The value of the PET scan in the diagnosis of autoimmune focal encephalitis is reported in a 22-month-old girl who presented with involuntary movements, hemiparesis, and behavioral changes at Juntendo University School of Medicine, Tokyo Metropolitan Institute for Neuroscience, Japan. On admission, she had low-grade fever, choreic movements in the right arm, and mild ataxia. MRI and EEG were unremarkable. CSF cell count was 19 per mm3 and protein 15 mg/dL; viral isolation was negative. Symptoms deteriorated and methylprednisolone therapy was ineffective. She developed hemiparesis, aphasia, agitation and temper outbursts. PET scan showed hypermetabolism in the left temporal lobe, caudate and lentiform nuclei, and prefrontal area. Autoimmune focal encephalitis was suspected and treatment with IV immunoglobulin begun. Behavior improved, phonation improved, and at 18 months after onset, hemiparesis and involuntary movements lessened. Repeat PET scan at 3 months after onset showed hypometabolism in the left temporal lobe. Aphasia and temper outbursts persisted, and her developmental quotient at 28 months was 54. Immunohistochemical analyses on sera showed immunoglobulin G autoantibodies that reacted specifically with cytoplasm of neurons in prefrontal and temporal lobes, globus pallidus, and putamen, corresponding with the neurologic symptoms. At 3 months after IVIG therapy, serum autoantibodies had disappeared and symptoms improved. Autoantibodies were important in the etiology of the focal encephalitis. [1]

COMMENT. PET scan is considered a useful adjunct in the diagnosis of autoimmune focal encephalitis, when MRI is unremarkable. FDG-PET detects inflammatory changes with more sensitivity than SPECT, which reflects regional cerebral blood flow. Serum immunohistochemical analyses are required to detect specific autoantibodies.

Autoimmune limbic encephalitis is discussed by Jerome Honnorat, Universite Claude Bernard, Bron, France [2]. Limbic encephalitis is an inflammatory disorder affecting the hippocampi, amygdalae, and fronto-basal and insular regions. Originally considered rare, paraneoplastic, and unresponsive to treatment, a new subtype is described with autoantibodies against a Gaba subunit receptor, that is not always associated with cancer and may be treatable [3]. The antibodies are directed against neuronal cell-surface antigens, and patients may improve with immunotherapy. Paraneoplastic limbic encephalitis with antibodies against intracytoplasmic antigens is not susceptible to immunotherapy.