Paraneoplastic Cerebellar Syndrome Revealing Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Breast Cancer: A Case Report.

Paraneoplastic cerebellar degeneration (PCD) is a rare neurological disorder that can precede the diagnosis of an underlying malignancy by several months or even years. We report the case of a 46-year-old woman who presented with progressive gait instability, dysarthria, and nystagmus. Brain magnetic resonance imaging (MRI) revealed cerebellar atrophy without any mass lesion, and serologic testing showed strong anti-Yo antibody positivity. Two years later, she was diagnosed with a human epidermal growth factor receptor 2 (HER2)-positive, hormone receptor-negative invasive ductal carcinoma of the breast. The patient underwent a modified radical mastectomy followed by adjuvant chemotherapy and trastuzumab. Despite appropriate oncologic treatment, her neurological symptoms persisted and gradually worsened. This case highlights the diagnostic challenge of PCD, particularly when neurological manifestations precede tumor detection, and suggests that HER2-positive breast cancer may also be associated with anti-Yo antibodies. Early recognition of this syndrome and systematic malignancy screening are essential to improve patient outcomes.