Neuropsychological effects of chronic asymptomatic increased lead absorption. A controlled study

Disparities in Toxic Chemical Exposures and Associated Neurodevelopmental Outcomes: A Scoping Review and Systematic Evidence Map of the Epidemiological Literature

BACKGROUND

Children are routinely exposed to chemicals known or suspected of harming brain development. Targeting Environmental Neuro-Development Risks (Project TENDR), an alliance of > 50 leading scientists, health professionals, and advocates, is working to protect children from these toxic chemicals and pollutants, especially the disproportionate exposures experienced by children from families with low incomes and families of color.

OBJECTIVE

This scoping review was initiated to map existing literature on disparities in neurodevelopmental outcomes for U.S. children from population groups who have been historically economically/socially marginalized and exposed to seven exemplar neurotoxicants: combustion-related air pollution (AP), lead (Pb), mercury (Hg), organophosphate pesticides (OPs), phthalates (Phth), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs).

METHODS

Systematic literature searches for the seven exemplar chemicals, informed by the Population, Exposure, Comparator, Outcome (PECO) framework, were conducted through 18 November 2022, using PubMed, CINAHL Plus (EBSCO), GreenFILE (EBSCO), and Web of Science sources. We examined these studies regarding authors' conceptualization and operationalization of race, ethnicity, and other indicators of sociodemographic and socioeconomic disadvantage; whether studies presented data on exposure and outcome disparities and the patterns of those disparities; and the evidence of effect modification by or interaction with race and ethnicity.

RESULTS

Two hundred twelve individual studies met the search criteria and were reviewed, resulting in 218 studies or investigations being included in this review. AP and Pb were the most commonly studied exposures. The most frequently identified neurodevelopmental outcomes were cognitive and behavioral/psychological. Approximately a third (74 studies) reported investigations of interactions or effect modification with 69% (51 of 74 studies) reporting the presence of interactions or effect modification. However, less than half of the studies presented data on disparities in the outcome or the exposure, and fewer conducted formal tests of heterogeneity. Ninety-two percent of the 165 articles that examined race and ethnicity did not provide an explanation of their constructs for these variables, creating an incomplete picture.

DISCUSSION

As a whole, the studies we reviewed indicated a complex story about how racial and ethnic minority and low-income children may be disproportionately harmed by exposures to neurotoxicants, and this has implications for targeting interventions, policy change, and other necessary investments to eliminate these health disparities. We provide recommendations on improving environmental epidemiological studies on environmental health disparities. To achieve environmental justice and health equity, we recommend concomitant strategies to eradicate both neurotoxic chemical exposures and systems that perpetuate social inequities. https://doi.org/10.1289/EHP11750.

Early lead exposure produces lasting changes in sustained attention, response initiation, and reactivity to errors

The present study tested the hypothesis that early lead (Pb) exposure causes lasting attentional dysfunction. Long-Evans dams were fed Pb-adulterated water during gestation and/or lactation; the offspring were tested as adults. The results of a visual discrimination task revealed no Pb effects on learning rate or information-processing speed. However, lasting effects of the early Pb exposure were seen in the subsequent vigilance tasks, particularly in the final task in which onset of the visual cue and cue duration varied randomly across trials. Exposure during both gestation and lactation impaired response initiation. In addition, animals exposed to Pb during lactation only or lactation+gestation committed significantly more omission errors than controls under two specific conditions: (1) trials in which a delay was imposed prior to cue presentation and (2) trials that followed an incorrect response. The pattern of treatment differences indicated that early Pb exposure produced lasting impairment of sustained attention and increased reactivity to errors. Both effects may contribute to the cognitive impairment, problematic classroom behaviors, and increased delinquency associated with early Pb exposure in children. These findings also demonstrate that the developmental timing of the exposure determines the pattern of effects. Thus, conclusions regarding whether or not a particular cognitive or affective function is impaired or spared by early Pb exposure must be limited to the specific timing and intensity of exposure.

Neurotoxicity in young adults 20 years after childhood exposure to lead: the Bunker Hill experience

OBJECTIVES

An epidemiological study of young adults was conducted to determine whether environmental exposure to lead during childhood was associated with current adverse neurobehavioural effects.

METHODS

The exposed group consisted of 281 young adults who had been exposed environmentally to lead as children and the unexposed referent group consisted of 287 age and sex frequency matched subjects. Information on demographics, past and current health, and past exposures to neurotoxicants, and responses to the Swedish Q16 questionnaire were collected by interview. Standard neurobehavioural and neurophysiological tests were administered by computer or trained technicians. K x ray fluorescence was used to estimate tibial bone lead concentrations among the exposed and unexposed groups. Associations were examined between the exposed group and referents and tibial bone lead concentration and the neurobehavioural and neurophysiological outcomes of interest.

