John M. Barkley
L. Anne Hayman
Pedro J. Diaz-Marchan
Shearing injuries, like parenchymal contusions, may be either hemorrhagic or nonhemorrhagic. They typically occur at the gray/white matter junction, in a subcortical location. These injuries result from acceleration or deceleration of the brain during trauma.
Shearing injuries may also occur in the brainstem, corpus callosum and septum pellucidum (2). Like any small abnormality in the brain, subtle lesions may not be appreciated with CT. Occasionally, shearing injuries are manifested as 1–5 mm petechial areas of hemorrhage. MRI has the advantage of gradient echo and magnetic susceptibility sequences that take advantage of the blooming phenomenon that occurs in areas of hemorrhage on MRI. Thus, MRI may detect a greater number of small hemorrhages that may not be seen with non-contrast CT (3).
Shearing injuries may manifest clinically as neurological, cognitive or behavioral impairment. Shearing injuries may also occur in infants and young children due to non-accidental trauma. Shaken baby syndrome classically produces shearing injuries within the brain as well as retinal hemorrhages (4). Shearing injuries may also result from the rotational forces and impact injuries that occur during certain types of athletic pursuits, most notably contact sports (5).
A normal CT scan in light of a neurologically devastated patient is a classic finding in diffuse axonal injury (DAI). It is the most common cause of a post-traumatic vegetative state (1). DAI consists of innumerable shearing injuries that are often beyond the resolution of CT (6).
The injuries typically occur at the gray-white matter junction in the frontal and temporal lobes. In very severe trauma, diffuse axonal injury may occur in the lobar white matter, corpus callosum and brainstem (Figure 11-4). In the brainstem, these shearing injuries are typically located in the dorsolateral aspects of the pons and midbrain.
DAI occurs after acute acceleration/deceleration forces disrupt neuronal axons. The mechanism of injury is typically severe such as that seen in high-speed motor vehicle accidents. If the axonal injuries are nonhemorrhagic, they will likely not be seen on CT. Because CT often misses these injuries, follow-up MRI in patients with DAI may demonstrate bright T2 foci at the gray-white interfaces or even delayed petechial hemorrhages (1).
MRI is much more sensitive in the detection of diffuse axonal injuries than is CT (Figure 11-5). Many of these injuries likely go undetected on any imaging modality. Gradient echo MR sequences are sensitive for subtle blood products by taking advantage of the blooming, or magnetic susceptibility artifact that iron containing compounds produce, as previously discussed. Gradient Recalled Echo (GRE) and fluid-attenuated inversion recovery (FLAIR) MRI sequences are very sensitive for the subtle findings seen in patients with petechial hemorrhage and diffuse axonal injury (7).
As mentioned earlier, CT markedly underestimates the extent of these injuries. The CT appearance may be normal or near normal. Patients who have sustained diffuse axonal injury have a worse long-term prognosis than those patients with gray matter contusions, thus it is helpful to employ MRI for diagnosis in these situations.
Contusions often occur due to direct impact injuries that occur in focal locations. Shearing injuries and DAI, on the other hand, occur from global rotational forces imparted to the brain. These types of forces result in more extensive and generalized injuries to the brain. MRI is also useful for imaging the long-term sequelae of patients with global brain injury. The long-term sequelae of DAI will be discussed in more detail at the end of the chapter.
Original: Brain Injury Medicine. Principles and Practice
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