Hope for Stroke Victims

Scientists have long believed that the irreversible damage caused by strokes results from cutting off oxygen to brain tissues that can survive only minutes without it. Now two neuroscientists have offered a more intricate theory that could explain some little-understood symptoms of strokes. It could also lead to treatments that could minimize the effects of strokes, which kill more than 200,000 Americans a year.

According to Dr. Richard Wurtman of M.I.T. and Dr. Nicholas Zervas of Beth Israel Hospital and Harvard Medical School, a large part of stroke injury may be caused by imbalances in the brain's neurotransmitters, the chemicals that carry nerve impulses from one neuron, or brain cell, to another. The doctors base their theory on experiments in which Neurosurgeon Zervas produced massive strokes in 13 monkeys by cutting off blood flow-and thus oxygen-to the left sides of their brains. Examining the brains afterward, he and Wurtman found that there were dramatic changes in the levels of dopamine, a substance that transmits nerve impulses among the brain cells that help coordinate movements. The amount of this chemical in the left halves of the brains was about half as much as it was in the unaffected right hemispheres. Tests on Mongolian gerbils reinforced the observations. When the brains of these deliberately "stroked" animals were analyzed, the affected hemispheres were found to have 46% less dopamine than the undamaged sides.

This discovery was the first proof that neurotransmitter levels change following a cutoff of blood to the brain; it forms the basis of the Wurtman-Zervas theory. The two neuroscientists speculate that cells starved of oxygen as the result of strokes die and allow their stored dopamine to escape. Dopamine is normally released only in minuscule amounts, and a sudden flood of the chemical can be lethal. Excess dopamine can cause nearby blood vessels to contract, cutting off oxygen to neighboring cells and thus spreading the stroke damage. After the flood has subsided, there is a serious shortage of the dopamine; the cells killed by the stroke are no longer producing it. Result: the lack of coordination that is typical of the stroke victims who survive.

Wurtman and Zervas concede that evidence in support of their theory is still fragmentary. But their hypothesis is consistent with the clinical picture of stroke, in which victims frequently develop worsening symptoms, and sometimes permanent paralysis, over a period of several hours after the initial episode. It also suggests a way in which the damage that follows a stroke may be lessened. Drugs are now available to restore proper neurotransmitter balances in patients suffering from depression, schizophrenia and Parkinson's disease. Proper use of these drugs after a stroke might restore the balance in survivors and reverse some of the damage.

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