What is an acquired brain injury?
An acquired brain injury (ABI) is any damage to the brain that occurs after birth, as opposed to conditions present from birth or developing as part of a degenerative disease. The causes are varied: a road traffic accident, a fall, a stroke (where blood supply to part of the brain is cut off), a brain tumour, an infection such as meningitis or encephalitis, poisoning, or a period of oxygen deprivation.
Within this broad category, there are two main types. A traumatic brain injury (TBI) results from an external force: a blow, jolt, or penetrating injury to the head.
A non-traumatic brain injury results from internal causes such as stroke, infection, tumour, or lack of oxygen. Both types can cause a wide range of effects depending on which parts of the brain are damaged.
Brain injuries are far more common than most people realise. In the United Kingdom, over one million people attend hospital each year with a head injury, and an estimated 1.3 million people are living with long-term difficulties as a result of a brain injury. It is one of the leading causes of disability in working-age adults.
What is cognitive decline?
The primal injury
At the moment of a traumatic brain injury, the immediate physical damage is called the primary injury. This can include bruising of brain tissue (a contusion), tearing of nerve fibres (called diffuse axonal injury), bleeding within or around the brain, and skull fractures.
Diffuse axonal injury deserves special attention because it is one of the most common and yet least visible forms of brain injury. When the head experiences a sudden jolt or rotation, the brain (which is softer than most people imagine, roughly the consistency of soft tofu) shifts inside the skull. This stretches and tears the long fibres connecting different parts of the brain. These fibres, called axons, are the brain's communication cables. When they are torn, the connections they carried are broken.
The effects of diffuse axonal injury can be widespread and subtle, which is partly why brain injuries do not always show up clearly on standard brain scans, even when someone is experiencing significant difficulties.
The secondary injury: what happens in the hours and days that follow
Immediately after the primary injury, a cascade of damaging processes begins inside the brain that can cause as much, or more, damage than the original event itself. This is called the secondary injury, and understanding it is one of the key focuses of brain injury medicine.
The damaged cells release a flood of glutamate (the brain's main excitatory chemical messenger) into the spaces between cells. This overexcites surrounding nerve cells, flooding them with calcium and triggering cellular energy failure, oxidative damage, and cell death. This process is called excitotoxicity, and it is the same destructive mechanism we see in conditions like ALS and after stroke.
Swelling of the brain, known as cerebral oedema, develops as the body's immune response kicks in. Because the brain sits inside the rigid casing of the skull, there is nowhere for this swelling to go. The resulting rise in pressure inside the skull can compress healthy brain tissue and reduce blood flow, causing further damage well beyond the original injury site.
Blood vessels may be damaged or disrupted, reducing oxygen delivery to areas of the brain that were initially uninjured. At the same time, the brain's own immune cells, the microglia, activate and begin releasing inflammatory substances.
This inflammation can be both protective (clearing debris) and damaging (harming surviving cells) depending on how prolonged and widespread it becomes.
Location matters
The brain is not one uniform organ. Different regions have quite specific jobs, and the effects of a brain injury depend heavily on where the damage occurs.
Injury to the frontal lobe, the large region behind the forehead, is among the most common in traumatic brain injury because the front of the brain hits the inside of the skull during sudden deceleration. The frontal lobes control planning, decision-making, impulse control, personality, and social behaviour. Frontal lobe damage can therefore produce changes sometimes described as a personality change: the person may become more impulsive, less able to plan ahead, more irritable, or less able to regulate their emotions, even when their memory and language are relatively intact.
The hippocampus, the brain's memory formation centre, is particularly vulnerable to the effects of oxygen deprivation and excitotoxicity. This is why memory difficulties, particularly difficulty forming new memories after the injury, are among the most common consequences of acquired brain injury.
The brainstem, which controls basic functions like breathing, heart rate, and consciousness, can be affected by severe injuries, diffuse axonal injury, or swelling. This is why severe brain injury can result in disorders of consciousness.
White matter, the brain's network of connecting fibres, is particularly vulnerable to diffuse axonal injury. When white matter pathways are disrupted, the speed and efficiency of communication between brain regions slows. This contributes to the cognitive fatigue, slowed processing speed, and difficulty with multitasking that many brain injury survivors describe, even when individual abilities such as memory or language seem relatively preserved.
brain recovery
What is neuroplasticity?
Perhaps the most important thing to understand about brain injury recovery is neuroplasticity: the brain's ability to physically reorganise itself in response to experience, damage, and learning.
The brain is not a fixed, hard-wired machine.
Throughout life, but especially in response to challenge and learning, it forms new connections between nerve cells, strengthens existing ones, and can sometimes recruit new regions to take over functions previously managed by damaged areas. After a brain injury, this process becomes one of the primary mechanisms of recovery.
Think of it like a road network after a major motorway closure. Traffic, in this case brain signals, cannot travel the same route as before. Over time, with use and effort, smaller roads get widened and improved to carry more traffic, and eventually a new effective route is established. It is rarely as fast or efficient as the original motorway, but it can restore a great deal of function.
Neuroplasticity is why rehabilitation matters so much after brain injury. Repetitive, purposeful practice, whether of movement, language, memory, or other functions, physically drives the formation of new connections.
