What are the epidemiology, clinical manifestations, and seizure pathophysiology of acute disseminated encephalomyelitis (ADEM)?

ADEM: Epidemiology, Clinical Manifestations, and Seizure Pathophysiology

Epidemiology

ADEM is a rare autoimmune demyelinating disease with an estimated incidence of 0.5-0.64 per 100,000 children per year, predominantly affecting young children with a median age of 4-5 years 1, 2. The disease shows no clear sex predominance, though some studies report slight male preponderance 2. ADEM occurs significantly more frequently in children than adults, with up to 70% of pediatric acquired demyelinating disease potentially being MOG-antibody associated in young children, declining with age 3. In adults, MOG-encephalomyelitis represents ≤1% of cases in Western countries and ≤5% in Asian populations 3.

The disease demonstrates seasonal variation with higher incidence during colder months 2. Most cases are preceded by an infectious trigger (occurring in approximately 40-60% of patients) or less commonly vaccination (within 4 days to 4 weeks post-immunization) 4, 2, 5. Human herpesvirus 6 has been specifically implicated in multiple cases 2.

Clinical Manifestations

The classic presentation includes acute onset of encephalopathy (altered mental status, confusion, behavioral changes) combined with multifocal neurological deficits 6, 5. The most common clinical features in order of frequency are:

• Motor deficits (75%) - including gait disturbances, weakness, ataxia 4
• Altered consciousness/encephalopathy (33-100%) - required for diagnosis 6, 4
• Seizures (33%) - a prominent feature that warrants MOG-IgG testing 3, 4
• Polyfocal neurological deficits - cranial nerve palsies, sensory changes 1
• Fever and headache - common prodromal symptoms 7

Seizures in ADEM are specifically highlighted as an indication for MOG-IgG antibody testing when accompanied by radiological signs of demyelination 3. Other severe manifestations include acute respiratory insufficiency, disturbance of consciousness, and behavioral changes 3.

Approximately 25% of children with ADEM require intensive care admission, with median ventilation duration of 3 days 1. Severe cases may present with persistent vegetative state, though full recovery remains possible even after prolonged encephalopathy 7.

Seizure Pathophysiology

Seizures in ADEM result from acute inflammatory demyelination causing cortical and subcortical dysfunction, with the pathophysiology involving multiple immune-mediated mechanisms:

Primary Mechanisms:

1. Molecular Mimicry and Autoimmunity 8, 5

   • Post-infectious immune response targets myelin antigens (particularly MOG)
   • Cross-reactivity between infectious epitopes and CNS myelin proteins
   • Loss of immune tolerance leading to autoreactive T-cell and B-cell activation

2. Blood-Brain Barrier Disruption 5

   • Inflammatory cascade compromises BBB integrity
   • Allows peripheral immune cell infiltration into CNS parenchyma
   • Creates environment for neuronal hyperexcitability

3. Neuroinflammatory Cascades 5

   • Cytokine release (IL-6, TNF-α, IFN-γ) alters neuronal excitability
   • Complement activation and IgG deposition cause direct tissue damage 3
   • Microglial activation perpetuates inflammatory response

4. Demyelination-Induced Cortical Irritability

   • Lesions predominantly affect deep white matter, basal ganglia, thalamus (58%), and cortical/subcortical areas (33%) 4
   • Disruption of normal inhibitory circuits
   • Altered ion channel function in demyelinated regions

The combination of acute inflammation, demyelination, and direct neuronal injury creates epileptogenic foci, particularly when lesions involve cortical gray matter or deep gray structures 4. Unlike chronic epilepsy, ADEM-related seizures typically resolve with immunotherapy as inflammation subsides.

Diagnostic Considerations for Seizures:

When seizures occur with ADEM, radiological signs of demyelination are required to distinguish from pure encephalopathy 3. MRI typically shows multiple T2/FLAIR hyperintense lesions in characteristic distributions, though CSF may be normal in up to 50% of cases 4, 2. The absence of CSF pleocytosis does not exclude ADEM 7.

Clinical Pitfalls:

• Do not confuse ADEM with encephalopathy alone - true ADEM requires evidence of CNS inflammation (CSF abnormalities, MRI changes, or pathologic confirmation) 6
• Seizures with symmetric white matter changes may represent atypical ADEM requiring extended immunotherapy trials 7
• Multiphasic presentations can mimic multiple sclerosis - recurrent ADEM or ADEM-ON patterns warrant MOG-IgG testing 3