What You Should Know About Subconcussion

Recently, people have been becoming more aware of the potential risks of repetitive head injuries. One class of injury, called subconcussion, is starting to get more attention. But what is subconcussion, and what do we need to be doing about it?

Definition

Subconcussion is a somewhat controversial term in medicine. Its exact meaning is still evolving. You might read or hear about subconcussion or any of the following closely related terms:

• Subconcussive symptoms
• Subconcussive injury
• Subconcussive brain trauma
• Subconcussive hits

These different terms underscore the fact that subconcussion is not a clearly defined category. It is also not well understood in terms of its short-term or long-term effects.

It may also be helpful to define subconcussion in terms of what it is not. Subconcussion results from some sort of direct or indirect force to the head that does not result in the full set of signs and symptoms used to diagnose a concussion. Concussion results in symptoms like the following:

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

Less commonly, a concussion might cause a loss of consciousness.

In some cases, a direct or indirect blow to the head does not result in symptoms. In other cases, a person might have very mild and temporary symptoms that don’t rise to the level of a concussion. Depending on the circumstances, this might be termed a “subconcussive hit” or “subconcussion.” Distinguishing a subconcussive hit from a hit that causes concussion can be tricky because the diagnosis of concussion is also not completely clear-cut.

Because concussions generate immediate symptoms, most people have assumed that concussive injuries are more dangerous and damaging than subconcussive injuries.

Are Subconcussive Hits Dangerous?

Recently, there has been growing awareness that subconcussive hits may actually pose a health concern. This may be true both in the short term (days and months) and in the long term (years later). This health risk is probably the greatest for people that receive many such hits over time. For example, this might apply to people in the military who are exposed to repeated explosions. American football players are another group of people who often receive many subconcussive hits.

Data from both animal and human studies suggest that repeated subconcussive hits may be more dangerous than previously thought. Recent evidence suggests that in some cases the brain may suffer real damage from subconcussive hits, even without any immediate signs or symptoms of concussion. These data come from both animal and human studies. For example, one study examined high school football players who had received many subconcussive hits but never had symptoms of a concussion. The researchers found that the athletes had subtle deficits in working memory. They also found subtle neurophysiological changes in the part of the brain when assessed by a type of imaging called fMRI.

In other words, at least some of the time, repetitive subconcussive hits may be resulting in subtle symptoms, even though these people never experience symptoms of a full concussion.

Diagnosis

Subconcussion is not usually diagnosed in a clinical setting. Typically, health professionals evaluate patients to see if signs and symptoms of a concussion are present after a head injury. At that point, they diagnose (or do not diagnose) a concussion and don’t worry about subconcussion effects.

However, in a laboratory setting, researchers can observe some alterations in brain physiology in animals exposed to head trauma. They can see these changes soon after this trauma, even if the animals don’t show any signs of having an actual concussion. People who have been exposed to repeated subconcussive hits also show subtle changes on specialized brain imaging (like fMRI). However, standard brain imaging tests (like head CT), generally cannot show such small changes.

Can the Brain Heal After a Subconcussion?

In some cases, the impact of a hit might not be enough to cause any sort of initial damage at all, so no healing is necessary. In other cases, there may be some initial damage, though small. This may vary based on a number of unknown factors, like the severity or angle of impact, age, or the number of previous impacts. But we don’t really understand this well yet.

In some cases, the brain may not have any long-term damage from a subconcussive hit, even if there is initial damage. You can think of a small cut on your skin that heals naturally with time. It's not a big deal. Researchers can find signs of temporarily increased inflammation in the brain of individuals who have received these subconcussive hits. But this may not always cause short-term or long-term problems. The inflammation may diminish on its own naturally, especially if it is given a chance to heal before becoming re-injured.

But one concern is the effect of repetitive subconcussive hits. There may be something about repetitive subconcussive hits that keeps the brain from healing properly. For example, this may result in an extended process of unresolved inflammation that contributes to brain problems over time.

Is Subconcussion a Form of Traumatic Brain Injury?

Depending on how you look at it, subconcussion can be considered a very mild form of traumatic brain injury. Concussion is considered a mild form of traumatic brain injury, and one might think of subconcussion as an even milder form. However, since some subconcussive hits might not cause damage, this is a controversial question.

The Relationship Between Subconcussion and CTE

Lately, researchers and advocates have become more concerned about the possible link between subconcussion and chronic traumatic encephalopathy (CTE). CTE is a chronic brain condition that causes damage or death to parts of the brain over time. It can lead to problems with memory, judgment, movement, mood, and even eventually dementia. Though the cause of CTE is not completely understood it has been linked to repetitive head trauma. For example, it seems to occur in some American football players years after they retire from the sport.

It was initially thought that hits leading to concussion would provide a good guide to the people at risk of developing CTE. However, scientific evidence suggests that subconcussive hits may also play a role in triggering CTE. This is concerning, since nonconcussive hits do not usually result in removal from gameplay in American football or other sports.

A Word From Verywell

There is much that is not known about the potential short-term and long-term consequences of subconcussion. However, the impacts of subconcussion appear to accumulate over time. A person who experiences a single subconcussive hit is unlikely to suffer any long term problems. However, the risk appears to increase with repeated hits. At this time, researchers are still learning about the safety risks posed by subconcussive hits, both in the short and long-term. Although it is important not to raise unnecessary alarm, it seems reasonable to take measures to limit the number and severity of such impacts.