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Diffuse Axonal Injury (DAI)

Summary

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  • Severe traumatic brain injury characterised by widespread damage to axons in white matter tracts
  • Caused by rapid acceleration/deceleration or rotational forces to the head
  • Imaging findings often subtle, with MRI being more sensitive than CT

Pathophysiology

  • Primary injury:
    • Shearing forces cause axonal stretching and disruption
    • Cytoskeletal damage leads to impaired axonal transport
  • Secondary injury:
    • Delayed axotomy occurs hours to days after initial trauma
    • Wallerian degeneration of affected axons
    • Microglial activation and neuroinflammation

Demographics

  • Most common in:
    • Young adults (15-35 years)
    • Males (3:1 male to female ratio)
  • Main causes:
    • Motor vehicle accidents
    • Falls from height
    • Assault
    • Sports-related injuries (e.g., boxing, football)

Diagnosis

  • Clinical presentation:
    • Loss of consciousness at time of injury
    • Prolonged coma or vegetative state
    • Varying degrees of cognitive and motor impairment
  • Glasgow Coma Scale (GCS) score:
    • Often <8 (severe head injury)
  • Biomarkers:
    • Elevated serum levels of neuron-specific enolase (NSE) and S100B protein

Imaging

  • Computed Tomography (CT):
    • Limited sensitivity for DAI
    • May show:
    • Petechial haemorrhages in white matter
    • Intraventricular or subarachnoid haemorrhage
    • Associated contusions or mass effect
  • Magnetic Resonance Imaging (MRI):
    • Gold standard for DAI diagnosis
    • Sequences:
    • T2-weighted and FLAIR: hyperintense lesions in white matter
    • Gradient Echo (GRE) or Susceptibility Weighted Imaging (SWI): haemorrhagic lesions
    • Diffusion Weighted Imaging (DWI): acute axonal injury
    • Adams grading system:
    • Grade I: Corpus callosum involvement
    • Grade II: Additional lesions in brainstem
    • Grade III: Additional lesions in rostral brainstem
  • Diffusion Tensor Imaging (DTI):
    • Advanced technique for assessing white matter tract integrity
    • Useful for prognostication and follow-up

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  • 20-year-old male suffered a severe head injury following a skiing accident.
  • T2-weighted imaging showed mature contusional damage at the vertex.
  • SWI shows microhaemorrhages at the grey-white matter interface, corpus callosum and brainstem.

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  • A 50-year-old patient suffered a severe head injury following a road traffic accident 3 years prior to presentation within worsening cognition.
  • MRI showed old parenchymal contusions in the left frontal and temporal lobes alongside an old contre-coup injury in the right occipital lobe.
  • SWI showed extensive superficial cortical siderosis secondary to traumatic subarachnoid haemorrhage.
  • There were many cortical or immediately subcortical microhaemorrhages. Some of the microhaemorrhages (e.g., in the right superior parietal lobe) were arranged linearly (red arrows).

Treatment

  • Acute management:
    • Intracranial pressure monitoring and management
    • Maintenance of cerebral perfusion pressure
    • Prevention of secondary injury (e.g., hypoxia, hypotension)
  • Neuroprotective strategies:
    • Hypothermia (controversial)
    • Pharmacological interventions (e.g., progesterone, still under investigation)
  • Rehabilitation:
    • Multidisciplinary approach involving physiotherapy, occupational therapy, and speech therapy
    • Cognitive rehabilitation
    • Psychosocial support for patients and families
  • Emerging therapies:
    • Stem cell therapy (experimental)
    • Neurotrophic factors to promote axonal regeneration

Differential diagnosis

Differential Diagnosis Distinguishing Feature
Cerebral Contusion Focal lesions on CT/MRI, typically in cortical areas
Subdural Haematoma Crescent-shaped extra-axial collection on imaging
Hypoxic-Ischaemic Injury More diffuse and symmetric brain involvement
Multiple Sclerosis Periventricular white matter lesions, clinical history
Toxic Leukoencephalopathy Exposure history, more symmetric white matter changes
Cerebral Oedema Diffuse brain swelling without shear injury pattern
Traumatic Subarachnoid Haemorrhage Blood in subarachnoid spaces on CT
Posterior Reversible Encephalopathy Syndrome (PRES) Predominant posterior circulation involvement, reversible
Cerebral Fat Embolism History of long bone fracture, starfield pattern on DWI