Semiparametric modeling of age at achieving developmental milestones after prenatal exposure to methylmercury in the Seychelles child development study

Principles of studying low-level neurotoxic exposures in children: using the Seychelles Child Development Study of methyl mercury as a prototype

Epidemiological studies to determine the impact of low level toxic exposure on child development are important in guiding clinical and public health action. However, carrying out such studies and interpreting their findings presents a number of significant challenges to the investigators. First, they must find a cohort with suitable exposure, select a biomarker that will accurately determine the level of exposure and determine the endpoints that are most likely to detect subtle differences in neurodevelopment. Following that, the logistics of the study must be organised and collaboration established with the local population and health authorities. To accurately interpret the data, they must also accurately determine covariates that impact child development. After the data are collected, interpreting the findings presents a further challenge. Throughout this process, the study must adhere to fundamental epidemiological principles and clearly defined statistical approaches. This paper discusses those principles and uses the Seychelles Child Development Study to show how one epidemiological study addressed them.

Neuropsychological function in school-age children with low mercury exposures

The EPA reference dose for methylmercury (MeHg) was established using data from populations with greater exposures than those typical of the US. Few data are available on potential adverse health effects at lower levels. We examined relationships between hair mercury (Hg) levels and neuropsychological outcomes in a population of US children. This study included data from 355 children ages 6-10 enrolled in the New England Children's Amalgam Trial. Data on total hair Hg levels, sociodemographic information and neuropsychological function were collected. We evaluated associations between hair Hg and neuropsychological test scores with linear regression methods and used generalized additive models to determine the shape of associations that departed from linearity. Models controlled for relevant covariates, including the potential beneficial effects of consuming fish. In adjusted models, we observed no significant linear relationships between hair Hg level and any test score. Significant departures from linearity were identified for WIAT Math Reasoning and WRAMVA Visual-Motor Composite scores. The association was positive for hair Hg levels below 0.5 microg/g and negative for levels between 0.5 and 1.0 microg/g. Overall, test scores of children with hair Hg levels 1.0 microg/g appeared to be lower than those of children with levels < 1.0 microg/g, but few children had levels in this upper range and these differences did not reach statistical significance. Hair Hg levels below 1.0 microg/g in US school-age children were not adversely related to neuropsychological function.

Mercury exposure and public health

Mercury is a metal that is a liquid at room temperature. Mercury has a long and interesting history deriving from its use in medicine and industry, with the resultant toxicity produced. In high enough doses, all forms of mercury can produce toxicity. The most devastating tragedies related to mercury toxicity in recent history include Minamata Bay and Niagata, Japan in the 1950s, and Iraq in the 1970s. More recent mercury toxicity issues include the extreme toxicity of the dimethylmercury compound noted in 1998, the possible toxicity related to dental amalgams, and the disproved relationship between vaccines and autism related to the presence of the mercury-containing preservative, thimerosal.

Prenatal methylmercury exposure and developmental outcomes: review of the evidence and discussion of future directions

I conducted a review of the published literature to assess the strength of the evidence for an association between prenatal exposure to methylmercury (MeHg) and subsequent child development. I identified 12 studies on this subject published since 1980. Of these, 3 were longitudinal studies--2 conducted in the Seychelle Islands, and 1 in the Faroe Islands. Nine were cross-sectional studies conducted in different countries where seafood, a source of MeHg, constituted a major part of the diet. The ages of the children studied ranged from 2 weeks to 12 years. The results of the longitudinal studies were contradictory. Researchers in the Faroe Islands identified an association between MeHg exposure and developmental effects, whereas those in the Seychelle Islands identified no such association. This inconsistency was mirrored in the results of the cross-sectional studies where there were some positive and some negative findings. It was concluded that it was not possible from currently available data to determine whether there is an association between prenatal MeHg exposure and adverse developmental effects in children. In advance of future research, consideration should be given to resolving the uncertainties surrounding exposure assessment and outcome measurement, as both elements varied between studies. It was suggested that questions of exposure assessment would benefit from the application of an expert review process. Outcome assessment would benefit from the development of theoretically based measures of specific aspects of cognitive functioning to replace the relatively crude measures of attainment and IQ currently employed in most studies. This would assist in the development of classic longitudinal studies by allowing repeated assessment over the full age range and providing data that are more readily interpretable and comparable between studies.

