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Sick sinus syndrome: Epidemiology, etiology, and natural history

Munther K Homoud, MD
Section Editor
Samuel Lévy, MD
Deputy Editor
Brian C Downey, MD, FACC


The sinoatrial (SA) node is normally the dominant pacemaker in the human heart. Originally described in 1907 as a subepicardial structure located at the junction of the right atrium and superior vena cava, the SA node represents the integrated activity of pacemaker cells in a compact region of the right atrium that depolarize and produce action potentials almost synchronously [1-3]. While the location of the primary pacemaker may move among groups of cells within the region of the SA node, only about 1 percent of the cells in the SA node act as the leading pacemaker [4].

Sick sinus syndrome (SSS) is the term used to describe the inability of the SA node to generate a heart rate that meets the physiologic needs of an individual. The initial clues to the diagnosis of SSS are often derived from taking the history and obtaining a routine electrocardiogram (ECG), though the symptoms and ECG findings are frequently vague and nonspecific. The diagnostic evaluation should initially include a search for remediable causes of SA nodal depression such as drugs (eg, beta blockers, calcium channel blockers, digoxin) and metabolic diseases (eg, hypothyroidism). Treatment of SSS is directed at symptoms and typically involves the implantation of a permanent pacemaker.

The epidemiology, potential etiologies, and the natural history of SSS will be reviewed here. The clinical manifestations, approach to diagnosis, and the treatment of SSS are discussed in detail separately. (See "Sick sinus syndrome: Clinical manifestations, diagnosis, and evaluation" and "Sick sinus syndrome: Treatment".)


Cellular physiology — Pacemaking activity that originates from the sinoatrial (SA) node is incompletely understood. There are two predominant mechanisms that are thought to serve as the initiation of sinus activity:

The funny current (If)

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Literature review current through: Nov 2017. | This topic last updated: Dec 08, 2017.
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