Seronegative Paraneoplastic Opsoclonus-Myoclonus-Ataxia Syndrome Secondary to Low Volume Endocrine-Sensitive Malignancy of Likely Breast Origin.

A 51-year-old female presented to the emergency department with vertigo, visual disturbances, involuntary rapid repetitive eye movements, incoordination, and imbalance. Physical examination revealed opsoclonus, myoclonus, and bilateral limb and gait ataxia. Initial workup was negative for intracranial abnormalities, and no abnormalities were noted on blood work or cerebrospinal fluid analysis. Tumor markers were within normal limits. As part of her diagnostic workup, a positron emission tomography (PET) scan was performed, which showed a highly FDG-avid solitary 7 mm left axillary lymph node. Ultrasound-guided percutaneous biopsy revealed metastatic poorly differentiated carcinoma. Histopathological examination could not conclusively distinguish between adenocarcinoma and squamous cell carcinoma. She was diagnosed with seronegative opsoclonus-myoclonus ataxia syndrome of paraneoplastic origin from an occult primary malignancy and started on pulsatile corticosteroids and intravenous immunoglobulin (IVIG), with only moderate symptomatic improvement. Given the anatomic location and immunohistochemical staining pattern of the lymph node, the malignancy was considered as being of primary breast origin. A left axillary lymph node dissection was performed, with 1/12 nodes testing positive for poorly differentiated carcinoma. The patient experienced significant improvement in her neurological symptoms 2-3 days following resection of the solitary malignant lymph node, largely regaining her functional independence. She went on to receive adjuvant radiotherapy to the breast and axilla, as well as adjuvant hormonal therapy.