Imaging description

Acute disseminated encephalomyelitis (ADEM) is an immune-mediated disorder of the CNS characterized by an inflammatory reaction and demyelination that predominantly involves the white matter of the brain and spinal cord [1]. ADEM is usually precipitated by a viral infection or vaccination. There is no specific biologic marker for ADEM; therefore the diagnosis is based on clinical and imaging features.

On MRI, ADEM lesions appear hyperintense on T2-weighted and FLAIR, and hypo- to isointense on T1-weighted imaging. Diffusion-weighted imaging (DWI) signal is variable, as is enhancement on post-contrast images, depending on the stage and degree of inflammation. The most common MRI pattern consists of multiple, 1–2cm, bilateral and asymmetric lesions scattered throughout the cerebral hemispheres, cerebellum, brainstem, and occasionally the spinal cord (Fig. 19.1) [2]. The cortex is spared in the majority of cases but the deep gray matter nuclei are frequently involved (Fig. 19.2).

Sometimes ADEM presents with “tumefactive” lesion(s) that show(s) perilesional edema. Solitary lesions are uncommon but they require differentiation from primary tumors and infarcts (Fig. 19.3). Acute hemorrhagic ADEM is uncommon and presents with multiple hemorrhagic lesions. Sequential MRIs are important in establishing the diagnosis of ADEM; in most cases lesions show partial and complete resolution within a few months. ADEM is not expected to develop new lesions within the short-term follow-up period, although recurrent forms of ADEM have been described.