Mild Encephalitis/Encephalopathy with a Reversible Splenial Lesion in an Adult with Cerebellar Ataxia: A Case Report.

Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is a clinicoradiological syndrome characterized by transient mild encephalopathy and magnetic resonance imaging (MRI) findings of a reversible lesion in the splenium of the corpus callosum (SCC). Patients with MERS generally present with central nervous system symptoms such as consciousness disturbance, headache, and seizure; adult-onset MERS with cerebellar ataxia is rare. A 53-year-old man was admitted to our hospital with fever of 1 week's duration, headache, neck stiffness, and gait disturbance. Neurological examination revealed bilateral intention tremor (predominantly affecting the right hand) and gait ataxia. Diffusion-weighted brain MRI showed a focal hyperintense lesion in the SCC. Cerebrospinal fluid analysis revealed elevated levels of mononuclear cells and proteins. Brain imaging with 123I-iofetamine single-photon emission computed tomography showed reduced cerebral blood flow in the left thalamus and right cerebellum. Several diseases, including cerebellar stroke and acute cerebellitis, develop as comorbidities in patients with acute cerebellar ataxia. This case suggests that MERS should be suspected in adults with cerebellar ataxia.