Adult-onset of mild encephalitis/encephalopathy with reversible splenial lesion (MERS): case report and systematic review

Background Mild encephalitis/encephalopathy with reversible splenial lesion (MERS) is a clinical-radiological syndrome characterized by transient central nervous system symptoms and a reversible lesion in the splenium of the corpus callosum (SCC) on magnetic resonance (MR). We reported a case of adult-onset MERS with uncommon presentation and reviewed the existing literature. Methods We described a case of adult-onset MERS with uncommon symptoms and signs and performed a systematic review of case series including more than four patients of adult-onset (> 14 years old) MERS, from January 2000 to December 2022. We summarized the clinical, laboratory, imaging and therapy data. Results We included seven eligible studies for a total of 51 adult-onset MERS patients. Neurological manifestations were preceded by prodromal symptoms in most of the patients (88%), mainly with fever (78%). Headache was the most common symptom (50%), followed by seizures (22%) and disturbance of consciousness (22%). Inflammatory changes on cerebrospinal fluid were present in a half of patient, so defining encephalitis cases. Clinal recovery, was achieved in all patients but two with severe disturbance of consciousness who required ventilator support in the acute phase. MR showed isolated lesion in the SCC in 92% of patients, while 8% of patients showed also extracallosal lesions, all the lesions resolved or improved on follow-up imaging. Conclusions MERS is a form of encephalitis/encephalopathy with a broad range of central nervous system manifestation, often with mild symptoms, such as headache alone, that can lead to overlooked some cases, with an excellent prognosis in most patient.