A case of suspected paraneoplastic nerve syndrome associated with prostate cancer or opsoclonus-myoclonus syndrome associated with COVID-19 infection, but symptoms improved after treatment of both.

Introduction: Paraneoplastic neurological syndrome is a type of neurological syndrome that occurs in patients with cancer; however, it is rarely associated with prostate cancer. We herein report a rare case of this condition. Case presentation: A 76-year-old man, treated conservatively for rotatory vertigo due to a subtype of Guillain-Barré syndrome after COVID-19, was referred to our neurology department. Magnetic resonance imaging showed no obvious findings; however, paraneoplastic neurological syndrome was suspected due to opsomyoclonus, and close examination of the primary tumor revealed a high prostate-specific antigen level and bone metastases, suggesting prostate cancer. Paraneoplastic nerve syndrome associated with prostate cancer was suspected. Since the possibility of OMS associated with COVID-19 infection was considered, bilateral orchiectomy and endocrine therapy, as well as pulse steroid therapy, were performed, and the patient's symptoms resolved. Conclusion: This rare case suggests the need for timely and aggressive treatment for prostate cancer-associated paraneoplastic nerve syndrome.