Sick Sinus Syndrome (Sinus Node Dysfunction)

How the heart's pacemaker works

The sinus (sinoatrial) node is a small group of specialized cells in the right upper chamber (atrium). It fires electrical impulses, normally 60 to 100 times a minute at rest, which spread across the atria and through the AV node to the lower chambers. In sick sinus syndrome, the node fires too slowly, irregularly, or stops for a few seconds, and sometimes the heart's backup systems also fail.

Rhythm problems in sick sinus syndrome

Causes

• Age-related fibrosis: gradual scarring of the sinus node and conduction system, the most common cause
• Medicines: beta-blockers, calcium channel blockers (verapamil, diltiazem), digoxin, antiarrhythmics, clonidine, donepezil, lithium. These can unmask or worsen sinus node dysfunction.
• Heart conditions: coronary artery disease, cardiomyopathy, infiltrative diseases such as amyloidosis and sarcoidosis, myocarditis
• Heart surgery, especially in children after congenital heart repairs
• Other reversible causes: hypothyroidism, high potassium, low body temperature, sleep apnea and increased vagal tone
• Inherited forms: rare gene changes affecting ion channels

Symptoms

Symptoms come from reduced blood flow to the brain and body during slow periods, or from fast rhythms:

• tiredness and weakness
• dizziness or lightheadedness
• fainting or near-fainting (syncope)
• confusion or memory problems, sometimes mistaken for dementia in older adults
• shortness of breath and reduced exercise tolerance
• palpitations or a racing heart
• chest pain
• poor sleep

Symptoms are often intermittent, which can make diagnosis difficult.

Diagnosis

• 12-lead ECG: may show a slow rate or pauses, but is often normal between episodes
• Holter monitor: records the heart for 24 to 48 hours
• Event monitor or patch monitor: for 2 to 4 weeks
• Implantable loop recorder: a small device under the skin records for up to several years, useful for infrequent fainting
• Exercise test: checks whether the heart rate rises normally
• Blood tests: thyroid function, potassium and other electrolytes
• Echocardiogram: checks heart structure and function
• Sleep study if sleep apnea is suspected

The key is linking symptoms to an abnormal rhythm.

Treatment

Reversible causes

• stop or reduce medicines that slow the heart, if possible
• treat hypothyroidism, electrolyte imbalances and sleep apnea

Pacemaker

A permanent pacemaker is the main treatment for people whose symptoms are caused by sinus node dysfunction. It is a small device placed under the skin below the collarbone, with leads into the heart. It prevents the heart rate from dropping too low. Sick sinus syndrome is one of the most common reasons for pacemaker implantation. A pacemaker also allows safe use of medicines needed to control fast rhythms in tachy-brady syndrome.

Managing fast rhythms and stroke risk

• rate- or rhythm-control medicines for atrial fibrillation, often after a pacemaker is in place
• catheter ablation in selected patients
• anticoagulation (blood thinners) for people with atrial fibrillation and stroke risk factors, because tachy-brady syndrome carries a significant risk of stroke

Without symptoms

People with a slow heart rate but no symptoms usually do not need a pacemaker; well-trained athletes and many people asleep normally have slow rates.

Complications

• falls and injuries from fainting
• stroke and blood clots in tachy-brady syndrome
• heart failure
• reduced quality of life

Outlook

Sick sinus syndrome is usually slowly progressive. Pacemakers are very effective at relieving symptoms from slow rhythms. Overall outlook depends mostly on other heart conditions and on stroke prevention when atrial fibrillation is present.

Frequently asked questions

What is sick sinus syndrome?

Sick sinus syndrome is a condition in which the heart's natural pacemaker, the sinus node, does not work properly. It causes slow heart rates, pauses or alternating slow and fast rhythms, leading to tiredness, dizziness and fainting.

What are the symptoms of sick sinus syndrome?

Common symptoms include fatigue, dizziness, fainting or near-fainting, confusion, shortness of breath, reduced exercise ability and palpitations. Symptoms often come and go, so heart monitoring over days or weeks may be needed.

Is sick sinus syndrome serious?

It can be. Fainting can cause injuries, and tachy-brady syndrome increases stroke risk. However, with a pacemaker and, when needed, blood thinners and rhythm medicines, most people manage well and have a good quality of life.

Do you always need a pacemaker for sick sinus syndrome?

No. A pacemaker is recommended when symptoms are caused by slow rhythms or pauses. If there are no symptoms, or the cause is reversible, such as a medicine or underactive thyroid, a pacemaker may not be needed.

What is tachy-brady syndrome?

Tachy-brady syndrome is a form of sick sinus syndrome in which fast rhythms, usually atrial fibrillation, alternate with slow rhythms or pauses. Medicines to slow fast rhythms can worsen slow ones, so a pacemaker is often needed first.