Diffuse Axonal Injury
Diffuse axonal injury (DAI) is one of the most common and devastating types of brain injury, occurring in almost half of all cases of severe head trauma. It is a type of diffuse brain injury, meaning that damage occurs over a more widespread area than in focal brain injury. DAI, which refers to extensive lesions in white matter tracts, is one of the major causes of unconsciousness and persistent vegetative state after head trauma.
Though diffuse axonal injury seldom leads to death, the outcome is frequently coma with over 90% of patients with severe DAI never regaining consciousness. Those that do wake up often remain significantly impaired.
Diffuse Axonal Injury Mechanism
Unlike brain trauma that occurs due to direct impact and deformation of the brain, DAI is the result of traumatic shearing forces that occur when the head is rapidly accelerated or decelerated, as may occur in auto accidents, falls, and assaults. It usually results from twisting or rotational forces (angular momentum), rather than forward and back impacts linear momentum. Car accidents are the most frequent causes of DAI, with sports accidents and child abuse also common causes.
The major cause of damage in DAI is the tearing of axons, the neural processes that allow one neuron to communicate with another. Tracts of axons, which appear white due to myelination, are referred to as white matter. Acceleration causes shearing injury, which refers to damage inflicted as tissue slides over other tissue. When the brain is accelerated, parts of differing densities and distances from the axis of rotation slide over one another, stretching axons that traverse junctions between areas of different density, especially gray-white matter junctions. Two thirds of DAI lesions occur in areas where grey and white matter meet.
Diffuse Axonal Injury Characteristics
Patients typically have several focal white matter lesions of variable size (1-15 mm) in a characteristic distribution. Areas most vulnerable to injury are the frontal and temporal lobes. Other common locations for DAI include the white matter in the cerebral cortex, the corpus callosum, the superior cerebral peduncles, basal ganglia, thalamus, and deep hemispheric nuclei. These areas may be more easily damaged because of the difference in density between them and the rest of the brain.
Diffuse Axonal Injury Diagnosis and Treatment
DAI is difficult to detect since it does not show up well on CT scans or with other macroscopic imaging techniques. However, Diffuse Axonal Injury does show up microscopically. Axonal damage in DAI is largely a result of secondary biochemical cascades, and has a delayed onset, so a person with DAI who initially appears well may deteriorate later. Thus injury is frequently more severe than is realized, and medical professionals must suspect DAI in any patients whose CT scans appear normal but who have symptoms like unconsciousness.
MRI is more sensitive and 30% of head injured patients with normal head CT scans have signs of DAI on MRI. But MRI may also miss DAI, because it identifies the injury using signs of edema, which may not be present.
DAI is classified in grades based on severity of the injury. In Grade I, widespread axonal damage is present but no focal abnormalities are seen. In Grade II, damage found in Grade I is present in addition to focal abnormalities, especially in the corpus callosum. Grade III damage encompasses both Grades I and II in addition to rostral brain stem injury and often tears in the tissue.
DAI currently lacks a specific treatment beyond what is done for any type of head injury, including stabilizing the patient and trying to limit increases in intracranial pressure (ICP).
