Acute disseminated encephalomyelitis (ADEM)
General description
Acute disseminated encephalomyelitis (ADEM) is an autoimmune inflammatory demyelinating disease that typically occurs following a viral infection or vaccination. It develops acutely, causing inflammation in the brain and spinal cord, leading to various neurological dysfunctions. ADEM is characterized by initial multifocal inflammatory demyelination of the central nervous system and generally follows a monophasic course. Common triggers include infections such as measles, rubella, influenza, human herpesvirus 6, mycoplasma, and campylobacter.
ADEM predominantly affects children, with the most common age of onset between 3 and 9 years. However, it can also occur in adults, although less frequently. The annual incidence ranges from 0.4 to 0.68 cases per 100,000 individuals. While ADEM can also develop following vaccination, the incidence is estimated to be 1 to 3.5 cases per million vaccinations.
After infection or vaccination, the disease typically develops rapidly, following an incubation period of 1 to 3 weeks. Common symptoms include fever, headache, vomiting, disorientation, and seizures, along with paralysis, visual impairment, and psychiatric symptoms. Symptoms usually progress quickly, often reaching their peak within a few days. In most cases, ADEM is monophasic and mild, and in some instances, it may resolve spontaneously.
Cerebral lesion
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CerebrumCerebral cortex
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CerebrumCerebral white matterSubcortical white matter
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CerebrumCerebral white matterDeep white matter
MRI typically shows bilateral asymmetric T2WI and FLAIR hyperintensity in the cerebral cortex and subcortical/deep white matter, with less frequent involvement of the periventricular white matter. The corpus callosum is usually spared. The basal ganglia and thalamus may also be affected. While white matter lesions tend to be asymmetric, lesions in the basal ganglia and thalamus are typically symmetric.
Basal ganglia and thalamus lesion
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Putamen
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Globus pallidus
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Caudate nucleus
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Thalamus
In contrast to white matter lesions, the lesions of the basal ganglia and thalamus are typically symmetric.
Brainstem and cerebellar lesion
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Brainstem
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Middle cerebellar peduncle
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Cerebellum
The brainstem and cerebellum are also commonly affected in ADEM. MRI typically reveals bilateral asymmetric T2WI and FLAIR hyperintensities, with multiple disseminated lesions in both the brainstem and cerebellum.
Spinal lesion
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Spinal cordSpinal gray matter
MRI sometimes reveals swelling of the spinal cord, which, when enlarged, tends to appear round rather than oval. T2WI typically show hyperintensity predominantly in the gray matter rather than the white matter. On sagittal T2WI, long cord involvement is often observed, serving as an important differential point from Multiple sclerosis (MS).
Reversible corpus callosum lesion
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Corpus callosumSplenium
ADEM occasionally presents with hyperintensity on DWI in the splenium of the corpus callosum, similar to what is seen in Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS). However, unlike the lesions in MERS, ADEM typically does not show a decreased apparent diffusion coefficient (ADC) value.