Sick sinus syndrome (SSS) is a group of signs and symptoms that, when they occur together, are an indication that an important part of the heart’s electrical system, called the sinus node, is not working well. The sinus node is a bundle of tissue, located in the upper right chamber of the heart (right atrium), that sets the pace at which the heart beats. The primary symptom of SSS is sinus bradycardia, a heart rhythm that is slower that 60 beats per minute. However, about half of patients with sick sinus syndrome may also experience rapid heartbeats or heart rates that alternate between abnormally fast and abnormally slow. 

Many patients with sick sinus syndrome will not feel any symptoms until their heart rate drops below 50 beats per minute. Therefore, the condition is often not diagnosed until its advanced stages. The most common treatment is the implantation of an artificial pacemaker followed by the prescription of antiarrhythmic medications.