Encephalopathy with a reversible splenial lesion is associated with hyponatremia

We have encountered several patients with clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) associated hyponatremia. In order to better understand this phenomenon, Na levels were evaluated in a series of patients with MERS. Na was 131.8 ± 4.1 mmol/l (mean ± SD, range 121–140) in 30 patients with MERS; 138.3 ± 2.7 mmol/l (range 134–144) in age-matched 21 patients with upper respiratory infection; 136.6 ± 2.5 mmol/l (range 132–140) in nine patients with other type of encephalopathy; and 136.2 ± 2.6 mmol/l (range 132–140) in 17 patients with febrile seizures. Twenty-five of the thirty patients with MERS had Na < 136 mmol/l. There were significant differences between the Na levels of patients with MERS and those with other groups. It is not possible, from the clinical perspective, to completely separate MERS from hyponatremic encephalopathy or to rule out hyponatremia as a contributing factor of MERS.