What is a diffuse axonal injury?

Unlike brain trauma that occurs due to direct impact and deformation of the brain, a diffuse axonal injury 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 rotational forces or severe deceleration.  Vehicle accidents are the most frequent cause of diffuse axonal injury. They can also occur as the result of child abuse such as in shaken baby syndrome.
Injury occurs because the unmoving brain lags behind the movement of the skull, causing brain structures to tear.
A diffuse axonal injury is characterized by axonal separation or tearing, in which the axon is torn at the site of stretch and the part distal to the tear degrades. The major cause of damage in a diffuse axonal injury is the disruption of axons, the neural processes that allow one neuron to communicate with another.  An axon or nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma.

The axon is particularly vulnerable to injury when the brain mass begins to move as a result of rotational forces placed upon it, because it typically will stretch across layers of different density within the brain. When rapid acceleration/deceleration forces are placed upon the brain, the different layers, at progressively further distances from the fulcrum of the rotation will move at different speeds, creating a sliding effect of these different layers across themselves.

The effect of this sliding is that the axon is rapidly stressed beyond its tolerance, which may result in the axon being torn or stretched. Even if the axon is not entirely severed as a result of such force, it may be significantly damaged. Like an electrical wire, to protect the axon from damage and to assure that its electrical impulse does not stray from the appropriate channel, the axon is protected by insulation, called the myelin sheath.  When this sheath, the axon’s insulation, is disrupted, the speed of information processing within the brain can be profoundly affected.

A diffuse axonal injury is difficult to detect since it does not show up well on CT or MRI scans or with other macroscopic imaging techniques, though it shows up microscopically.

Newer studies such as Diffusion Tensor Imaging are able to demonstrate the degree of white matter fiber tract injury even when the standard MRI is negative. Since axonal damage in a diffuse axonal injury is largely a result of secondary biochemical cascades, it has a delayed onset, so a person with a diffuse axonal injury who initially appears well may deteriorate later.