Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner
Low-level mercury exposure is unlikely to contribute to autism, although further work will be needed for a definitive conclusion to this controversy, a new British study suggested.
Mercury levels in urine did not differ between children with autism and controls, regardless of whether values below the limit of detection, which was 0.07 µg/L, were counted as zero (P=0.162) or as 0.07 µg/L (P=0.156), Barry Wright, MD, of the North Yorkshire and York Primary Care Trust in York, and colleagues reported online in PLoS One.
Exposure to environmental mercury in high doses can be highly toxic, and even lethal.
Certain groups have persistently argued that mercury in thimerosal-containing vaccines also could exert neurotoxic effects on young children, with the result that vaccination rates have fallen and the incidence of measles and mumps has risen.
"This was proposed despite the fact that the mercury compound in vaccines, ethylmercury, cannot easily pass through the blood-brain barrier as methylmercury can and is associated with few central nervous system problems in environmental health research," wrote Wright and colleagues.
Further arguing against the mercury-autism link is the observation that rates of autism have continued to increase even after thimerosal was dropped from vaccines more than a decade ago.
Nonetheless, some researchers have hypothesized that autistic children have an impairment in the ability to excrete mercury, after analysis of hair revealed low concentrations of various heavy metals.
To examine this possibility, the researchers enrolled 54 children with autism spectrum disorders, 121 healthy children attending mainstream schools, 34 who attended special schools for learning disabilities, and 42 siblings of autistic children.
Because some degree of mercury exposure could be expected in children who had amalgam fillings in their teeth, Wright's group controlled for the number of fillings, which was slightly lower in the autism group.
They also adjusted for urine concentration and body mass by correcting for creatinine, and as with the unadjusted analysis, found no difference among the groups of children in urinary excretion of mercury.
Further adjustment for age, sex, and number of fillings once again revealed no differences, whether values below the limit of detection were calculated as zero or 0.07 µg/L (P=0.56).
Tests for other heavy metals such as lithium (P=0.344), manganese (P=0.613), and copper (P=0.391) also turned up no differences.
The investigators noted that their findings should be interpreted with caution, however, because of the possible effects of a small number of children in the autism and special school groups whose levels were considerably higher.
They also noted that the study population was somewhat small, and that 24-hour urine collections were not done because of difficulties undertaking this in children with difficulties such as autism.
"This study does not lend support for widespread mercury metabolism problems in autism, but given small numbers of outliers it does suggest further research is warranted to better understand whether a subgroup with autism and/or learning disabilities have mercury poisoning or excretion difficulties," they wrote.