Diffuse Axonal Injury

Epidemiology

In DAI, the outcome is rarely death, but frequently it is coma. Up to 90% of patients with severe DAI remain in a persistent vegetative state. Those patients who do wake up often remain profoundly impaired. DAI has hence been found to be the most significant cause of morbidity in patients with TBI. With its high degree of chronic debilitation, DAI potentially becomes a highly stressful family problem exacerbated by staggering medical costs.

Anatomy

The brain has three main components: the cerebrum, the cerebellum, and the brainstem. The cerebrum comprises a left and right hemisphere connected by a large bundle of nerve fibers, the corpus callosum. The cerebral hemispheres are divided into lobes corresponding to the section of the overlying skull: temporal, parietal, and occipital. The cerebellum and brainstem are located posterior and inferior to the cerebral hemispheres.

Brain tissue consists of 40% gray matter and 60% white matter. Gray matter is dark in color because of the relatively high proportion of nerve cell nuclei present. White matter consists mostly of axons covered with lightly colored myelin, an insulating protein sheath. Brain cells include neurons and glial cells. Glial cells make up 90% of the brain cells and exist to provide essential support functions to neurons. Neurons are nerve cells that send and receive electrochemical signals to and from the brain and the distal nervous system. Each consists of a cell body with branching dendrites (signal receivers) and a long projection called an axon, which conducts the nerve signal. Bundles of axons called nerves are in the body's periphery and are referred to as nerve tracts within the central nervous system.

Primary Injury

As the name suggests, DAI is a widespread insult to the brain axons, in contrast to a focal brain injury such as a contusion, laceration, or hematoma. The injury is the result of traumatic shearing forces that typically occur when the head is subjected to rapid acceleration/deceleration or severe rotational forces. Motor vehicle accidents are the most frequent cause of DAI. Severe DAI is almost always due to vehicular injury. DAI has been documented in practically every sporting event that potentially involves a sudden head deceleration from a high-speed collision with an immovable object.

The major cause of damage in DAI is the disruption of axons through shear injury. Shear injury refers to the damage sustained to brain matter as tissues of varying density slide over each other in response to forces produced in a sudden acceleration/deceleration impact. Axonal tissue disruption is greatest in those areas where the density difference is greatest. For this reason, the majority (two thirds) of DAI lesions occur at the gray/white-matter junction. The regions of the brain most commonly injured are the frontal and temporal lobes, followed by the corpus callosum. The DAI affecting the corpus callosum is more frequently unilateral and may be hemorrhagic due to close proximity to blood vessels. Consequently, the involvement of the corpus callosum carries a relatively poor prognosis. The brainstem typically remains unaffected, which helps explain why DAI rarely causes death.

...