Autoimmune Limbic Encephalitis Detected on FDG Brain Scan Performed for the Evaluation of Dementia Abraham C. Thomas, MD,*† Richard K.J. Brown, MD, FACR,* Rukhsana Begum, MD,‡ Ashok Srinivasan, MD,* Nicolaas I. Bohnen, MD, PhD,*† and Kirk A. Frey, MD, PhD*† Abstract: Most diagnostic imagers are familiar with the classic patterns of altered metabolism seen in Alzheimer disease, frontal temporal dementia, and dementia with Lewy bodies. However, awareness of other conditions that can cause cognitive impairment is crucial when interpreting FDG brain PET studies. This is particularly true of infectious and inflammatory processes, such as viral encephalitis, which should be diagnosed and treated on an emergent basis. In addition, altered metabolism can be caused by autoimmune paraneoplastic and encephalitic syndromes. We report a case of autoimmune limbic encephalitis encountered as part of evaluation for atypical onset cognitive impairment. Key Words: autoimmune limbic encephalitis, PET, brain (Clin Nucl Med 2015;40: 358–359) Received for publication October 06, 2014; revision accepted November 21, 2014. From the Departments of *Radiology, and †Neurology, University of Michigan and Ann Arbor VAMC, Ann Arbor; and ‡Midwest Neurology, Howell, MI. Conflicts of interest and sources of funding: none declared. Reprints: Richard K.J. Brown, MD, Department of Radiology, University of Michigan Health System, 1500 E, Medical Center Dr, UH B1G505, Ann Arbor, MI 481095030. E-mail: [email protected]
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Clinical Nuclear Medicine • Volume 40, Number 4, April 2015 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Clinical Nuclear Medicine • Volume 40, Number 4, April 2015
Autoimmune Limbic Encephalitis on FDG PET
FIGURE 1. A 72-year-old male patient presented with acute onset cognitive impairment. Axial 18F-FDG PET images through the midbrain at 4-mm slice thickness demonstrate 2 foci of hypermetabolism in the right mesial temporal lobe in the hippocampal region (arrows). An MRI and electroencephalogram were performed but revealed no structural or physiologic abnormalities. Because viral encephalitis was a consideration, an emergent lumbar puncture was performed. The lumbar puncture was negative for infection, but a panel of antibodies associated with limbic encephalitis was also obtained. The combination of imaging findings and cerebral spinal fluid biomarkers confirmed the diagnosis of autoimmune limbic encephalitis. Autoimmune limbic encephalitis is often secondary to occult neoplasms expressing neuronal antigens.1–6 Nonparaneoplastic limbic encephalitis is an autoimmune reaction to the expression of neuronal antigens without known underlying neoplasm. However, the distinction can be subtle as paraneoplastic limbic encephalitis may present years before the actual detection of the occult cancer, and repeated screening for occult neoplasm is suggested for at least 4 years if initial screening is negative.7 Classic patterns of altered metabolism seen in Alzheimer disease, frontal temporal dementia, and dementia with Lewy bodies are often recognized in FDG PET/CT imaging.8,9 This case highlights the fact that although uncommon, etiologies such as autoimmune limbic encephalitis should be recognized during evaluation of dementia by FDG PET/CT imaging.
© 2015 Wolters Kluwer Health, Inc. All rights reserved. Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.