Breast Cancer with Subacute Progressive Neurological Symptoms due to Anti-Yo Antibody-Associated Paraneoplastic Neurological Syndrome: A Case Report.

INTRODUCTION: Anti-Yo antibody-associated paraneoplastic neurological syndrome (PNS) is a rare immune-mediated disorder that often presents with subacute cerebellar symptoms and is frequently associated with breast and gynecological malignancies. CASE PRESENTATION: A woman in her 50s developed subacute progressive neurological symptoms, including dizziness, diplopia, dysphagia, gait disturbance, ataxia, and nystagmus. Brain MRI showed no obvious abnormalities. Although Miller-Fisher syndrome was initially suspected, intravenous immunoglobulin therapy had limited efficacy, and serum anti-Yo antibodies were subsequently detected. Further investigation revealed human epidermal growth factor receptor 2-positive breast cancer with axillary lymph node metastasis. Because her neurological symptoms rapidly progressed and her performance status deteriorated, neoadjuvant chemotherapy was considered difficult. Left mastectomy with axillary lymph node dissection was therefore performed as prompt tumor-directed treatment. The final pathological stage was pT1aN1aM0, stage IIA. Postoperatively, additional immunomodulatory treatments and rehabilitation were provided for the neurological deficits. CONCLUSIONS: This case highlights the importance of considering anti-Yo antibody-associated PNS in patients with unexplained subacute progressive neurological symptoms, even when brain imaging shows no clear abnormalities. Early recognition of PNS and prompt screening for an underlying malignancy, including breast cancer, are essential to allow timely tumor-directed treatment before severe neurological deficits become established.