What Is Sick Sinus Syndrome?

Sick sinus syndrome occurs when the sinus node becomes diseased enough to cause bradycardia (slow heart rate) that produces symptoms. This disorder is most commonly seen in people over 70 years of age. You may also hear it referred to as sinus node dysfunction (SND).

People who have sick sinus syndrome often require treatment with a permanent pacemaker to relieve their symptoms. In addition to bradycardia symptoms, sick sinus syndrome often occurs with atrial fibrillation, which may require additional treatment.

Causes

The most common cause of sick sinus syndrome is age-related change, like 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, sinus bradycardia can result.

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, fibrosis can also affect the AV node. If it does, sinus bradycardia may be accompanied by episodes of heart block.

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

• Amyloidosis
• Sarcoidosis
• Chagas disease
• Hypothyroidism
• Cardiac trauma 

However, aging-related changes are by far the most common cause of sick sinus syndrome.

Sick Sinus Syndrome Symptoms

The most prominent symptoms are usually those due to a slow heart rate. They include:

• Easy fatiguability  
• Lightheadedness
• Syncope
• Dyspnea
• Confusion

In some people with sick sinus syndrome, these symptoms will only occur when they attempt to exert themselves; 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. 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 "brachy-tachy syndrome."

The most troublesome symptom associated with this is syncope. The loss of consciousness usually occurs immediately after an episode of atrial fibrillation suddenly ends, 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 suppress its function even further. 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.

Diagnosis

Sick sinus syndrome may be diagnosed when a person who has typical symptoms has an electrocardiogram (ECG) that indicates significant sinus bradycardia. Ideally, the symptoms and ECG reading will occur at the same time.

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 bradycardia-tachycardia syndrome may also have a partial heart block and, therefore, a relatively slow heart rate when they are in atrial fibrillation.

Doctors can make the diagnosis of chronotropic incompetence 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 self-advocate for proper evaluation.

Treatment

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.

1. These people have a relatively high risk of experiencing syncope (from those prolonged pauses when the atrial fibrillation terminates).
2. 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 allow the doctor to treat atrial fibrillation much more safely.