Diffuse axonal injury: a widespread type of traumatic brain injury

Nerve cells send signals along long fibres called axons, which run through the white matter of the brain. Headway defines diffuse axonal injury as widespread tearing of these nerve fibres across the whole of the brain. Leeds Teaching Hospitals NHS Trust describes it as a more diffuse type of head injury, which means that instead of happening in a specific area of the brain it occurs over a more widespread area.

A review of the research describes DAI as an integral process in all grades of traumatic brain injury, with two main kinds of axonal damage: swellings along the length of axons and large terminal bulbs at the ends of broken axons. It adds that although "diffuse" implies that damage is spread throughout, DAI is better described as lesions in multiple but common sites in the white matter tracts, such as the corpus callosum, cerebral hemispheres and brainstem. Damage in the brainstem and corpus callosum has been linked to worse outcomes.

The injury can therefore co-exist with other head injuries such as bleeding or bruising, which are separate problems that may need different treatment, including surgery.

DAI results from the brain being moved violently within the skull. The Leeds leaflet says it results from the brain moving back and forth in the skull and is common after road traffic accidents, sports-related accidents and falls. The research review explains that the rotational acceleration and deceleration of the head is the typical mechanism, and that sudden shear forces are linked with damage at the junction between grey and white matter.

It does not require a blow to the head; high-speed changes of direction or rotation can produce it. The NHS head injury page lists hitting the head at high speed, such as in a road accident, and falling from a height of more than 1 metre or 5 stairs among situations in which 999 should be called.

The immediate picture can range from a period of unconsciousness to prolonged coma. The NHS head injury page lists signs needing a 999 call: being knocked out and not waking, being unable to stay awake or keep the eyes open, a fit, problems with vision or hearing, clear fluid from the ears or nose, new numbness or weakness, problems with walking, balance, understanding, speaking or writing, and changes in behaviour.

After the early period, the Leeds leaflet describes a wide range of possible effects depending on the type, location and severity of the injury. It lists fatigue, mobility and balance problems, sensory impairment, difficulties with speech, epilepsy, weakness often affecting one side, hormonal imbalance from damage to the hypothalamus or pituitary, and cognitive effects such as memory problems, reduced concentration and slower information processing.

Any significant head injury, particularly after a road accident or fall from height, needs emergency assessment. The NHS says to call 999 for the signs listed above, not to drive yourself to A&E, and to call NHS 111 if you have recently had a head injury and are being sick, feel dizzy, take medicine that thins the blood, were drinking alcohol or taking drugs at the time or have any other symptoms you are worried about.

The Leeds leaflet lists symptoms that need urgent medical attention after discharge, including increasing drowsiness, difficulty walking or weakness in the legs, arms or face, confusion or memory problems, double or blurred vision, stiff neck or raised temperature, severe sensitivity to light, severe headache, seizure, and persistent vomiting.

The research review says diagnosis in humans can currently only be made definitively through post-mortem examination, with growing interest in diffusion tensor imaging to identify the severity of axonal damage, an area that remains controversial. In practice, hospital teams use CT scanning to look for bleeding or swelling that needs urgent treatment, MRI to look for patterns of white matter injury and the clinical course, including the length of unconsciousness and recovery.

The brain may look near-normal on an early CT scan despite significant diffuse injury, which is why the clinical picture matters and why repeat imaging or MRI may be used. This point about scans is general clinical practice rather than a statement from the cited pages.

Leeds says DAI is managed medically as surgery is not an option, which means treatment aims to protect the brain from further damage, for example by controlling swelling and pressure, blood pressure, oxygen levels and fits in an intensive care or neurosurgical setting, and to support recovery. Surgery may still be needed for any coexisting bleeding.

Rehabilitation is central. The Leeds leaflet describes therapy to improve mobility, equipment such as walking aids, speech therapy, and strategies for fatigue, memory and concentration, and notes that recovery needs rest periods through the day and support from family and friends. Rehabilitation may continue after discharge, through NHS community neurorehabilitation teams, brain injury charities and, where used, private neurorehabilitation services.

Leeds says this type of head injury often has the most serious complications and in some cases these can be permanent. Outcomes vary widely, from good recovery to severe long-term disability, and are hard to predict early on. The research review reports that axonal lesions in the brainstem and genu of the corpus callosum are negative prognostic indicators.

Headway, the UK brain injury association, provides information and support for people with brain injury and their families. Support from the hospital team, GP, rehabilitation services and charities can help with returning to work, driving decisions and the emotional effects, which are often as important as the physical ones.