Acute disseminated encephalomyelitis (ADEM)

Acute disseminated encephalomyelitis is, as the name would suggest, acute inflammation and demyelination of white matter typically following a recent (1 - 2 weeks prior) viral infection or vaccination. Grey matter, especially that of the basal ganglia, is also often involved, but to a lesser extent, as is the spinal cord. It is usually a monophasic illness, although within the episode individual lesions may be of varying stages of evolution.

Typically ADEM presents in children, however cases in all ages have been reported.

Appearances vary from small 'punctate' lesions to tumefactive regions, which however have less mass effect than one would expect for their size. Compared to multiple sclerosis, involvement of the callososeptal interface interface is unusual. Lesions are usually bilateral but asymmetrical.

On CT the lesions are indistinct regions of low density within the white matter, which may demonstrate ring enhancement.

MRI is far more sensitive, demonstrating regions of high T2 signal, with surrounding oedema. Punctuate, ring or arc enhancement is often demonstrated along the leading edge of inflammation. The center of the lesion, although high on T2 and low on T1 does not have increased restriction on DWI (c.f. cerebral abscess), nor however does it demonstrate no signal as one would expect from a cyst. This is due to increase in extracellular water in the region of demyelination.

Absence of enhancement does not exclude the diagnosis.

Complete recovery within one month is most common (50-60%) with sequelae (most commonly seizures) seen in a significant proportion of cases (20-30%). Death during the acute phase is seen in 10 - 30% of patients. Once recovered relapses are rare.

In a small proportion (2%) the course is more fulminant, usually resulting in death. In such cases the lesions demonstrated haemorrhage and the condition is then known as Acute hemorrhagic leukoencephalopathy.

Relapsing forms of ADEM are certainly described (Relapsing Disseminated Encephalomyelitis - RDEM, and Multiphasic Disseminated Encephalomyelitis - MDEM), although the demarcation between these and relapsing-remmitting MS is contentious.