Definition of Sick sinus syndrome:

Sick sinus syndrome is a collection of heart rhythm disorders that include:

• Sinus bradycardia -- slow heart rates from the natural pacemaker of the heart
• Tachycardias -- fast heart rates
• Bradycardia-tachycardia -- alternating slow and fast heart rhythms

Causes, incidence, and risk factors:

Sick sinus syndrome is relatively uncommon. Sinus bradycardia occurs more often than the other types.

Tachycardias that start in the upper chambers of the heart are also common forms. These include atrial fibrillation, atrial flutter/tachycardia, and supraventricular tachycardia. A period of elevated heart rates is typically followed by very slow heart rates when the tachycardia ends.

Abnormal heart rhythms are often made worse by medications such as digitalis, calcium channel blockers, beta-blockers, and anti-arrhythmics. Disorders that cause scarring, degeneration, or damage to the conduction system of the heart can cause sick sinus syndrome.

Sick sinus syndrome usually occurs in people older than 50, in whom the cause is often a nonspecific, scar-like degeneration of the heart's conduction system.

In children, a common cause of sick sinus syndrome is heart surgery, especially on the upper chambers.

Coronary artery disease, high blood pressure, and aortic and mitral valve diseases may be associated with sick sinus syndrome, although those diseases may have nothing to do with the syndrome.