RESULTS

Among the measures of peripheral nerve function, after controlling for confounders, sural sensory nerve evoked response amplitude, peroneal motor nerve compound motor action potential amplitude, vibrotactile thresholds of fingers and toes, and standing steadiness were significantly associated with exposure group. Among the neurobehavioural tests, hand-eye coordination, simple reaction time latency, trails B latency, symbol digit latency, serial digit, and learning error score were also significantly associated with exposure group after controlling for confounders. Exposed subjects had significantly more neuropsychiatric symptoms than the referents. Associations between tibial bone lead concentration and scores for vocabulary, vibrotactile thresholds of the fingers, and vibrotactile thresholds of the toes approached significance.

CONCLUSIONS

Significant adverse central and peripheral neurological effects were found in a group of young adults 20 years after childhood environmental exposure to lead when compared with non-exposed controls. The absence of a significant association between neurological outcomes and tibial bone lead concentration, and the presence of significant associations between neurological outcomes and exposure group may be due to either the magnitude of measurement uncertainty in K x ray films relative to the actual tibial bone lead concentration in these young non-occupationally exposed subjects, or uncontrolled confounding of the exposure group.

Residual cognitive deficits 50 years after lead poisoning during childhood

The long term neurobehavioural consequences of childhood lead poisoning are not known. In this study adult subjects with a documented history of lead poisoning before age 4 and matched controls were examined with an abbreviated battery of neuropsychological tests including measures of attention, reasoning, memory, motor speed, and current mood. The subjects exposed to lead were inferior to controls on almost all of the cognitive tasks. This pattern of widespread deficits resembles that found in children evaluated at the time of acute exposure to lead rather than the more circumscribed pattern typically seen in adults exposed to lead. Despite having completed as many years of schooling as controls, the subjects exposed to lead were lower in lifetime occupational status. Within the exposed group, performance on the neuropsychological battery and occupational status were related, consistent with the presumed impact of limitations in neuropsychological functioning on everyday life. The results suggest that many subjects exposed to lead suffered acute encephalopathy in childhood which resolved into a chronic subclinical encephalopathy with associated cognitive dysfunction still evident in adulthood. These findings lend support to efforts to limit exposure to lead in childhood.

Effect of acute pretreatment of lead on picrotoxin-induced convulsions in rats

The effect of acute exposure to lead acetate (LA)/lead nitrate (LN) on onset and severity of convulsions induced by a low dose of picrotoxin was examined in rats. Both LA and LN reduced the time of onset and exacerbated the severity of convulsions, with a resultant high lethality. On comparison, it was noted that in the LA-pretreated group, convulsion scores and incidence of tonus and mortality were much higher; the appearance of tonus was more delayed than in the LN-pretreated group. In lead-pretreated animals, the potentiation of picrotoxin-induced convulsions was accompanied by higher lead levels in blood (p < 0.001). However, the whole-brain lead levels were not significantly different in these animals compared to the controls. The difference in the degree of potentiation by the two forms of lead could possibly be attributed either to the role of a combination of anions and cations or to the variable cerebral uptake and regional distribution of lead or due partly to the extent of competitive interaction involving d-aminolaevulinic acid--whose level is known to be elevated consequent to lead-induced disruption of haem biosynthesis--at GABA receptors.

Clinical presentation of the lead-poisoned child on mental ability tests

Current literature on effects of childhood lead poisoning and high blood lead level on psychological test performance was reviewed. A pathognomic configuration of mental ability test scores is described. A sample of 18 moderately to severely lead poisoned children was found to have a significantly higher frequency of this score configuration than either a comparison sample or the Black subsample of the original WISC-R standardization group. Implications for clinical practice and research are discussed.

A longitudinal study of dentine lead levels, intelligence, school performance and behaviour. Part III. Dentine lead levels and attention/activity

The relationship between dentine lead levels and maternal/teacher ratings of inattentive/restless behaviour in children was examined for a birth cohort of New Zealand children. There were small but relatively consistent and stable correlations between dentine lead values and behaviour ratings. After correction for errors of measurement in dentine lead values and behaviour ratings it was estimated that the correlation between lead levels and inattentive/restless behaviour was in the region of +0.18. However, after control for various sources of confounding there was only a small, but statistically significant, correlation of +0.08 between lead levels and inattention/restless in children. It is concluded that the weight of the evidence favours the view that there is a very weak causal association between lead levels and attention and activity levels in children.

