Failure to find hyperactivity in preschool children with moderately elevated lead burden

Neurotoxicants, Micronutrients, and Social Environments

SUMMARY—Systematic research evaluating the separate and interacting impacts of neurotoxicants, micronutrients, and social environments on children's cognition and behavior has only recently been initiated. Years of extensive human epidemiologic and animal experimental research document the deleterious impact of lead and other metals on the nervous system. However, discrepancies among human studies and between animal and human studies underscore the importance of variations in child nutrition as well as social and behavioral aspects of children's environments that mitigate or exacerbate the effects of neurotoxicants.

In this monograph, we review existing research on the impact of neurotoxic metals, nutrients, and social environments and interactions across the three domains. We examine the literature on lead, mercury, manganese, and cadmium in terms of dispersal, epidemiology, experimental animal studies, effects of social environments, and effects of nutrition.

Research documenting the negative impact of lead on cognition and behavior influenced reductions by the Center for Disease Control in child lead-screening guidelines from 30 micrograms per deciliter (μg/dL) in 1975 to 25 μg/dL in 1985 and to 10 μg/dL in 1991. A further reduction is currently being considered. Experimental animal research documents lead's alteration of glutamate-neurotransmitter (particularly N-methyl-D-aspartate) activity vital to learning and memory. In addition, lead induces changes in cholinergic and dopaminergic activity. Elevated lead concentrations in the blood are more common among children living in poverty and there is some evidence that socioeconomic status influences associations between lead and child outcomes. Micronutrients that influence the effects of lead include iron and zinc.

Research documenting the negative impact of mercury on children (as well as adults) has resulted in a reference dose (RfD) of 0.1 microgram per kilogram of body weight per day (μg/kg/day). In animal studies, mercury interferes with glutamatergic, cholinergic, and dopaminergic activity. Although evidence for interactions of mercury with children's social contexts is minimal, researchers are examining interactions of mercury with several nutrients.

Research on the effects of cadmium and manganese on child cognition and behavior is just beginning. Experimental animal research links cadmium to learning deficits, manganese to behaviors characteristic of Parkinson's disease, and both to altered dopaminergic functioning.

We close our review with a discussion of policy implications, and we recommend interdisciplinary research that will enable us to bridge gaps within and across domains.

Biopsychosoziale Einflüsse auf die kognitive und soziale Entwicklung von Klein- und Vorschulkindern

Zusammenfassung. Für die Analyse biopsychosozialer Einflüsse auf die Entwicklung der Kinder wurde ein Kindergarten aus einem blei- und umweltbelasteten Gebiet mit den Kindern aus einem Kindergarten ohne nennenswerte Umweltrisiken verglichen. Der Entwicklungsstand der Kinder wurde mit sieben entwicklungsdiagnostischen Verfahren erfaßt. Die Lebensgeschichte und der Gesundheitszustand der Kinder wurden explorativ erhoben und zu spezifischen Risikoindices zusammengefasst. Die geringer belastete Gruppe erwies sich in nahezu allen Entwicklungstests überlegen. Gleichzeitig führte die Kumulation biologischer und sozialer Entwicklungsrisiken zu ungünstigeren kognitiven und sozialen Entwicklungsergebnissen.

Lead, attention, and impulsive behavior: changes in a fixed-ratio waiting-for-reward paradigm

Similar to the effects observed in children diagnosed with attention deficit hyperactivity disorder (ADHD), experimental animals exposed to lead (Pb) exhibit behaviors attributed to "impulsivity" and "inability to inhibit inappropriate responding." Such behaviors have led some to suggest that Pb exposure is associated with attention deficit. Based on the hypothesis that attention deficits are related to an ineffectiveness of delayed reinforcement, this study examined the effects of chronic postweaning Pb exposure on an FR waiting-for-reward paradigm. Rats were exposed chronically from weaning to 0, 50, or 150 ppm Pb acetate in water and following 40 days of exposure, trained on a fixed-ratio (FR) wait behavioral baseline. A total of 50 lever press responses (FR 50) produced food delivery. After earning an FR pellet, "free" pellets could be obtained by waiting; emission of another lever press reinitiated the FR requirement. "Free" pellets were delivered at increasing intervals (2 s, 4 s, 6 s, etc.). Pb exposure increased response rates on the FR schedule and decreased the mean longest waiting time, but also resulted in a higher number of responses per reinforcer than exhibited by controls. These Pb-induced differences are consistent with an inability to manage delays of reinforcement.

Low-level lead exposure and behavior in early childhood

OBJECTIVE

To assess whether small elevations in blood lead level were associated with measurable behavioral changes in a group of poor children between 1 and 3 years old.

