Concussions in Sports: Brain Injury, Symptoms, and Long-Term Risks

What Is a Concussion?

A concussion is a mild traumatic brain injury (mTBI) caused by a blow or jolt to the head, or by an impact elsewhere on the body that transmits force to the head. The defining characteristic is a functional disturbance of the brain — a temporary disruption in normal neurological function — rather than a structural injury visible on standard imaging. CT scans and MRIs typically appear normal after a concussion, which is one reason the injury was historically minimized: if nothing shows up on a scan, the brain must be fine. We now understand that appearance and function are different things.

Concussions are extraordinarily common in contact and collision sports. The American Academy of Neurology estimates that 1.6–3.8 million sports-related concussions occur in the United States annually, though this is likely an underestimate because many athletes do not report symptoms. American football, ice hockey, rugby, soccer, lacrosse, and wrestling carry the highest per-athlete concussion rates. Even sports not traditionally associated with head injury — basketball, cheerleading, gymnastics — contribute substantially to the overall burden.

What Happens Inside the Brain

At the moment of impact, the brain moves within the skull — stretching, compressing, and shearing the axons (communication fibers) of neurons. This mechanical deformation triggers a neurochemical cascade that disrupts normal brain function. Potassium floods out of neurons while calcium floods in, disrupting the electrical gradients that allow cells to fire normally. Neurotransmitter levels become dysregulated. The energy-demanding processes needed to restore ionic balance — primarily the sodium-potassium pump — increase dramatically, but simultaneously, blood flow to the injured areas decreases, creating a mismatch between energy demand and energy supply. This metabolic crisis is thought to underlie many acute concussion symptoms and also explains why the brain is particularly vulnerable to a second injury during the recovery period.

Diffuse axonal injury — the stretching and tearing of axonal fibers throughout the brain — is the primary structural change in concussion, even when it is not visible on standard imaging. Advanced imaging techniques such as diffusion tensor imaging (DTI) can detect microstructural white matter changes that conventional MRI misses, providing a more accurate window into the biological reality of these injuries. These changes in white matter connectivity may persist well beyond the resolution of clinical symptoms, a finding with significant implications for return-to-play decisions.

Recognizing Concussion Symptoms

Concussion symptoms span four domains: somatic (physical), cognitive, sleep-related, and emotional. Somatic symptoms include headache (the most common, occurring in 80–90 percent of cases), nausea, vomiting, dizziness, sensitivity to light (photophobia), and sensitivity to noise (phonophobia). Cognitive symptoms include feeling mentally foggy or slowed, difficulty concentrating, memory problems (including retrograde amnesia for events before the injury and anterograde amnesia for events afterward), and slowed reaction time.

Sleep disturbances are common and include sleeping more than usual, difficulty falling asleep, and disrupted sleep architecture. Emotional symptoms — irritability, sadness, anxiety, and emotional lability — often emerge in the days following injury and can persist into post-concussion syndrome. In younger athletes, behavioral changes may be more prominent than the headache and cognitive symptoms seen in adults.

Identification on the field of play is complicated by the fact that athletes are often reluctant to report symptoms. Studies consistently find that between 50 and 70 percent of concussions in youth and high school sports go unreported. Athletes cite fear of losing playing time, not wanting to let teammates down, not recognizing their own symptoms, and culture of toughness as barriers to reporting. Sideline assessment tools like the Sport Concussion Assessment Tool (SCAT6) and the King-Devick rapid eye movement test help trained medical personnel identify concussed athletes, but their accuracy depends on pre-season baseline testing and experienced evaluators.

Return to Play: The Graduated Protocol

The cardinal principle of concussion management is that no athlete should return to competition the same day they sustain a concussion, and no athlete should return while still symptomatic. The danger of premature return is second-impact syndrome — a rare but potentially catastrophic condition in which a second concussion, sustained before the first has resolved, triggers massive cerebral edema (brain swelling) due to loss of autoregulation of cerebral blood flow. While second-impact syndrome is more common in younger athletes, the principle of symptom-free return applies across all ages.

The internationally recognized return-to-play protocol, endorsed by the Concussion in Sport Group, consists of six graduated steps: complete rest until asymptomatic; light aerobic exercise (walking, swimming); sport-specific exercise; non-contact training drills; full-contact practice (medical clearance required); and finally, return to competition. Each step takes a minimum of 24 hours, meaning the fastest possible return is six days from injury. If symptoms return at any step, the athlete drops back to the previous symptom-free level. Research shows that most recreational athletes recover within 10–14 days, though high school athletes may take longer, and a small percentage develop post-concussion syndrome with symptoms persisting for months.

Chronic Traumatic Encephalopathy: The Long-Term Question

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disease associated with repeated head impacts, including subconcussive blows that do not produce concussion symptoms. First described in boxers in the 1920s under the name "punch drunk syndrome" or dementia pugilistica, CTE gained widespread attention after neuropathologist Bennet Omalu published his 2005 findings in the brain of former NFL player Mike Webster, who died after years of cognitive and behavioral deterioration.

CTE is characterized at autopsy by abnormal accumulations of tau protein — the same protein implicated in Alzheimer's disease — in a distinctive distribution around small blood vessels in the brain. Clinically, it is associated with progressive problems in cognition, behavior, mood, and eventually motor function. Critically, CTE can currently only be diagnosed postmortem — there is no validated biomarker or imaging test that can identify it in living individuals, though research into blood-based tau markers and advanced imaging is advancing rapidly.

The prevalence of CTE in athletes is not well established. A widely cited 2017 study found CTE in 99 of 111 donated NFL player brains, but this figure reflects profound ascertainment bias — the brains were donated precisely because the athletes were symptomatic, meaning the most severely affected cases are vastly overrepresented. Estimating population risk from these samples is not scientifically valid, but the consistent finding of CTE across contact sport athletes — football, hockey, soccer, rugby, and boxing — with exposure to repeated head impacts has driven major rule changes across all levels of these sports.

Prevention and the Future of Concussion Science

Prevention strategies span equipment, rule changes, and culture. Helmet technology continues to advance, with newer designs better attenuating peak acceleration forces, though no helmet can fully prevent concussion — the brain can be damaged by rapid rotational acceleration even without a direct head impact. The development of mouthguard-based sensors and helmet accelerometers allows real-time measurement of head impact exposure, giving teams data to identify players with unusually high impact burdens for removal from play or technique correction.

Rule changes — eliminating helmet-to-helmet contact in football, restricting heading in youth soccer, enforcing checking rules in youth hockey — are perhaps the most impactful interventions because they address the root cause. Research suggests that delaying full-contact practices until high school, rather than youth sports, significantly reduces cumulative brain impact exposure during the most vulnerable developmental years.

The science of concussion is evolving rapidly. Blood biomarkers including GFAP (glial fibrillary acidic protein) and UCH-L1 (ubiquitin C-terminal hydrolase-L1) are showing promise for objective diagnosis and recovery tracking. Neuroimaging advances continue to reveal the invisible structural changes underlying clinical symptoms. And the development of a validated, in-vivo diagnostic test for CTE — perhaps the field's most important outstanding challenge — would transform both research and clinical care for athletes concerned about their long-term brain health.