What is a silent stroke?

During a typical ischemic stroke, a clot blocks a blood vessel that feeds part of the brain. Without blood, cells in that area malfunction and may die, causing symptoms such as numbness or weakness on one side of your face, trouble speaking, difficulty walking, or vision problems, which reflect the functions that were controlled by the affected part of the brain.

During a silent stroke, an interruption in blood flow destroys areas of cells in a part of the brain that is "silent," meaning that it doesn't control any vital functions. Although the damage will show up on an MRI or CT scan, it's too small to produce any obvious symptoms. "A blood vessel can get blocked off, the tissue supplied by that vessel can die, but the person doesn't experience symptoms so they don't know they've had a stroke," explains Karen Furie, associate professor at Harvard Medical School and director of the Massachusetts General Hospital Stroke Service.

Most people who've had a silent stroke have no idea it occurred. "That doesn't mean it's insignificant, though," Dr. Furie says. Silent strokes can cause subtle signs, such as cognitive impairment, she adds.

Silent strokes and your memory

Though a woman may not notice any immediate effects, a silent stroke could interrupt the flow of information in her brain needed for memory, especially if several of these strokes occur over time (which is the most common scenario).

In a study published in the January 3 issue of the journal Neurology, researchers looked at more than 650 people without a history of dementia. Using MRI scans, the study authors tracked interruptions in blood supply to the participants' brains. More than 170 of the participants were found to have small areas of dead tissue from a lack of blood supply (called infarcts) in the brain, even though only 66 of them reported having had symptoms of a stroke. People with these brain infarcts had difficulties with memory and mental processes (cognition). The memory issues occurred independent of any shrinkage of the hippocampus (the part of the brain responsible for memory)—which is typically seen with Alzheimer's and other forms of age-related memory loss.

Researchers say that over time, the damage from silent strokes can accumulate, leading to more and more memory problems. "The more brain damage or injury that you have due to these silent strokes, the more difficult it is for the brain to function normally," Dr. Furie says.