Acute Disseminated Encephalomyelitis (ADEM)
Incidence
In the pre-vaccination era, ADEM occurred in one out of 1000 measles infections, and 10 to 20 times less frequently with chickenpox and rubella infections. ADEM may also occur following vaccines, especially the measles, mumps, and rubella immunizations. The incidence of ADEM after the measles vaccine is estimated to be less than one per million, much less than the incidence in children infected with the measles virus itself. Therefore, the risk of developing ADEM after vaccination does not justify refusing immunization. ADEM affects younger children most commonly, though adolescents and adults may develop ADEM also. Boys and girls are affected equally.
Clinical Characteristics
Acute disseminated encephalomyelitis (ADEM) is a disorder affecting the white matter of the brain and spinal cord, usually occurring after an infection or immunization. ADEM previously occurred most commonly following such infections as measles, smallpox, and chickenpox. However, the institution of widespread immunizations in developed countries has significantly reduced the incidence of such childhood infections, and ADEM is now seen most frequently after non-specific upper respiratory tract infections. ADEM usually presents within three weeks of a preceding viral infection or vaccine with such non-specific symptoms as fever, vomiting, headache, stiff neck, muscle aches, and decreased appetite. Patients subsequently may develop confusion, stupor, or delirium; in severe cases, coma may follow. Specific neurological signs are often present on examination, such as clumsy walking, paralysis on one side of the body, or seizures. These symptoms may last for days, weeks, or even months in rare cases. ADEM is most likely an autoimmune disease caused by a virus. Viruses are believed to trigger a hyperactive immune state, attacking not just the virus, but also the patient's own cells. In ADEM, the myelin sheath is damaged, resulting in inflammation of the brain.
Precipitants
ADEM may follow a number of different viral infections, including measles, mumps, chickenpox, influenza, herpes, HIV, Epstein-Barr virus, and cytomegalovirus. ADEM may occur more rarely after vaccines, however, the specific cause often remains unidentifiable.
Provocation Tests
None
Diagnostic Procedures
It is a clinical diagnosis. The spinal fluid of patients with ADEM usually shows abnormally increased white blood cells and protein and increased viral antigens. An electroencephalogram (EEG), or brain wave test, often shows non-specific changes. Magnetic resonance imaging (MRI) is the most informative study, revealing white matter lesions in the brain characteristic of ADEM and only a few other rare conditions in childhood.