Lung cancer masquerading as a paraneoplastic neurologic syndrome without a primary lung mass: Case report and review of literature.

Paraneoplastic and autoimmune encephalitis (AIE) syndromes describe a range of inflammatory disorders of the brain. "Classic" paraneoplastic encephalitis syndromes occur due to a remote neoplasm and are associated with antibodies that target intracellular neuronal proteins while the more recently described AIE syndromes are not always paraneoplastic and occur in association with antibodies that target cell-surface neuronal receptors (e.g., anti-NMDA receptor, anti-LGI1, anti-GABA receptor). Diagnosis can be difficult and delayed due to nonspecific clinical, imaging, and laboratory findings, and in those syndromes associated with a neoplasm, the neurologic syndromes often precede the cancer diagnosis. We present a case of a 64-year-old patient diagnosed with anti-GABA receptor encephalitis that subsequently revealed an underlying small cell lung cancer without a primary lung tumor. This case highlights the clinical challenge in diagnosing immune-mediated encephalitis, its methodical work up, and subsequent management.