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Acute Disseminated Encephalomyelitis (ADEM)

Summary

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  • Acute inflammatory demyelinating disorder of the central nervous system
  • Typically follows viral infection or vaccination
  • Characterised by multifocal white matter lesions

Pathophysiology

  • Autoimmune-mediated demyelination
  • T-cell mediated response against myelin basic protein
  • Perivascular inflammation and oedema in white matter
  • Grey matter involvement can occur, particularly in the thalamus and basal ganglia

Demographics

  • More common in children and young adults
  • Peak incidence between 3-10 years of age
  • Slight male predominance (1.3:1)
  • Incidence: 0.3-0.6 per 100,000 person-years

Diagnosis

  • Clinical presentation:
    • Acute onset of neurological symptoms (encephalopathy, motor deficits, ataxia)
    • Often preceded by viral illness or vaccination (1-4 weeks prior)
  • Cerebrospinal fluid analysis:
    • Mild pleocytosis
    • Elevated protein
    • Oligoclonal bands (less common than in multiple sclerosis)
  • Neuroimaging
    • Characteristic lesions as described below
    • Exclusion of other neurological disorders

Imaging

  • CT:
    • Hypodensities in the same distribution as the T2-weighted hyperintensities described below.

  • MRI

    • T2: Multiple, large (>1-2 cm) hyperintense lesion white matter (deep grey nuclei may be involved)
    • T1+C: Variable. May be absent initially. When present, usually peripheral or leading edge enhancement.

  • Spinal cord (short segment lesions) involvement in 20-30% of cases

  • Follow-up imaging:

    • resolution or significant improvement of lesions
    • Development of new lesions are atypical as classically a monophasic process
    • If new lesions continue to develop, then other demyelinating conditions should be considered

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  • One month after a viral upper respiratory tract infection, a 25-year-old patient presented with confusion left sided weakness.
  • MRI showed multifocal white matter lesions with incomplete rim enhancement.
  • Some of the lesions had a leading edge of diffusion restriction.

Treatment

  • High-dose intravenous corticosteroids (first-line treatment)
  • Intravenous immunoglobulin (IVIG) for steroid-resistant cases
  • Plasma exchange for severe cases not responding to steroids or IVIG
  • Supportive care and symptomatic management

Differential diagnosis

Differential Diagnosis Distinguishing Feature
Multiple Sclerosis Ovoid periventricular plaques with Dawson's fingers on sagittal FLAIR; calloso-septal interface lesions; no basal ganglia involvement typical for MS
Acute Haemorrhagic Leukoencephalitis (AHLE) Haemorrhagic foci on GRE/SWI within T2 lesions; more confluent white matter involvement; mass effect
Neuromyelitis Optica Longitudinally extensive spinal cord lesion (>3 vertebral segments); bilateral optic nerve enhancement; area postrema involvement
Posterior Reversible Encephalopathy Syndrome Posterior-predominant parieto-occipital vasogenic oedema with elevated ADC; no basal ganglia or brainstem involvement typical of PRES