Encephalitis, Myelitis, and Acute Disseminated Encephalomyelitis (ADEM) are rare but serious inflammatory conditions of the central nervous system. Encephalitis affects the brain, myelitis affects the spinal cord, and ADEM is an immune-mediated condition that damages myelin, the protective coating around nerve fibers. Because their clinical presentations can be very similar, particularly early in the illness, they are considered together.
Recognizing these conditions and classifying cases, based on diagnostic certainty, consistently is important for both clinical care and vaccine safety monitoring. To support this, the Brighton Collaboration developed the standardized case definitions for encephalitis, myelitis, and ADEM.
What Are Encephalitis, Myelitis, and ADEM?
Encephalitis is inflammation of the brain, and Myelitis is inflammation of the spinal cord. When both are affected, the condition is called Encephalomyelitis. Brain involvement typically presents as encephalopathy, and may also present as focal neurologic deficits. Spinal cord involvement (myelopathy) can cause limb weakness, loss of or altered sensation, loss of bladder or bowel control, and sexual dysfunction.
ADEM mainly affects the white matter of the brain but can also involve the spinal cord, and it is usually a single (monophasic) episode. Its clinical features can be identical to those of encephalitis, and the key distinguishing feature is demyelination, usually seen on brain MRI.
Very rarely, vaccines have been associated with these conditions, particularly live-attenuated viral vaccines.
When Should Encephalitis, Myelitis, or ADEM Be Suspected?
Encephalitis, myelitis, or ADEM should be suspected when a person develops:
- Encephalopathy, such as a depressed or altered level of consciousness, lethargy, or personality change lasting more than 24 hours
- Focal neurologic deficits referable to the brain, such as speech or language difficulties, cranial nerve abnormalities, visual field defects, weakness, or loss of coordination
- Signs of spinal cord dysfunction, such as limb weakness, a sensory level, or bladder or bowel dysfunction
In such cases, evidence of inflammation should be sought, including fever (38°C or higher), an increased number of white blood cells in the cerebrospinal fluid (CSF pleocytosis), and abnormal EEG or neuroimaging findings. A brain MRI showing diffuse or multifocal white matter lesions suggests ADEM.
The time interval between vaccination and onset of illness is not part of these case definitions; it is assessed separately when evaluating cases.
Why Standardized Case Definitions Matter
The Brighton Collaboration case definitions classify cases by level of diagnostic certainty. Level 3 is a possible case, Level 2 a probable case and level 1 a definite case. Level 4 is when there is insufficient information to meet Level 1-3, and level 5 is not a case. The video describes levels of diagnostic certainty for all three conditions.
Supporting Global Vaccine Safety Monitoring
Neurologic events are among the most serious and challenging adverse events reported following immunization. An event that follows vaccination may be temporally associated with, but is not necessarily caused by, the vaccine, which makes consistent case identification essential.
Standardized case definitions allow researchers, clinicians, and regulators to classify cases consistently across countries, study settings, strengthening global vaccine safety surveillance and improving confidence in vaccine safety systems.
Educational resources like this video help clinicians and public health professionals better understand encephalitis, myelitis, and ADEM and apply standardized definitions.
