Aspects of aging, such as specific age-related conditions, arise from shared root causes. If an individual exhibits one given outcome of aging, then they are more likely to also exhibit others that arise from the same underlying processes. Thus we should not be surprised to see that incidence of stroke is correlated with incidence of dementia. We don't have to suggest that stroke-induced damage to the brain, and the inflammatory and other reactions to that damage, can accelerate the onset of dementia. We can instead argue that both stroke and dementia are consequences of the aging of the cardiovascular system, and there likely to occur in close proximity to one another. In fact both of these explanations are likely to be true.
A new study analysed data on stroke and dementia risk from 3.2 million people across the world, finding that people who have had a stroke are around twice as likely to develop dementia. The link between stroke and dementia persisted even after taking into account other dementia risk factors such as blood pressure, diabetes, and cardiovascular disease. Their findings give the strongest evidence to date that having a stroke significantly increases the risk of dementia.
The researchers analysed 36 studies where participants had a history of stroke, totalling data from 1.9 million people. In addition, they analysed a further 12 studies that looked at whether participants had a recent stroke over the study period, adding a further 1.3 million people. "We found that a history of stroke increases dementia risk by around 70%, and recent strokes more than doubled the risk. Given how common both stroke and dementia are, this strong link is an important finding. Improvements in stroke prevention and post-stroke care may therefore play a key role in dementia prevention."
Stroke characteristics such as the location and extent of brain damage may help to explain variation in dementia risk observed between studies, and there was some suggestion that dementia risk may be higher for men following stroke. "Around a third of dementia cases are thought to be potentially preventable, though this estimate does not take into account the risk associated with stroke. Our findings indicate that this figure could be even higher, and reinforce the importance of protecting the blood supply to the brain when attempting to reduce the global burden of dementia."