[A Case of Breast Cancer Initially Presenting with Opsoclonus-Myoclonus Syndrome]

A 41-year-old woman presented with persistent dizziness. A neurological exam revealed rapid, multidirectional saccades, suggestive of opsoclonus-myoclonus syndrome(OMS). CT incidentally revealed a right breast mass, and further evaluation confirmed Stage ⅢC(cT2N3aM0), HR-positive/HER2-negative breast cancer, considered the underlying cause of OMS. Neoadjuvant chemotherapy with AC followed by weekly paclitaxel improved OMS and achieved partial tumor response. She underwent breast-conserving surgery with axillary dissection. Postoperatively, tamoxifen, a CDK4/6 inhibitor, and radiotherapy were administered. Five months after surgery, multiple liver metastases developed. Despite initiating weekly paclitaxel plus bevacizumab, the disease progressed rapidly, and she died 4 months later. Notably, OMS symptoms did not recur. This case highlights paraneoplastic OMS as an initial manifestation of breast cancer, with neurological improvement following systemic therapy.