Can brainstem herniation occur in Diffuse Axonal Injury (DAI)?

Can Brainstem Herniation Occur in DAI?

Yes, brainstem herniation can occur in patients with diffuse axonal injury (DAI), but it is not a direct consequence of the DAI itself—rather, it results from associated mass effect, edema, or other space-occupying lesions that commonly co-occur with DAI.

Understanding the Relationship Between DAI and Brainstem Pathology

DAI Classification and Brainstem Involvement

DAI is classified into three grades based on anatomical distribution, with Grade III specifically involving the brainstem 1:

• Grade I: Axonal lesions confined to cerebral hemispheres 1
• Grade II: Focal axonal lesions in the corpus callosum 1
• Grade III: Focal or multiple axonal lesions in the brainstem, specifically the dorsolateral quadrant 1

The presence of dorsolateral brainstem lesions (DAI Stage 3) is associated with poor outcomes 2, 3.

Mechanisms of Brainstem Injury in TBI

The key distinction is that brainstem herniation and primary brainstem DAI are separate pathological processes:

• Primary DAI in brainstem: Results from shearing forces during acceleration-deceleration injury, causing direct axonal damage in the dorsolateral pons and midbrain 1, 4
• Secondary brainstem herniation: Occurs due to increased intracranial pressure from mass effect (subdural hematoma, contusions, cerebral edema) causing downward displacement and compression 5

Co-occurrence of Pathologies

DAI rarely occurs in isolation. Large-scale studies demonstrate that traumatic brain injuries present with multiple concurrent pathoanatomical findings 2:

• Traumatic subarachnoid hemorrhage, skull fractures, intraparenchymal hemorrhage, and acute subdural hematoma frequently co-occur with DAI 2
• Brain herniation is documented as a distinct common data element separate from DAI in both the CENTER-TBI and TRACK-TBI studies 2
• The co-occurrence of abnormalities is dependent on initial injury severity 2

Clinical Implications

When evaluating a patient with DAI, brainstem herniation should be suspected if:

• There is evidence of mass effect from associated hematomas or significant cerebral edema 5
• Cisternal compression is present on imaging 2
• Midline shift is documented 2
• The patient demonstrates signs of increased intracranial pressure requiring ICP monitoring (threshold >20 mmHg) 6

Prognostic Considerations

The location and extent of brainstem involvement significantly impacts outcomes 4:

• Patients with large, numerous pontine lesions (particularly in the dorsal pons) demonstrate worse neurological outcomes with average discharge GCS of 7.1 4
• However, some patients with Grade 3 DAI involving the brainstem can achieve unexpectedly good recovery with discharge GCS of 14-15 4
• The number of discrete DAI lesions and their size are more predictive than simply the presence of brainstem involvement 4

Common Pitfalls to Avoid

Do not assume that brainstem pathology in TBI is always herniation-related 5:

• Focal traumatic brainstem injury can occur from direct impact or stretching forces without increased intracranial pressure 5
• Brainstem hemorrhage has multiple etiologies: Duret hemorrhage (from herniation), DAI-related hemorrhage (dorsolateral), and diffuse vascular injury (periventricular) 5
• The absence of pathological evidence of increased intracranial pressure does not exclude brainstem injury 5

CT has significant limitations in detecting DAI 3, 6:

• Non-contrast CT is first-line for acute TBI but may miss microhemorrhages associated with axonal injury 3
• MRI with T2*-weighted GRE and susceptibility-weighted imaging is 3-6 times more sensitive for detecting hemorrhagic axonal injuries 3
• MRI is indicated when CT is normal but unexplained neurologic findings persist 3, 6