According to a study published in 2008, the estimated incidence of ADEM 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.¹ 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 syndrome and neurological symptoms often begin within a couple of weeks after the viral or bacterial illness. No infectious agent is isolated in most cases.

Post-immunization: Less than 5 percent of ADEM cases follow immunization.¹ Although a temporal association between immunization and inflammatory demyelination has been reported, a direct causal relationship has not been proven. Post-vaccinal 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-vaccinal ADEM. 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.