A concussion is a type of traumatic brain injury caused by a blow to the head, neck, or body that sends force into the brain. It triggers a cascade of chemical and metabolic disruptions inside brain cells, but it doesn’t cause visible damage on standard brain scans. Less than 1% of concussions show acute injury on MRI, which is why concussions are diagnosed based on symptoms and clinical evaluation rather than imaging. Globally, there were roughly 20.8 million new traumatic brain injuries in 2021, and concussions make up a significant share of that total.
What Happens Inside the Brain
A concussion isn’t a bruise on the brain. It’s a disruption at the cellular level. When force hits the brain, cell membranes develop tiny defects, and ions rush in and out of cells in the wrong directions. Potassium floods out, while sodium and calcium flood in. This triggers a wave of abnormal electrical activity across the brain, sometimes described as a “spreading depression” that may explain why people feel immediately dazed, confused, or slow after a hit.
To fix this ionic imbalance, brain cells crank their energy-consuming pumps into overdrive. The brain burns through its glucose reserves rapidly, but blood flow to the brain stays the same or actually decreases. This creates an energy crisis: the brain needs more fuel than it can get. Meanwhile, excess calcium gets shunted into mitochondria (the energy-producing structures inside cells), which can impair them further and deepen the energy shortage. This mismatch between energy supply and demand is a key reason why rest matters so much in the early days after a concussion. The brain is working harder than normal just to restore its basic chemistry.
Symptoms Across Four Categories
Concussion symptoms can appear immediately or develop over minutes to hours. The CDC groups them into four domains.
Physical: headaches, dizziness, balance problems, nausea or vomiting, sensitivity to light or noise, fatigue, and vision problems. Headache is the single most common complaint.
Thinking and memory: difficulty concentrating, feeling mentally “foggy” or slowed down, trouble thinking clearly, and problems with short- or long-term memory.
Emotional: irritability, sadness, anxiety, and feeling more emotional than usual.
Sleep: sleeping more or less than normal, or difficulty falling asleep.
Not everyone gets all of these. Some people experience mainly headaches and fogginess, while others notice emotional changes or sleep disruption first. The combination and severity vary widely from person to person, and even from one concussion to the next in the same person.
How Concussions Are Diagnosed
There is no blood test or brain scan that confirms a concussion. CT scans and MRIs are sometimes ordered to rule out more serious injuries like bleeding or skull fractures, but the concussion itself is invisible on these images. One large study found that 79% of CT scans and 75% of MRIs in concussion patients came back completely normal.
Instead, clinicians diagnose concussions through symptom evaluation, cognitive testing, and physical examination. In sports settings, the most widely used tool is the Sport Concussion Assessment Tool (now in its sixth version, SCAT6). It checks for observable warning signs like confusion or a blank stare, tests memory with sport-specific questions (“What venue are we at?” or “Who scored last?”), evaluates balance through several standing positions, and has the athlete rate 22 possible symptoms on a severity scale. Cognitive screening includes tasks like recalling a list of words, reciting months in reverse order, and repeating number sequences backward.
Outside of sports, emergency departments and primary care doctors use similar symptom checklists and neurological exams, focusing on mental status, balance, coordination, and eye movement.
Recovery Timeline
Most concussion symptoms resolve within days to a few weeks. Children typically feel better within two to four weeks. Adults often recover on a similar timeline, though individual variation is significant.
The current approach to recovery has shifted away from strict bed rest in a dark room. Brief initial rest (24 to 48 hours) is still recommended, but after that, light activity that doesn’t worsen symptoms is encouraged. For athletes, recovery follows a structured six-step return-to-play progression, with each step taking at least 24 hours:
- Step 1: Return to regular daily activities like school or work
- Step 2: Light aerobic exercise (5 to 10 minutes of walking, light jogging, or stationary cycling)
- Step 3: Moderate activity with more head and body movement
- Step 4: Heavy non-contact activity like sprinting and full weightlifting
- Step 5: Full-contact practice
- Step 6: Return to competition
If symptoms return at any step, you go back to the previous one and rest before trying again. The entire progression requires medical clearance before it begins and monitoring throughout.
When Symptoms Don’t Go Away
A subset of people develop what’s called post-concussion syndrome, where symptoms persist well beyond the expected recovery window. Under one set of diagnostic criteria, this requires cognitive problems with attention or memory plus at least three additional symptoms (fatigue, sleep disturbance, headache, dizziness, irritability, mood changes, or personality changes) lasting three months or longer. A broader definition captures symptoms persisting beyond three weeks.
Post-concussion syndrome can include the same headaches, fogginess, fatigue, balance issues, insomnia, and emotional changes seen in the acute phase, but they simply don’t resolve. The reasons are not fully understood, and treatment typically involves managing individual symptoms while gradually increasing activity.
Why a Second Hit Is Dangerous
Sustaining another concussion before the brain has recovered from the first one is particularly risky. In rare cases, a condition called second impact syndrome can occur, where the brain loses its ability to regulate blood flow. Pressure inside the skull rises rapidly, and the brain can herniate (shift out of its normal position) within minutes. The condition is often fatal. This is the primary reason return-to-play protocols exist and why athletes are pulled from competition at the first sign of a possible concussion. The brain’s energy crisis from the first injury leaves it uniquely vulnerable to a second insult, even one that might seem minor.