Chronic Traumatic Encephalopathy and the Risk of Repeated Hits

Concussion May Not Be a Good Indicator of Risk

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Awareness has been growing about the potential risk of chronic traumatic encephalopathy (CTE), a chronic brain illness. CTE seems at least partly to be caused by various forms of repeated head injury. Such repeated injuries may result from military service or from contact sports, like American football.

There is much that researchers still do not understand the exact causes of CTE, and the particular factors that put people most at risk.

However, there is a growing consensus that even relatively low-impact injuries that initially cause relatively minor symptoms may be a source of damage.

Medical Syndromes Resulting from Head Trauma

To understand the role of repeated head injury in triggering CTE, it can be useful to distinguish different syndromes and categories of injury. These include:

  • Traumatic Brain Injury
  • Concussion
  • Post-Concussion Syndrome
  • Subconcussion (also called subconcussive injuries)
  • Chronic Traumatic Encephalopathy

These syndromes are related, and in some cases may overlap. However, they may also involve distinct physiological processes in the brain.

What Is Traumatic Brain Injury?

Traumatic brain injury (TBI) refers to a type of brain injury that occurs due to some sort of bump, blow, or other physical injury. The damage can either be done through directly touching brain tissue (as in a penetrating traumatic brain injury) or indirectly, as the brain shakes within the skull.

It means the injury is due to some sort of outside force (as opposed to a medical problem like a stroke).

TBIs occur on a spectrum of severity, depending on exactly what parts of the brain are damaged and how extreme the damage is. The worst of these types of injuries can lead to permanent injury or even death.

But even mild TBIs can lead to problems, both in the short-term and in the long-term. In recent years, researchers have been learning more about the long-term consequences for some people who experience repeated mild TBIs.

Researchers are still learning a lot about what happens in the brain in the days, weeks, and months after a TBI. Though in some cases the brain may return to normal, in other cases there may be long-term changes in the brain, especially in people who are exposed to repeated injuries.

What Is a Concussion?

Concussion can be considered a mild form of TBI. Symptoms of a concussion usually appear either right after injury or within a few hours. There are no universal definitions on what a concussion is, but some possible symptoms of concussion include:

  • Headache
  • Dizziness
  • Balance disturbances
  • Disorientation
  • Drowsiness
  • Difficulty concentrating or remembering

Loss of consciousness sometimes occurs with concussion, but it is less common. Concussion is diagnosed based on a person’s symptoms and history of injury. Most of the time symptoms of concussion don’t last longer than a week to 10 days (although this may be longer in children and adolescents).

What is Post-Concussive Syndrome

A certain number of people who had a concussion continue to experience some sort of symptoms.

Instead of going away, symptoms continue after the initial injury. These may persist for a few months and even sometimes for a year or more. This is called post-concussive syndrome. Such people may have continued symptoms from their concussion, and also may experience additional symptoms like depression and anxiety.

The diagnosis of post-concussive syndrome is a somewhat controversial—one that researchers are still trying to understand. However, it is important to understand that post-concussive syndrome is distinct from CTE. In post-concussive syndrome, symptoms of concussion persist for several weeks or more.

This contrasts with CTE, in which symptoms are not apparent for several years. At this time it is not clear what the relationship is (if any) between post-concussive syndrome and future development of CTE.  

What is Subconcussion?

Sometimes the brain suffers a mild traumatic injury but no easily observable symptoms of concussion are seen. This is can be categorized as something called a “subconcussion.” Such injuries do not meet the criteria for a diagnosis of a concussion. A person might only have one or two temporary symptoms, or no symptoms at all.  However, laboratory evidence and advanced neuroimaging findings suggest that in some cases the brain may suffer real physiological damage (and potentially long-term injury) but without any immediate signs or symptoms. Such injuries may particularly damage the brain if they repeatedly occur over time.

Both concussion and subconcussion can occur in many sports and outside of the sporting arena. However, American football has a relatively high rate and thus has been a particular source of scrutiny. Subconcussive injuries, in particular, may occur quite frequently in contact or collision sports. One of the concerns about subconcussion is that such injuries usually do not result in removal from game play.

What is CTE?

CTE  is a condition that causes damage or death to parts of the brain over time. It leads to symptoms like

  • Memory impairment
  • Poor judgment
  • Poor impulse control
  • Slowed, slurred speech
  • Parkinsonism (causing tremor, rigidity, and slow movements)
  • Depression (and sometimes suicide)
  • Dementia (later in the disease)

The causes of CTE are not well understood. However repetitive head injury is thought to play a role. Microscopically, certain proteins begin to accumulate abnormally in the brain (such as tau and TDP-43). Currently, there is no test that can be used to diagnose CTE in living people. It can only be diagnosed by examining the brain after death.

Notably, symptoms of CTE appear years after the physical trauma, for example, in retired football players. However, it is important to note that not everyone who experiences repeated head impacts seems to get CTE.

Is Concussion a Good Guide for CTE Risk?

Currently, sporting guidelines put much greater emphasis on concussion than on subconcussive injuries. For example, the National Football League has established a post-concussion protocol to help determine when players are allowed to return to the game. Players diagnosed with concussion are removed from game play for the day. This is important for proper recovery from concussion symptoms.

However, it is not clear that such protective measures adequately protect players. There is evidence that repetitive, subconcussive injuries (that don’t result in removal from games) may also pose a risk for CTE in the long term.

For example, a 2018 study published in the academic journal Brain studied the link between subconcussion symptoms and CTE. Dr Lee Goldstein, an associated professor at Boston University's School of Medicine, worked with a team of researchers from several institutions. The team examined the post-mortem brains of student-athletes who had experienced sports-related head-impact injuries. They also used a mouse model to study the effects of different kinds of head trauma on later CTE findings (when examined under a microscope).

They found that some mice that showed symptoms of concussion after an initial powerful blow did not later go on to develop CTE. However, other mice exposed to repeated (but less intense) blows did not show any concussion-type symptoms. But some of these mice later developed signs of CTE.

The team concluded that some of the hits that lead to concussion may contribute to CTE. However, concussion itself does not seem to be necessary to trigger the process. In a press release, Dr Goldstein noted, “These findings provide strong evidence—the best evidence we have so far—that subconcussive impacts are not only dangerous but also causally linked to CTE.”

Impacts on Sporting

Sporting organization may need to consider the impact of these subconcussive impacts when developing guidelines, in addition to following existing precautions on concussions. The damage from subconcussive injuries appears to accumulate over time. For the present, we lack information about the number of subconcussive impacts that are safe for athletes before they should end their game, season, or career. However, for the safety of players, changes are needed to limit the overall number of head impacts for players. Players, too, should be educated that even non-concussive hits may increase their long-term risk of CTE.


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