What happens in the brain after a head injury?

Emergency numbers
United States: 911
United Kingdom: 999 or 112
European Union: 112
Elsewhere: your local emergency number

Short answer. In a head injury, an external force may cause rapid head movements, strain on brain tissue and disruption of communication between nerve cells. Depending on the intensity and circumstances, this may result in concussion with symptoms but no visible abnormality on CT, or injuries such as a contusion, bleeding or more diffuse damage. Initial effects and those that develop afterwards are different.

Important. After a recent impact, someone who is difficult to wake, has a seizure, weakness in a limb, new difficulty speaking, increasing confusion, vomiting after the injury or a worsening headache needs emergency medical help: call emergency services immediately. The vomiting alert preserves the French public guidance used here. Anticoagulant medication or loss of consciousness requires appropriate assessment even without dramatic signs. The mechanisms described here can never establish remotely whether an impact was harmless.

Why can the brain be affected even if the skull remains intact?

The skull protects the brain but does not make it immune to sudden movement. A fall, collision or acceleration followed by a sudden stop can transmit forces to tissues inside the skull. An impact does not have to leave a wound or fracture to warrant medical assessment.

Several components are generally distinguished. Acceleration and deceleration rapidly change the head’s speed. Rotational movements may strain connections between brain regions. The actual mechanism depends on many variables; knowing that someone fell from standing height or hit a car door cannot precisely identify the injuries present.

It would therefore be misleading to attribute a memory difficulty automatically to a blow to the back of the head, or dizziness to a balance area that must have been damaged. The brain works through networks, and clinical examination remains essential to interpreting symptoms.

What are primary and secondary injuries?

This distinction helps explain many decisions made in hospital.

• Primary injury: damage occurring at the time of the accident. Depending on the case, this may be a contusion, bleeding, a fracture or diffuse damage to nerve connections.

• Secondary injury: possible worsening after the initial impact, for example if the brain lacks oxygen, pressure inside the skull rises or other complications develop.

Healthcare staff cannot turn back time to erase the initial mechanism. In a severe injury, however, a substantial part of care involves preventing and treating secondary complications. This explains monitoring of oxygenation, circulation, alertness and sometimes intracranial pressure in specialist settings.

This does not mean everyone with a concussion will develop severe brain swelling: these complications occur in situations where care depends on actual severity and examination findings.

What changes in communication between neurons?

The brain does not work solely through isolated anatomical areas. Neurons exchange information within networks involved in perception, movement, attention, memory and many other activities. An injury can disrupt this communication, including through mechanical and neurochemical changes.

After concussion, someone may understand another person’s words but struggle when several conversations overlap. It would be an overstatement to conclude that their neurons have been destroyed because noise makes them tired. A functional symptom alone cannot identify the nature or location of an injury.

Educational resources also describe brain plasticity: the nervous system’s ability to change its organisation and some aspects of how it works. This helps explain the possibility of progress over time and with rehabilitation. It cannot, however, predict an individual’s recovery time or promise the return of every ability.

Why can a CT scan be normal when the person feels different?

CT is particularly useful in emergencies for detecting significant structural abnormalities: certain haemorrhages, contusions, fractures or effects of swelling. It was not designed to measure directly the effort needed to pay attention in a meeting or tolerate noise in a shop.

Consider two distinct situations:

• Someone has had a concussion. Their CT scan shows no injury requiring acute treatment, but they still have light sensitivity and concentration difficulties. The absence of a visible abnormality does not make these symptoms fictitious.

• Someone else has bleeding visible on imaging. The medical priority differs, but the image alone cannot describe all their future abilities or the help they will need day to day.

A normal CT scan must therefore be used neither as proof that everything will be fine whatever happens nor as grounds to claim that a serious invisible injury necessarily exists. Medical reasoning combines the history of the accident, examination and, when indicated, imaging.

How can you understand symptoms affecting several functions?

Difficulty reading may involve attention, visual processing, cognitive effort, pain or fatigue. Dizziness may have several explanations, including vestibular causes. Feeling slowed down may be worsened by poor sleep or certain medicines. These factors must neither all be automatically attributed to the injury nor dismissed because the CT scan was normal.

At follow-up, describing the specific situation is often more useful than assuming a brain mechanism. Being unable to follow a restaurant conversation describes a different difficulty from forgetting what you have just done. Depending on the circumstances, the professional may decide to assess memory, vision, balance, sleep or other aspects.

Those close to the person can help by reporting what they observed before, during and after the accident. Their notes may clarify loss of consciousness, the period of confusion or behaviour changes, without replacing the person’s own account.

Why does recovery sometimes require adjustments?

The biological course of recovery does not always match the return of everyday demands. At home, someone may manage a simple meal without difficulty; the next day, a long working day, noisy transport and a string of decisions may demand much greater effort. This does not establish a new injury every time fatigue occurs.

Instead of assuming that a particular brain region must be damaged, ask which activities become difficult, what makes things worse and what assessment would help. This avoids appealing but false mechanical interpretations and directs follow-up towards the functions actually causing difficulty.

For practical return-to-activity measures, timings and persistent symptoms, the articles devoted to concussion and recovery are more appropriate: this article explains what may happen in the brain rather than offering an individual protocol.

A common question: do symptoms necessarily indicate irreversible injury?

No. A symptom after an impact cannot, in isolation, establish irreversible injury. Some difficulties improve, sometimes with targeted support. Others may last and require ongoing care. Only medical assessment can address their cause, severity and possibilities for recovery.