The lead connection
Lead is a known neurotoxin and, although its commercial use has been restricted, it can still be found in trace amounts in everything from children's costume jewellery and old housepaint to soil and drinking water. It can be inhaled or ingested, and nearly all children have been found to have detectable levels of lead in their bodies (Curr Direct Psychol Sci, 2010; 19: 24-9).
In a study that has only just been published, researchers in the US have reported convincing evidence of a link between levels of lead found in the blood circulation system and a diagnosis of ADHD.
A total of 236 children-both with and without ADHD-took part in the Portland, Oregon-based study, which measured blood lead levels in relation to symptoms. The results showed that the children with ADHD had higher lead levels than those without the disorder. In particular, blood lead was associated with the hyperactive-impulsive type of ADHD (J Child Psychol Psychiatry, 2010; 51: 58-65).
Similar findings have been reported before, but what this new study revealed for the first time was that even very low lead levels can increase the risk of ADHD. The children had a mean level of only 0.73 mcg (microgrammes, or one-thousandth of a gramme) per dL (decilitre, or one-tenth of a litre) of blood, which is below the average population exposures found in both the US and Western Europe.
Although this does not constitute proof that low-level lead exposure causes ADHD, earlier animal studies have shown that lead can affect neurodevelopment, making it a plausible culprit. While it's unlikely that lead is solely responsible for ADHD, it could be that exposure to the toxin interacts with other factors-such as genetic influences or other toxic exposures-to bring about the condition.
In fact, another US study has recently reported that children exposed prenatally to tobacco smoke and to lead during childhood face a particularly high risk of being diagnosed with ADHD. More than 2500 children, aged 8 to 15 years, were included in the study, which measured prenatal tobacco expos-ures, based on reports of maternal cigarette use during pregnancy, while lead exposure was assessed by measuring the children's current blood lead levels.
The authors, from the Cincinnati Children's Hospital Medical Center in Ohio, found that children exposed to tobacco smoke before birth were 2.4 times more likely to have ADHD, and that those whose blood lead levels were in the top third of the general population had a 2.3-fold greater risk of ADHD. But most alarming was the finding that the combined effects of these toxicants were synergistic. Those children who had both exposures had a more than eightfold higher risk of having ADHD compared with the children who had been exposed to neither pollutant.
The authors estimated that up to 35 per cent of ADHD cases in children aged between 8 and 15 years could be reduced simply by eliminating their exposure to both tobacco smoke and lead (Pediatrics, 2009; 124: e1054-63).
Some of the chemicals commonly found in plastic might also be causing behavioural problems in children. Phthalates-components of numerous plastic consumer products such as toys, cleaning materials and vinyl flooring-have been linked to a variety of health problems in recent years and, now, it appears that they could also be contributing to ADHD.
Scientists from the Seoul National University College of Medicine in Korea analyzed urine samples from 261 children, aged between eight and 11 years, and assessed symptoms of ADHD using reports from teachers and results from computerized tests.
The researchers found a "strong positive association" between phthalate exposure and ADHD. In other words, the higher the concentration of phthalate metab-olites in the urine, the worse were the ADHD symptoms and/or test scores (Biol Psychiatry, 2009; 66: 958-63).
Likewise, US scientists at the Mount Sinai School of Medicine in New York found a correlation between prenatal phthalate exposure and behavioural problems in children aged four to nine years (Environ Health Perspect, 2010 Jan 8; Epub ahead of print).
Although much more research is needed, it already seems clear that environmental toxicants are an important piece in the ADHD puzzle.
Other possible causes
In addition to toxic exposures, there are other environmental factors that may well be involved in ADHD. The role of diet is well known-particularly the influence of food additives and essential fatty acids (see box, page 26)-but less familiar is the potential part that sleep plays in ADHD.
Mounting research suggests that sleep problems, such as snoring and sleep apnoea-when breathing is reduced or even stops for brief periods during sleep-may be the unsuspected cause of learning difficulties and behaviours, such as hyperactivity and inattention, in some children.
Some of the most compelling evidence comes from a study led by Dr Ronald Chervin of the University of Michigan in Ann Arbor. It found that 22 of 78 children (28 per cent) thought to have sleep-disordered breathing (SDB)-a term describing a spectrum of abnormal breathing during sleep-also had ADHD, as determined by a child psychiatrist. However, one year after undergoing surgery to relieve their SDB, half of these children no longer met the criteria for an ADHD diagnosis (Pediatrics, 2006; 117: e769-78).
"These findings help support the idea that sleep-disordered breathing is actually helping to cause behav-ioural problems in children," said Chervin. In fact, the study findings also suggest that a potentially significant proportion of children with SDB are being wrongly diag-nosed with ADHD.
For more information on the link between sleep problems and ADHD, see WDDTY vol 18 no 7.
A drug-free future
So far, much of the research on ADHD has focused on the inheritability of the condition. But now, the latest evidence indicates that environmental factors, such as sleep problems and toxic exposures, can play a crucial role in a significant proportion of cases.
Clearly, more research is needed, but these insights suggest that it may be possible to prevent ADHD or at least treat it without resorting to dangerous drugs.
WDDTY VOL 21 NO 2