"Over the years, any number of coincidental findings have suggested that exposure to a particular substance may cause a certain illness. But under the critical eye of careful research, most of these apparent associations turn out to have no cause-and-effect relationship....While the final "i" has yet to be dotted on this question, overwhelming evidence so far suggests that thimerosal poses no significant threat to the developing brain....
Claims vs. Facts
Typically, children who are found to be autistic appear normal for their first months or year, then they seem to lose natural developmental landmarks. Many parents of affected children have noted that this reversal occurs suddenly or gradually soon after the vaccinations received in the child's first 15 months. But Dr. Nelson and Dr. Bauman point out that "age of onset of symptoms can be highly misleading as an indicator that some environmental event has caused or precipitated a disorder."
For example, in two disorders known to be caused solely by a defect in a single gene, a period of normal development occurs before symptoms appear — about 18 months in Rett syndrome and 45 years in Huntington's chorea, the disease that killed the songwriter Woody Guthrie.
Those who have focused on mercury as a cause of autism list nearly 100 clinical signs that they say are shared by autism and mercury poisoning. But Dr. Nelson and Dr. Bauman say this list does not distinguish between "typical and characteristic manifestations" and those that are "rare, unusual and highly atypical."
For example, the scientists said, in mercury poisoning, including the few cases of poisoning caused by ethyl mercury, the form in thimerosal, characteristic motor symptoms are lack of coordination, unsteadiness and difficulty speaking because of poor muscle control, along with tremors, muscle pains and weakness and, in severe cases, spasticity. But in autism, they said, the only common motor symptoms are "repetitive behaviors such as flapping, circling or rocking."
As for sensory symptoms, mercury poisoning is associated with extreme sensitivity to touch, a function of peripheral nerve damage. But in autism a reduced sensitivity to pain accompanies a hypersensitivity to sound, suggesting involvement of the central nervous system, not peripheral nerves.
There is even debate over ethyl mercury and whether it gets into the brain. Methyl mercury has an "active transport system" to carry it across the blood-brain barrier, but there is no such transport for ethyl mercury, which is further hindered from entering the brain by being a larger molecule that is rapidly decomposed once in the body.
Also striking, the scientists report, are differences in the brains, including size and the kinds of cells and areas that are damaged in autism and mercury poisoning. Children with mercury poisoning experience a shrinkage of the brain; those with autism tend to have abnormally large brains, with an enlarged cortex and white matter and more nerve processes than normal...."