Sick Sinus Syndrome

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Introduction

Sick sinus syndrome, also known as sinus node dysfunction, occurs when sinoatrial node (SAN) dysfunction causes bradyarrhythmias or tachyarrhythmias.1,2

The condition predominantly affects older adults, although it can occur at any age. In the United States, sick sinus syndrome accounts for more than half of pacemaker implantations. Due to an ageing population, the incidence continues to rise.


Aetiology

The SAN contains two types of specialised cells. Pacemaker cells have intrinsic pacemaker activity and initiate action potentials at regular intervals. Transitional cells facilitate the propagation of the impulse across the atria (Figure 1).3

For more information on the cardiac conduction system, see the Geeky Medics article here

In sick sinus syndrome, dysfunction of the SAN leads to an atrial rate that is inappropriate for normal requirements.

The electrical conduction system of the heart relevant to sick sinus syndrome
Figure 1. The electrical conduction system of the heart.

The causes of sick sinus syndrome can be divided into intrinsic and extrinsic causes.1,3

Intrinsic causes include:

  • Idiopathic fibrosis: age-related degeneration of the SAN is the most common cause of sinus node dysfunction
  • Ischaemic heart disease (e.g. myocardial infarction, ischaemia)
  • Myocarditis
  • Pericarditis
  • Rheumatic heart disease
  • Infiltrative diseases (e.g. sarcoidosis, amyloidosis, haemochromatosis)
  • Congenital abnormalities
  • Iatrogenic: for example, damage to the SA node during open-heart surgery

Extrinsic causes include:

  • Drugs: digoxin, beta-blockers, calcium channel blockers, anti-arrhythmics
  • ‘Hypos’: hypothermia, hypothyroidism, hypoxia
  • ‘Hypers’: hyperkalaemia, hyperthyroidism
  • Autonomic dysfunction

The exact cause is often difficult to identify and there may be multifactorial precipitants. For example, there may be a degree of age-related fibrosis of the SA node which is exacerbated by infarction of the SA node after an inferior myocardial infarct, or by the addition of medications such as beta-blockers.

Pathophysiology

Several different arrhythmias may arise according to the specific dysfunction of the SAN (e.g. dysfunction of pacemaker cells vs. transitional cells).

Arrhythmias seen may change over time and are often intermittent. Some important examples of arrhythmias include tachycardia-bradycardia syndrome, sinus bradycardia and sinus arrest, sinoatrial exit block and slow atrial fibrillation.3

Tachycardia-bradycardia (“tachy-brady”) syndrome

Tachy-brady syndrome is identified by periods of bradycardia or sinus arrest interspersed with periods of tachycardia, most commonly atrial fibrillation.

This is caused by abnormal conduction within the atrial tissue and is the most common manifestation of sick sinus syndrome, affecting at least 50% of patients.

Figure 2a and 2b are two ECGs taken from a patient with tachy-brady syndrome.

Sinus bradycardia and sinus arrest

Severe, inappropriate sinus bradycardia is often a feature of sick sinus syndrome (Figure 3) 

A pause of three seconds or more without any atrial activity (p waves) is known as sinus arrest. This reflects the failure of pacemaker cells to generate an action potential (Figure 4).

To prevent asystole and syncopal episodes, the heart may ‘rescue’ a severe sinus bradycardia or sinus arrest by utilising pacemaker tissue that is outside of the SAN to generate a new action potential and allow systole to occur. This is called an escape rhythm and it acts to preserve cardiac output.

Escape rhythms may arise from atrial tissue (atrial escape rhythm) the AVN (junctional escape rhythm) or the ventricular myocytes (ventricular escape rhythm) and give characteristic appearances on the ECG.4,5

Sinoatrial exit block

Sinoatrial exit block is similar to atrioventricular heart blocks but instead of affecting the atrioventricular node, here the cause is a failure of the sinoatrial node transitional cells to propagate the impulse across the atria. There are multiple different grades of sinoatrial exit block according to severity.

Atrial fibrillation with a slow ventricular response

Atrial fibrillation with a slow ventricular response may be seen in the absence of beta-blocker therapy. The ECG will show a chaotic but bradycardic ventricular rhythm (irregularly irregular) with no evidence of any organised atrial activity in the form of p waves.


