Legionella-Associated Encephalopathy With a Reversible Splenial Lesion and Additional Reversible Cortical and White Matter Lesions.

Cytotoxic lesions of the corpus callosum (CLOCCs) are diffusion-restricted callosal lesions associated with various infectious and metabolic conditions. The clinicoradiological syndrome traditionally termed mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) substantially overlaps with this imaging concept. We describe the case of a 60-year-old man with Legionella pneumonia and marked hyponatremia (serum sodium, 121 mEq/L) whose neurological symptoms progressed to incoherent speech and gait disturbance. Brain magnetic resonance imaging (MRI) on hospital day 3 demonstrated a diffusion-restricted lesion in the splenium of the corpus callosum together with additional unilateral lesions involving the right temporo-occipital cortex and white matter. All three lesions showed restricted diffusion with reduced apparent diffusion coefficient values. With treatment of Legionella pneumonia and correction of hyponatremia, the neurological abnormalities resolved, and follow-up MRI demonstrated complete resolution of all lesions. The splenial lesion was consistent with a CLOCC, whereas the unilateral cortical and white matter lesions were atypical of the conventional pattern described as MERS type 2. This case illustrates that reversible callosal, cortical, and white matter MRI abnormalities may occur in the setting of Legionella pneumonia and marked hyponatremia, although a common pathophysiology cannot be established.