About IHRD’s

ICU cardiac monitorInherited Heart Rhythm Disorder Information


Inherited Heart Rhythm Disorders (IHRD’s) are a group of genetic diseases the effect the heart and make it susceptible to sudden cardiac arrest. Taken together these diseases represent one of the leading causes of death in young people.* Sudden cardiac arrest resulting from an underlying IHRD claims the lives of 700 young Canadians each year.

The similarities between the various IHRD’s make it practical and useful to consider them as group:

  • The preponderance of IHRD’s are acquired genetically from a gene carrying parent. The diseases are typically transmitted by an autosomal dominant gene meaning if one parent has the gene on average half of their children will have the disease, with no gender bias.
    • IHRD’s or versions of them may be acquired in various other ways including:
      • As a result of certain viral or bacterial infections such as Myocarditis an inflammation of the heart muscle or Pericarditis an inflammation of the sac around the heart
      • Through a de novo gene mutation which occurs at conception in the sperm, the egg or the embryo, is not transmitted by a parent and therefore only affects the child
      • Through a recessive gene, both parents must provide a copy
      • As a result of intense exercise or certain traumas which can lead to remodelling of the heart
  • All IHRD’s leave the patient susceptible to tachyarrhythmia (rapid, perhaps chaotic heart beats) which often devolve into cardiac arrest. These life threatening events may occur at any time in the patient’s life from birth through to old age and may have an obvious trigger such as exercise or emotional distress or no obvious trigger occurring during relative inactivity or even sleep.
    • From the onset of puberty through the late twenties to early thirties IHRD’s tend to particularly lethal
    • Many gene positive people will never experience tachyarrhythmia in their lifetime
  • Typically unseen problems with the heart’s structure or electrical system are the only manifestation of IHRD’s. In every other way the patient seems healthy and fit and therefore there is no obvious reason to suspect or test for an underlying heart condition.
    • For many patients the first symptom that they have the disease is death
    • For many other patients there are warning signs which if heeded can lead to diagnosis and effective therapy

The core group of IHRD’s divide into two categories Cardiomyopathies which affect the structure of heart muscle and Channelopathies which affect the heart’s electrical system. There is also a third group of diseases and conditions which are not IHRD’s but have some similarities and are known to cause sudden cardiac arrest in young people.

Cardiomyopathies affect the structure of heart muscle. If heart muscle becomes excessively thick of excessively weak it will impair the muscle’s ability to transmit the electrical signals which cause the heart to contract and relax in a rhythmic pattern.

  • Hypertrophic Cardiomyopathy (HCM) is the most prevalent of all of the IHRD’s with some experts suggesting as many as 1 in 500 people have the gene for the disease. It is the most common finding for cause of death on autopsy when young athletes die suddenly. HCM causes a thickening (hypertrophy) of the muscles most notably on the left side of the heart.
  • Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) is less far common at 1 in 5,000 to 7,500 but seems to claim a disproportionate number of young lives. The disease causes weakening (dysplasia, the replacement of healthy muscle tissue with fatty tissue) of the muscle on the right side of the heart. The disease is often referred to as Arrhythmogenic Right Ventricular Dysplasia (ARVD)

Channelopathies affect the function of the pathways which conduct electricity through heart muscle. These pathways are called calcium-ion channels, hence channelopathy. When the elements (calcium, potassium, sodium) that generate rhythmic electrical impulses get out of balance, perhaps as result of physical or emotional stress, a tachyarrhythmia leading to cardiac arrest may result.

  • Long QT Syndrome (LQTS) is the most prevalent – a recent study suggests 1 in 2,000 – and well known of the channelopathies. The first genetic association was identified in the early 1990’s and there are now no less than 13 variations of the disease, LQTS1-13. The common clinical presentation is an ECG that shows a resting QT interval of greater than .460 milliseconds in males
  • Brugada Syndrome may be as common as 1 in 2,000 worldwide. It has a complex genetic profile with over 160 possible gene mutations being identified to date, however it does often present very distinctly on a simple ECG test. The disease claims the lives of far more men than women and tachyarrhythmia events often occur without warning even when the patient is at rest.
  • Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) affects as many as 1 in 10,000 and is estimated to be the cause for 15% of unexplained cardiac related deaths of young people. Intense physical and emotional stress is a common trigger for tachyarrhythmia events. CPVT is difficult to detect on a resting ECG but may show up on an exercise/stress ECG
  • Short QT Syndrome is quite rare and as of now not well understood. In Long QT, the QT intervals on the ECG are often longer than .460 milliseconds while in Short QT they are often less than .370 milliseconds. The disease has a known association with sudden cardiac arrest.

Non-IHRD’s are diseases which may leave the patient at increased risk for sudden cardiac arrest but do not have all of the characteristics of an IHRD. They may or may not be genetic or familial in origin, they do not affect heart muscle or calcium-ion channels but do in some way affect heart function. These diseases may include other manifestations or symptoms which make the patient more readily identifiable. The types of awareness and screening programs that PACED endorses have the potential to benefit people living with these diseases. A partial list includes:

  • Wolff-Parkinson-White Syndrome (WPW) is thought to affect up to 1 in 1,000 people. There is often no clear genetic transmission. WPW is characterized by an accessory or extra electrical conduction pathway in the heart. If electrical impulses travel down the accessory pathway it can set-up the conditions for cardiac arrest. Many people have the extra pathway but never have a life threatening event.
  • Congenital Heart Defects (CHD) are problems with the heart’s structure (walls, valves, arteries etc.) that are present at birth and impair normal heart function, often leaving the patient susceptible to cardiac arrest. Many patients with minor, but perhaps worsening CHD’s may be identified when communities are more sensitive to the warning signs for IHRD’s.
  • Marfan Syndrome is a genetic disorder which affects skeletal and tissue development. The penetrance of the gene is highly variable so the affects can vary from mild to severe. Many Marfan patients are at increased risk for cardiac arrest
  • Kawasaki Disease is an immune system disease often caused by a viral infection usually seen in children under five. It may affect many organ systems but its rarest and most serious effect is on the heart, where it can cause lethal cardiac events in untreated children.

* Unless otherwise stated the term young people refers to everyone under the age of 35. This definition is commonly used in medical research and literature. Prior to the age of 35 very few people die or are severely affected by the processes of chronic diseases.