Risk factors

Risk factors for the development of sick sinus syndrome include:1,3

  • Advancing age: the most important risk factor
  • Cardiac disease: ischaemic, inflammatory, structural or congenital
  • Electrolyte derangement: especially hyperkalaemia (e.g. in patients with renal impairment or in those on medications which cause hyperkalaemia)
  • Thyroid disease
  • Medication: particularly negative chronotropes/antiarrhythmics which may unmask subclinical sinus node dysfunction in those with additional risk factors

Clinical features

Sick sinus syndrome often develops over a long time course and many patients are asymptomatic.

Symptoms and signs arise due to arrhythmias which reduce cardiac output and lead to end-organ hypoperfusion. Therefore, the clinical presentation is not specific to sinus node dysfunction.1

History

Typical symptoms of sick sinus syndrome may include:

  • Fatigue
  • Dizziness
  • Palpitations during periods of arrhythmia
  • Pre-syncope or syncope due to cerebral hypoperfusion
  • Angina: often caused by rate-related myocardial ischaemia

Other important areas to cover in the history include:

  • Past medical history: underlying cardiac disease, multi-system infiltrative disorder (haemochromatosis, sarcoidosis, amyloidosis) and thyroid disease
  • Drug history: negative chronotropes including beta-blockers, rate-limiting calcium channel blockers and other antiarrhythmics may all contribute to the pathogenesis of sick sinus syndrome and precipitate its onset in those with other predisposing conditions

Clinical examination

There are no specific clinical signs seen in sick sinus syndrome other than tachycardia and bradycardia. 


Differential diagnoses

The differential diagnosis of sick sinus syndrome is diverse and may include other tachyarrhythmias and bradyarrhythmias. 

Other tachyarrhythmias to consider include the supraventricular tachycardias:

  • Atrial fibrillation
  • AV-nodal re-entrant tachycardia (AVNRT)
  • Atrioventricular re-entrant tachycardia (e.g. Wolff-Parkinson-White syndrome)
  • Atrial tachycardia

Other bradyarrhythmias to consider include:

  • Atrioventricular node blockade (heart block)
  • Sinus bradycardia: due to beta-blockers or other drugs
  • Physiological bradycardia: for example amongst endurance athletes

Investigations

Sick sinus syndrome can be challenging to diagnose because the arrhythmias seen are often intermittent and can change over time.

For a diagnosis of sick sinus syndrome, there must be correlation between ECG evidence of bradyarrhythmias alongside typical clinical symptoms.

Bedside investigations

Relevant bedside investigations include:1,3

  • Resting 12-lead ECG: diagnosis based on a resting ECG is unusual because the arrhythmias are often episodic

Laboratory investigations

Relevant laboratory investigations include:

  • U&Es: to assess plasma electrolyte levels. Hyperkalaemia may be a cause of sinus node dysfunction.
  • Thyroid function tests: hyperthyroidism and hypothyroidism can both cause sinus node dysfunction
  • Drug levels: for example, digoxin levels (a recognised cause of sinus node dysfunction)

Other investigations

Ambulatory ECG (‘Holter’ monitor)

Ambulatory ECG monitoring for 24 – 48 hours can demonstrate episodes of bradycardia and correlate these with the patient’s symptoms.

The patient wears a device that captures an ECG rhythm strip over one or two days. The data is then captured and analysed.

Sometimes a wearable device is required for longer periods (an event recorder). The patient can activate the device following symptoms such that the previous 5-minute recording is then saved for analysis.

Implantable loop recorder

If ambulatory ECG fails to reveal the diagnosis, then an implantable loop recorder may be placed to monitor the heart rhythm over a longer period of time.

These small battery-operated devices (approximately 4-5cm long and less than 1cm wide) which are inserted under the skin in the left parasternal region under local anaesthetic.  This is a minor procedure, and they can record for up to three years.


Management

The management of sick sinus syndrome first involves removal of extrinsic causes (see aetiology). For example, correcting electrolyte abnormalities, stopping unnecessary medications and treating any metabolic disturbance.3

Definitive management involves implanting a pacemaker to prevent episodes of symptomatic bradycardia (Figure 5).

A dual-chamber pacemaker with both an atrial lead and a ventricular lead (rather than a ventricular lead alone) is preferred. This provides a more physiological form of pacing and protects against heart block due to AVN fibrosis. Dual-chamber pacing has also been shown to reduce the risk of AF.6

Patients with tachy-brady syndrome will require beta-blocker therapy and pacemaker implantation.

