Head injury and dementia risk explained

Research shows that TBI can make someone more likely to develop dementia. The risk may be higher:

• in people who have repeated head injuries.
• in people who have more severe head injuries.

Some studies suggest that people with TBI may develop dementia earlier. This could be around two to three years sooner than expected.

Scientists do not yet fully understand why TBI increases dementia risk. But there are a few possible explanations:

• TBI may produce similar molecular changes in the brain to those seen in diseases like Alzheimer’s, including abnormal clumping together of proteins called ‘amyloid’ and ‘tau’.
• TBI can directly cause problems with memory and thinking, meaning that there is less ‘reserve’ or spare capacity as people age, or experience other neurodegenerative problems. This may mean that symptoms manifest more quickly than they otherwise would have done.

Some of the increased risk of dementia seen after TBI may be linked to falls or accidents in people who are developing dementia (so-called ‘reverse causation’) or shared factors which can both cause TBI and dementia, such as alcohol use.

For every 100 cases of dementia worldwide, almost half (45%) could be prevented or delayed. Three of those 45 cases are linked to traumatic brain injury in midlife. 

So, if nobody had a traumatic brain injury, there'd be three fewer cases of dementia for every 100 people who develop it now.

Find out more: Addressing 14 health and lifestyle factors could prevent nearly half of global dementia cases - Alzheimer's Research UK

Even though concussion is a less severe form of TBI, it has been linked to an increased risk of dementia. However, the risk appears to be higher in people who have repeated concussions or blows to the head.

Research shows that professional sports players who experience repeated mild TBI, or “blows to the head”, have a higher risk of developing dementia than those not involved in any sports.

There is a rare form of dementia called chronic traumatic encephalopathy (CTE) which is often caused by repeated blows to the head.

However, research shows that any TBI, not just those caused in sports can increase our risk of other more common types of dementia too, such as Alzheimer’s disease.

More research is needed to further understand the complex relationship between head injuries and dementia risk.

Most of what we know about dementia risk comes from observational studies, which follow groups of people over time to look for patterns in health and lifestyle.

For example, studies like Insight 46 have followed people for decades to understand how health across life is linked to brain health and later on.

This is often the most practical way to study dementia risk. Many risk factors cannot be tested in trials in a fair or ethical way. Dementia also develops over many years, so studies can take a long time to show results.

Observational studies can show when something like our diet or occupation are linked to a higher or lower chance of dementia. But they cannot prove that something directly causes dementia, or that changing it will definitely reduce risk. These studies look at patterns across large groups, not what will happen to any one person.

Randomised controlled trials give stronger evidence because participants are assigned to different groups at random. This design makes it more likely that differences in outcomes, for example our risk of dementia, are caused by the intervention, for example a diet change, rather than by other factors.

But these studies are harder to do in dementia research. They often take many years and may test several changes at once, making results harder to interpret.

Newer trials are helping to build a clearer picture of what may support brain health. No single study can give a definite answer, but it is all research findings together that help us understand what is important for our brain health and risk of dementia.