Imaging of Acute Disseminated Encephalomyelitis
Section snippets
Pathogenesis
The pathogenesis of ADEM is debated despite the large number of theories proposed, even though there is general consensus that ADEM is an immune-mediated disorder resulting from an autoimmune reaction to myelin. The most compelling evidence perhaps is represented by the similarities of ADEM to experimental allergic encephalomyelitis, in which damaged myelin exposes segregated antigens that in turn induce an immune response. Two basic mechanisms currently are under consideration [2]:
- 1.
Inflammatory
Clinical and laboratory findings
ADEM most commonly affects young and adolescent children, with a 0.8 per 100,000 population per year incidence and no gender preponderance [2]. A greater prevalence during the winter months is recorded, probably in association with a higher incidence of upper airway infections in winter [5]. It commonly is accepted that the incidence in adult and elderly age groups is significantly lower [2].
The clinical presentation of ADEM is widely variable, ranging from an acute presentation with seizures
Neuroimaging
The neuroradiologic findings of ADEM are not specific and typically do not allow differentiation of ADEM from MS or encephalitis. CT typically is unrevealing unless large lesions are present, in which case a faint hypodensity may be detected (Fig. 1). As a consequence, in presence of suggestive clinical findings MRI should be obtained.
Conventional MRI findings typically include multiple, asymmetrically distributed, poorly marginated, hyperintense areas on T2-weighted and fluid-attenuated
Variants of acute disseminated encephalomyelitis and classification issues
Although ADEM is, by definition, a monophasic disease, studies show that as many as one third of patients experience relapses in the future [8]. It still is problematic to distinguish these cases from a relapsing-remitting form of MS and to predict which patients will suffer from recurrent disease bouts. Moreover, although lesions of ADEM typically are reversible, more aggressive variants exist. Therefore, it has become clear that there is a broad spectrum of central nervous inflammatory and
Acute relapsing disseminated encephalomyelitis, biphasic disseminated encephalomyelitis, and multiphasic disseminated encephalomyelitis
The real significance of the relapsing-remitting variants of ADEM still is questioned in that they may represent intermediate forms between ADEM and MS; this would strengthen the concept of a common pathophysiologic spectrum [20], [21]. According to the proposals of the International Pediatric MS Study Group [22], the definition of acute relapsing disseminated encephalomyelitis (ARDEM) requires a second attack more than 3 months after the initial event (1 or more months after steroid
Acute hemorrhagic encephalomyelitis
Lesions of ADEM may be explained prevalently by a microvascular inflammatory pathologic mechanism. When the degree of inflammation leads to a stage of necrotizing vasculitis, a hyperacute form of ADEM characterized by ball, ring-shaped, or confluent hemorrhages and necrosis of vascular wall with fibrin deposition and neutrophilic infiltration. Clinical presentation of acute hemorrhagic encephalomyelitis (AHEM) is severe with reduced consciousness, hemiparesis, aphasia, brainstem dysfunction,
Acute necrotizing encephalopathy
AHEM seen in Western populations probably has some relationship with acute necrotizing encephalopathy of childhood (ANEC), a rare devastating complication of influenza and other viral infections that prevails in Eastern Asia and has a bleak prognosis with elevated mortality. ANEC has been related to intracranial cytokine storms causing blood-brain barrier damage that results in edema and necrosis, without signs of direct viral invasion or parainfectious demyelination. MRI findings involve a
Differential diagnosis
When assessing patients who have suspected ADEM, the most obvious concern is the differentiation from an onset of MS. This is a major problem for which there seems to be no established solution based on current knowledge. Studies show that approximately one fourth of patients who have ADEM develop a classical relapsing form of MS in the subsequent 2 to 5 years [27], but these patients are difficult to identify on initial presentation. Several clinical, biologic, and radiologic differences
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