A short story of the short QT syndrome
ABSTRACTShort QT syndrome is a recently recognized cause of cardiac rhythm disorders, including sudden cardiac death. Although the syndrome is rare, its potential lethality justifies routinely screening the electrocardiograms of patients with syncope or unexplained atrial or ventricular arrhythmias to look for this diagnosis. This review discusses recent advances in the understanding of the pathogenesis of this syndrome and outlines some of the challenges in establishing the diagnosis.
KEY POINTS
- Short QT syndrome is a genetic disease described initially in young patients who had atrial fibrillation or who died suddenly with no apparent structural heart disease.
- The diagnosis is established by the finding of a short QT interval. However, other factors including personal and family history are also important in establishing the diagnosis.
- The current recommendations for managing patients with short QT syndrome are not evidence-based. We encourage consultation with centers that have special interest in QT-interval-related disorders.
- Placement of an implantable cardioverter-defibrillator is considered the standard of care, especially in survivors of sudden cardiac death, ventricular fibrillation, or ventricular tachycardia. Unfortunately, a higher incidence of inappropriate shocks adds to the challenges of managing this potentially deadly disease.
Five types of short QT syndrome
Short QT syndrome 1. In 2004, Brugada et al25 identified the first mutation that causes abnormal shortening of the action potential duration. In contrast to the mutations that underlie long QT syndrome, this mutation actually causes a gain of function in the gene coding the rapidly acting delayed potassium current (IKr) channel proteins KCNH2 or HERG. Potassium leaving at a more rapid rate causes the cell to repolarize more quickly and shortens the QT interval. The clinical syndrome associated with KCNH2 gene gain-of-function mutation is called short QT syndrome 1.
Short QT syndromes 2 and 3. Other IK (potassium channel) proteins have been implicated as well. Gain-of-function mutations in the KCNQ1 and KCNJ2 genes are believed to account for short QT syndromes 2 and 3, respectively. KCNQ1 codes for the IKs protein, and KCNJ2 codes for the IK1 protein.26,27
Short QT syndromes 4 and 5 were identified by Antzelevitch et al,28 who described several patients who had a combination of channel abnormalities and ECG findings. Their ECGs showed “Brugada-syndrome-like” ST elevation in the right precordial leads, but with a short QT interval. These new syndromes were found to be associated with genetic abnormalities distinct from those of Brugada syndrome and other short QT syndromes. These abnormalities involved loss-of-function mutations in the CACNA1C gene (which codes for the alpha-1 subunit of the L-type cardiac calcium channel) and in the CACNB2 gene (which codes for the beta-2b subunit of the same channel). The two defects correspond to the clinical syndromes short QT syndrome 4 and short QT syndrome 5, respectively.28
MECHANISM OF ARRHYTHMOGENESIS IN SHORT QT SYNDROME
The myocardium is made of different layers: the epicardium, the endocardium, and the middle layer of myocytes composed mainly of M cells. Cells in the different layers differ in the concentration of their channels and can be affected differently in various syndromes. When cells in one or two of the layers repolarize at a rate different from cells in another layer, they create different degrees of refractoriness, which establishes the potential for reentry circuits to form.
It is believed that in short QT syndrome the endocardial cells and M cells repolarize faster than the epicardial cells, predisposing to reentry and arrhythmias. This accentuation of “transmural dispersion of repolarization” accounts for arrhythmogenesis in short QT syndrome as well as in long QT syndrome and the Brugada syndromes. The difference between these syndromes appears to be the layer or area of the myocardium that is affected more by the channelopathy (the M cells in long QT syndrome and the epicardium of the right ventricle in the Brugada syndrome).29
WHEN TO THINK OF SHORT QT SYNDROME
In any survivor of sudden cardiac death, the QT interval should be thoroughly scrutinized, and family members should undergo ECG. Patients in whom a short QT interval is incidentally discovered and for which other reasons are ruled out (see differential diagnosis) should be encouraged to have family members undergo ECG. Other potential patients are young people who develop atrial fibrillation and patients who have idiopathic ventricular fibrillation.4
TREATMENT AND PROGNOSIS
Evidence-based recommendations for the management of short QT syndrome do not yet exist, mainly because the number of patients identified to date is small.
Implantable cardioverter-defibrillators
Although placing an implantable cardioverter-defibrillator (ICD) seems to be warranted in patients who experience ventricular fibrillation, ventricular tachycardia, or aborted cardiac death, or in patients who have a family history of the same symptoms, the best management option is less clear for patients who have no symptoms and no family history.30 In addition, some patients may not want an ICD or may even not qualify for this therapy.
A unique problem with ICDs in short QT syndrome stems from one of the syndrome’s main features on ECG: the tall and peaked T wave that closely follows the R wave can sometimes be interpreted as a short R-R interval, provoking an inappropriate shock from the ICD.31
For the above reasons, we strongly encourage consulting a center with expertise in QT-interval-related disorders before placing an ICD in a patient suspected of having short QT syndrome.
Antiarrhythmic drugs
Prolongation of the QT interval (and the effective refractory period) with drugs has been an interesting area of research. Gaita et al32 studied the effect of four antiarrhythmics—flecainide (Tambocor), sotalol (Betapace), ibutilide (Corvert), and quinidine—in six patients with short QT syndrome. Only quinidine was associated with significant QT prolongation, from 263 ± 12 ms to 362 ms ± 25 ms. This resulted in a longer ventricular effective refractory period (> 200 ms), and ventricular fibrillation was no longer inducible during provocative testing.
In a recent study of long-term outcomes of 53 patients with short QT syndrome, Giustetto et al33 noticed that none of the patients taking quinidine, including those with a history of cardiac arrest, had any further arrhythmsic events. On the other hand, the incidence of arrhythmic events during the follow-up was 4.9% per year in patients not taking this drug. Quinidine had a stronger effect on the QT interval in patients with the HERG mutation than in those without.
RESEARCH MAY LEAD TO A BETTER UNDERSTANDING OF OTHER DISEASES
The short QT syndrome is one of the most recently recognized cardiac channelopathies associated with malignant arrhythmias. As with long QT syndrome, research in short QT syndrome may lead to a better understanding of the pathogenesis of more common but still poorly understood arrhythmias such as lone atrial fibrillation and idiopathic ventricular fibrillation.
