Case of clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) due to Legionella pneumonia

Clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is a clinicoradiologic syndrome diagnosed by temporary hyperintense lesion in the area, including the splenium of the corpus callosum, on diffusion-weighted imaging and neuropsychiatric symptoms that recover without sequelae. MERS is rare in adults, especially elderly people. We herein report a man in his 60s diagnosed with MERS caused by Legionella pneumonia. He completely recovered with only the administration of levofloxacin and azithromycin despite the risk factors of an advanced age, medical history of untreated hypertension, bilateral spontaneous pneumothoraxes, smoking and drinking habits and pulmonary emphysema. To our knowledge, this is the oldest case of MERS due to Legionella pneumonia and extremely old among total MERS cases. Our research revealed that Legionella species are the most common pathogens of adult-onset MERS, while viruses are the main causative factors in children. This case helps clarify the features of MERS in high-risk adults.