Circulation: The Great Brain Robbery

Doctors have long been as puzzled as their patients have been alarmed when some unaccustomed exercise causes not only numbness in an arm but faintness and even temporary blindness. Now artery researchers at West Virginia University School of Medicine have an explanation for what investigators call "the subclavian-steal syndrome."

As Irish-born Dr. Robert James Marshall explained it last week to the American Heart Association in Atlantic City, the steal involves one of the arteries that normally help to supply blood to the brain. Besides the well-publicized carotid arteries, there are two lesser-known vertebral arteries, each of which branches off from one of the subclavian arteries in the shoulders and ascends to the brain (see diagram). These arteries unite at the base of the brain to form the basilar artery, and in a healthy person they supply up to 20% of the brain's blood. Normally, the blood in the vertebral arteries flows in one direction: upward, to the brain.

But in older people who have arteriosclerosis, Dr. Marshall explained, there may be a clot in, say, the left subclavian artery. Then the blood pressure beyond the clot, and in the left arm, falls below normal, lower than the pressure in the right ascending vertebral artery. This sets up the steal. If the left arm demands extra blood because of unwonted exercise, it gets some by drawing it in a reverse flow down the left vertebral artery, stealing it from the right vertebral artery at their junction just below the brain.

In some cases, said Dr. Marshall, a small steal produces no obvious ill effects; this has been dubbed "the subclavian snitch." But Dr. Marshall suggested that a truly massive steal, in which both carotid arteries are also robbed of blood, might well be called "the great brain robbery."