stroke (cerebrovascular accident)

A stroke is what happens when part of the brain suddenly loses its blood supply and the brain tissue there begins to die within minutes. Brain cells are astonishingly hungry: they cannot store fuel, so they depend on a constant river of blood bringing oxygen and sugar every single second. Cut off that river, and the cells in the starved region stop working almost at once, the way a city block goes dark the instant its power line is severed. Because each patch of brain runs a particular job, the symptoms point to where the damage is: a face droops on one side, an arm goes weak, speech turns to garble, vision blurs, or the person cannot keep their balance. This is a medical emergency, and the old phrase doctors repeat captures why: time is brain.

There are two main ways the blood supply fails, and they are nearly opposites. In an ischemic stroke, by far the more common kind, a clot plugs an artery feeding the brain, like a cork jammed in a pipe, and everything downstream is left dry. In a hemorrhagic stroke, a blood vessel instead bursts and bleeds into or around the brain, so blood is pumped to the wrong place, drowning and crushing the tissue while the region beyond the leak is starved as well. Telling the two apart matters enormously, because the treatments are mirror images: a clot may be dissolved with a drug or pulled out with a tiny catheter, but giving that same clot-busting drug to someone who is bleeding would be catastrophic, which is why a brain scan comes first.

The damage from a stroke can be devastating, yet it is far from a fixed sentence. A core of tissue usually dies quickly, but a surrounding rim of cells often hovers in a stunned, still-rescuable state for a precious window of hours, which is why rushing to hospital can save brain that would otherwise be lost. In the months afterward, the brain's remarkable capacity to rewire itself, with surviving regions gradually taking over jobs the dead tissue once handled, underlies much of the recovery seen in rehabilitation. And many strokes can be prevented altogether by controlling the things that damage blood vessels over years, chiefly high blood pressure, smoking, diabetes, and an irregular heartbeat that breeds clots.