Severe Diffuse Axonal Injury: Severe TBI Primer Five


Severe Diffuse Axonal Injury

By Gordon S. Johnson, Jr.

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We have discussed Severe Diffuse Axonal Injury (DAI) for years on these pages, but more typically within the context of its milder forms.

However, the type of Severe Diffuse Axonal Injury (DAI) which occurs in a severe brain injury, also referred to as Type Three Diffuse Axonal Injury, does not involve the damage over time typically seen in a concussion, but an immediate injury where the shear forces actually sever large portions of the brain’s axonal tracts.

A shear injury occurs when layers of different density within the brain slide across each other when the brain is forcefully rotated in a traumatic event.  Most injury after concussion occurs as a result of this acceleration/deceleration phenomenon. However, in a severe DAI, the force literally tears the brain apart at its roots, the axonal tracts.

The axonal tracts are the collections of the neurons extensions, the axons, which cross from the gray matter into the white matter.  For example, the corpus callosum is the white matter (axonal) tract which connects the two hemispheres of the brain together, where the white matter crisscrosses between the two sides of the brain.

Diffuse axonal injury is graded Type One thru Type Three, with Type One being the most mild and Type Three the most severe.  Even though Type Three will result in immediate unconsciousness and often death, it does not necessarily show up on either CT or MRI, acutely.  The majority of cases of severe brain injury which involve immediate coma, involve severe diffuse axonal injury.

Elsewhere I make the analogy of the stretching of the axons in Type One DAI, as comparable to spraining an ankle. Severe Diffuse Axonal Injury, or Type Three DAI, is the equivalent to having broken the ankle. The axon is literally sheared off by the force.

For my most detailed explanation of the neuropathology of shear injuries, click here.
Brain Stem Injury

Gordon S. Johnson, Jr., the author of this page is a lawyer, not a doctor, who practices law with the Brain Injury Law Group, S.C. Click here for more on the firm.