Improvement of Steroid-Dependent Encephalopathy Clinically Diagnosed as Paraneoplastic Neurological Syndrome in Onconeural Antibody-Negative Sigmoid Colon Cancer after Laparoscopic Tumor Resection

INTRODUCTION: Paraneoplastic neurological syndrome (PNS) is a rare complication of malignancy caused by immune-mediated mechanisms. Although characteristic onconeural antibodies are useful for the diagnosis of PNS, 30%–40% of cases are seronegative, which makes recognition difficult. We present a case of sigmoid colon cancer complicated by onconeural autoantibody-negative PNS which caused impaired consciousness. The patient’s symptoms recurred with steroid tapering but improved after resection of the primary tumor and radiofrequency ablation (RFA) of the metastasis. This case highlights the importance of considering PNS, even when onconeural antibodies are negative.