F-18 FDG positron emission tomography in a reversible form of dementia: Unexpected patterns of brain glucose metabolism in autoimmune limbic encephalitis

Objectives To present two distinct metabolic patterns on FDG PET associated with non-paraneoplastic autoimmune limbic encephalitis (NPALE), an increasingly recognized and potentially reversible form of dementia that can mimic neurodegenerative disease on FDG PET.

Methods Case series performed on seven patients diagnosed with NPALE who recovered completely or partially following therapy with corticosteroids. All patients received at least one FDG PET scan during the course of their illness.

Results Three patients had metabolic abnormalities mimicking advanced neurodegenerative disease (e.g., Alzheimer’s disease) while a fourth patient showed similar but milder abnormalities. In contrast, three other patients demonstrated a very different and unique metabolic pattern on FDG PET, with increased temporal lobe metabolism and strikingly reduced uptake in the primary sensory and motor cortex, particularly the visual cortex. Mild or improving PET scan findings predicted an excellent clinical recovery in two patients.

Conclusions Our results suggest FDG PET can be useful in the diagnosis of NPALE, but the interpreting physician must be aware of the variety of scan patterns, including one pattern that has not been described previously for any neurological disorder. It should be kept in mind when interpreting brain PET scans in patients with subacute cognitive decline that an apparent neurodegenerative pattern of cortical hypometabolism may instead represent NPALE, which is potentially reversible. Additionally, limited data suggest that mild or improving scan findings predict a good clinical outcome