Perinatal and lifetime exposure to methylmercury in the mouse: behavioral effects

This project was undertaken to more completely understand the consequences of lifetime exposure to methylmercury. A series of experiments examined how perinatal or lifetime exposure to methylmercury affected behavioral performances in the adult mouse at different ages. One hundred female B6C3F1/HSD mice were assigned to one of three dose groups, 0 ppm, 1 ppm, or 3 ppm methylmercury chloride administered in a 5 nM sodium carbonate drinking solution. Four weeks after initiating dosing, the females were bred with male CBA/J HSD mice to produce the trihybrid offspring B6C3F1/HSD x CBA/J HSD. The methylmercury-treated litters were split into two subgroups, one exposed throughout its lifetime to the original dose, the other exposed through postnatal day 13. Altogether, then, five groups were studied: Control, 1 ppm perinatal, 1 ppm lifetime, 3 ppm perinatal, and 3 ppm lifetime. Three neurobehavioral indices were evaluated: (1) delayed spatial alternation (a test of memory) and (2) running in a wheel to earn food pellets (schedule-controlled operant behavior) were assessed starting at 5 and 15 months of age; (3) hindlimb splay, a measure of motor function, was assessed at 5, 15, and 26 months of age. Subjects tested at one age were littermates of those tested at the other ages. MeHg altered the hindlimb splay distance; control mice differed from methylmercury-exposed mice, the 1 ppm lifetime and 3 ppm lifetime groups differed from each other, and the analysis yielded an age by dose interaction. MeHg exposure altered different measures of wheel running under the 3 ppm lifetime condition. In the delayed alternation procedure, the mouse was required to respond to one of two locations in a strictly alternating sequence. More mice from the treated groups, except for the 1 ppm perinatal group, failed to meet the criterion at longer delay values. Overall, the results show that exposure to low levels of methylmercury produces behavioral effects that depend on the test procedure, the dose, the duration of exposure, and the age. Lifetime evaluations of exposure to toxicants, beginning with early development, should be a component of the risk assessment process for neurotoxicity.

Assessing the Effects of Environmental Toxicant Exposure in Developmental Epidemiological Studies: Issues for Risk Assessment

Epidemiological studies have been of critical importance to the understanding of the effects of environmental chemical exposure during development on the behavior of infants and children. The ultimate goal of these studies should be to provide information that may be used directly for the protection of public health. The strategies for the assessment endpoints include development of domain-specific tests based on knowledge concerning effects of the chemicals being assessed, or use of standard clinical instruments that sample a range of functions. Discussion of an overall strategy for choice of endpoints would allow more straightforward comparisons across studies. There is increasing recognition of the importance of measuring a number of chemicals relevant to the population under study; however, different investigators make different decisions concerning which and how many chemicals to measure, as well as how to include them in the statistical analysis, particularly when there is a high degree of collinearity. Chemicals that are highly correlated with the "chemical of interest" are sometimes not included in the statistical analysis, resulting in missed opportunity to derive important information from the study. In addition, the shape of the relationship between exposure and effect is usually not explored in epidemiological studies, even though such information is critical for risk assessment. Opportunity for discussion among investigators, statisticians, and risk assessors potentially would result in human developmental toxicity studies being maximally useful for public health decisions.

Exploring nonlinear association between prenatal methylmercury exposure from fish consumption and child development: evaluation of the Seychelles Child Development Study nine-year data using semiparametric additive models

Studies of the association between prenatal methylmercury exposure from maternal fish consumption and neurodevelopmental test scores in the Seychelles Child Development Study have not found adverse effects through age 9 years. The analysis for the most recent 9-year data (Lancet 361 (2003) 1686) employed conventional linear regression models. In this study we reanalyzed the same Seychelles 9-year data using semiparametric additive models with different degrees of smoothing to explore whether nonlinear effects of prenatal exposure were present. Of 21 endpoints in the linear analysis, we chose only those with a two-tailed P value less than 0.2 for the effect of prenatal exposure. Six endpoints met the criterion. A nonlinear effect was identified with the more smooth model for only one endpoint. The test for an overall effect of prenatal exposure was also significant, with a P value of 0.04, while the corresponding P value in the linear regression analysis was 0.08. The nonlinear curve appeared to be nearly flat when the level was below approximately 12 ppm in maternal hair, with a linear trend above that level, suggesting a possible adverse effect in the uppermost range of prenatal exposure included in this cohort. Because of the descriptive nature of semiparametric additive models, the P values are not precise, and certainly there are fewer data above 12 ppm. We conclude that this reanalysis supports the primary linear analysis, showing little evidence for a prenatal adverse effect.

