Sinus node disease was previously thought to be a disease limited to the sinus node and its atrial connections. However, recent reports have demonstrated sinus node disease as a disease of the entire right atrial myocardium. These patients have widespread electrophysiological abnormalities of their atria, including prolonged refractory periods and slowed conduction. In addition to these electrical changes, there are significant structural changes, such as fibrosis and fatty infiltration, which can be detected endocardially as regions of fractionated signals, low-voltage electrograms, and electrically silent areas. In most cases, the etiology of these changes is unknown. These changes may contribute to the high prevalence of atrial fibrillation seen in patients with sinus node disease.