Diffuse brain injuries include diffuse axonal injury, diffuse cerebral swelling, and edema. Diffuse axonal injury is produced by high shearing stresses that occur at different parts of the brain, including at the gray matter-white matter interface. These shearing stresses cause axonal stretching commonly involving the corpus callosum, anterior commissure, and upper brain stem. Blood vessels may or may not be disrupted. When vessels are uninterrupted, the scattered small areas of edema are best demonstrated by T1-weighted MR images as slightly hypointense or isointense regions that become hyperintense on T2-weighted images. When vessel disruption produces hemorrhages, they appear early on CT as multiple sites of hyperdensity (Fig. 6-91).
FIGURE 6-91. Diffuse axonal injury. Nonenhanced CT scan shows hemorrhagic foci at the genu of the corpus callosum (arrows).
Diffuse cerebral swelling occurs with many types of head injury. It is thought to be produced by a rapidly increased volume of circulating blood. By MRI and CT, the general brain enlargement is visualized by an obliteration or encroachment of the normal CSF spaces: the cortical sulci, the perimesencephalic and basal cisterns, and the ventricles (85). By CT, the enlarged brain may show slightly increased density.
In generalized cerebral edema, the enlarged brain also encroaches on the CSF spaces, but by CT the edema produces a generalized hypodensity that usually takes longer to develop than diffuse cerebral swelling (Fig. 6-92). The edema may obscure gray matter-white matter boundaries.
Both diffuse brain swelling and generalized cerebral edema are emergencies, because if not treated promptly they may lead to brain herniation sometimes with fatal outcomes.