When the elderly decline mentally — they complain of slowing down or lose interest in their bridge club, for example — family, friends, and even their doctors may blame it on the natural effects of aging. Indeed, many of the most common ailments of the elderly are written off as signs of getting old, including urinary incontinence, poor balance, depression, apathy and confusion. But new studies suggest that the true cause of some age-related changes in mood may be so-called silent strokes: tiny blockages of blood vessels deep inside the brain which cause real damage but don’t show obvious symptoms, such as muscle paralysis or garbled speech. In addition, imaging studies of people who have had silent strokes are giving us valuable new insights into the workings of the brain’s mood and memory circuits.
How can someone have a stroke and not notice? If you’ve never heard the term “silent stroke” you aren’t alone; scientists are just beginning to realize their importance. Although the term “silent” is a bit of a misnomer, they are called silent because the regions of the brain that are most likely to suffer these kinds of strokes may not cause much change in a person’s motor functions, speech, pain, or sense of touch. They are involved more with thinking and mood regulation — and thus harder to detect. Hence, strokes in these regions of the brain are referred to as silent. Silent strokes mostly happen in parts of the brain that are dependent on single, tiny blood vessels for their supply of oxygen and nutrients.
These regions, such as the frontal cortex and the deep white-matter tracts crisscrossing inside, lack alternative supply lines, called collaterals, if the tiny vessel is blocked. People who are otherwise healthy don’t usually assume they may have had a little stroke while they were asleep or driving, even if they experience subtle emotional changes. As a result, millions of people are literally walking around without knowing they’ve have had a stroke. They usually attribute changes in their mood or memory to something else, such as age, fatigue, or a recent social event. Many elderly people live alone, suffer from multiple medical conditions, and have faced personal losses — so new signs of depression can often be mistaken as a normal reaction and dismissed.
Globally, about 10 percent of healthy middle-aged people are thought to have had silent strokes without being aware of it. And silent strokes are nearly five times more common than regular strokes. In the United States alone, each year some 11 million people are estimated to have endured a silent stroke — a shockingly large number. In a population study called the Cardiovascular Health Study, 3,660 adults, in their 60s and older, underwent MRI scans of their brains. Of these, 31 percent showed evidence of a stroke — yet most denied ever having had one. The symptoms, when present, were often vague; not even their doctors attributed them to a stroke. Are these a new phenomenon? Not at all. Doctors have long noted such findings postmortem. What is new is that, with the greater availability of higher-resolution brain MRI scans, more of these strokes are being detected in living individuals.
Silent but Hardly Benign
While these little strokes may not cause major paralysis, they do produce more subtle symptoms — ones we usually attribute to normal aging and may not link to brain damage due to a stroke. Impairment of a person’s balance leading to more falls, loss of bladder control causing urine leakage, changes in mood and personality, and the loss of cognitive abilities are some of the common symptoms associated with silent strokes. Silent strokes can also increase the probability of more serious strokes in the future, and eventually death. Depression is a telling example.
Studies conducted by our group at Duke University in the 1990s found that older adults who experienced depression for the first time in their older years were more likely to display evidence of silent strokes on brain MRI scans than an age-matched control group who did not have depression. The term vascular depression was coined to describe such individuals. At that time, depression in older adults was generally being attributed to life changes, such as loss of a loved one or retirement. Studies conducted at Duke and Cornell universities (among others) also found that such individuals are more likely to respond poorly to antidepressant medications and are at greater risk for having recurrent depressive episodes and functioning less well in daily activities. People with vascular depression also tend to be more impaired in their attention, information-processing speed, and memory, all of which can put them at greater risk for developing dementia. Indeed, vascular causes are the second most common cause of dementia (vascular dementia) globally, which is a tragedy since many of these cases are probably preventable with early detection and risk-modification.