Ictal and interictal FDG-PET in anti-NMDAR encephalitis with mutism

We report a case of a 27-year-old right-handed gentleman with mutism and seizures diagnosed with Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis without evidence of underlying malignancy. Brain MRI was unremarkable. Clinical seizures were controlled but long-term video-EEG monitoring was needed for better characterization of his clinical manifestations especially that language partially improved. It was crucial to identify whether this mutism was ictal in origin or not. Ictal brain Positron Emission Tomography with 18 F-fluorodeoxyglucose (FDGPET) scan combined with EEG was done. It revealed left fronto-temporal, parietal, and crossed cerebellar hypermetabolism (or diaschisis) concomitant to the underlying rhythmic focal delta activity on EEG. Beside anti-epileptic drugs he was treated with escalating immunotherapy (intravenous solumedrol then immunoglobulins then full rituximab course). Six months later, EEG combined to FDG-PET scan were repeated, and were normal. At 3 years follow up the patient remains neurologically stable and seizure-free, off anti-epileptics drugs. Performing the FDGPET scan combined to EEG was useful to identify non-convulsive status epilepticus and should be performed early in anti-NMDAR encephalitis to guide treatment.