Recovery is not passive. It is an active process that the brain undertakes in response to being used.
Recovery takes time, often much more than people expect
There is a widespread and harmful myth that recovery from brain injury stops after six months to a year. The evidence does not support this.
While the fastest period of recovery is often in the first weeks and months, neuroplasticity continues for years, and meaningful improvements can and do occur well beyond the timeframes traditionally discussed with patients and families.
What changes over time is not the brain's ability to adapt, but the intensity of rehabilitation, the support available, and the specific areas where progress becomes possible.
Ongoing engagement, cognitive stimulation, and physical activity all continue to support brain recovery long after the acute phase.
Common effects of brain injury
Fatigue
Brain injury fatigue is one of the most consistently reported and least understood consequences of acquired brain injury. It is not the same as ordinary tiredness.
It is a profound, often sudden exhaustion that can be triggered by tasks that previously required no effort at all, such as having a conversation or going to the supermarket.
The brain basis relates partly to the extra neural effort required when damaged or disrupted pathways force the brain to work harder to accomplish the same task. When the motorway is closed and traffic is being routed down back roads, the journey takes more fuel.
Cognitive and emotional processing after brain injury works the same way: more neural resources are required, and they run out faster.
Emotional and behavioural changes
Many people and families are unprepared for the emotional and behavioural changes that can follow brain injury, and these are often the most difficult to live with. Irritability, anxiety, depression, emotional lability (laughing or crying suddenly and unexpectedly), reduced empathy, and impulsivity are all common.
It is crucial to understand that these changes are not character flaws or deliberate behaviour. They reflect direct damage to the brain systems that regulate emotion and social behaviour, particularly the frontal lobes and their connections to the limbic system (the brain's emotional processing centre).
The person has not become a different person by choice. Their brain is working differently, and with the right support, many of these changes can improve significantly over time.
Memory and concentration
Difficulty forming new memories after an injury is extremely common and reflects the vulnerability of the hippocampus. Difficulty with concentration and being easily distracted reflects disruption to the prefrontal circuits that regulate attention, similar in some ways to what happens in ADHD, but arising from acquired damage rather than developmental difference.
Chronic Traumatic Encephalopathy (CTE)
While most acquired brain injuries are single events, there is a related condition caused by repeated head impacts over time.
Chronic traumatic encephalopathy (CTE) is a progressive brain disease linked to repeated concussions and smaller sub-concussive hits, the kind that may not cause obvious symptoms at the time but accumulate over months and years.
CTE has been found in the brains of former contact sports players, military veterans, and others who have experienced repeated head trauma.
It cannot currently be diagnosed during a person's lifetime. It is only confirmed by examining brain tissue after death, which is one of the reasons it has taken so long to understand.
What happens in the brain in CTE?
Each impact, even a small one, can cause microscopic damage to the long connecting fibres of nerve cells.
Over time, a protein called tau, which we also see playing a damaging role in Alzheimer's disease, begins to misfold and accumulate inside nerve cells at these sites of repeated injury.
These tau deposits spread gradually through the brain, disrupting and eventually killing nerve cells in the regions they reach.
Crucially, the tau deposits in chronic traumatic encephalopathy (CTE) form in a distinctive pattern, starting around the small blood vessels deep in the folds of the brain, which sets it apart from Alzheimer's and other conditions where tau is also involved.
This unique pattern is one of the key ways researchers can identify CTE in post-mortem brain tissue.
What are the symptoms?
Symptoms of CTE typically appear years or even decades after the period of repeated head impacts, which makes the link easy to miss.
They can include changes in mood and behaviour (depression, irritability, impulsivity), problems with memory and thinking, and in some cases a progressive dementia in later life.
Because these symptoms overlap with many other conditions, CTE is very difficult to identify in a living person based on symptoms alone.
Why does this matter for brain injury care?
The existence of CTE reinforces something important about brain injury: damage does not have to be dramatic or immediately obvious to have lasting consequences.
The cumulative effect of repeated impacts, each one seemingly minor, can set a slow process in motion that only becomes apparent much later.
This is one of the reasons that proper assessment, appropriate rest, and careful return-to-activity protocols after any head injury matter.
current research
Protecting the brain in the critical window after injury
One of the biggest priorities in brain injury research is reducing the secondary injury that unfolds in the hours and days after the initial event. Treatments aimed at reducing excitotoxicity, controlling brain swelling, and limiting inflammation could significantly reduce long-term disability. Several neuroprotective agents are currently in clinical trials.
Enhancing neuroplasticity
Researchers are exploring ways to actively boost the brain's neuroplasticity during recovery, including non-invasive brain stimulation techniques, targeted rehabilitation programmes, and pharmacological approaches that increase the brain's receptiveness to forming new connections.
Better detection of hidden injury
Advanced imaging techniques, including diffusion tensor imaging (DTI), which can visualise white matter pathways, and blood biomarkers such as neurofilament light chain (NfL), are improving the ability to detect the full extent of brain injury that standard scans miss. This is particularly important for mild traumatic brain injury, where invisible damage can have very real and lasting consequences.