Heart Block Caused by Cardiac Metastasis From Merkel Cell Carcinoma: A Case Report.

Merkel cell carcinoma (MCC) is a rare, rapidly proliferating skin cancer that commonly metastasizes to regional lymph nodes. We present the case of a 73-year-old woman with a history of MCC and non-Hodgkin B-cell lymphoma who presented with second-degree heart block (Mobitz type II) caused by an interatrial mass. Temporary pacing was required before biopsy, which revealed metastatic MCC. Treatment included permanent pacing, anti-programmed cell death ligand 1 immunotherapy, and radiation to the heart resulting in notable decrease in tumor size and normalized cardiac rhythm.