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We retrospectively collected three patients with clinically mild encephalitis with a reversible splenial lesion (MERS) after mumps vaccination, and reviewed five patients, including two patients previously reported. The five patients (all males, aged 1 to 9) presented with fever, vomiting, or headache as the initial symptoms (day 0), suggesting meningitis, at 13 to 21 days after mumps vaccination. Consciousness disturbance, delirious behavior, seizures, or dysarthria was observed on days 1 to 3, which had completely resolved before day 11. Hyponatremia was observed in all patients. A cerebrospinal fluid study showed pleocytosis, and confirmed the vaccine strain genome. MRI revealed reduced diffusion in the splenium of the corpus callosum on days 2 to 4, which had completely disappeared on the follow-up studies performed on days 7-15. EEG showed high voltage slow wave in three patients, which later normalized. These findings led to a diagnosis of MERS after mumps vaccination. MERS after mumps vaccination may be more common than previously considered. MERS is suspected when a male patient after mumps vaccination presents with neurological symptoms with hyponatremia, following symptoms of aseptic meningitis, and MRI would be performed to examine the splenium of the corpus callosum., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Four patients with encephalitis/encephalopathy and parenchymal lesions accompanying reversible splenial lesions were retrospectively evaluated. In 3 patients, reversible lesions with transiently reduced diffusion were seen in the splenium and symmetrically in the peripheral frontoparietal white matter, clinical signs and symptoms were mild, and recovery was complete. These and previous observations suggest a less severe course and outcome for patients with reversible lesions isolated to the splenium or to the splenium and peripheral frontoparietal white matter.