Clinical meaning
Acute disseminated encephalomyelitis (ADEM) is an immune-mediated inflammatory demyelinating disease of the central nervous system (CNS) that typically occurs following a viral infection or, rarely, vaccination. It predominantly affects children aged 5 to 8 years, although it can occur at any age. ADEM is characterized by a monophasic (single-episode) course of widespread demyelination affecting the brain and spinal cord, producing multifocal neurological deficits, encephalopathy (altered consciousness), and often dramatic clinical presentations that require urgent neurological evaluation and aggressive immunosuppressive therapy.
The pathogenesis of ADEM involves molecular mimicry and post-infectious autoimmunity. During a preceding viral infection (most commonly upper respiratory tract infections, influenza, Epstein-Barr virus, cytomegalovirus, herpes simplex virus, measles, mumps, rubella, or varicella), the immune system generates T-lymphocytes and antibodies directed against viral antigens. Due to structural similarity (molecular mimicry) between certain viral epitopes and myelin proteins (particularly myelin basic protein [MBP], myelin oligodendrocyte glycoprotein [MOG], and proteolipid protein [PLP]), these immune cells cross-react with the host's own myelin sheaths. The activated autoreactive T-cells cross the blood-brain barrier, recruit macrophages and additional inflammatory cells, and initiate a widespread inflammatory attack on CNS myelin.
The histopathology of ADEM reveals perivenous (around small veins and venules) sleeves of demyelination with dense inflammatory infiltrates consisting of activated T-lymphocytes, macrophages laden with myelin debris, and reactive astrocytes. Unlike multiple sclerosis (MS), which produces well-demarcated plaques of demyelination at different stages (some old, some new), ADEM lesions are characteristically all of the same age (reflecting a single immunological event) and are widely distributed throughout the white matter of both cerebral hemispheres, brainstem, cerebellum, and spinal cord. Gray matter involvement (basal ganglia, thalamus, cortex) can also occur in ADEM, which helps distinguish it from MS where gray matter involvement is less prominent on MRI.