ADD and head injuries

A recent study from the British Medical Journal concerning the causes of Attention Deficit Disorder (ADD) illustrates the power of using epidemiologic studies to determine the cause of a disease.

Epidemiology looks at factors in a population that might or might not be associated with illness. Researchers look at large groups of people with and without a certain disease, and try to tease out what wakes the two populations different. Does one group exercise more? Or eat more of a certain food? Does one group have more of a family history of that disease? Or maybe a certain environmental exposure? Studies like these can get quite complex, because human lives themselves are so complex.

As an example, we know that lung cancer used to be very rare—almost unheard of, in fact. It started becoming more common in men during the 1930s. Factors that may have correlated with the rise of lung cancer could have been increased living in cities, increased use of automobiles, or increased reading of newspapers. But careful observations of these and other factors found that it was cigarette smoking that contributed most heavily to lung cancer. Over the following years, as more women started to smoke, their rates of lung cancer rose to about the same level as men. No clinical trials have ever been done in people proving that smoking causes lung cancer—that is, no one has deliberately exposed people to cigarette smoke to see if they get cancer—but the overwhelming weight of epidemiology has been instrumental in demonstrating the risks.

ADD affects 5-8% of schoolchildren, and contributes to poor school performance, delinquency, and substance abuse. We know that genetics plays a part, but environmental influences also seem to be important. One observation that has been made is that many children with ADD have a history of some sort of head injury in early childhood—so is it possible that minor brain damage from these kinds of injuries is a cause of ADD? That’s what the BMJ study tried to figure out.

The study looked at 62,000 children in the United Kingdom, using a heath database that records diagnoses and medical problems. They found that in this group, children with ADD were about twice as likely as children without ADD to have had been seen at a medical facility for some kind of head injury in the past.

Does that mean that head injuries cause ADD? Maybe. But perhaps it’s the other way around. After all, we know kids with ADD are more impulsive and hyperactive—maybe they’re more likely to hurt themselves. Which comes first, the ADD or the head injury?

To answer that question, the authors looked at another health observation among the children: a history of any burn injury. A burn on some other part of the body wouldn’t cause injury the brain, but would be another way of showing that the kids with ADD are more injury-prone in general. Sure enough, in the database the children with ADD were also about twice as likely to have had a burn injury than kids without ADD. So it’s not just head injuries that are associated with ADD, it’s injuries of any kind—which fits the hypothesis that kids with ADD are reckless, and hurt themselves more. That’s why they more often have a history of a head injury. It’s not the head injury that caused the ADD, but rather the ADD that caused the head injury.

So: don’t worry about the inevitable minor head bonks. They’re a part of childhood that can’t always been avoided. But children who are especially reckless and get injured a lot might just be telling their parents what kind of person they are.

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