A new study of more than 5,400 Danish polio patients throughout the last century has found that people with the crippling virus were roughly twice as likely as those without the infection to develop Parkinson's disease over time.
The findings, reported as a research letter in tomorrow's issue of the Journal of the American Medical Association, are merely an association, and the researchers said there's no direct link between the two conditions. However, they said, there is a tantalizing connection. Poliomyelitis -- which has been all but eradicated from the planet -- is well known to destroy nerves, and Parkinson's is a degenerative ailment resulting from the destruction of brain cells that produce the signaling molecule dopamine.So it's possible, the scientists argue, that a case of polio might damage enough brain cells to aggravate the loss of neurons that normally occurs with age -- leaving the organ particularly vulnerable to Parkinson's.
Dr. Richard Bruno, director of the Post-Polio Institute at Englewood Hospital and Medical Center in Englewood, N.J., said the Danish finding "makes perfect sense."
The trouble is, he added, no one else has been able to show a similar connection -- but not for a lack of trying.
In the 1940s and 1950s, Dr. David Bodian, a Baltimore polio expert, found that the virus attacks the brain, starting low in the organ's stem and working its way up. Indeed, he declared, the microbe kills neurons even if it never infects the spinal cord and causes paralysis.
That discovery sparked the obvious question of why there didn't seem to be an increase in Parkinson's disease and other neurological disorders in polio survivors. This, in turn, led to the following conclusion: A case of polio strong enough to knock out dopamine neurons was almost certainly a deadly infection, so patients didn't have time to develop the second illness.
Yet if polio suppresses dopamine, there should be evidence of a link between the two diseases. After all, there are more than 1.6 million Americans alive today who contracted polio, more than have Parkinson's and multiple sclerosis combined. So far, though, no study in this country has shown that they have a higher risk of the degenerative brain disease, Bruno said.
On the other hand, he said, it's quite clear that polio survivors are prone to a complex syndrome that's also associated with Parkinson's. It is marked by fatigue, weakness, slower brain waves, decreased attention and concentration, and even the "tip of the tongue" phenomenon -- where words and ideas seem just out of reach.
This disorder, which Bruno called "brain brownout," seems to be related to an increase in the brain of a substance called prolactin, which sits at one end of a molecular seesaw whose opposite number is dopamine. Thus, as prolactin increases, dopamine drops.
"When you put all of that together, what you have is evidence that a lower amount of dopamine is associated with both the signs and symptoms of post-polio brain fatigue," Bruno said. And insufficient dopamine is the hallmark of Parkinson's disease. Conspicuously missing in the catalog of post-polio symptoms, at least so far, is the kind of muscle and movement dysfunction that accompanies Parkinson's.
What's more, Bruno noted, polio isn't the only virus that attacks "brain activating neurons," including those that produce dopamine. Other microbes, do, too, and may in fact be more likely to cause the kind of damage that might lead to Parkinson's disease and similar maladies.
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