Population studies of head injury and dementia

Please note - This information reflects the evidence at the time of the roundtable in April 2017. This is an active area of research, and new findings are emerging that may change the level of evidence available. Please contact [email protected] if you have any questions related to this report. 

• Severe head injury (of any cause) is an established risk factor for dementia. For example, a recent study of nearly 200,000 US veterans suggested that there was a 60% increase in the risk for developing dementia in those people who had a history of head injury (Barnes et al, 2014). 
• Not every study though has found a strong link between head injury and risk of dementia, for instance a study of 7,500 individuals who experienced loss of consciousness found no link to dementia but did to Parkinson’s (Crane et al, 2014).
• The issue is also subject to a possible ‘reverse causality’ effect, where an increased risk of head injury in the years before a diagnosis of dementia might be a consequence of early dementia causes falls, not a cause.  
•  From published research on all types of head injury, it appears that severity of the head injury and/or number of times a person has a head injury may play a role in determining their risk of dementia. 

The problems with population studies 

• There are only a small number of studies that have compared professional athletes with a control population or compared athletes who have sustained head injuries with those who have not. As such, the evidence is unclear either way although new studies are emerging with increasing interest in the field. 
• Epidemiological or population studies of large numbers of sportsmen and woman were considered to be complex, since a number of factors other than sports-related head injury may also influence risk of dementia and act as confounds in any analysis. This includes protective factors such as exercise and the better general health of professional sportspeople and risk factors such as heavy alcohol intake.
• Another unknown is whether there is a ‘minimum’ level of head injury that establishes risk, and, more specifically, whether minor impacts, such as those sustained when heading a football are enough to increase risk. 
• Minor head injuries are rarely recorded by doctors or healthcare professionals.  As a result we do not have access to an appropriately large volume of routinely collected data that would help explore any potential epidemiological links between mild head injuries and late life dementia.     
• Anecdotally, clinicians report that there do not appear to be a disproportionally large number of professional sports people visiting memory services or reporting memory problems. However, a significant proportion of people with dementia are never diagnosed and patients’ sporting history may not be explored even if they are diagnosed.