A case of clinically mild encephalitis with a reversible splenial lesion (MERS) after mumps vaccination.

We describe for the first time an 8-year-old male patient who demonstrated clinically mild encephalitis with a reversible splenial lesion after mumps vaccination. He suffered from transient hallucinations, nuchal rigidity, and inappropriate antidiuretic hormone secretion syndrome. On the 5th day of admission, his head MRI showed symmetrical high-signal-intensity lesions on T2, FLAIR, and diffusion-weighted images in the splenium of the corpus callosum and in the periventricular white matter, while an apparent diffusion coefficient map showed reduced diffusion. The images were not enhanced by gadolinium. Follow-up MRI on the 16th day of admission revealed none of these abnormalities. His serum IgM and IgG antibodies against the mumps virus were positive according to an enzyme immunoassay. Mumps Torii vaccine strain was isolated from the patient's cerebrospinal fluid. Previous reports demonstrated that transient delirious behavior, the syndrome of inappropriate antidiuretic hormone secretion, and good prognosis were the main clinical features of mild encephalitis with a reversible splenial lesion. This case shows that mild encephalitis with a reversible splenial lesion could occur after mumps vaccination.