The medical condition that reportedly struck Cuban President Fidel Castro is a serious, even life- threatening event, but it can be reversed quickly, leaving no permanent damage.

Hypertensive encephalopathy happens when blood pressure soars dangerously out of control, causing blood vessels to narrow and depriving the brain of the fuel it needs to run properly.

``It's definitely an emergency, a serious medical event,'' said Dr. David Kushner, an assistant professor of neurology at the University of Miami medical school. ``And if it's untreated, it could result in permanent injuries not only to the brain but also to the kidneys and the heart.''

The condition most often happens in people who suffer from chronically high blood pressure, an ailment known as hypertension, among the most common of medical maladies. In other cases, it can be related to kidney problems or other diseases.

When blood pressure rises too high, blood vessels in the brain respond as if they're under attack -- and, in effect, try to close the gates to keep the invaders out. That's why they suddenly, and dramatically, constrict.

But that blood flow is what keeps the brain healthy, supplying it with the oxygen that is the body's manna. Stanch the river of blood, and the brain goes into crisis. People with hypertensive encephalopathy can begin experiencing headaches, nausea, vomiting, convulsions, visual problems, stupor, even strokes and coma.

And when blood is cut off to a section of the brain, that means the functions controlled by that region also cease -- which could explain, for instance, facial paralysis.

Dr. Richard Perlman has seen such cases in the emergency room of Mount Sinai Medical Center in Miami Beach.

``People can act very strangely, with memory loss, dementia,'' said Perlman, an emergency-room physician. ``And afterward you could have some deficits, like weakness on one side of the face, or difficulty with balance.''

As in any medical emergency, speed is critical in treating people with hypertensive encephalopathy. It's imperative that treatment be provided promptly so that the brain is not damaged permanently. Typically, patients are given medications that reduce blood pressure to a safe range, but not so low that other problems develop.

If treatment is successful and blood begins flowing properly to the deprived region of the brain, then paralysis should disappear. Thus, UM's Kushner said, it would not be surprising that Castro could experience these attacks last fall and then be able to maintain a grueling pace during the Pope's visit to Cuba in January.

``It's possible,'' the neurologist said. ``There is a standard treatment for this, and while it is definitely a medical emergency, it is also completely reversible.''

Still, for anyone suffering such an episode, it should serve as a resounding wake-up call: Once patients have an attack, there's a greater likelihood they will experience another in the future. That's why they are prescribed daily doses of medicine to control blood pressure and advised to make lifestyle changes, such as adhering to a low-salt diet, exercising regularly and controlling stress.