Effects of 1.2 microM lead in the laboratory mouse: developmental and behavioural consequences of chronic treatment

Lead acetate, as a 0.025% (1.2 microM) solution in the drinking fluid, did not adversely affect the reproductive success in breeding mice, weights of pups at birth or cause delays in development. At 21 days, the treated offspring showed reduced weights but these returned to normal values in adulthood. Offspring of treated mice were given 1.2 microM lead acetate as drinking fluid after weaning. The mean daily intake of lead amounted to 2.4 and 2.6 mg/100 g body wt in females and males respectively and caused significant increases in the concentrations of lead in bone and brain. Sequestration of bone was greater in females than males. The behaviour of the offspring was examined by ethological analysis of social encounters between pairs of unfamiliar mice of similar treatment groups in a neutral cage. Evidence of enhanced reactivity to the test situation was seen in treated males at age 3-4 weeks by increased social investigation during the first 3 min of the 20 min encounter, at 7-8 weeks by decreased immobility in the final 5 min of the test and at 18-19 weeks by an increased duration of exploratory behaviour. However, when encountering females at 30-31 weeks, the treated males showed more immobility than did the controls. Treated females at 3-4 weeks, in the first 3 min of the test, showed a decreased frequency of exploratory behaviour and increased immobility, but at 7-8 and 18-19 weeks they showed enhancement of social and sexual investigation. Behaviour was unaltered at 30-31 weeks in encounters with unfamiliar males.(ABSTRACT TRUNCATED AT 250 WORDS)

Automated testing of reaction time and its association with lead in children

Following Needleman et al.'s (1979) report of a correlation between tooth lead estimates in children and reaction time as measured by Rodnick and Shakow's (1940) delayed reaction time paradigm, a version of the procedure with two delay periods of 3 s and 12 s was developed for automated presentation and scoring on a VIC-20 microcomputer. Data are presented from a study of 300 children aged 6-14 years. Mean reaction time over six trials for each delay period related in a curvilinear fashion with age, but no relationships were found with sex or intelligence. Age-adjusted reaction time related significantly with blood-lead levels, but accounted for only about 1 per cent of the variance. The effect was mainly observed in younger (6-10 years) children in whom higher lead was associated with slower reaction time.

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.

Low-level lead exposure and the time-dependent organ-tissue distribution of essential elements in the neonatal rat

Effects of low level lead (Pb) exposure on the time-dependent organ-tissue distribution of zinc (Zn), iron (Fe), copper (Cu), and calcium (Ca) were investigated in the neonatal rat. Pups were exposed to Pb acetate (50 mg/kg) via intragastric intubation at 3-d intervals from day 6 postpartum to day 18. This level of Pb exposure had no significant effect on body weight. Exposure to Pb resulted in an alteration of the normal, time-dependent organ distribution of the essential elements and significantly lower concentrations of Zn were found in brain, liver, kidney, and femur of Pb-treated animals.

Behavioral deficit (delayed matching to sample) in monkeys exposed from birth to low levels of lead

Monkeys (Macaca fascicularis) were dosed po from birth with 500 micrograms/kg/day of lead as lead acetate. Blood levels peaked at an average of 55 micrograms/dl by 100 days of age, and dropped after 200 days of age to a steady-state level averaging 33 micrograms/dl. No overt signs of lead toxicity were observed. Between 3 and 3.5 years of age, monkeys were tested on both a nonspatial (color) and a spatial delayed matching to sample paradigm. For the nonspatial paradigm, the monkey was required to press a button that was lit with one of three colors a specified number of times, which turned the light off. After a specified delay period (varying from 0 seconds to several minutes), three test buttons were lit, one with each of the three colors. The monkey pressed the button corresponding to the color that had appeared on the sample button in order to receive a fruit juice reward. For the spatial matching to sample, one of the three test buttons was lit. The monkey responded on this button a specified number of times, which turned the light off. After a predetermined delay of variable duration, all three test buttons were lit and the monkey responded on the previously lit one in order to be reinforced. The treated monkeys were impaired relative to controls on both the spatial and nonspatial matching tasks, even though they learned the matching tasks as readily as controls and performed as well as 0 sec delay.

Effects of low lead exposure on neuro-behavioral function in the rat

Small doses (45-180 micrograms/g) of lead acetate were administered to male rats by gavage every day during the first 3 wk of life. A blood concentration of approximately 59 micrograms/100 ml blood produced signs of disturbances in reflex development and some changes in emotional behavior. Larger doses resulted in subtle changes in the neuromotor coordination function. The effect of low levels of lead exposure on the cognitive function in operant conditioning could not clearly be observed. Brain lead concentration tended to be higher than in other tissues examined. At approximately 10 months following cessation of lead acetate administration, the brain lead concentration had decreased to almost the same level found in control rats, and no distinguishable differences were observed between the lead-treated rats and controls in emotional behavior and neuromotor coordination.

