CTE is a brain disease that can only be diagnosed after death. It has been linked to specific changes in the brain that affect how the brain works. The research to date suggests that CTE is caused in part by repeated traumatic brain injuries, including concussions, and repeated hits to the head, called subconcussive head impacts. However, understanding among researchers about the causes of CTE is currently limited. Researchers do not know the number and types of head impacts that increase the risk for CTE. It is possible that biological, environmental, or lifestyle factors could also contribute to the brain changes found in people with CTE diagnosed after death.

More studies are needed to learn about the causes of CTE, its symptoms, and how it affects the brain. In addition, research on the role of genetics, a person’s medical history, and other factors (such as environmental or lifestyle factors) is needed to better understand the risk factors for CTE.

Occasional hits to the head, such as the bumps and tumbles that children experience when learning to walk, do not cause CTE.

Researchers do not know how many people in the United States have CTE. Most studies on CTE have focused on a small group of people who experienced head or brain injuries over many years. People in this group had their brains donated for research, and according to reports from family members, they often had problems with thinking, emotions, or behavior while they were alive.

CTE has been diagnosed in people with and without a history of head or brain injuries. However, most people with a history of head or brain injuries do not develop CTE.

Brain injuries can arise from hits to the head during contact sports like football or soccer. Repeated brain injuries can lead to a condition called chronic traumatic encephalopathy, or CTE. CTE is a serious issue for American football players. It can lead to dementia later in life and even premature death.

An NIH-funded research team is working to learn which types of head impacts are most risky. They looked at data from earlier studies that used specialized helmets to measure head impacts during football games. These studies included youth, high school, and college games.

From these data, the scientists estimated the number and types of head impacts that different players might have each season. They compared this data with CTE diagnoses from more than 600 former football players.

The team found that the risk of having CTE varied by the person’s highest level of play. It also increased the longer a person played football. For each additional year of play, the risk of CTE rose by 15%. The type and force of head impacts were also stronger predictors of CTE risk than the number of hits to the head a player experienced. In contrast, the number of reported concussions were not linked with CTE. Concussions are hits to the head that result in signs or symptoms of brain injury.

“This study suggests that we could reduce CTE risk through changes to how football players practice and play,” says study co-lead Dr. Daniel Daneshvar of Mass General Brigham. “If we cut both the number of head impacts and the force of those hits in practice and games, we could lower the odds that athletes develop CTE.”

Subconcussive head impacts are bumps, blows, or jolts to the head. Unlike concussions, which cause symptoms, subconcussive head impacts do not cause symptoms. A collision while playing sports is one way a person can get a subconcussive head impact.

Researchers are not certain what symptoms are directly linked to CTE. Family members have reported noticing changes in thinking, feeling, behavior, and movement among people who are later diagnosed with CTE after death. Some people diagnosed with CTE first had problems with depression or anxiety. Some later developed memory and other thinking problems. Over time, some of these people had mood or personality changes. Family members of people who were later diagnosed with CTE have reported that their family member had problems that became serious enough to get in the way of normal daily activities (such as social or work-related activities).

The symptoms described by family members are similar to those of other health problems (e.g., Alzheimer’s Disease, Parkinson’s Disease), so having these symptoms does not mean a person has CTE.

In addition, while there is increasing media attention on suicide among former professional athletes, the link between CTE and suicide is unclear.

If you or a family member or friend have any questions or concerns, it is important to talk to a doctor. Treatments are available to help with many of these symptoms.

To diagnose CTE, doctors check the brain of a person after he or she dies. Doctors look for changes in the brain that happen in people with CTE. Through this process, doctors confirm whether the person had CTE or another disease, such as Alzheimer’s disease, or no disease at all. Given the limited understanding of CTE and its causes, doctors cannot diagnose CTE in a living person.