Short QT syndrome

Short QT syndrome
Schematic representation of normal ECG trace (sinus rhythm), with waves, segments, and intervals labeled.
DiseasesDB	11105

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Synonyms and keywords: SQTS; short QT; short QTc; QT interval shortening

Overview

Short QT syndrome is a genetic disease of the electrical system of the heart. It consists of a constellation of signs and symptoms, consisting of a short QT interval on an EKG (≤ 300 ms) that does not significantly change with heart rate, tall and peaked T waves, and a structurally normal heart. Short QT syndrome appears to be inherited in an autosomal dominant pattern, and a few affected families have been identified.

Diagnosis

Diagnostic Criteria

Recent diagnostic criteria have been published out of the Arrhythmia Research Laboratory at the University of Ottawa Heart Institute from Drs. Michael H Gollob and Jason D Roberts.

The Short QT Syndrome diagnostic criteria is based on a point system as follows:

• QTc in milliseconds

<370 = 1 point

<350 = 2 points

<330 = 3 points

• J point - T peak interval in milliseconds

<120 = 1 point

• Clinical History

Sudden cardiac arrest = 2 points

Polymorphic VT or VF = 2 points

Unexplained syncope = 1 point

Atrial fibrillation = 1 point

• Family History

1st or 2nd degree relative with SQTS = 2 points

1st or 2nd degree relative with sudden death = 1 point

Sudden infant death syndrome = 1 point

• Genotype

Genotype positive = 2 points

Mutation of undetermined significance in a culprit gene = 1 point

The points are summed and interpreted as follows:

• > or equal to 4 points: High-probability of SQTS
• 3 Points: Intermediate probability of SQTS
• 2 points or less: Low probability of SQTS

Symptoms

Some individuals with short QT syndrome frequently complain of palpitations and may have unexplained syncope (loss of consciousness).

Genetic Testing

Mutations in the KCNH2, KCNJ2, and KCNQ1 genes cause short QT syndrome. These genes provide instructions for making proteins that act as channels across the cell membrane. These channels transport positively charged atoms (ions) of potassium into and out of cells. In cardiac muscle, these ion channels play critical roles in maintaining the heart's normal rhythm. Mutations in the KCNH2, KCNJ2, or KCNQ1 gene increase the activity of the channels, which changes the flow of potassium ions between cells. This disruption in ion transport alters the way the heart beats, leading to the abnormal heart rhythm characteristic of short QT syndrome. Short QT syndrome appears to have an autosomal dominant pattern of inheritance.

Short QT syndrome is associated with an increased risk of sudden cardiac death, most likely due to ventricular fibrillation.

Etiology

The etiology of short QT syndrome is unclear at this time. A current hypothesis is that short QT syndrome is due to increased activity of outward potassium currents in phase 2 and 3 of the cardiac action potential. This would cause a shortening of the plateau phase of the action potential (phase 2), causing a shortening of the overall action potential, leading to an overall shortening of refractory periods and the QT interval.

In the families afflicted by short QT syndrome, mutations have been described in three genes, KvLQT1, the human ether-a-go-go gene (HERG), and KCNJ2.

Treatment

Currently, some individuals with short QT syndrome have had implantation of an implantable cardioverter-defibrillator (ICD) as a preventive action, although it has not been demonstrated that cardiac problems have occurred before deciding to implant an ICD.

A recent study has suggested the use of certain antiarrhythmic agents, particularly quinidine, may be of benefit in individuals with short QT syndrome due to their effects on prolonging the action potential and by their action on the I_K channels. Some Trial are currently under way but do not show a longer QT statistically. Short QT syndrome is a genetic disease of the electrical system of the heart. It consists of a constellation of signs and symptoms, consisting of a short QT interval interval on EKG (≤ 300 ms) that doesn't significantly change with heart rate. Tall and peaked T waves are often present, and the heart is structurally normal. Short QT syndrome appears to be inherited in an autosomal dominant pattern, and a few affected families have been identified.

Symptoms and signs

Individuals with short QT syndrome frequently complain of palpitations and may have syncope (loss of consciousness) that is unexplained. Mutations in the KCNH2, KCNJ2, and KCNQ1 genes cause short QT syndrome. These genes provide instructions for making proteins that act as channels across the cell membrane. These channels transport positively charged atoms (ions) of potassium into and out of cells. In cardiac muscle, these ion channels play critical roles in maintaining the heart's normal rhythm. Mutations in the KCNH2, KCNJ2, or KCNQ1 gene increase the activity of the channels, which changes the flow of potassium ions between cells. This disruption in ion transport alters the way the heart beats, leading to the abnormal heart rhythm characteristic of short QT syndrome. Short QT syndrome appears to have an autosomal dominant pattern of inheritance.

Due to the autosomal dominant inheritance pattern, most individuals will have family members with a history of unexplained or sudden death at a young age (even in infancy), palpitations, or atrial fibrillation.

Short QT syndrome is associated with an increased risk of sudden cardiac death, most likely due to ventricular fibrillation.

Diagnosis

The diagnosis of short QT syndrome consists of characteristic history and findings on EKG and electrophysiologic testing. There are currently no set guidelines for the diagnosis of short QT syndrome.

• 
  Short QT syndrome is inherited in an autosomal dominant pattern.

Electrocardiogam

The characteristic findings of short QT syndrome on EKG are a short QT interval, typically ≤ 300 ms, that doesn't significantly change with the heart rate. Tall, peaked T waves may also be noted. Individuals may also have an underlying atrial rhythm of atrial fibrillation.

Electrophysiologic Studies

In the electrophysiology lab, individuals with short QT syndrome are noted to have short refractory periods, both in the atria as well as in the ventricles. Also, ventricular fibrillation is frequently induced on programmed stimulation.

Etiology

The etiology of short QT syndrome is unclear at this time. A current hypothesis is that short QT syndrome is due to increased activity of outward potassium currents in phase 2 and 3 of the cardiac action potential. This would cause a shortening of the plateau phase of the action potential (phase 2), causing a shortening of the overall action potential, leading to an overall shortening of refractory periods and the QT interval.

In the families afflicted by short QT syndrome, two different missense mutations have been described in the human ether-a-go-go gene (HERG). These mutations result in expression of the same amino acid change in the cardiac I_Kr ion channel. This mutated I_Kr has increased activity compared to the normal ion channel, and would theoretically explain the above hypothesis.

Treatment

Currently, the only effective treatment option for individuals with short QT syndrome is implantation of an implantable cardioverter-defibrillator (ICD).

A recent study has suggested that the use of certain antiarrhythmic agents, particularly quinidine, may be of benefit in individuals with short QT syndrome due to their effects on prolonging the action potential and by their action on the I_K channels. While the use of these agents alone is not indicated at present, there may be benefit of adding these agents to individuals who have already had ICD implantation to reduce the number of arrhythmic events.

See also

• Long QT syndrome

Additional resource

• Short QT syndrome