What happened, in summary

This 40-year-old woman was admitted 1 month after sudden vertigo with unsteady gait. Ten months earlier, she had developed unexplained left-sided hearing loss that later affected both ears, worse on the right. Oral prednisone did not help, and she underwent cochlear implantation 1 month before admission. Neurologic examination showed bilateral nystagmus, cerebellar ataxia, and abnormal speech. Hearing tests showed bilateral sensorineural hearing loss, while temporal bone CT, brain MRI, and magnetic resonance angiography did not show structural causes. Cerebrospinal fluid and serum tissue-based assays suggested neuronal autoantibodies, although specific antibodies were not identified. She received IVIG plus intravenous methylprednisolone. Dizziness improved slightly, but hearing did not recover. At 3-month follow-up, chest CT for respiratory symptoms showed multiple enlarged right axillary lymph nodes. Breast ultrasound then found a deep hypoechoic calcified right nipple-region nodule categorized as BI-RADS 4a. Pathology confirmed poorly differentiated adenocarcinoma. Axillary node biopsy also showed poorly differentiated adenocarcinoma. Immunohistochemistry supported mammary origin, with about 100% ER positivity, HER2 2+, CK, CK7, and GATA3 positivity, and negative CgA, Syn, CD56, P40, SOX2, GCDFP-15, and PAX8. PET/CT and pathology confirmed right axillary lymph-node involvement. She declined surgery and began endocrine therapy. As of December 2025, at 1-year follow-up, she remained on endocrine therapy and serial chest CT showed reduced right axillary lymph-node size, while hearing outcome after cochlear implantation could not be assessed. Her presentation fit paraneoplastic cochleovestibulopathy because neurologic and hearing symptoms came before the breast tumor was discovered, structural imaging was unrevealing, immune treatment produced only limited dizziness relief, and cancer treatment became the central long-term therapy.