What is ADEM (Acute Disseminated Encephalomyelitis)?
ADEM is a rare, immune-mediated inflammatory demyelinating disorder of the central nervous system that typically presents as a monophasic illness with acute, multifocal neurological deficits, often following an infection or vaccination. 1, 2
Core Pathophysiology
ADEM results from an autoimmune attack on CNS myelin, characterized by:
- Perivenular demyelination and inflammation - the histological hallmark that distinguishes it from other demyelinating diseases 3
- Post-infectious immune dysregulation through molecular mimicry, where the immune system mistakenly targets CNS antigens that resemble infectious agents 1, 4
- Blood-brain barrier disruption and loss of immune tolerance 1
- Inflammation affecting cerebral hemispheres, brainstem, cerebellum, and spinal cord 5
Clinical Presentation
ADEM presents with multiple simultaneous neurological deficits accompanied by encephalopathy 6. Key features include:
- Encephalopathy (altered consciousness, behavioral changes, seizures) - a distinguishing feature from MS 7
- Motor deficits (weakness, ataxia, postural imbalance) 6
- Sensory disturbances
- Visual symptoms (optic neuritis)
- Brainstem symptoms (cranial nerve palsies, respiratory insufficiency) 7
- Prodromal illness 4 days to 4 weeks before onset (viral/bacterial infection or vaccination) 7, 1
Diagnostic Features
MRI Characteristics
Large, confluent T2 hyperintense white matter lesions are the radiological signature 7. Specific patterns include:
- Bilateral, asymmetric supratentorial and infratentorial lesions 6
- Deep gray matter involvement (thalamus, basal ganglia)
- Brainstem and cerebellar involvement
- Absence of periventricular ovoid lesions and Dawson's fingers (helps distinguish from MS) 7
CSF Analysis
- Lymphocytic pleocytosis (may include neutrophils) 7, 6
- Elevated protein 6
- Absence of oligoclonal bands in most cases 7, 6
- Normal glucose 6
Age Distribution and Prognosis
ADEM predominantly affects children and young adults 1, 2, though adult cases occur. Critical prognostic differences exist:
- Pediatric patients generally have favorable outcomes with treatment 5
- Adult-onset ADEM carries a less favorable prognosis, with higher risk of progressive disease and mortality 5
- Elderly patients face particular diagnostic and treatment challenges with worse outcomes 5
MOG-IgG Association
Consider MOG-IgG testing in all ADEM cases, particularly those with 7:
- Recurrent ADEM, multiphasic ADEM, or ADEM-ON (ADEM followed by optic neuritis)
- Post-vaccination onset
- Large confluent brain lesions with disturbance of consciousness
- Steroid-dependent symptoms
MOG-IgG positivity is found in up to 70% of young children with ADEM, declining with age 7.
Key Distinguishing Features from MS
ADEM differs from multiple sclerosis by:
- Monophasic course (single episode) vs. relapsing-remitting 4
- Encephalopathy at presentation 7
- Absence of periventricular ovoid lesions and Dawson's fingers 7
- Typically no oligoclonal bands 7, 6
- Temporal relationship to infection/vaccination 7
Common Pitfalls
Do not confuse multiphasic presentations with MS - true ADEM is monophasic; recurrent demyelination suggests MOG-encephalomyelitis or MS 7. In adults, maintain high suspicion for alternative diagnoses given the worse prognosis and diagnostic complexity 5. Always exclude infectious encephalitis through appropriate CSF studies before attributing symptoms to ADEM 6.