Aggressive Paraneoplastic Seronegative Dermatomyositis Associated With Head and Neck Squamous Cell Carcinoma of Unknown Primary: A Case Report.

Dermatomyositis is a recognized paraneoplastic syndrome, with malignancy risk being an important consideration during evaluation. Its association with head and neck squamous cell carcinoma (SCC) is uncommon, and its occurrence with a presumed unknown primary has not been well documented. We report the case of an 80-year-old man who presented with dysphagia, proximal weakness, and a characteristic rash. Serology was negative, but multiple malignancy alarm features (age, dysphagia, leukocytoclastic vasculitis on biopsy) prompted aggressive screening. Fluorodeoxyglucose (FDG)-PET/CT revealed a single hypermetabolic cervical lymph node; biopsy confirmed p16-positive SCC. No mucosal primary was identified despite laryngoscopy, and further biopsies could not be performed due to rapid functional decline. The patient proved refractory to immunosuppression and was unfit for oncologic therapy. He died three months after symptom onset. This case illustrates the utility of clinical alarm features and early PET/CT in seronegative dermatomyositis, while highlighting how quickly the window for diagnosis and treatment can close.