Acute Disseminated Encephalomyelitis (ADEM) is a brief but intense attack of inflammation (swelling) in the brain and spinal cord and occasionally the optic nerves that damages the brain’s myelin (the white coating of nerve fibers). Other terms used to refer to ADEM include post-infectious encephalomyelitis and immune-mediated encephalomyelitis.
ADEM is sometimes difficult to distinguish from multiple sclerosis (MS) because the symptoms common to both "demyelinating" disorders include loss of vision, weakness, numbness and loss of balance. Both ADEM and MS involve immune-mediated responses to myelin in the brain and spinal cord.
The cause of ADEM is not clear but in more than half of the cases, symptoms appear following a viral or bacterial infection, usually a sore throat or cough and very rarely following vaccination. ADEM is thought to be an autoimmune condition where the body’s immune system mistakenly identifies its own healthy cells and tissues as foreign and mounts an attack against them. This attack results in inflammation. Auto-antibodies directed against a component of myelin called myelin oligodendrocyte glycoprotein (MOG) are found in 36-64% of children with ADEM. Most cases of ADEM begin about 4 to 14 days after an infection. In some cases of ADEM, no preceding event is identified.
Although ADEM can appear at any age, children are more likely than adults to develop it. More than 80 percent of childhood cases occur in patients younger than 10 years. Most of the remaining cases occur between the ages of 10 and 20 but ADEM is sometimes (rarely) diagnosed in adults. ADEM is thought to affect 1 in every 125,000 to 250,000 individuals in a given year. Five percent of these cases could be associated with vaccination.
- ADEM appears a little more frequently in boys than girls (about 60 percent of the time) with a male to female ratio of 1.3:1.
- ADEM appears more often in winter and spring.
- ADEM occurs in all parts of the world and in all ethnic groups.