A new study from the American Academy of Neurology suggests that traumatic brain injury could be one of the many factors associated with the risk of ischaemic stroke. A traumatic brain injury (TBI) is any head injury that disrupts the normal function of the brain
Ischaemic stroke is the most common type, accounting for 80% of all strokes. This kind of stroke is caused by lack of blood flow to the brain. Abnormalities like carotid arteries, embolism and perforated arteries are all causes.
The research, which surveyed more than a million people, found that people who had suffered traumatic brain injuries were 30% more likely to develop an ischaemic stroke than those who did not. This figure was adjusted to account for known stroke risk factors, so researchers could identify an independent link between the two.
This seems like a significant number, but there is reason to beliece this risk is not as severe as first impressions suggest. Assessing the risk of stroke is a complicated procedure, and there is already a huge number of known and hypothesised risk factors.
“Environmental factors such as high blood pressure, diabetes, high fat and an irregular heart beat, among others, account for between 50% and 60% of stroke risk.”, explained Dr Steve Bevan, Reader in Cerebrovascular Genetics at St George’s, University of London. Some sources put these figures even higher.
Doctors are still trying to answer the question of how to account for the rest of the risk. “A substantial proportion of risk is unexplained,” study co-author Dr James Burke said. “Our findings suggest that TBI may explain some of that unexplained risk, but it’s hard to say how much.”
But Bevan had his own take. “The remaining 40% of stroke risk is thought to be due to a person’s genes,” he said. “Several genes have already been identified which predispose to stroke risk, but many more remain.” He pointed to research published last year in Nature Genetics and The Lancet that showed the association between genetics and stroke.
The study found that the absolute risk of stroke for brain injury patients is 1.1%, compared to 0.9% for other patients, accounting for other risk factors. Though there is a relatively large difference between the groups, the risk levels involved are really quite small.
Dr Caroline Lovelock, also of St George’s, had some reservations about the study. “The results are very much reliant on the accuracy of the diagnoses that were given to people at discharge from hospital, and previous studies have shown a relatively high frequency of errors in hospital administration data,” she said.
This could mean that some of the 1,173,353 people whose data was used may not have suffered stroke at all. “It would have been nice to see them take a sample group to assess the accuracy of the diagnoses they used,” Dr Lovelock suggested. “It is an interesting study, which shows an association between traumatic brain injury and subsequent stroke, but this does not prove causation.”
Dr Burke agreed. “Our study is not clear evidence of a causal link between TBI and stroke,” he said.
He acknowledged that more work needs to be done to find out how much of a risk brain injury poses compared to other factors. “To answer the question you would need to take a population-representative sample of patients without stroke and follow them over time to determine the rate at which they develop stroke.”