Cost effectiveness of lead screening

Lead-screening programs may reduce childhood disabilities, but at what cost? Through a review of the literature, we performed a cost-effectiveness analysis in which the costs, savings, and health benefits of two lead-screening strategies--employing either a free erythrocyte protoporphyrin assay or blood lead measurement--were compared with each other and with a strategy of no screening in a population of three-year-old children. When the prevalence of lead poisoning among the children screened is 7 per cent or more, we estimate that free erythrocyte protoporphyrin screening averts morbidity and results in net savings: It is both better and cheaper than no screening. At prevalences below 7 per cent, the net positive costs from screening and early treatment must be weighed against the noneconomic benefits of improved quality of life and considered in relation to other investments that could be made to benefit society. At all prevalence rates, free erythrocyte protoporphyrin screening is more cost effective than blood lead screening.

Effect of early inorganic lead exposure on rat blood-brain barrier permeability to tyrosine or choline

The primary objective of this research was to test the hypothesis that low level lead (Pb) exposure during early life leads to disruption in blood-brain barrier (BBB) function in the young rat. Newborn rats received lead via milk from lactating dams that were drinking water containing 0.1% lead acetate Pb(Ac)2. Pups were weaned to, and maintained on, 0.1% Pb(Ac)2-containing solution up to 70 days of age. Growth was no different from that of coetaneous controls. Experimental animals displayed elevated blood lead (15 microgram/dl) within 2 days from the onset of exposure, and it increased to 35-40 microgram/dl between 13 and 22 days of age. Following weaning to the higher lead source, blood lead values continued to increase (55 microgram/dl) but, then, appeared to decline after 55 days of age. Control animals consistently possessed blood lead values of less than 5 microgram/dl. The brain capillary (BBB) transport of the neurotransmitter precursors, choline and tyrosine, was studied at 55 and 70 days of age using intracarotid injections of a bolus containing 14C-labeled substrate and 3HOH as a diffusable reference (Brain Uptake Index). There was no difference in the transport of either choline of tyrosine in lead-intoxicated rats compared to controls. Suspected adverse psychoneurological effects of low level inorganic lead probably relate to the parenchymal cells of the CNS and not to the brain capillary endothelial cells.

Blood lead stability

Blood lead levels, the single most useful and authoritative index of lead toxicity, has heretofore been faulted for the following weaknesses: (1) it is too responsive to evanescent environmental changes, thereby putting its stability into question; (2) it does not provide sufficient insight into a total body burden; and (3) it may be normal at a time when toxicity is still occurring or has occurred recently. Each "weakness" is addressed herein, and hopefully, put into a useful perspective, i.e., at a clinical research and treatment level, none of the above is severe enough, if extant at all, to outweigh its manifold usefulness.

Task dependent neurobehavioral effects of lead in rats

In order to test neurobehavioral effects of low-level lead exposure during early development, Wistar-rats were given pre- and postnatal dietary lead as lead acetate in four concentrations: 0, 80, 250, and 750 ppm. These diets, known to give rise to blood lead-levels (PbB) of less than 5, 11, 18, and 31 micrograms/dl, resulted in erythrocyte ALAD-inhibition of 40, 73, and 83%, respectively. The animals were tested first at 70 to 100 days postnatal (PN 70-100) in a 2-way active avoidance-task, and then at PN 190-250 in a visual discrimination-task. Lead-exposure was associated with performance-disruption in the discrimination-task, significant (p less than 0.001) already for the 250 ppm-exposure, but with significant (p less than 0.05) performance-facilitation in the avoidance-task. If learning and retention in animals is taken as a measure of cognitive performance neurobehavioral lead-toxicity cannot easily be explained in terms of cognitive deficit. An interpretation in terms of emotional reactivity or behavioral disinhibition would seem more convincing, which covers the results from activity-studies as well. 70% ALAD-inhibition corresponds to PbBs of about 20 micrograms/dl in rats, and to about 40 micrograms/dl in children. Some implications of animal studies for neuropsychological results from Pb-exposed children are discussed.

