Acute disseminated encephalomyelitis pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
Pathophysiology
- Pathology shows scattered foci of demyelination and perivenular inflammation throughout the brain and spinal cord.
- The foci may be 0.1 to several millimeters, and usually surround small and medium sized veins.
- The nerves cells and axons are generally intact
- Perivenular inflammation is manifested by cellular reaction by pleomorphic microglia, in the region of demyelination, and lymphocytes and mononuclear cells about the vessel. Multifocal meningeal cellular infiltration may also be present.
- Peripheral nervous system demyelination is present in a subset
- Postvaccinial disease may occur after:
- Rabies vaccine (also known as a neuroparalytic accident)
- Noted mostly only after use of the Semple vaccine, which uses phenol-inactivated virus propagated in brains of adult rats, and occurs in 1-10 per 5000 given this vaccine; affected patients have a mortality rate of 25%. This vaccine has been replaced in most developed countries such as the U.S. by vaccine grown in duck embryos or human diploid tissue, which has largely eliminated this complication.
- Smallpox vaccine
- Eradication of smallpox has eliminated need for prophylaxis
- Occurred in 1 in 4000 vaccinations
- Measles vaccine
- ADEM may occur after live measles vaccination very rarely, in approximately one in one million vaccinations
- Japanese B encephalitis vaccine
- Rare complication, perhaps in one in one million
- Postinfectious encephalomyelitis may be seen after a number of infectious processes, most commonly the viral exanthems of childhood.
- Infectious versus post-infectious viral associated encephalomyelitis is not always easy to distinguish clinically, though the sequence of presentation is usually helpful. Viral associated encephalomyelitis pathologically shows viral invasion of neurons, and the virus may be cultured. Postinfectious encephalomyelitis spares neurons, but demyelination and perivenular inflammation is prominent.
- Measles viral infection is one of the most common associations with ADEM: ADEM occurs in about 1 in a 1000 cases. 10-20% of those affected die, and 10-20% are left with persistent neurologic sequelae.
- Vaccination with a live measles vaccine in developed countries has made this uncommon in the developed countries. ADEM may occur after live measles vaccination very rarely, in approximately one in one million vaccinations.
- Varicella-zoster virus (VZV)/Chickenpox
- 0.1-0.7% of chickenpox infections may be associated with neurologic manifestations; many of these are mild and self-limited, though these patients may present with ADEM
- Prominent cerebellar features are common
- Temporal relationship between skin and neurologic findings is quite variable
- Mumps
- Rubella
- Smallpox
- Infectious mononucleosis – Epstein-Barr virus’’’
- Mycoplasma
- Cytomegalovirus
- Influenza
- Parainfluenza
- Rabies vaccine (also known as a neuroparalytic accident)
- The pathogenesis of ADEM has not been completely worked out, but the demyelination and inflammation appear to occur due to immune reactivity to self-antigens, induced either via mimicry or adjuvant effects, in association with exposure to infectious or other environmental antigens.
- Myelin basic protein (MBP) is one of the antigens involved. After measles and rabies vaccinations, MBP-reactive T cells and antibodies have been documented in the cerebrospinal fluid (CSF) of some patients. Induction of immune response to other central nervous system constituents has also been documented following measles infection, but only development of antibodies to MBP appears thus far to correlate with the development of ADEM.
- An animal model has been developed, experimental allergic encephalomyelitis, produced by inoculating animals with a mixture of sterile brain tissue and adjuvants. About 8-15 days after inoculation, a similar neurologic disease develops, with similar pathologic findings.
- Some cases of postvaccinial enchephalomyelitis may occur as a consequence of exposure to brain material that contaminates viral vaccines.
- Other cases may occur as a consequence of direct viral central nervous system (CNS) invasion, though attempts to document viral invasion have been unsuccessful, and the pathology is different from that seen in infectious encephalomyelitis.