ACUTE DISSEMINATED ENCEPHALOMYELITIS
Acute disseminated encephalomyelitis (ADEM) is an immune-mediated central nervous system (CNS) disorder. Patients with ADEM have multifocal inflammatory (but noninfectious) lesions within the brain or spinal cord. There is usually a history of a preceding upper respiratory infection or immunization within a few weeks of onset of the neurological manifestations. The pathophysiology likely involves an excessive immunological response that attacks myelin. The histological features include infiltration of monocytoid cells and perivenous demyelination. Patients with ADEM usually experience a sudden onset of multifocal neurological signs and symptoms. The CNS lesions may completely subside within a few weeks, or progress to glial scars. Important considerations in the diagnosis of ADEM include the clinical presentation, cerebrospinal fluid (CSF) analysis, neuroimaging studies, and the absence of another definable cause of the symptoms. The clinical differential diagnosis includes multiple sclerosis, leukodystrophy, and mitochondrial encephalomyopathy. ADEM is a monophasic disorder. Multiphasic disseminated encephalomyelitis refers to a relapsing form.1–4
In temperate climates, ADEM most often occurs in the winter and spring. Because the precipitating infection has usually resolved by the time neurological symptoms occur, microbiological examinations are often negative. ADEM can follow infections with various organisms, such as Epstein-Barr virus, Mycoplasma pneumoniae, influenza, measles, pertussis, cytomegalovirus, and varicella-zoster virus. CSF evaluation often demonstrates mild pleocytosis and elevation of protein. The most common neurological features of ADEM are motor deficits such as ataxia, paraparesis, hemiparesis, or monoparesis. Altered consciousness is also common. Patients with spinal cord involvement may have paraparesis, sensory deficits, and/or urinary retention. Complete clinical recovery occurs in most patients with ADEM. The most commonly employed treatment is intravenous corticosteroids. Intravenous immune globulin is an alternative therapy.
MRI of patients with ADEM demonstrates multifocal hyperintense lesions on T2-weighted and fluid-attenuated inversion recovery (FLAIR) images (Figure 17-1). These hyperintense foci predominantly represent inflammatory edema. Because there is usually little or no myelin breakdown, proton MR spectroscopy (MRS) demonstrates normal choline levels in ADEM lesions. This is in contradistinction to some of the important considerations in the differential diagnosis that are associated with demyelination and have elevated choline levels, such as acute-phase multiple sclerosis and various leukodystrophies. Lactate levels are also usually normal in ADEM; lactate is often elevated in mitochondrial encephalopathies and in some acute plaques of multiple sclerosis. N-Acetylaspartate (NAA) is sometimes decreased in the active phase of ADEM, with subsequent complete or partial normalization. The lesions of ADEM usually have increased water motion on diffusion-weighted images, that is, high signal intensity on apparent diffusion coefficient (ADC) images. The presence of extensive areas of restricted diffusion in a patient with ADEM indicates a high likelihood for progression to encephalomalacia and gliosis. There is usually restricted diffusion in acute multiple sclerosis plaques and in brain lesions due to vasculitis.1,5,6
ADEM in 4 different children.
A. A FLAIR image of a 2-year-old child with seizures shows ...