[Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) with a family history: an adult case].

We present an adult case of mild encephalitis/encephalopathy with a reversible splenial lesion (MERS). A 26-year-old man with a family history of MERS presented to our hospital owing to dysarthria and dysesthesia of the right side of his body. The duration of these symptoms was approximately 4 hours. T and diffusion weighted MRI showed high signal intensity lesions in the bilateral deep white matter. On a 3-week follow-up MRI, the lesions had completely disappeared. We attributed this clinical course and image findings to MERS. The patient had experienced similar symptoms at the age of 8 years old. Furthermore, his younger brother showed a similar clinical history and experienced a few recurrence events during the age of 9-16 years old. The mechanism of MERS remains controversial. However, similar to our case, there are some case reports with a family history. In addition, a previous report has confirmed the existence of a heterozygous variant in the myelin regulatory factor gene in patients with MERS. Some genetic factors may induce MERS, especially with extensive white matter lesions.