What are the claims?
Although researchers have reported a link between sports-related traumatic brain injuries and dementia, the exact relationship between the two is not clear. There is limited evidence that genetic predisposition, age at which the head is exposed to trauma, and other factors influence risk of this disease. Larger scale studies of athletes are needed to clarify the causes of dementia.
Thousands of athletes experience traumatic brain injuries every year around the world, particularly in contact and collision sports such as boxing, American football, rugby, and hockey.
There is increasing evidence of a link between such brain injuries and dementia. As far back as 1928, the term 'punch drunk' was introduced to describe a disorder of progressive dementia that was first seen in boxers. It was later called 'dementia pugilistica' and is known today as chronic traumatic encephalopathy (CTE).
What is CTE?
CTE is a progressive change in the brain characterised by inflammation in the nervous system and the abnormal build-up of a protein called tau. The condition can lead to memory loss, confusion, impaired judgment, aggression, depression, and, eventually, full-blown dementia. CTE can start years or even decades after the head injury and can only be diagnosed after the person has died.
Although scientists have linked CTE with a history of repetitive brain traumas, the exact relationship between the two is unclear. While all cases of CTE show a history of brain injuries, not all individuals who experience repetitive brain traumas develop CTE. It is unknown whether brain injuries on their own can lead to dementia. Other factors, such as genetic predisposition and age at which the brain is exposed to trauma, may play a role in its development.
What research has been done?
The risks of dementia in boxing and American football have been studied considerably more than in any other sports, but the reports have often been contradictory. While some researchers have seen a correlation between sports-related brain injuries and dementia, others have not. The confusion has not been helped by poor data - many of the studies used unreliable self-reporting, interviews with family members, and assessments of athletes during their lifetime in place of clinical evidence from post-mortem examinations.
One of the key studies dates back to 1969, when researchers from the Royal College of Physicians examined 224 randomly selected retired boxers and found clinical evidence of severe neurological disorders, such as dementia, in 17 per cent of them. More recent studies suggest that a longer boxing career, older age at retirement from boxing, participation in more bouts, and higher numbers of knockouts increase the risk of CTE. Boxers who took significant head blows as part of their fighting style were at a higher risk of developing progressive dementia than other fighters.
A link between CTE and a gene called apolipoprotein E (ApoE), which is known to increase the risk of developing Alzheimer's disease, is often discussed. A study of 30 professional boxers aged 23 to 76 years reported a correlation between this gene and the severity of neurological problems, although CTE cases in these athletes were not confirmed. The ApoE frequencies in the subjects were similar to those found in the general population.
Some studies reported a significant difference between professional and amateur boxers in terms of dementia risks. Researchers reviewed the performance of amateur boxers in different psychometric tests, made neurological examinations, and compared their brain activity to that of people who were not boxers. Many amateur boxers did not show abnormal results, although these findings were not consistent. The differences between professional and amateur boxers could be explained by a higher level of protection worn by amateurs, lower number of rounds, and more referee stops to save an amateur boxer from a knockout.
Professional Fighters Brain Health Study is an ongoing five-year research assessing progressive changes in brain structure and function. The scientists expect enrollment of more than 400 boxers and mixed martial arts athletes and hope to be able to predict early stages of CTE based on the new results of cognitive assessments, speech analysis, and surveys of mood and impulsivity.
In recent years, chronic brain damage in American football players has received increasing attention.
Researchers from Boston University studied the brains of 165 people who played American football at high school, college, or professionally. They found evidence of CTE in 131 athletes, including 87 National Football League players. The numbers are overwhelming, but these figures may not be an accurate representation of CTE cases across the whole of American football. There may be selection bias as athletes and their families are more likely to donate brains for post-mortem examination if they notice some symptoms of the disease during a player's life.
Other studies confirmed CTE cases in smaller groups of American football players and suggested that retired players with three or more concussions were at higher risk of developing memory problems compared with retirees with no history of concussion.
A study of 34 American football players reported that the position played had a correlation with the development of CTE. Positions that experienced repetitive collisions, such as offensive linemen and running backs, received more brain injuries than others. Researchers have also found CTE in American football players with no history of reported concussions but who played in high speed positions and experienced multiple hits to the head. This suggests that not just symptomatic concussions, but milder brain traumas may lead to the development of this disorder.
Studies of the influence of the ApoE gene on the risk of dementia have been contradictory. In one of the largest studies to date, with 42 American football players who had CTE, the proportion of them with different versions of the ApoE gene in was similar to the general population, which suggests that ApoE is not a risk factor for CTE. Another study reported that older professional American football players who had the gene scored lower on cognitive tests than the players without this gene, but the link to CTE was not assessed in this study.
What does this mean for sportspeople worried about dementia?
Although there is evidence that dementia is linked to repeated brain injuries in boxing and American football, there is a shortage of confirmed CTE cases across all sports, and the results in rugby, hockey, football (soccer), wrestling, and other sports are considerably less conclusive.
Almost a hundred years after the term 'punch drunk' was coined, there is still no consensus on the link between brain trauma and dementia. Large scale studies of contact and collision sports involving amateur and professional athletes with varying career lengths, age, and genetic predisposition are needed to clarify the causes of this condition.