Whilst this may seem counter-intuitive, the pacemaker allows the use of sufficiently high doses of beta-blockers to control episodes of tachycardia, whilst protecting from symptomatic bradycardia.3

Pacemakers are not appropriate for patients with sick sinus syndrome who are asymptomatic, or for those in whom a reversible cause can be identified and corrected (e.g. non-essential drug therapy).2

For more information on cardiac pacemakers see the Geeky Medics article here.


Complications

Complications of sick sinus syndrome include:

  • Syncope and pre-syncope (sometimes with injury)
  • During periods of profound tachycardia (e.g. fast AF) patients may experience rate-related myocardial ischaemia, which can cause a troponin rise.
  • Tachyarrhythmias may also precipitate acute heart failure
  • Heart block due to AVN fibrosis
  • Patients with tachy-brady syndrome (paroxysmal atrial fibrillation) are at risk of thromboembolic events, such as stroke.
  • Sudden cardiac death (rare, more common in AVN disease/heart block)

Key points

  • Sick sinus syndrome is caused by dysfunction of the SAN, most commonly resulting in interspersed periods of tachycardia and bradycardia (tachy-brady syndrome).
  • The condition predominantly affects older adults (due to age-related fibrosis), although it can occur at any age.
  • The causes can be split into intrinsic and extrinsic causes however the exact cause is often difficult to identify and there may be multifactorial precipitants.
  • Arrhythmias seen may change over time and are often intermittent. Some important examples of arrhythmias include tachy-brady syndrome, sinus bradycardia and sinus arrest, sinoatrial exit block and slow atrial fibrillation.
  • Symptoms and signs occur due to arrhythmias which reduce cardiac output and lead to end-organ hypoperfusion.
  • Diagnosis requires correlation of ECG features and symptoms of bradycardia, which can be challenging given the intermittent nature of the disease.
  • Treatment involves removing any precipitating causes, implantation of a pacemaker and managing thromboembolic risk in those with atrial fibrillation or tachy-brady syndrome.
  • Complications of sick sinus syndrome include syncope (with possible injury), rate-related ischaemia, heart failure, thromboembolic events and sudden cardiac death. 

Reviewer

Dr Hazel White

Consultant Cardiologist with a special interest in complex device implantation

Mid Yorkshire Hospitals NHS Trust


Editor

Dr Chris Jefferies


References

  1. Tidy C (Patient.info Professional Reference). Sick Sinus Syndrome. Published in 2014. Available from: [LINK]
  2. Burns E (LITFL.com). Sinus Node Dysfunction (Sick Sinus Syndrome). Published in 2019. Available from: [LINK]
  3. Jabbour F (StatPearls). Sinus Node Dysfunction. Published in 2020. Available from: [LINK]
  4. Burns E (LITFL.com). Ventricular Escape Rhythm. Published in 2019. Available from: [LINK]
  5. Burns E (LITFL.com). Junctional Escape Rhythm. Published in 2019. Available from: [LINK]
  6. Lamas GA et al., Ventricular Pacing or Dual-Chamber Pacing for Sinus-Node Dysfunction. New England Journal of Medicine. Published in 2002. Available from: [LINK]

Figures

  • Figure 1. Madhero. The Electrical Conduction System of the Heart. License: [CC BY-SA]. Available from: [LINK]
  • Figure 2a. Fruitsmaak S. Two ECGs taken from a Patient with Tachy-Brady Syndrome. License: [CC BY-SA]. Available from: [LINK]
  • Figure 2b. Fruitsmaak S. Two ECGs taken from a Patient with Tachy-Brady Syndrome. License: [CC BY-SA]. Available from: [LINK]
  • Figure 3. Meyerson A. ECG Showing Sinus Bradycardia. License: [CC BY-SA]. Available from: [LINK]
  • Figure 4. CardioNetworks. ECG demonstrating a period of sinus arrest. License: [CC BY-SA]. Available from: [LINK]
  • Figure 5. Fruitsmaak S. St Jude Medical Pacemaker Device. License: [CC BY]. Available from: [LINK]
  • Figure 6. CardioNetworks. PA Chest Radiograph Demonstrating Left-Sided Dual-Chamber Permanent Pacemaker in situ. License: [CC BY-SA]. Available from: [LINK]

 

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