Acute Disseminated Encephalomyelitis
Acute Disseminated Encephalomyelitis (ADEM) is a rare inflammatory demyelinating disease of the central nervous system. ADEM is thought to be an autoimmune disorder in which the body’s immune system mistakenly attacks its own brain tissue, triggered by an environmental stimulus in genetically susceptible individuals. More often it is believed to be triggered by a response to an infection or to a vaccination. For this reason, ADEM is sometimes referred to as post-infectious or post-immunization acute disseminated encephalomyelitis.
According to a study published in 2008, the estimated incidence in California is 0.4 per 100,000 population per year, and there are approximately 3 to 6 ADEM cases seen each year at regional medical centers in the US, UK, and Australia. ADEM is more common in children and adolescents than it is in adults, and there does not seem to be a higher incidence of ADEM among males or females, nor does there seem to be a higher frequency among any particular ethnic group.
Post-infectious In approximately 50-75 percent of ADEM cases, the inflammatory attack is preceded by a viral or bacterial infection. There have been a large number of viruses associated with these infections, including but not limited to: measles, mumps, rubella, varicella zoster, Epstein-Barr, cytomegalovirus, herpes simplex, hepatitis A, influenza, and enterovirus infections. A seasonal distribution has been observed showing that most ADEM cases occur in the winter and spring. The inflammatory attack and neurological symptoms often begin within a couple of weeks after the viral or bacterial illness.
Post-immunization Less than 5 percent of ADEM cases follow immunization. The association between an inflammatory attack following an immunization has been temporal and the direct connection between a vaccination and an immune attack has not been established. Post-vaccinial ADEM has been associated with immunization for: rabies, hepatitis B, influenza, Japanese B encephalitis, diphtheria/pertussis/tetanus, measles, mumps, rubella, pneumococcus, polio, smallpox, and varicella. Currently, the measles, mumps, and rubella vaccinations are most commonly associated with post-vaccinial ADEM. No infectious agent is isolated in most cases. The incidence of ADEM associated with the live measles vaccination is 1 to 2 per million. Neurologic symptoms typically appear 4 to 13 days after a vaccination.