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Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is a clinicoradiologic syndrome which consists of a transient lesion usually located in the midline of the corpus callosum that shows restricted diffusion. The exact pathophysiology of this entity is still unknown. We present the case of a 9-year-old boy with a history of 3 days of headache and vomiting associated with an episode of syncope and mild hyponatremia. Magnetic resonance imaging (MRI) findings included a focal lesion with well-defined margins on the splenium of the corpus callosum with hyperintensity on T2 and diffusion-weighted images. The patient underwent full recovery, and a follow-up MRI 5 months later showed no signs of the initial lesion. In conclusion, MERS is part of the spectrum of cytotoxic lesions of the corpus callosum, which must be recognized as secondary to a several causes, to avoid an erroneous diagnosis and treatment of the patient. It should be emphasized the importance of continuing research on this type of process to clarify its pathophysiology as well as the long-term effects.

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