Diffuse Axonal Injury (DAI)

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Diagnosis Definition

DAI (also referred to as traumatic axonal injury, or TAI) is a type of traumatic brain injury (TBI) resulting from accelerating and decelerating motion, with a rotational component, that leads to shearing of the white matter tracts of the brain
Clinical presentation ranges from clinically insignificant findings to a comatose state
The cause is usually high-speed motor vehicle accident but DAI can also result from child abuse (e.g., shaken baby syndrome)
The outcome is frequently coma, with over 90% of patients with severe DAI never regaining consciousness; those who do wake up often remain significantly impaired

Imaging Findings

DAI can be inferred when MRI, especially SWI or GRE sequences that are exquisitely sensitive to paramagnetic blood products, shows several small 1-15 mm ovoid or linear regions of susceptibility artifact (i.e., hemorrhage) at the grey-white matter junction (most commonly the frontal and temporal lobes), in the corpus callosum, and in more severe cases in the brainstem and pons, surrounded by FLAIR hyperintensity
Surrounding edema increases in the first few days and usually resolves in 3 months on FLAIR images, whereas hemorrhagic changes on SWI may take 12 months to resolve; this is to be expected as edema is faster to resolve than hemorrhage

Pearls

There is no association between diffuse axonal injury and underlying skull fractures
MRI is more sensitive than CT but MRI may also be normal in cases of DAI if edema isn’t present
Diffusion tensor imaging can show findings of DAI even when MRI is negative

References

Davis PC. Head trauma. AJNR Am J Neuroradiol. 2007; 28(8):1619-21
Scheid R, Preul C, Gruber O, Wiggins C, von Cramon DY. Diffuse axonal injury associated with chronic traumatic brain injury; evidence from T2*-weighted gradient-echo imaging at 3T. AJNR Am J Neuroradiol. 2003; 24(6):1049-1056

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