Table of Contents
The regular cycle of atrial and ventricular contractions results in blood being pumped effectively out of the heart. These contractions and relaxations of the heart are controlled by an electrical system. This electrical system may fail to function properly thereby interfering with effective pumping of blood. The heart may then beat faster than normal (tachycardia), slower (bradycardia), or irregularly. The abnormalities are called arrhythmias. The arrhythmia may occur from either the top or the bottom heart chambers and during the arrhythmia, a person may experience different sensations, feel nothing, the heart pounding, heaviness in chest, shortness in breath. However, under server circumstances of arrhythmia, the heart is not able to pump the blood efficiently and the person may experience a fainting episode and even die. This can if the heart beat is too slow or too rapid.
To find the type of arrhythmia, the heart's electrical system may be studied and the treatment that will ensue depends upon the type of arrhythmia and any difficulty it presents. Treatment includes medication, using a pacemaker, cardiac catheterization, or surgery. Some arrhythmias are congenital such as Long QT Syndrome (LQTS), Brugada Syndrome (BrS), atrial fibrillation e.t.c. Ideally, this means that the patient’s family members have a chance of having the disease.
Congenital Long Qt Syndrome (Lqts)
The period of activation and recovery of the myocardium is represented by the QT interval on the ECG. Extended recovery from ventricular electrical excitation increases the chances of refractory dispersion when parts of the myocardium may be refractory upon subsequent depolarization. Dispersion and repolarization occurs between three layers of the heart, and repolarization phase appear to be extended in the myocardium. This is the explanation to the normal wideness of the T wave and the interval characterizes transmural dispersion of repolarization (TDR).
LQTS is a genetic disorder of the cardiac electrical system presented by a longer QT interval on the ECG and an increased likelihood of sudden cardiac death. The electrical activities of the heart are produced by the flow of electrically charged ions of sodium, potassium, calcium, and chloride. LQTS manifests in response to defects in the heart’s electrical system, which cause delay in time taken for the electrical system to recharge after every beat. Longer Q-T intervals than normal increase torsade de pointes risks which is the life threatening type of ventricular tachycardia.
Studies indicate that “at least 10-15% of LQTS gene carries have a normal QTc duration.” consequently, LQTS remains a largely under-diagnosed disorder. Its prevalence is also difficult to determine but it is estimated to be 1 in 10,000 individuals. Future increase in frequency of diagnosis may provide more accurate estimates.
“Mortality, morbidity, and responses to pharmacologic treatment differ in the various types of long QT syndrome (LQTS). This issue is under investigation. LQTS may result in syncope and lead to sudden cardiac death, which usually occurs in otherwise healthy young individuals. LQTS is thought to cause about 4000 deaths in the United States each year. The cumulative mortality rate reaches approximately 6% by the age of 40 years. Although sudden death usually occurs in symptomatic patients, it happens with the first episode of syncope in about 30% of the patients. This occurrence emphasizes the importance of diagnosing LQTS in the presymptomatic period.”
While there is no differences relating to race have been found, there appears to be differences related to sex. Available data indicate that 60-70% of female patients are presented with LQTS more than males. Female patients generally have a longer QTc than male patience hence predominance of LQTS. LQTS usually manifests in children, adolescents, and young adults. However, the risk of LQTS is less in girls than in boys of less than 10 years.
Etiology and Genetics
QTS is broadly classified as either acquired or congenital LQTS. While acquired LQTS is caused by many medications, congenital LQTS is hereditary whereby the responsible gene code for the ion the ion channel is abnormal. There are various forms of congenital LQTS; the most common multiple ion abnormalities discovered include the LQT1, LQT2, LQT3, LQT4, LQT5, which are described by the type of channel that causes Long QT Syndrome. The LQT1, LQT2, and LQT3 accounts for the majority of LQTS at 45%, 45%, and 7%, respectively.
Romano-Ward Syndrome and Jervell, Lang-Nielsen Syndrome are the two main types of LQTS. In the Romano-Ward Syndrome, only one parent has LQTS and every child has 50% chance of inheriting the abnormal gene. Chances of the children developing LQTS are greater. In the Jervell, Lange-Nielsen Syndrome however, both parents are carriers of the abnormal gene. For further information, refer to table 1 below.
