MRI features of acute disseminated encephalomyelitis

Acute disseminated encephalomyelitis (ADEM) is an inflammatory demyelinating central nervous system disorder. Unlike multiple sclerosis (MS), ADEM is considered to be a monophasic inflammatory disorder (although early relapses are possible). Whether ADEM and MS are immunopathologically related or distinct has not been established. ADEM is an important model of hyperacute, post-infectious inflammatory demyelination. As there is increasing emphasis on early treatment of MS in children, it is increasingly important to discriminate between monophasic ADEM and chronic relapsing MS. As a consequence, there have been a number of studies attempting to differentiate ADEM from MS. This chapter will discuss the characteristic MRI features of ADEM, with particular emphasis on distinguishing it from the typical MRI of clinically isolated syndromes (CIS) and MS. Early reports were flawed due to the variable and inconsistent clinical definitions of ADEM, CIS, and MS. Indeed, ADEM was previously diagnosed when any child presented with acute polysymptomatic encephalitis with evidence of MRI disseminated hyperintense lesions [1–5]. Despite these limitations, these studies were able to describe features characteristic of monophasic demyelination (ADEM) and how it differed from MS. The features that were validated by at least two reports from these important early studies are presented in Table 18.1. Many of these characteristics are still considered typical of ADEM and are discussed in more detail later.