Diffuse axonal injury (DAI), also known as traumatic axonal injury (TAI), is a severe form of traumatic brain injury due to shearing forces. It is a potentially difficult diagnosis to make on imaging alone, especially on CT as the finding can be subtle, however, it has the potential to result in severe neurological impairment.
The diagnosis is best made on MRI where it is characterised by several small regions of susceptibility artifact at the grey-white matter junction, in the corpus callosum, and in more severe cases in the brainstem, surrounded by FLAIR hyperintensity.
The patients at risk of diffuse axonal injury belong to the same cohort as those who suffer traumatic brain injury and as such young men are very much over-represented.
Typically, patients who are shown to have diffuse axonal injury have loss of consciousness at the time of the accident. Post-traumatic coma may last a considerable time and is often attributed to coexistent more visible injury (e.g. cerebral contusions). As such the diagnosis is often not suspected until later when patients fail to recover neurologically as expected.
Diffuse axonal injury is the result of shearing forces, typically from rotational acceleration (most often a deceleration). Due to the slightly different specific gravities (relative mass per unit volume) of white and grey matter, shearing due to change in velocity has a predilection for axons at the grey-white matter junction, as the name implies. In the majority of cases, these forces result in damage to the cells and result in oedema. Actual complete tearing of the axons is only seen in severe cases. It is also known that some neurones may undergo degeneration in the weeks or months after trauma, it is called secondary axonotmesis.
Diffuse axonal injury is characterised by multiple focal lesions with a characteristic distribution: typically located at the grey-white matter junction, in the corpus callosum and in more severe cases in the brainstem (see: grading of diffuse axonal injury).
Non-contrast CT of the brain is routine in patients presenting with head injuries. Unfortunately, it is not sensitive to subtle diffuse axonal injury and as such, some patients with relatively normal CT scans may have significant unexplained neurological deficit 4,5.
The appearance depends on whether or not the lesions are overtly haemorrhagic. Haemorrhagic lesions will be hyperdense and range in size from a few millimetres to a few centimetres in diameter. Non-haemorrhagic lesions are hypodense. They typically become more evident over the first few days as oedema develops around them. They may be associated with significant and disproportionate cerebral swelling.
CT is particularly insensitive to non-haemorrhagic lesions (as defined by CT) only able to detect 19% of such lesions, compared to 92% using T2 weighted imaging 4. When lesions are haemorrhagic, and especially when they are large, then CT is quite sensitive. As such, it is usually a safe assumption that if a couple of small haemorrhagic lesions are visible on CT, the degree of damage is much greater.
MRI is the modality of choice for assessing suspected diffuse axonal injury even in patients with entirely normal CT of the brain 5,6. MRI, especially SWI or GRE sequences, exquisitely sensitive to paramagnetic blood products may demonstrate small regions of susceptibility artefact at the grey-white matter junction, in the corpus callosum or the brain stem. Some lesions may be entirely non-haemorrhagic (even using high field strength SWI sequences). These will, however, be visible as regions of high FLAIR signal.
Over the first few days, the degree of surrounding oedema will typically increase, although by 3-months post-injury FLAIR changes will have largely resolved 7. In contrast, SWI changes will usually take longer to resolve, although by 12-months post-injury there will have been substantial resolution 7. This is to be expected as oedema is faster to resolve than haemorrhage.
In the months that follow the trauma, there is accelerated brain volume reduction, which can sometimes be detected by visual inspection, but sometimes only by volumetric studies8.
Importantly, it should be noted, that even with high field strength modern scanners, the absence of findings does not categorically exclude the presence of axonal injury.
Treatment and prognosis
Unfortunately little can be done for patients with diffuse axonal injury other than trying to minimise secondary damage caused by cerebral oedema, hypoxia, etc.
Depending on the severity and distribution of injury (see: grading of diffuse axonal injury) patients can vary from minimally affected to being in a persistent vegetative state 1,2. The amount of axonal injury in the brainstem is predictive of long-term vegetative state, whereas supratentorial injury can result in focal neurological or neuropsychiatric deficits 1.
On imaging consider:
- cortical contusions
- diffuse vascular injuries
- particularly on T2* sequences
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