Two different 18F-FDG brain PET metabolic patterns in autoimmune limbic encephalitis

Autoimmune limbic encephalitis (ALE) is a severe, but treatable, neuropsychiatric disorder that is difficult to diagnose clinically. With the goal of improving diagnosis of this disorder, we retrospectively evaluated the cerebral FDG PET pattern in a group of patients with ALE.

Nine adult patients with subacute cognitive decline were eventually diagnosed with ALE based on clinical presentation, cerebrospinal fluid inflammatory markers, and response to immunosuppressive therapy. All patients received FDG PET brain scanning during their diagnostic evaluation, which were retrospectively reviewed for this study.

Our patients' scans fell into 2 readily separable patterns. Five younger patients had a mixed metabolic pattern most easily recognized by pronounced occipital hypometabolism, accentuated by hypermetabolism in the temporal and orbitofrontal cortex. Other, milder findings were also present. Once this unusual pattern was established as corresponding to ALE, it helped lead to the correct clinical diagnosis in the last 2 patients. Four older patients had scans that closely resembled diffuse neurodegenerative disease.

We found 2 different PET scan patterns in patients with ALE. One is an easily recognizable mixture of hyper- and hypometabolism that has also been described in a few recent case reports and is potentially specific for ALE. The other is indistinguishable from neurodegenerative disease. We propose that awareness of these patterns may contribute to the diagnosis of this elusive, but treatable, neurologic disorder.