Sick Sinus Syndrome

sinus rhythm
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Sick sinus syndrome occurs when the sinus node becomes diseased enough to cause bradycardia (slow heart rate) that produces symptoms. People who have sick sinus syndrome require treatment with a permanent pacemaker to relieve their symptoms.

In addition to symptomatic bradycardia, sick sinus syndrome also is very often accompanied by episodes of atrial fibrillation, which may require additional treatment. Sick sinus syndrome is a disorder of older people and is most commonly seen in people over 70 years of age.

What Causes Sick Sinus Syndrome?

The most common cause of sick sinus syndrome is age-related fibrosis affecting the sinus node (the tiny structure in the right atrium that generates the heart’s electrical impulse). “Fibrosis” means that normal tissue is replaced by a form of scar tissue. When fibrosis affects the sinus node, bradycardia can result. And when bradycardia is caused by a problem with the sinus node, it is termed “sinus bradycardia.”

The same, age-related fibrosis that affects the sinus node can also affect the atrial muscle itself. This generalized atrial fibrosis leads to the atrial fibrillation that often accompanies sick sinus syndrome.

Furthermore, this fibrosis can also affect the AV node. If it does, the sinus bradycardia may be accompanied by episodes of heart block. So in sick sinus syndrome, there may actually be two causes for bradycardia — sinus bradycardia, and heart block.

In some cases other medical conditions can affect the sinus node, producing sinus bradycardia. These conditions include:

However, fibrosis associated with aging is by far the most common cause of sick sinus syndrome.

What Symptoms Are Associated With Sick Sinus Syndrome?

The most prominent symptoms are usually those due to a slow heart rate, and include:

In some people with sick sinus syndrome, these symptoms will only occur when they attempt to exert themselves, and they will feel perfectly well while at rest. In these cases, the chief problem is an inability to increase the heart rate appropriately during activity, a condition called "chronotropic incompetence."

Sick Sinus Syndrome and Atrial Fibrillation

People with sinus node disease who also have episodes of atrial fibrillation will frequently experience symptoms caused by sinus bradycardia, and in addition, they may have symptoms of tachycardia (fast heart rate), especially palpitations. People who have episodes of both slow and fast heart rates are said to have bradycardia-tachycardia syndrome, or "brady-tachy syndrome."

The most troublesome symptom associated with brachy-tachy syndrome is syncope. The loss of consciousness usually occurs immediately after an episode of atrial fibrillation suddenly terminates, leading to a prolonged pause in the heart rate.

This prolonged pause occurs because, when the sinus node is already "sick," an episode of atrial fibrillation tends to even further suppress its function. So, when the atrial fibrillation suddenly stops, the sinus node may require several seconds to "wake up" and begin generating electrical impulses again. During this interval, there may be no heartbeat at all for 10 or more seconds — leading to extreme lightheadedness, or syncope.

How Is Sick Sinus Syndrome Diagnosed?

Diagnosing sick sinus syndrome is usually not difficult. The correct diagnosis is most often pretty apparent when a person who complains of typical symptoms is found to have significant sinus bradycardia on their electrocardiogram (ECG). The "brachy-tachy" variety of sick sinus syndrome is diagnosed when a patient with sinus node disease is also discovered to have episodes of atrial fibrillation.

Because the fibrosis that causes sinus node disease sometimes affects the AV node, people with brachy-tachy syndrome may also have a partial heart block, and therefore, a relatively slow heart rate when they are in atrial fibrillation. So, whenever a person with atrial fibrillation is discovered to have a relatively slow heart rate (in the absence of medication aimed at slowing the heart rate), that should give the doctor a strong clue that sick sinus syndrome is likely also present.

Doctors can make the diagnosis of chronotropic incompetence simply by observing the patient's heart rate during exercise — for instance, during a  stress test. Because chronotropic incompetence is a fairly common condition in the elderly and is readily treatable (with a rate-responsive pacemaker), it is important for older people who are experiencing fatigue with mild or moderate exertion to make sure their doctors do the proper evaluation.

How Is Sick Sinus Syndrome Treated?

Virtually all people with sick sinus syndrome should be treated with a permanent pacemaker.

A pacemaker is especially important for people who have the brachy-tachy form of sick sinus syndrome, for two reasons. First, these people have a relatively high risk of experiencing syncope (from those prolonged pauses when the atrial fibrillation terminates). And second, many of the drugs that are often used to treat atrial fibrillation —  beta blockers,  calcium channel blockers, and antiarrhythmic drugs  — may make sinus node disease much worse. Implanting a pacemaker will prevent syncope, and will allow the doctor to treat atrial fibrillation much more safely.

A Word From Verywell

In sick sinus syndrome, a disease of the sinus node causes sufficient bradycardia to lead to symptoms — most typically, easy fatiguability or lightheadedness. This condition may also be accompanied by atrial fibrillation which, in combination with sinus node disease, makes episodes of syncope likely. Sick sinus syndrome is treated with a permanent pacemaker.

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Article Sources
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