Pills or skills for hyperactive children

The controversial nature of drug treatment of hyperactivity, the incidence and sequelae of hyperactivity, and problems of differential diagnosis of hyperactivity versus aggression were discussed. The effects of psychostimulant medication and behavior therapy on hyperactive children were reviewed with regard to effects on their social and academic behavior. Both treatments have resulted in clear short-term changes in social behavior but neither long-term academic nor long-term social effects have been shown with either treatment. Short-term effects on academic behavior have resulted from behavioral interventions but not from psychostimulants. However, the interventions have been too brief to allow one to draw unequivocal conclusions about the clinical efficacy of behavioral treatments. Although there have been long-term evaluations of psychostimulant therapy, there have not been any evaluations of long-term behavioral treatment programs for hyperactive children. Given the salutary short-term effects of behavior therapy with hyperactive children, extended clinical trials of behavior therapy need to be conducted. Finally, specific directions are suggested for future research.

Deficits in psychologic and classroom performance of children with elevated dentine lead levels

To measure the neuropsychologic effects of unidentified childhood exposure to lead, the performance of 58 children with high and 100 with low dentine lead levels was compared. Children with lead levels scored significantly less well on the Wechsler Intelligence Scale for Children (Revised) than those with low lead levels. This difference was also apparent on verbal subtests, on three other measures of auditory or speech processing and on a measure of attention. Analysis of variance showed that none of these differences could be explained by any of the 39 other variables studied. Also evaluated by a teachers' questionnaire was the classroom behavior of all children (2146 in number) whose teeth were analyzed. The frequency of non-adaptive classroom behavior increased in a dose-related fashion to dentine lead level. Lead exposure, at doses below those producing symptoms severe enough to be diagnosed clinically, appears to be associated with neuropsychologic deficits that may interfere with classroom performance.

Behavioral and neurological effects of symptomatic and asymptomatic lead exposure in children

Forty-five children 4 to 8 yr of age who had been exposed to environmental lead were studied, consisting of an acute encephalopathy group and groups with short- and long-term exposure but without encephalopathy. Control children were matched for age, sex, race, and socioeconomic status with lead-exposed subjects, but lived in post-1945 housing and had negative neurological history and blood tests. The encephalopathy group had a significantly higher incidence of neurological deficits, retarded mental development, and higher hyperactivity than control subjects. Children with short- and long-term exposure short of encephalopathy were somewhat inferior to matched control subjects, but not to a statistically significant extent.

IQ following treatment of lead poisoning: a patient-sibling comparison

Forty-seven children treated for lead poisoning (PbB 50 to 365 microgram/dl) were compared to siblings next in age (PbB less than 40 microgram/dl) by a battery of psychologic tests. Symptoms were present in 18 but none had frank encephalopathy. Physical and neurologic examinations revealed no residual damage. Mean psychologic test scores showed no significant difference between patients and controls except in the arithmetic subtest, in which patients' scores were not related to lead concentration. Intelligence tests failed to distinguish children successfully treated from their sibling controls.

Lead exposure during infancy permanently increases lithium-induced polydipsia

Lead (200 milligrams per kilogram) was administered daily by intubation to Long-Evans rats on days 3 through 30 of life. Thirty to 180 days after cessation of lead administration, the lead-treated rats were consistently more polydipsic after lithium administration (2 millimoles per kilogram per day) than were pair-treated controls. Lithium increased the plasma renin activity equally in both the lead treated and the control groups. These data are evidence that there may be permanent neural changes induced by postnatal exposure to lead that are manifested by pharmacological challenge with lithium.

Dopamine and noradrenaline levels in the brains of lead and zinc poisoned rats

Acute plumbism include recurrent seizures, cerebral palsy and mental retardations. The impairment of the central nervous system (CNS) with increased lead absorption is of paramount concern which remains unsolved because of the lack of specific and sensitive neurochemical/biochemical indicators of the effect of lead on the CNS. In our experimental acute lead-zinc poisoning, significant increase in noradrenaline and slight decrease in dopamine have been found in the brains of rats, which suggest that there is change in neurotransmitter metabolism in lead poisoning.

Effect of tetraethyl lead and restricted food intake on locomotor activity in the rat

The effect of tetraethyl lead (TEL) and restricted food intake on spontaneous locomotor activity in male albino rats was investigated. Forty animals were injected intraperitoneally with 4, 7, 10 or 13 mg/kg body weight of TEL in peanut oil, or a peanut oil placebo. Forty additional animals were food yoked to lead treated animals as a control procedure to hold food intake constant between lead treated and lead free animals. A comparison of pre- and posttreatment measures revealed significant decreases in food intake and increases in activity levels at dosages of 7, 10 and 13 mg/kg of TEL. In addition, food intake and activity were significantly correlated in both lead treated and yoked groups. The issue of factors associated with reduced food intake playing a role in observed activity level increases was raised.