What is the primary pathophysiology of diffuse axonal injury (DAI) and how does it commonly appear on CT?

• A. Localized edema with mass effect; CT shows large contusions
• B. Widespread axonal shearing due to rotational forces; CT often shows few early abnormalities, with possible small petechial hemorrhages in white matter tracts
• C. Demyelination due to hypoxia; CT shows diffuse low density
• D. Ischemic penumbra; CT shows early diffusion restriction

Answer: (B)

Diffuse axonal injury results from rapid brain movement that creates widespread shear forces on axons, especially along white matter tracts and at gray–white matter junctions like the corpus callosum and brainstem. This mechanical disruption of the axonal cytoskeleton leads to diffuse disconnection of brain networks, rather than a single focal lesion, so the injury is “diffuse” in nature.

Because these injuries are largely microscopic, the acute CT often looks normal or only shows subtle findings. When CT does show abnormalities, they’re typically small, punctate hemorrhages (petechial) within white matter tracts, rather than large contusions with mass effect. MRI is far more sensitive for detecting DAI, revealing white matter pathology and microhemorrhages that CT can miss.