Teaching Case

The patient was admitted to the emergency department in an unconscious state (GCS=6), after sustaining a closed head injury in a motor vehicle accident.

The patient was admitted to the emergency department in an unconscious state (GCS=6), after sustaining a closed head injury in a motor vehicle accident. An emergency brain CT scan without contrast (not presented here) was performed, which demonstrated diffuse cerebral oedema along with several punctate, bilateral hemispheric hyperdense foci suggestive of diffuse axonal injury. A brain MRI consisting of sagittal T1-weighted images, axial TSE-T2-weighted images, and axial GE images was performed two weeks after the patient's admission. The diagnosis of haemorrhagic diffuse axonal injury (DAI) was confirmed on these images.

Diffuse axonal injury (DAI - also known as white matter shearing) is common in patients with severe head trauma. It is claimed to result from acute rotational acceleration and decelaration forces (which are most commonly encountered in motor vehicle accidents), and can occur without anything actually striking the head. The patient is unconscious from the moment of impact and remains unconscious, vegetative or severely disabled until death.
The DAI lesions are usually multiple, have an elliptical configuration (with the long axis parallel to the disrupted axonal tracts), and range in size from 5mm to 15mm; they can be either haemorrhagic or nonhaemorrhagic. DAI occurs in three major anatomic areas: lobar white matter, corpus callosum and dorsolateral upper brainstem (the triad of DAI). It tends to occur in these areas in successive stages, progressively involving deeper structures with increasing severity of trauma.
The three stages of DAI are as follows:

• Stage 1: Lobar white matter lesions (commonly at the grey-white junction); usually located in the parasagittal frontal and the periventricular parts of the temporal lobes, followed by the peripheral parietal and occipital lobes, cerebellum and internal/external capsules. These are the most frequently encountered DAI lesions.
• Stage 2: Corpus callosal lesions, which are usually seen in the posterior half of this structure and are frequently associated with intraventricular haemmorhage. Almost all callosal lesions are associated with lobar DAI.
• Stage 3: Brain stem lesions (usually with coexisting lobar and corpus callosal lesions), typically seen in the dorsolateral aspect of the midbrain and upper pons with frequent involvement of the superior cerebellar peduncles.

It must be emphasised that DAI lesions can also be observed in the basal ganglia, and the cortical grey matter (the latter due to secondary extension of larger grey-white junction lesions).

MRI is more sensitive than CT in identifying DAI lesions; the latter modality often fails to detect them. T2*-weighted gradient echo images, due to magnetic susceptibility effects, are useful in showing small hemorrhagic DAI lesions. However, FLAIR images have been reported to be very useful for detecting non-hemorrhagic DAI, seen only as small areas of edema, and in a later stage, gliosis. The combination of FLAIR and T2*- weighted GRE is important for the detection of both hemorrhagic and nonhemorrhagic lesions in such patients.