|Type of LQTS||Chromosomal Loci||Mutated Gene||Ion Current Affected|
|LQT1||11p15.5||KVLQT1 or KCNQ1 (heterozygotes)||Potassium (IKs)|
|LQT2||7q35-36||HERG, KCNH2||Potassium (IKr)|
|LQT4||4q25-27||ANK2, ANKB||Sodium, potassium and calcium|
|LQT5||21q22.1-22.2||KCNE1 (heterozygotes)||Potassium (IKs)|
|LQT6||21q22.1-22.2||MiRP1, KNCE2||Potassium (IKr)|
|LQT7 (Anderson syndrome)||17q23.1-q24.2||KCNJ2||Potassium (IK1)|
|LQT8 (Timothy syndrome)||12q13.3||CACNA1C||Calcium (ICa-Lalpha)|
|JLN1||11p15.5||KVLQT1 or KCNQ1 (homozygotes)||Potassium (IKs)|
|JLN2||21q22.1-22.2||KCNE1 (homozygotes)||Potassium (IKs)|
Table 1. Long QT syndrome Subtypes
Children and young people are at the highest risk of congenital LQTS. In addition, family members of people with congenital LQTS are also at risk of the disease. Congenital LQTS manifests by syncope (fainting), seizures, and sudden cardiac failure. It occurs in response to certain trigger including emotional stress, loud noise and exercise. These symptoms relate to torsade de pointes whereby the heart beat is very fast and irregular. The heart is unable to furnish the brain with adequate blood supply, hence the syncope and seizures. However, most congenital LQTS patients are asymptomatic and symptoms may only appear during the early teenage years. QT prolongation in LQTS is due to excess positively charged ions in the myocardial cells when ventricular repolarization takes place.
Prolongation of QT in LQTS may lead to torsade de pointes or polymorphic tachycardia. This may in turn lead to ventricular fibrillation and sudden death. It is stipulated that torsade de pointes is triggered by calcium channel reactivation, reactivation of delayed sodium current, and reduced outward potassium current which results in early afterdepolarizaton (EAD). Any condition that hastens reactivation of calcium channels increase risk for EAD.
History, Diagnosis and Prognosis
The first case of Long QT syndrome was observed in 1856, when a deaf girl died after a teacher yelled at her. The elder brother who was also deaf was reported to have died after a terrible fright.[ Tranebjaerg L, Bathen J, Tyson J et al (1999). "Jervell and Lange-Nielsen syndrome: a Norwegian perspective". Am J Med Genet 89 (89): 137–46.] this was described before the invention of the ECG. The first ECG-documented case was in 1957 by Anton Jervell and Fred Lange-Nielsenin 1964,[ Ward OC (April 1964). "A new familial cardiac syndrome in children". J Ir Med Assoc 54: 103–6] Owen Conor Ward, separately described the more common variant of LQTS with normal hearing, later called Romano-Ward syndrome.
Statistics show that 2.5% of the healthy have extended QT interval and over 10% of Long QT Syndrome patients have normal QT interval. It is therefore difficult to diagnose LQTS. The LQTS diagnostic score is the common criterion used to diagnose LQTS which is calculated by using QTc, Torsades de Pointes, T-wave in at least three leads, heart rate,syncope, congenital deafness, and family history. Four or more points indicate high probability of LQTS and one or less shows the probability is low.
The risk for LQTS patients who have not been treated and have events such as syncope and cardiac arrest can be effectively predicted from their gender, corrected QT interval, and their genotype (LQT1-8). Ellinor PT, Milan DJ, MacRae CA (2003). "Risk stratification in the long-QT syndrome". N. Engl. J. Med. 349 (9): 908–9.
Short Qt Syndrome
Until recently, emphasis was placed upon congenital LQTS. This is because it was easier to fathom the implications of an extended QT interval of an electrocardiogram. However, there is increasing evidence that a shorter than normal QT interval can also lead to increased risk of life threatening disease, Short QT Syndrome (SQTS). SQTS is a newly discovered congenital disease, which is “characterized by a shortened QT interval and by episodes of syncope, paroxysmal atrial fibrillation, or life-threatening cardiac arrhythmia. The syndrome usually affects young and healthy people with no structural heart disease and may be present in sporadic cases as well as in families.” Three genes associated with the condition have been identified and the biophysical basis already described. The mutation of the gene encoding the potassium ion channels result in SQTS.
BrS is an inherited arrhythmia which causes the ventricles to beat abnormally faster (a situation called ventricular fibrillation (VF)). This can thwart the efficient circulation of blood in the body and the patient will faint and or even die in a few minutes if the heart continues beating so fast. This syndrome has been described in all ages even though it is more prevalent in younger people. Therefore, it is recommended that everyone in a family is screened of the disease.
BrS is a disease of right bundle branch block. It is characterized by the ST segment elevation in V1 to V3 as well as sudden death. The clinical-ECG diagnosis is based on syncope or sudden death episodes in individuals with structurally normal heart with a typical electrocardiographic pattern: The ECG indicates ST segment elevation in the precordial leads V1 to V3, with morphology of the QRS complex resembling a right bundle branch block. This pattern of right bundle branch block is also known as J point elevation.
Syncope episodes and (aborted) sudden death are caused by the fast polymorphic ventricular tachycardias (PVT) or VF. These arrhythmias are asymptomatic, show no prolonged QT interval, and only in rare cases are there alternations of long and short sequences before PVT, a phenomenon common with other arrhythmias such as "torsade de pointes" in the LQTS. Unlike in the case of cathecolamine-dependent polymorphic ventricular tachycardia, there is no preceding acceleration in the heart rate.