Östliche Pferdeenzephalomyelitis

Magnetic
resonance images (MRIs) and computed tomography (CT) neuroradiographs showing lesions in brains of 3 children with eastern equine encephalitis. A) Results of noncontrast CT scan of the brain of patient 12 on hospital day 2; the neuroradiograph shows subtle hypoattenuation of the left caudate head (arrow) and diencephalic region. B) Axial fluid attenuated inversion recovery (FLAIR) image from brain MRI scan of patient 14 on hospital day 2; the image shows abnormal T2 hyperintense regions of the bimesial temporal regions (thick arrows) with accompanying abnormal T2 hyperintense regions of the dorsal pontomesencephalic regions (thin arrows). C, D) FLAIR images from brain MRI scan of patient 15 on hospital day 3. C) Abnormal T2 hyperintense caudate and thalamic nuclei, most prominent on the right (arrow). D) Abnormal T2 hyperintense regions are most prominent in the right parietotemporal gray matter (arrow) and subcortical white matter but are also seen scattered throughout.
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Eastern equine encephalitis (EEE) is one of many viral encephalitides and results from infection with the eastern equine encephalitis virus.

Clinical presentation

Most patients have non-specific viral prodromal symptoms for approximately one week, including fevers, headache, nausea and vomiting, and malaise . This is followed by the emergence of neurological features, such as confusion, focal neurological signs, generalized or focal seizures, meningism, and somnolence . These neurological features rapidly progress and the patient's condition typically deteriorates within days to become stuporous or comatose .

Pathology

EEE is caused by the eastern equine encephalitis virus (EEEV), a single-stranded RNA alphavirus . Mosquitos and wild birds, especially in the east and Gulf coasts of the Unites States of America, are reservoirs for the virus, which is then spread by mosquitoes . Importantly, only approximately 5% of patients with EEEV infection develop EEE .

Radiographic features

Asymmetric bilateral basal ganglia and thalami involvement is classical regardless of CT or MR imaging . Less commonly, the internal and external capsules, brain stem, periventricular white matter, cortex, and leptomeninges may also be involved .

CT

Lesions may be subtle on CT, but are most often hypodense involving the basal ganglia and thalami. In large lesions, local mass-effect may also be appreciated .

MRI

MRI is the imaging modality of choice and is nearly always abnormal in patients with EEE . The most striking feature are asymmetric high T2-weighted signal intensities classically affecting the basal ganglia and thalami . It is thought that these lesions represent a combination of edema, inflammation, and ischemia . As with CT, in large lesions, local mass-effect may also be appreciated .

Uncommonly, and as aforementioned, there may be additional involvement of the adjacent internal and external capsules, the brainstem (especially the midbrain), the periventricular white matter, the cortex (especially the medial temporal lobes), and the leptomeninges (see leptomeningeal enhancement) . Particular involvement of the internal and external capsules, visualized as linear T2-weighted regions of high signal, has emerged as a sign that may be unique to EEE compared to other viral encephalitides, and has been described as the ‘parentheses sign’ .

In regards to other MR sequences, all of these lesions regardless of location may demonstrate low signal on T1 with variable contrast enhancement, and usually demonstrate iso-intense diffusion signal on DWI .

Treatment and prognosis

There is no specific antiviral treatment available for EEE, and thus supportive management (e.g. anti-epileptics, corticosteroids) is encouraged . Mortality is thought to be between 50-75%, however surviving infection with EEEV does confers lifelong immunity .

History and etymology

EEE was first recognized in the United States of America in horses from Virginia, Delaware, New Jersey, and Maryland during an equine epizootic in 1933 . The first human cases followed during a 1938 outbreak in Massachusetts, when there were 25 deaths from 34 cases .

Differential diagnosis

Other infectious causes that can cause a similar imaging pattern include the Flavivirus encephalitides:

Furthermore, other causes of increased T2 signal in the basal ganglia should also be considered, such as:

Siehe auch:
und weiter:
Assoziationen und Differentialdiagnosen zu Östliche Pferdeenzephalomyelitis:
Virusenzephalitis
 
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