Contractile fibroblasts (myofibroblasts) in a painful pacemaker pocket.

Electron microscopy of a painful fibrotic capsule around a cardiac pacemaker revealed classic myofibroblasts. These contractile fibroblasts share characteristics of both smooth muscle cells and fibroblasts and are thought to be the cellular cause of wound and scar contraction. The occurrence of pain and tenderness around a sterile pacemaker probably indicates contraction of scar tissue as the basis of the symptoms. This scar tissue is similar to that seen around other large inert implants placed in the thoracic tissue.