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. CASE PRESENTATION: A 79-year-old man was referred for evaluation of sigmoid colon cancer detected during the investigation of bloody stools. He had been receiving 20 mg of prednisolone (PSL) for suspected arthritis and cellulitis and was later diagnosed with gout. After tapering and discontinuation of PSL, the patient developed recurrent arthritis, impaired consciousness with somnolence, and hypoxemia. Cardiac, infectious, cerebrovascular, and metabolic causes were excluded, raising the suspicion of PNS despite negative onconeural antibody testing. The patient's symptoms improved with 40 mg of PSL but worsened when the dose was reduced. An iliopsoas abscess and disseminated intravascular coagulation were identified and treated with antibiotics and PSL. However, balancing the risk of infection against recurrent neurological decline was difficult. For this reason, the patient underwent a laparoscopic Hartmann procedure. Postoperatively, PSL was tapered to 2.5 mg; however, impaired consciousness recurred with evidence of early liver metastasis. RFA was then performed, leading to the resolution of symptoms. CONCLUSIONS: This case demonstrates an onconeural antibody-negative PNS associated with sigmoid colon cancer. Neurological symptoms emerged in association with both the primary tumor and subsequent liver metastasis and improved after treatment of each lesion, with milder symptoms observed at metastatic recurrence, consistent with a smaller tumor burden. Clinicians should consider PNS even in seronegative cases and emphasize tumor-directed therapy when immunosuppression is constrained by infection risk.