METHODS

The study population consisted of children presenting for routine well-child care to the pediatric clinic at Bellevue Hospital Center, a large urban public hospital. The following inclusion criteria were used for entry into the study: age 12 to 36 months; capillary lead screening result <1.21 micromol/L (25 microg/dL); no known prior history either of blood lead level >1.21 micromol/L (25 microg/dL) or lead exposure requiring chelation therapy; Latino or African-American; English or Spanish spoken in the home; biological mother as primary caretaker; child not presently attending day care; full-term, singleton gestation; birth weight at least 2500 g; no known neurologic or developmental disorder; and no severe chronic disease, including human immunodeficiency virus infection. Study enrollment was simultaneously stratified by capillary lead level and age. All children between 12 and 36 months attending the pediatric clinic during the study period received screening capillary blood measures of lead level following the recommendations of the Centers for Disease Control and Prevention and the American Academy of Pediatrics as part of routine primary care. During periods of enrollment, consecutive lead measurements performed in the pediatric clinic were reviewed by one of the researchers. For those children meeting entry criteria based on lead level and age, further eligibility based on the remainder of the inclusion criteria was determined through parental interview and review of the medical record. Lead exposure was assessed with a single capillary blood specimen, using atomic absorption spectrophotometry. Subjects were considered to be lead-exposed if their lead level was between 0.48 and 1.20 micromol/L (10 and 24.9 microg/dL) and nonexposed if their lead level was between 0 and 0.48 micromol/L (0 and 9.9 microg/dL). Behavior was assessed using the Behavior Rating Scale (BRS) of the Bayley Scales of Infant Development, second edition. The BRS in this age group consists of three components: an Emotional Regulation Factor that measures hyperactive/distractible/easy-frustration behaviors; an Orientation-Engagement Factor that measures fear/withdrawal/disinterest behaviors; and a Motor Quality Factor that assesses the appropriateness of movement and tone. The BRS is scored as a percentile; lower scores reflect more problematic behaviors. Researchers performing the BRS were blinded to capillary lead results. Information was collected concerning factors that might confound the relationship between lead and behavior. Demographic factors were collected, including: child's age, gender, and country of origin; mother's age, marital status, parity, country of origin, and primary language spoken; parental education, and occupation and receipt of public assistance. Socioeconomic status was determined using the Hollingshead Two-Factor Index of Social Position. Maternal verbal IQ was assessed using the Peabody Picture Vocabulary Test-Revised. Maternal depression was assessed using the Center for Epidemiologic Studies-Depression Scale. Cognitive stimulation provided in the home was assessed using a new office-based instrument, the StimQ, which measures the quantity and quality of play materials and parent-toddler activities in the child's home. To assess the child for iron deficiency, we performed a hematocrit and mean corpuscular volume at the time of the capillary lead evaluation. A presumptive diagnosis of iron deficiency was made if the child was either anemic (defined as a hematocrit <32) or had a mean corpuscular volume <72.

RESULTS

The study sample consisted of 72 children. Children in the lead-exposed group (n = 41) had a mean BRS behavior score that was 15.8 points lower than that of children in the nonexposed group (n = 31), which was significant by the Stu

The comparative developmental neurotoxicity of lead in humans and animals

The effects of lead on neurobehavioral development have been extensively investigated in humans as well as animals. This valuable lode of research findings offers a basis for comparing the developmental neurobehavioral toxicity of lead across species and for assessing the validity of animal models of developmental neurotoxicity. Comparisons of human and animal findings suggest that the greatest qualitative similarities involve relatively complex behavioral processes such as cognition and learning. Quantitative comparisons based on dose-response relationships for these endpoints are difficult to make because the relationships are sometimes nonmonotonic (U-shaped) and because blood lead levels may not be directly comparable between species. However, the lowest levels of exposure at which developmental neurobehavioral effects have been observed are similar: 10-15 micrograms/dl in children, less than 15 micrograms/dl in primates, and less than 20 micrograms/dl in rodents. Although the convergence between animal and human findings for other neurobehavioral endpoints is not as striking, sensory-evoked potentials and communicative processes offer two promising areas for continued investigation and cross-species comparison.

Intellectual and behavioural consequences of low level lead exposure: a review of recent studies

Recent studies investigating the association between low levels of lead and children's IQ, behaviour and educational attainment are reviewed. The main emphasis is on the methodological issues and problems which face researchers carrying out these cross-sectional epidemiological studies, and in particular the problem of confounding social factors. It is concluded that body lead levels in children do to some extent act as a marker for socially disadvantageous factors, and that when these are controlled adequately, if there are any functional effects due to lead, then these are so small that they cannot be detected with any certainty, and they may not exist at all.