[Mild encephalitis/encephalopathy with a reversible splenial corpus callosum lesion (MERS)]

Mild encephalitis/encephalopathy with a reversible isolated splenial corpus callosum lesion (MERS) is a clinical-radiological syndrome characterized by a lesion in the center of the splenium identified by magnetic resonance imaging (MRI), the imaging test of choice.

A 31-year-old man is admitted with fever, intense hemicranial headache, disorientation, dysarthria and paresthesia in the lips and both upper extremities is presented; and that he is admitted for a suspected diagnosis of viral encephalitis. The cerebrospinal fluid analysis shows an elevation of proteins and the electroencephalogram shows generalized slowing, predominantly on the left. MRI shows a well-defined ovoid lesion, isolated in the splenium of the corpus callosum, homogeneous and hyperintense on T2 and FLAIR, with restriction to fluid diffusion and without uptake after gadolinium administration. The patient received empirical treatment with acyclovir and corticosteroids, with good subsequent evolution and disappearance of the lesion described in the control MRI at 3 weeks.

MERS is a benign, infrequent entity of unknown pathogenesis, which must be differentiated from other pathologies that present with lesions of the corpus callosum, but with an unfavorable prognosis.