Introduction
Acute disseminated encephalomyelitis (ADEM), also called secondary or perivenous encephalitis, is an acute, immune-mediated inflammatory demyelinating disease of the brain and spinal cord. It appears days to a few weeks after an infection or a vaccination and is usually a single, monophasic episode. A good answer defines ADEM and places it among the post-infectious encephalitides, then covers its triggers and pathogenesis, clinical picture, diagnosis and differential diagnosis, treatment, and prognosis.
Definition and classification
ADEM is not a direct infection of neural tissue. The triggering virus or vaccine antigen is usually cleared or absent from the CNS, and an aberrant immune response attacks myelin. Pathologically, there is perivenous inflammation with demyelination of the white matter, oedema, and sometimes small haemorrhages. Classification is based on the trigger:
- Post-infectious ADEM: after measles, varicella, mumps, rubella, influenza, Epstein-Barr virus, Mycoplasma pneumoniae and streptococcal infections, among others.
- Post-vaccinal ADEM: reported after rabies vaccines (especially older nerve-tissue preparations), measles-containing, influenza, pertussis and rarely other vaccines. The causal link is rare and much less frequent than the risk after natural infection.
- Monophasic course (typical) versus a rare recurrent form; the fulminant acute haemorrhagic leukoencephalitis (Hurst) is a severe variant.