ABSTRACT

DAI is diagnosed by documenting widespread axonal injury throughout the brain, along with appropriate clinical history. It has been divided into a three-tiered grading system based upon the following gross and microscopic findings (Adams et al., 1989):

• Grade 1 – microscopic damage to axons without gross hemorrhage • Grade 2 – microscopic damage to axons with hemorrhage in the corpus callosum • Grade 3 – microscopic damage to axons with hemorrhage in the dorsal aspect of the brainstem

Severe DAI is accompanied by hemorrhages in characteristic locations. They are readily identified upon sectioning of a fixed brain. Axonal damage is most easily documented in microscopic sections of long white matter tracts where there is a high density of axons. Examples include the corpus callosum, internal capsules, and cerebellar peduncles (Geddes et al., 2000). Damage is typically most severe in midline and paramidline structures and increases in an anterior-to-posterior gradient (Leclercq, 2001). Microscopic examination of injured axons has shown the posterior aspect of the corpus callosum to be the most sensitive region. The posterior limbs of the internal capsules and the superior and middle cerebellar peduncles have also been demonstrated to be sensitive to axonal injury (Geddes et al., 2000). As will be discussed later in this chapter, traumatic axonal injury must be differentiated from ischemic axonal injury or other etiologies of axonal injury. See Appendix page 233 for a guide to histological sampling of the brain in cases of suspected DAI.