What we know and do not know about chronic traumatic encephalopathy in sport

• Chronic traumatic encephalopathy is associated with a clinical features, so diagnosis can only be made post-mortem by identifying typical pathological changes in the brain.
• Playing professional contact sport (in males) is associated with the development of CTE, and with neurodegenerative diseases in general.
• Sports-related concussion should be taken seriously and treated in line with current recommendations.

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The possibility of long-term effects from sports-related concussion hit the headlines in 2014 when the US National Football League agreed to pay US$765 million to settle a legal action brought by 4500 ex-NFL players for serious medical conditions associated with repetitive brain injury (with the final settlement up to US$1 billion). In return, it was agreed that there would be no investigation into the conduct of the NFL. As of 2020, US$760 million had been paid out – US$300 million for dementia claims; however, 3000 dementia claims have been declined.

Since that time, there has been a lot of interest in this topic, both in the popular press and the scientific literature, with much of this concentrated on chronic traumatic encephalopathy (CTE).

Late last year, it was announced that seven professional rugby players intended to bring a lawsuit against World Rugby, the Rugby Football Union and the Welsh Rugby Union for long-term effects of concussion. This really caught the attention of the New Zealand press. The involvement of the popular press has been very helpful in spreading the message that concussion should be treated seriously.

However, the long-term effects of concussion is a complex topic, and some of the facts have not been accurately reported, leading to confusion and an impression that if you play contact sport, then CTE is almost inevitable. The prime example is a highly emotive piece published in The New York Times in 2017, showing brain sections from 111 ex-NFL players, 110 of which were diagnosed with CTE (https://nyti.ms/2OdjsMV).

The term CTE was first used by Macdonald Critchley in 1947 to describe a clinical syndrome that had been recognised in ex-professional boxers – first described by Harrison Martland in 1928 and called “punch drunk syndrome”. The syndrome was characterised by a gradual onset of neuropsychological, psychiatric, behavioural and motor disturbances with progressive cognitive decline.

The seminal paper describing the neuropathological changes of CTE was written by Corsellis and colleagues in 1973.¹ However, very little else was published in the scientific literature until it was recognised that CTE changes could occur in sports other than boxing – presumably, there was little scientific interest in investigating a group who continued to participate in an activity with known consequences.

In 2005, Omalu and colleagues were the first to recognise CTE changes in the brain of an ex-NFL player (Mike Webster).² The story around this was made into the movie Concussion, starring Will Smith and released in 2015 – the year after the NFL settlement.

As you are now well aware, pathological changes of CTE have since been described in several other ex-contact-sport players, including those who played football, rugby union, rugby league and ice hockey. Further work on the pathological changes of CTE was carried out, resulting in the 2016 publication of a consensus statement by the National Institute of Neurological Diseases and Stroke.³

The predominant feature is the deposition of phosphorylated tau protein in a distinctive pattern not seen in other neurological conditions associated with tau protein deposition – a perivascular distribution in the depths of cortical sulci. However, these diagnostic criteria were based on only 10 cases of suspected CTE, so these must be con¬sidered preliminary criteria.³

It is important to note that this deposition of tau protein is not the only pathological change associated with repetitive brain injury. Deposition of beta amyloid and TAR DNA-binding protein 43 (a messenger protein) has also been reported, and both these proteins (along with tau) have been implicated in other neurodegenerative diseases.

The clinical picture reported in ex-contact-sport players with CTE pathological changes appears to be slightly different to that seen in the early reports of boxers.

Increased suicidality, aggression and disinhibition are prominent features in ex-contact-sport players, whereas Parkinsonian symptoms featured in the boxer reports. Whether this represents a slightly different condition or is simply a result of the massive reporting bias in the current studies remains to be seen.

There are no diagnostic clinical features of CTE, so, at this stage, the diagnosis of CTE can only be made post-mortem by identifying typical CTE pathological changes in the brain. This makes understanding the link between contact sport and CTE very challenging. However, there have been some long-term cohort studies in ex-professional contact-sport players looking at the incidence of neurodegenerative diseases in general. This helps give an idea of the scale of the problem.

A study by Mackay and colleagues compared 7676 Scottish ex-professional football players born before 1977 with 23,028 age, sex and social deprivation-matched controls. It was found that in the 1180 soccer players who died (with a mean 18-year follow-up), there was:⁴

• 3.45 times increased risk of death from neurodegenerative disorders (p<0.001)
• 5.1 times increased risk of death from Alzheimer disease (p<0.001)
• 4.3 times increased risk of death from motor neurone disease (p<0.001).

For each 1000 deaths, there were:

• 29 deaths from neurodegenerative disorders in soccer players versus 10 in the control group
• 8.4 deaths from Alzheimer disease in soccer players versus 4.7 in the control group
• 2.7 deaths from motor neurone disease in soccer players versus 0.7 in the control group.

Interestingly, these figures are similar to those seen in a cohort study of ex-American football players.

In contrast, another study of the same ex-Scottish football players found the soccer-player cohort had a significantly lower incidence of admission to hospital for anxiety, depression, bipolar disease and drug or alcohol abuse than the control group, and no significant difference in suicide rate was found between the two groups.⁵

1. We know that playing professional contact sport (in males) is associated with the development of pathological changes in the brain now classified as CTE.

We do not know:

• the incidence of CTE changes in ex-contact-sport players
• whether CTE also occurs in amateur players, although it seems likely that this would be the case
• whether young athletes (who take longer to recover from concussion than adults) or female athletes (who have a high¬er incidence of concussion than male athletes) are more or less susceptible to the development of CTE changes
• what it is about a certain concussion (severity, symptoms, absolute number, time between concussions) that leads to the development of CTE changes, or even whether subconcussive blows (eg, heading a ball at football) can lead to CTE changes – from a management point of view, it would be nice if CTE changes only resulted after symptomatic head injuries, but this is unlikely to be the case.

2. We know that CTE pathological changes in the brain are associated with a clinical syndrome (see above).

We do not know:

• why some with CTE changes develop symptoms and others do not
• the link between CTE and other neurodegenerative diseases.

As with Alzheimer disease,⁶ there is possibly a whole host of other factors that influence the development of symptoms in those with CTE pathological changes, such as genetics, exercise, mental health disorders, alcohol intake, hypertension, smoking and social isolation.

3. Playing professional contact sport (in males) is associated with an increased risk of neurodegenerative diseases in general – not just CTE – including Alzheimer disease, motor neurone disease and Parkinson disease. The absolute inci¬dence is low but relative incidence is quite high (see above).

In contrast, studies show ex-contact-sport players have a lower incidence of mental health disorders and a lower mortality rate (up to the age of 70) compared with controls.

While more information is gathered, it is important to educate stakeholders (players, medical staff, administrators, coaches, parents) about the importance of treating sports-re¬lated concussion in line with recommendations (see “Sports medicine”, New Zealand Doctor, 11 December 2019).

Steve Targett is a sports physician in Doha, Qatar

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