25 March 2003
and Autism, Beyond the Fear Factors
By Jane E. Brody
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.
suspicion that vaccines given to infants and children can cause
autism is one such association, with attention directed at the vaccines
that use the preservative thimerosal, which contains mercury.
have poked many holes in this theory, which arose because of two
facts: that mercury is a known neurotoxin and that symptoms of autism
typically appeared soon after children were given vaccines containing
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.
on the side of caution three years ago, vaccine makers stopped using
thimerosal to prevent microbial contamination of certain vaccines
given to infants and small children, although none of the existing
batches were recalled and remained in use until supplies ran out.
Also, thimerosal is still used in flu vaccine, which is now recommended
for otherwise healthy children ages 6 months to 2 years.
It is easy for parents to become convinced that thimerosal is the
culprit in the current rise in autism rates if they consider only
the arguments presented by those who believe in this association.
it is important to review the evidence that experts have marshaled
against it, which is described in the current issue of the journal
Pediatrics by two neuropathologists, Dr. Karin B. Nelson of the
National Institute of Neurological Disorders and Stroke and Dr.
Margaret L. Bauman of Harvard Medical School.
October 2001, a 15-member expert panel of the Institute of Medicine,
an arm of the National Academy of Sciences, concluded that there
was not enough evidence to prove or disprove a link between thimerosal
Dr. Marie C. McCormick, the Harvard professor of maternal and child
health who led the panel, suggested that parents ask their doctors
to use mercury-free vaccines. But, she said, even if they are not
available, parents should still have their children vaccinated.
diseases that vaccines prevent can cause severe illness, brain damage
and even death, and these real risks are far greater than anything
that may be imagined to result from the vaccines themselves.
be sure, the numbers of reported cases of autism and its milder
forms are alarming: an increase from about 1 child in 2,000 before
1970 to a prevalence today of 1 in 500, 1 in 250 and possibly even
1 in 150, according to various estimates.
accounts for the change remains a mystery: some environmental agent
may be acting on the brains of susceptible children, or the change
may simply reflect better recognition and diagnosis.
has been used to protect vaccines since the 1930's. But the change
in diagnosis rates of autism has coincided with an increased exposure
of American infants to vaccines preserved with thimerosal, which
have included Haemophilus influenzae (HiB), hepatitis B, diphtheria
and tetanus. This preservative is also used in some immunoglobulins
given to pregnant Rh-negative women and some over-the-counter eardrops
and nasal sprays.
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
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
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."
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."
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.
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
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.
an interview, Dr. Nelson suggested that the normal "pruning
process" — the loss of extraneous neurons and connections
that occur as children mature and gain experience — fails
to happen in autism, leading to an enlarged brain. More studies
should be done, she said, to reveal why autistic children seem unable
to focus and are overwhelmed by environmental stimuli.
further brain difference, she noted, lies in the brain cells: those
that are relatively spared in mercury poisoning are damaged in autism,
and vice versa.
is known to have hereditary influences, and Dr. Nelson, among others,
says that it "probably begins in the womb," the result
of abnormal brain development. Although the symptoms usually do
not become apparent until months or longer after birth, some mothers
of autistic children have reported noticing abnormalities in their
newborns before any mercury-containing vaccine was administered.
the years, several cases of serious mercury poisoning have occurred,
including one involving a mercury-containing teething powder and
another related to the contamination of fish consumed by millions
of people in Japan. But still, no increase was seen in the incidence
of autism or in behavior changes suggestive of autism.
of children get vaccines every year, but fewer than 1 percent end
up with autism. If vaccines are a factor, why would so many be spared?
The hypothesis put forth by Sallie Bernard and colleagues at the
parent organization Safe Minds in Cranford, N.J., is that some children
are either born sensitive to mercury or fail to eliminate mercury
normally and thus accumulate large amounts that can damage the brain.
this matter deserves thorough scientific attention. Autism is a
devastating condition that is very costly for child and family and,
if cases can be prevented, they should be. If thimerosal is an important
cause, the phasing out of its use in vaccines that is now occurring
should result in a significant decline in cases in the next few