Mercury in the environment: sources, toxicities, and prevention of exposure

Acute and chronic exposure to mercury can significantly affect the health of a population, specifically the children. Methylmercury may pose the highest threat, as it is ubiquitous in the environment and it is a potent neurotoxicant. Methylmercury easily passes through the placenta to the developing fetus. Elemental mercury, or quicksilver, also poses a threat to children because it may be found readily in schools, hospitals, and medicine cabinets, and its intriguing liquid nature may be enticing to children. Pediatricians must be diligent in informing patients of possible exposure sources, and alerting them to new government advisories and recommendations. They should also be knowledgeable regarding classic clinical presentations of mercury toxicity. It is only in cases involving a knowledge of mercury that appropriate historical information is obtained and correct diagnoses are made. Preventing mercury exposure and consequent toxicity is of importance because therapies are controversial and long-term consequences may be significant.

Prenatal methylmercury exposure from ocean fish consumption in the Seychelles child development study

INTRODUCTION

Exposure to methylmercury (MeHg) before birth can adversely affect children's neurodevelopment. The most common form of prenatal exposure is maternal fish consumption, but whether such exposure harms the fetus is unknown. We aimed to identify adverse neurodevelopmental effects in a fish-consuming population.

METHODS

We investigated 779 mother-infant pairs residing in the Republic of Seychelles. Mothers reported consuming fish on average 12 meals per week. Fish in Seychelles contain much the same concentrations of MeHg as commercial ocean fish elsewhere. Prenatal MeHg exposure was determined from maternal hair growing during pregnancy. We assessed neurocognitive, language, memory, motor, perceptual-motor, and behavioural functions in children at age 9 years. The association between prenatal MeHg exposure and the primary endpoints was investigated with multiple linear regression with adjustment for covariates that affect child development.

FINDINGS

Mean prenatal MeHg exposure was 6.9 parts per million (SD 4.5 ppm). Only two endpoints were associated with prenatal MeHg exposure. Increased exposure was associated with decreased performance in the grooved pegboard using the non-dominant hand in males and improved scores in the hyperactivity index of the Conner's teacher rating scale. Covariates affecting child development were appropriately associated with endpoints.

INTERPRETATION

These data do not support the hypothesis that there is a neurodevelopmental risk from prenatal MeHg exposure resulting solely from ocean fish consumption.

Prenatal exposure to methylmercury changes dopamine-modulated motor activity during early ontogeny: age and gender-dependent effects

We have shown previously that prenatal exposure of rats to 0.5 mg/kg/day of methylmercury (MeHg) produces gender-dependent changes in motor activity in adulthood. In the present study we have investigated whether changes in motor activity could also be found during early ontogeny of the offspring. Pregnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lactation. The habituation to a novel environment (spontaneous activity) and the response to stimulation of the dopaminergic system were studied on postnatal day 14 and 21. Measures of spontaneous activity showed a slight increase in MeHg-prenatal exposed male and female rats at 14 days, but not at 21 days. Following administration of U91356A, a selective dopamine D(2) receptor agonist, a significantly lower dopamine-mediated locomotor activity was observed in the 21 day old MeHg-treated males, but not in females. These results show that prenatal exposure to MeHg alters postjunctional dopaminergic activity during the period of maturation of the dopamine system in the brain. Moreover, the gender-dependent susceptibility previously found in adulthood is already evident at the prepubertal stage.

