The Elephant in the Room: GABAB Receptor Autoantibody-Associated Paraneoplastic Neurological Syndrome Masquerading as Small Cell Lung Cancer

Abstract

SESSION TITLE: Lung Cancer 3 SESSION TYPE: Affiliate Case Report Slide PRESENTED ON: Wednesday, November 1, 2017 at 11:00 AM - 12:15 PM INTRODUCTION: Paraneoplastic neurological syndrome (PNS) is rare in daily clinical practice, and may be the heralding sign of malignancy. Early recognition can lead to prompt diagnosis and potential reversal of neurologic manifestations with treatment of the underlying cancer. We describe a woman who initially presented with Limbic Encephalitis (LE) and was subsequently diagnosed with small cell lung cancer (SCLC). CASE PRESENTATION: A 64-year-old woman with an 80-pack-year smoking history presented with acute onset seizures, confusion, and memory loss. On physical exam, she was oriented to person only. CT Head demonstrated encephalomalacia of the right temporal and frontal lobe, and an MRI brain confirmed the same. An EEG showed continuous diffuse mixed theta and alpha slowing of the background suggestive of mild diffuse cerebral dysfunction. A lumbar puncture was performed demonstrating 6 WBC (85% lymphocytes), 49 protein, 61 glucose. CSF studies were negative for cytology, VDRL, EBV, and HSV. Autoimmune serologies were also negative, but CSF subsequently returned positive for GABAB receptor autoantibody (GABAB R Ab). PET CT (Fig.1) showed increased FDG uptake in enlarged left hilar and subcarinal lymph nodes, without a primary lung mass. An EBUS-TBNA of the hilar lymph node confirmed SCLC. (Fig.2) DISCUSSION: GABAB R Ab resulting in paraneoplastic LE is a relatively recent clinical entity first described in 20111. In a group of 3989 patients with suspected autoimmune encephalopathy, the incidence of GABAB R Ab was 0.2%1. Most of these patients had SCLC and showed improvement of neurologic symptoms with treatment. Most recently, a case series of patients with anti-GABAB R Ab LE emphasized the typical triad of clinical presentation, including memory alteration, seizures, and SCLC2. CONCLUSIONS: Although a rare clinical entity, GABAB R Ab is closely associated with LE and SCLC. The misdiagnosis rate of anti-GABAB receptor encephalitis is high as testing is not always readily available. GABAB R Ab LE with concomitant SCLC is associated with a median survival of less than one year. The diagnosis requires strong clinical suspicion, and prompt treatment for improved outcomes. Reference #1: Jefferey et al. GABAB receptor autoantibody frequency in service serologic evaluation. Neurology 2013. 81;882-7. Reference #2: Liu KQ, Yan SQ, Lou M. Gamma-aminobutyric-acid-B receptor antibodies in limbic encephalitis with small cell lung cancer. Neuroimmunol Neuroinflammation 2015;2(3):187-9. DISCLOSURE: The following authors have nothing to disclose: Nikita Desai, Marina Mosunjac, Munish Luthra No Product/Research Disclosure Information