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Brain Injury Basics

What is an Acquired Brain Injury?

Acquired brain injuries (ABI) are the medical conditions that happen to the brain (usually after childhood) that change how it works. There are two kinds of ABI:

  • traumatic brain injury
  • non-traumatic brain injury

Brain swelling (cerebral edema)

Just like if you hit your thumb with a hammer and it swells up, swelling in your brain happens as a normal response to an injury (trauma) or irritation (non-trauma).

There is a balance of brain, cerebrospinal fluid (CSF), and blood within the vessels of the brain. The skull is a fixed compartment; this means that any increase in either the size of the brain or the amount of blood or CSF puts pressure inside the skull (called intracranial pressure or ICP), which can be dangerous. Pressure on the brain tissue reduces blood supply, which means the brain gets less oxygen, damaging the brain.

Traumatic brain injury

Traumatic brain injuries involve physical trauma like an accident, brain surgery, or a head injury.

The brain may hit the skull at the point of impact, bounce against the other side of the skull, while also twisting quickly. This is also called a closed head injury. A brain contusion is a bruise to the brain, just like if you were to bump your knee and got a bruise.

An open head injury is a head injury in which the dura mater, the outer layer of the meninges, is damaged. If there is a fracture in the skull, especially at the base of the skull (a basal skull fracture) the CSF fluid can leak.

A penetrating injury is one that’s caused by objects such as bullets, knives, and other objects that push the skull bone into the brain. If the skull pieces push into the brain covering it’s called a depressed skull fracture.

Diffuse Axonal Injury (DAI)

Diffuse axonal injury is one of the most common types of traumatic brain injury, where the damage occurs over a more widespread area of the brain.

When the brain moves around the skull upon impact the brain tissue stretches and tears. This causes the brain cells (the axons) to twist, stretch, and snap (Figure 6). As some of the badly injured brain cells die, they release chemicals that irritate the surrounding brain tissue and cause the brain tissue to swell. This is called cerebral edema.

Since axons connect brain cells, when they are injured, it can slow how fast messages are sent from one part of the brain to the other.

For example, if the main road through a city is closed, you can still drive across the city but it takes much longer because you have to travel on smaller, slower routes. You also need to learn the new route! This is the same effect as DAI.


Figure 6: This diagram shows the axon twisting when the brain moves around the skull after an impact.


Whenever the brain is shaken hard or twisted the brain tissue tears. If blood vessels are broken, bleeding may continue until a pool of blood (hematoma or hemorrhage) builds up, causing pressure on the brain (Figure 7).

  • An epidural hematoma is blood between the dura and the skull.
  • A subdural hematoma is bleeding below the dura layer.
  • A subarachnoid hematoma is bleeding under the arachnoid layer of the brain.
  • An intracerebral hemorrhage is bleeding inside the brain.
  • An intraventricular hemorrhage is bleeding into the ventricles.

Figure 7: Two types of bleeding in the brain 


Figure 8: Where the different types of bleeding are found.

What are some causes of non-traumatic brain injuries?

Some causes include:

  • When the brain doesn’t get enough oxygen (hypoxia/anoxia)

    Hypoxia is when the brain cells aren’t getting enough blood (therefore oxygen). Anoxia is when the brain cells aren’t getting any oxygen.

  • Infection of the brain (encephalitis/meningitis)

    An infection or inflammation of the brain that affects the membranes and CSF surrounding the brain.

  • Changes in the body’s chemistry or toxins that affect the way the brain normally works (metabolic/toxic encephalopathy)

    The abnormal brain function is caused by abnormalities in the balance of electrolytes and other chemicals in the body (for example, blood sugar levels that are too high or too low).

    Toxic changes caused by poisons such as carbon monoxide, alcohol, or drugs can also injure the brain.

  • Stroke

    An ischemic stroke is when the blood supply to a part of the brain is interrupted. The artery may be blocked by a clot (embolism) or become too narrow for blood to flow through it (thrombosis, or hardening of the arteries).

    A hemorrhagic stroke is when there is bleeding in the brain. This type of bleeding is usually caused by high blood pressure, which weakens the blood vessel’s walls until the wall breaks and blood enters the surrounding brain tissue.

  • Aneurysm

    This is when an artery wall has a weak section; this weak section balloons out under the pressure of blood flow and may burst or leak, causing bleeding inside the brain (intracerebral hemorrhage).

  • Some kinds of brain tumours

    This is an abnormal growth of cells within the brain tissue. As the tumour grows, it takes up more space in the skull, pushes on the brain, causing swelling, which can affect the blood and oxygen supply to healthy brain tissue.

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