A motorcyclist wearing no helmet sustains a high-speed collision and remains comatose until death, though CT shows no large haematoma. Autopsy reveals small haemorrhages in the corpus callosum and dorsolateral brainstem, with microscopic axonal retraction balls in the parasagittal white matter. The mechanism responsible is:

• A Direct impact contusion of the cortex
• B Shearing of axons by rotational acceleration and deceleration forces ✓
• C Venous sinus thrombosis with venous infarction
• D Secondary brain oedema with tonsillar herniation

Explanation

Diffuse axonal injury results from angular acceleration and deceleration of the head, which shears axons at the interface of tissues of differing density, particularly in the parasagittal white matter, corpus callosum and dorsolateral brainstem. Coma is immediate and prolonged, often without a mass lesion on imaging. Retraction balls, seen with silver stains, represent disrupted axons with proximal bulbous swelling. Direct contusions would produce cortical surface lesions, which are absent here.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.