and GO was not heterogeneous (p = 0.89), but some BPb and GCI associations varied in magnitude and direction across the cohorts. Additional adjustment for child hemoglobin, birth weight, gestational age, gestational BPb concentrations, or test examiner did not change the pattern of associations. Conclusions: Higher BPb concentrations at 2 years of age were most predictive of decreased cognitive abilities among these Mexico City children; however, the observed pattern may be due to exposure, outcome, or cohort related factors. These results may help developing countries more efficiently implement childhood Pb prevention strategies. (C) 2012 Elsevier Inc. All rights reserved.