| Print Date: | May 19, 2013, 7:28 am |
| Title | 1. Severe diffuse axonal injury (shearing injury) of the lobar white matter, corpus callosum, and upper brain stem. |
| Text | Patients with DAI characteristically present with severe impairment of consciousness that begins at the moment of injury. The degree of coma is typically quite profound, although the level will vary somewhat depending upon the severity and extent of injury. As a group, these patients usually have significantly greater impairment of consciousness than do those with cortical contusions, intracerebral hematomas, or extra-axial hematomas. One of the most striking characteristics of patients with DAI is the marked discrepancy between the initial CT scan findings and the clinical status of the patient. The CT scan at first glance may seem "surprisingly normal" when contrasted with the patient's markedly abnormal neurologic condition. Very small DAI lesions have profound clinical sequelae since they affect densely packed bundles of axons located deep within the brain. Cortical contusions are usually several times larger than DAI lesions but, owing to their superficial location, typically cause much less neurologic damage.
The small size and primarily nonhemorrhagic nature of DAI lesions often render them quite inconspicuous on the first CT scan. Initially, the lesions are almost isodense to surrounding brain since they exhibit only minimal edema. About 20-30% of lesions do have small central petechial hemorrhages that slightly increase their conspicuity. These tiny (1-3 mm) hemorrhages should be specifically searched for on the initial CT scan, since they may provide the only indication of severe head injury. DAI lesions usually become lower in density and more conspicuous on CT scans over the first week as edema begins to develop. Edema arises because of axoplasmic leakage from disrupted neurons, plasma leakage from associated capillary injury, and traumatically induced intracellular edema. MRI, because of its greater ability to detect subtle areas of edema, is considerably more sensitive than CT for detecting DAI. Even MRI, however, probably greatly underestimates the true extent of DAI since many autopsy studies have shown that much of the axonal disruption is at a microscopic level. MRI probably detects only those regions where axonal disruption is confluent enough to allow visualization. Nevertheless, MR is usually much better than CT for demonstrating these primarily nonhemorrhagic lesions. This case also illustrates the classic triad of anatomic locations for severe DAI: 1. lobar white matter, 2. corpus callosum, and 3. brain stem. Brain stem DAI, as depicted here, typically affects the dorsolateral aspect of the upper pons and midbrain (Film .5 - scans 1 and 4, curved arrows). This location is quite different from that seen with secondary brain stem injury due to transtentorial herniation. The lesions in the latter group of patients typically involve the ventral pons and midbrain. Also, the brain stem is usually markedly deformed in patients with secondary brain stem injury. Survivors of severe DAI generally have very poor neurologic outcomes. Most patients will have moderate to severe disability at long-term follow-up. Some patients, in fact, never regain consciousness but remain in a "persistent vegetative state" for the rest of their lives. |
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| Contributor | Neuroradiology Learning File - © ACR (ACR Learning File®) |
| Author | Lindell R. Gentry, MD |
| Peer Reviewer | James G. Smirniotopoulos, M.D. (Uniformed Services University) |
| Record Number | : 1548 |
| Created | 2001-03-27 13:39:59-05 |
| Modified | 2005-05-19 15:22:52-04 |
| Category: | Trauma |
| Location: | Brain and Neuro |
| Sublocation: | Brainstem (all parts) |
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