Reversible Splenial Lesion Syndrome (Resles) in a Patient with Clinically Mild Tick-Borne Encephalitis and Hyponatremia

Introduction: Reversible splenial lesion syndrome (RESLES) is a clinicoradiological syndrome of varied etiology, characterized by transient lesions involving the splenium of the corpus callosum (SCC). Clinical presentation is nonspecific and depends on etiology. In the case of infectious disease the syndrome is also called mild encephalitis with reversible splenial lesion (MERS). Hyponatremia is often described in patients with RESLES. Here we present a patient case of RESLES/MERS in a patient with tick-borne encephalitis (TBE) accompanied by mild hyponatremia. Case Presentation: A 46-year-old man presented with malaise, fever, headache, discrete nuchal rigidity, brain stem symptoms (disturbance of ocular movements, gait ataxia) and mild cognitive and psychomotor impairment. Cerebrospinal fluid analysis at two different time points showed a lymphocytic pleocytosis and seroconversion for anti-TBEIgM/- IgG, serum biochemical analysis a mild hyponatremia. Magnetic resonance imaging (MRI) of the brain on day four after admission revealed a distinct signal hyperintensity on T2/FLAIR sequences in the SCC associated with diffusion restriction and low apparent diffusion coefficient (ADC) values on diffusion-weighted sequences. On T1-weighted images no contrast enhancement was detectable. Until the diagnosis of TBE the patient was treated with intravenous ceftriaxone, ampicillin and acyclovir. The patient recovered completely within three weeks. The T2/FLAIR hyperintense and diffusion-restricted lesion of the SCC was completely resolved ten days after the first MRI. Conclusion: TBE accompanied by hyponatremia may lead to RESLES/MERS, a clinicoradiological syndrome with reversible non-enhancing lesion of the SCC and excellent prognosis. Neuroradiological findings in RESLES are very similar to findings described in patients with osmotic demyelination syndromes like central pontine myelinolysis or extrapontine myelinolysis.