Association between methylmercury exposure from fish consumption and child development at five and a half years of age in the Seychelles Child Development Study: an evaluation of nonlinear relationships

Studies to date of the developmental effects of pre- and postnatal methylmercury exposure from fish consumption in the Seychelles Islands, using linear regression models for analysis, have not shown adverse effects on neurodevelopmental test scores. In this study we evaluated whether nonlinear effects of methylmercury exposure were present, using scores on six tests administered to cohort children in the Seychelles Child Development Study at 66 months of age. Prenatal exposure was determined by measuring mercury in a segment of maternal scalp hair representing growth during pregnancy. Postnatal exposure was measured in a segment of the child's hair taken at 66-months of age. Generalized additive models (GAMs), which make no assumptions about the functional form of the relationship between exposure and test score, were used in the analysis. GAMs similar to the original linear regression models were used to reanalyze the six primary developmental endpoints from the 66-month test battery. Small nonlinearities were identified in the relationships between prenatal exposure and the Preschool Language Scale (PLS) Total score and Child Behavior Check List (CBCL) and between postnatal exposure and the McCarthy General Cognitive Index (GCI) test scores. The effects are best described graphically but can be summarized by computing the change in the predicted test score from 0 to either 10 or 15 ppm and then above this point. For the PLS the trend involved a decline of 0.8 points between 0 and 10 ppm followed by an increase (representing improvement) of 1.3 points above 10 ppm. For the CBCL there was an increase of 1 point from 0 to 15 ppm, and then a decline (improvement) of 4 points above 15 ppm. The GCI increased by 1.8 points through 10 ppm and then declined 3.2 points (representing worse performance) above 10 ppm. These results are not entirely consistent. Two of the trends involve what appear to be beneficial effects of prenatal exposure. The one possibly adverse trend involves postnatal exposure. In every case the trend changes direction, so that an effect in one direction is followed by an effect in the opposite direction. Because of the descriptive nature of GAMs it is difficult to provide a precise level of statistical significance for the estimated trends. Certainly above 10 ppm there is less data and trends above this level are estimated less precisely. Overall there was no clear evidence for consistent (across the entire range of exposure levels) adverse effects of exposure on the six developmental outcomes. Further nonlinear modeling of these data may be appropriate, but there is also the risk of fitting complex models without a clear biological rationale.

Secondary analysis from the Seychelles Child Development Study: the child behavior checklist

Human exposure to methylmercury (MeHg), a known neurotoxin, is primarily from fish consumption. As part of a large study examining the association between MeHg exposure and child development in a population with high fish consumption we examined school-age behavior using the Achenbach Child Behavior Checklist (CBCL). The CBCL Total T score was a primary endpoint and was reported earlier to show no adverse association with prenatal or postnatal MeHg exposure. In this study we analyzed the T scores of the CBCL subscales to determine if more discrete aspects of measured behavior were associated with exposure. The Seychelles Child Development Study (SCDS) is a prospective, double blind, longitudinal evaluation of over 700 children. The index of prenatal exposure was maternal hair total mercury (T-Hg) in a segment growing during gestation. Postnatal exposure was T-Hg in the child's hair taken at 66 months of age. The child's primary caregiver completed the CBCL during the 66-month evaluation. No association between prenatal or postnatal MeHg exposure and the CBCL subscales was found. In Seychellois children exposed to MeHg from consumption of ocean fish we found no association between either prenatal or postnatal MeHg exposure and behavior as measured by the CBCL subscales.

Twenty-seven years studying the human neurotoxicity of methylmercury exposure

Research at the University of Rochester (U of R) has been focused on mercury for nearly half a century. Initially studies focused on dosimetry, especially the accuracy of measuring exposure, and experimental work with animal models. Clinical studies in human populations started when the U of R mercury group was asked to assist with dosimetry in the Iraq epidemic of 1971-1972. Initial clinical studies described the effects of methylmercury (MeHg) poisoning on adults and children. A dose-response curve for prenatal exposure was determined and it suggested that relatively low exposures might be harmful to the fetus. Since most human exposure to MeHg is dietary from fish consumption, these theoretical dangers had far-reaching implications. After Iraq, the Rochester team pursued exposure from fish consumption in both adults and children. Populations with high fish consumption were identified in Samoa and Peru for studying adults and in Peru and the Seychelles islands for studying children. The possible health threat to the fetus from maternal fish consumption quickly became the focus of research efforts. This paper reviews the Rochester experience in studying human exposure to MeHg from fish consumption.

Uncertainty exists in recommendations for reducing prenatal exposure to mercury

The Environmnetal Working Group, Health Care Without Harm (1999) recommends that pregnant women avoid eating tuna, swordfish, or shark and that young children avoid eating more than one serving of tuna per week. Childbirth educators are in a unique position to educate women about the dangers of methylmercury exposure during pregnancy.