Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is an infection-associated encephalitis/encephalopathy syndrome that is predominately caused by a virus. MERS has no direct association with central nervous system (CNS) infections or inflammation. Non-CNS infections may cause reversible lesion in the splenium of corpus callosum. Recently, there have been reports of many patients with hyponatremia related MERS. Interleukin-6 (IL-6) was also found elevated in serum and in cerebrospinal fluid (CSF) in patients with MERS. The role of IL-6 in the non-osmotic release of vasopressin is crucial. Persistent hyponatremia may be linked to this effect. The following is a case report of MERS secondary to encephalitis, complicated by hyponatremia. We will summarize the latest research and progress regarding MERS.

A 31-year-old man was admitted to our department with a 5-day history of fever and headache. His initial diagnosis was encephalitis and hyponatremia; during this period the patient also developed MERS secondary to the encephalitis.

Encephalitis was diagnosed by reviewing the history of fever, headache, neck rigidity and Kerning sign (+) on clinical examination. Lab tests revealed: serum VCA IgG (+), EBNA-1 IgG (−), EBV IgM (−), and inflammation in the analysis of CSF. Cranial MRI+C showed that the blood vessels on the surface of the brain were obviously increasing and thickening and diffuse slow waves were detected on the electroencephalogram (EEG). The patient's hyponatremia aggravated on the third day of hospitalization. On the fourth day of hospitalization, the patient was somnolent, apathetic, and slow. Magnetic resonance imaging (MRI) of the brain, with a T2-weighted fluid attenuated inversion recovery image, showed high-signal intensity in the splenium of the corpus callosum (SCC) on the fifth day of hospitalization. Diffusion-weighted imaging (DWI) showed splenial hyperintensity as a “boomerang sign” and reduced diffusion on apparent diffusion coefficient (ADC) maps. Cranial MRI findings returned to normal after 1 month. The diagnosis of MERS was confirmed.

We administered an intravenous drip infusion of acyclovir and prescribed oral sodium supplementation.

The patient's neurological symptoms gradually improved. The MRI lesion in the SCC disappeared on the 30th day.

In patients with encephalitis accompanied by hyponatremia, elevated IL-6 or urinary β2-microglobulin (β2MG), and exacerbations such as sudden somnolence, delirium, confusion, and seizures, the possibility of secondary MERS should be investigated, in addition to the progression of encephalitis.