Histologically, the area of myelin breakdown is sharply demarcated and displays extensive loss of oligodendrocytes, infiltration with foamy macrophages and reactive astrocytosis.

At MR the lesion is characterized by T2 hyperintensity in a central pontine location (Fig.1); gadolinium enhancement may sometimes be present. Extrapontine sites of myelinolysis in the basal ganglia and midbrain, and subcortical white matter may also occur.

Many entities can be considered in the differential diagnosis, including ischaemia, multiple sclerosis, encephalitis, toxic exposures, radiation therapy effects and brain stem gliomas. When present, the key to the MRI diagnosis is the bilateral involvement of the deep and capsular white matter, often accompanied by abnormalities in the thalami and basal ganglia.