Specific Imaging Findings

Diffuse axonal injury (DAI, shear injury) on CT presents as small hypodense (nonhemorrhagic) or hyperdense (hemorrhagic) foci; however, the majority of DAI lesions are not detected on CT. Gray–white matter junction (especially paramedial), dorsolateral midbrain, and corpus callosum (especially the splenium) are the most typical DAI locations. Multiple oval lesions 1–15 mm in diameter are detected on T2WI and FLAIR images. T2 signal intensity depends on the presence of hemorrhage: hemorrhagic lesions show low signal, while the nonhemorrhagic ones are T2 hyperintense. On T1WI the lesions are usually hypointense and not well seen, hyperintensity is present in subacute hemorrhagic lesions. T2* sequences detect susceptibility effects of hemoglobin degradation products as areas of signal loss in hemorrhagic lesions. The detection of acute or chronic hemorrhagic DAI is improved by heavily T2*WI, such as with higher field strength and a longer echo time (TE), and even further with susceptibility-weighted imaging (SWI). Diffusion MR imaging is the most sensitive modality for DAI detection with bright signal on DWI in the acute phase. Some lesions may only be detected with DWI, some with T2* and a few with FLAIR. Presence of hemorrhage in the interpeduncular cistern on initial CT is a marker for possible brainstem DAI.

Pertinent Clinical Information

The main and classic DAI symptom is lack of consciousness; however, this is not always present. A conscious patient may show other signs of brain damage, depending on lesion location. Most patients (> 90%) with severe DAI remain in a persistent vegetative state. Milder forms of DAI in the chronic phase may cause residual neuropsychiatric problems and cognitive deficits, focal neurologic lesions, memory loss, concentration difficulties, intellectual decline, psychiatric disturbances, headaches, and seizures.