Yes, a concussion is a brain injury. It is classified medically as a mild traumatic brain injury (mild TBI), and it is the most common type of traumatic brain injury. The CDC defines it as an injury caused by a bump, blow, or jolt to the head, or by a hit to the body that causes the head and brain to move rapidly back and forth. The word “mild” refers to the fact that concussions are usually not life-threatening, not that the injury itself is trivial or unimportant.
What Happens Inside the Brain
A concussion doesn’t typically cause visible bleeding or structural damage that shows up on a scan. What it does cause is a chemical and metabolic disruption at the cellular level. When the brain shifts rapidly inside the skull, brain cells release a flood of charged particles. Potassium rushes out of cells while sodium and calcium rush in, throwing off the normal electrical balance that neurons depend on to function.
Restoring that balance requires a surge of energy, but the brain’s blood supply can drop significantly after impact. In animal studies, cerebral blood flow has been shown to decrease by up to 50%. So the brain needs more fuel at the exact moment it has less available. This mismatch between energy demand and energy supply is what drives many concussion symptoms: the fog, the fatigue, the difficulty concentrating. It’s a real, measurable injury to brain function, even though the brain’s physical structure may look intact on imaging.
Why Scans Often Look Normal
One reason people question whether a concussion is a “real” brain injury is that standard CT scans and MRIs frequently come back normal. This doesn’t mean nothing happened. CT scans are designed to detect bleeding and fractures. The cellular disruption from a concussion is microscopic, not structural. Even advanced MRI sequences miss the majority of diffuse injuries because over 80% of the affected areas don’t involve visible hemorrhage. A concussion is diagnosed clinically, based on symptoms, history, and neurological assessment, not by imaging.
Symptoms Across Four Categories
Concussion symptoms extend well beyond a headache. They span physical, cognitive, emotional, and sleep-related categories, which reflects just how broadly the injury affects brain function.
Physical symptoms include headaches, dizziness, balance problems, nausea or vomiting (especially early on), sensitivity to light or noise, vision problems, and persistent fatigue. Cognitive symptoms include trouble thinking clearly, feeling mentally foggy or slowed down, difficulty concentrating, and problems with short- or long-term memory.
Emotional changes are common too: increased irritability, anxiety, sadness, or feeling more emotional than usual. Sleep disruption rounds out the picture, with some people sleeping far more than normal and others unable to fall asleep at all. These symptoms can appear immediately or develop over hours and days following the injury.
Recovery Timeline and Persistent Symptoms
Most concussions resolve within a few weeks. Current guidelines recommend early, light physical activity and aerobic exercise as part of recovery, a shift from the old advice of resting in a dark room until all symptoms disappeared. Structured return-to-learn and return-to-sport protocols help people gradually increase activity while monitoring for symptom flare-ups. Rehabilitation targeting the neck, balance system, and vestibular function is recommended for people dealing with neck pain, headaches, dizziness, or balance issues.
Not everyone recovers on that timeline. Roughly 50% of people with a mild head injury still have symptoms at one month. About 15% continue to experience symptoms at one year. In children and adolescents, between 14% and 31% still have ongoing symptoms at three months. When symptoms persist beyond four weeks, a comprehensive clinical evaluation is recommended to identify what’s driving the prolonged recovery and guide targeted treatment.
The Risk of a Second Impact
Sustaining another concussion before the first one has fully healed carries serious risks. Second impact syndrome occurs when a still-recovering brain loses its ability to regulate its own blood supply. This can cause rapid, dangerous swelling inside the skull. The condition carries a mortality rate approaching 50%, and nearly all survivors experience lasting disability. This is the core reason why return-to-play protocols exist and why no one should go back to contact sports or high-risk activities while still symptomatic.
Warning Signs That Need Emergency Care
While most concussions resolve safely, certain symptoms after a head impact signal something more dangerous, such as bleeding inside the skull. Call 911 or go to an emergency department if you notice any of the following:
- Seizures or convulsions
- Repeated vomiting
- One pupil larger than the other
- Slurred speech, weakness, numbness, or loss of coordination
- A headache that keeps getting worse
- Inability to recognize people or places
- Increasing confusion, agitation, or unusual behavior
- Loss of consciousness, extreme drowsiness, or inability to stay awake
In infants and toddlers, the same warning signs apply, along with inconsolable crying and refusal to eat or nurse.