Risk factors for long QT syndrome include the following:

- female sex

- increasing age

- liver or renal impairment

- family history of congenital long QT syndrome

- pre-existing cardiovascular disease

- electrolyte imbalance: especially hypokalemia, hypocalcemia, hypomagnesemia

- concurrent administration of interacting drugs

Anorexia nervosa has been associated with sudden death, possibly due to QT prolongation. It can lead a person to have dangerous electrolyte imbalances, leading to acquired long QT syndrome and can in turn result in sudden cardiac death. This can develop over a prolonged period of time, and the risk is further heightened when feeding resumes after a period of abstaining from consumption. Care must be taken under such circumstances to avoid complications of refeeding syndrome.

The risk for untreated LQTS patients having events (syncopes or cardiac arrest) can be predicted from their genotype (LQT1-8), gender, and corrected QT interval.

- High risk (> 50%) - QTc > 500 ms, LQT1, LQT2, and LQT3 (males)

- Intermediate risk (30-50%) - QTc > 500 ms, LQT3 (females) or QTc < 500 ms, LQT2 (females) and LQT3

- Low risk (< 30%) - QTc < 500 ms, LQT1 and LQT2 (males)

A 1992 study reported that mortality for symptomatic, untreated patients was 20% within the first year and 50% within the first 10 years after the initial syncope.

The number of people affected by Brugada ECG is higher in Asia than in the United States and Europe. Specifically, Brugada Type 1 ECG appears more frequently in Asia (0%–0.36% of the population) and Europe (0%–0.25%) than in the United States (0.03%). Type 2 and Type 3 ECG is more prevalent in Asia (0.12%–2.23%) than in Europe (0.0%–0.6%) or the United States (0.02%).

It is the most common cause of sudden death in young men without known underlying cardiac disease in Thailand and Laos.

The cause 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.

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

Brugada syndrome (BrS) is a genetic condition that results in abnormal electrical activity within the heart, increasing the risk of sudden cardiac death. Those affected may have episodes of passing out. Typically this occurs when a person is at rest.

It is often inherited from a person's parent with about a quarter of people having a family history. Some cases may be due to a new mutation or certain medications. The abnormal heart rhythms can be triggered by a fever or increased vagal tone. Diagnosis is typically by electrocardiogram (ECG), however, the abnormalities may not be consistently present.

Treatment may be with an implantable cardioverter defibrillator (ICD). Isoproterenol may be used in those who are acutely unstable. In those without symptoms the risk of death is much lower, and how to treat this group is unclear. Testing people's family members may be recommended.

Between 1 and 30 per 10,000 people are affected. Onset of symptoms is usually in adulthood. It is more common in people of Asian descent. Males are more commonly affected than females. It is named after the Spanish cardiologists Pedro and Josep Brugada who described the condition in 1992. Their brother Ramon Brugada described the underlying genetics in 1998.

Romano–Ward syndrome is inherited in an autosomal dominant pattern. It is the most common form of inherited long QT syndrome, affecting an estimated 1 in 7,000 people worldwide. It should be mentioned that "long QT syndrome" has 6 different variations, therefore Romano–Ward syndrome is one of many

This condition is incredibly rare, with only 100 cases reported worldwide, however there are thought to be many cases that have been left undiagnosed. It is either inherited from at least one parent containing the mutated gene. or it can be gained through the mutation of the KCNJ2 gene.

Romano–Ward syndrome is the major variant of "long QT syndrome". It is a condition that causes a disruption of the heart's normal rhythm. This disorder is a form of long QT syndrome, which is a heart condition that causes the cardiac muscle to take longer than usual to recharge between beats; if untreated, the irregular heartbeats can lead to fainting, seizures, or sudden death

The prognosis for patients diagnosed with Timothy syndrome is very poor. Of 17 children analyzed in one study, 10 died at an average age of 2.5 years. Of those that did survive, 3 were diagnosed with autism, one with an autism spectrum disorder, and the last had severe delays in language development. One patient with atypical Timothy syndrome was largely normal with the exception of heart arrhythmia. Likewise, the mother of two Timothy syndrome patients also carried the mutation but lacked any obvious phenotype. In both of these cases, however, the lack of severity of the disorder was due to mosaicism.

Andersen–Tawil syndrome, also called Andersen syndrome and Long QT syndrome 7, is a form of long QT syndrome. It is a rare genetic disorder, and is inherited in an autosomal dominant pattern and predisposes patients to cardiac arrhythmias. Jervell and Lange-Nielsen Syndrome is a similar disorder which is also associated with sensorineural hearing loss. It was first described by Ellen Damgaard Andersen.

JLNS patients with "KCNQ1" mutations are particularly prone to pathological lengthening of the QT interval, which predisposes them to episodes of "torsades de pointes" and sudden cardiac death. In this context, if the patient has had syncopal episodes or history of cardiac arrest, an implantable cardiac defibrillator should be used in addition to a beta blocker such as propranolol.

Jervell and Lange-Nielsen syndrome (JLNS) is a type of long QT syndrome associated with severe, bilateral sensorineural hearing loss. Long QT syndrome causes the cardiac muscle to take longer than usual to recharge between beats. If untreated, the irregular heartbeats, called arrhythmias, can lead to fainting, seizures, or sudden death. It was first described by Anton Jervell and Fred Lange-Nielsen in 1957.

Timothy syndrome is a rare autosomal dominant disorder characterized by physical malformations, as well as neurological and developmental defects, including heart QT-prolongation, heart arrhythmias, structural heart defects, syndactyly (webbing of fingers and toes) and autism spectrum disorders.

Timothy syndrome often ends in early childhood death.

Knowledge that TdP may occur in patients taking certain prescription drugs has been both a major liability and reason for retirement of these medications from the marketplace. Examples of compounds linked to clinical observations of TdP include amiodarone, fluoroquinolones, methadone, lithium, chloroquine, erythromycin, amphetamine, ephedrine, pseudoephedrine, methylphenidate, and phenothiazines. It has also been shown as a side effect of certain anti-arrhythmic medications, such as sotalol, procainamide, and quinidine. The gastrokinetic drug cisapride (Propulsid) was withdrawn from the US market in 2000 after it was linked to deaths caused by long QT syndrome-induced torsades de pointes. In many cases, this effect can be directly linked to QT prolongation mediated predominantly by inhibition of the hERG channel.

In September 2011 (subsequently updated in March 2012 and February 2013), the FDA issued a warning concerning increased incidence of QT prolongation in patients prescribed doses of the antidepressant Celexa (citalopram) above 40 mg per day, considered the maximum allowable dosage, thereby increasing the risk of Torsades. However, a study, "Evaluation of the FDA Warning Against Prescribing Citalopram at Doses Exceeding 40 mg," reported no increased risk of abnormal arrhythmias, thus questioning the validity of the FDA's warning.

Common causes for torsades de pointes include diarrhea, low blood magnesium, and low blood potassium. It is commonly seen in malnourished individuals and chronic alcoholics, due to a deficiency in potassium and/or magnesium. Certain combinations of drugs resulting in drug interactions can contribute to torsades de pointes risk. QT prolonging medications such as clarithromycin, levofloxacin, or haloperidol, when taken concurrently with cytochrome P450 inhibitors, such as fluoxetine, cimetidine, or particular foods including grapefruit, can result in higher-than-normal levels of medications that prolong the QT interval in the bloodstream and therefore increase a person's risk of developing torsades de pointes. In addition, inherited long QT syndrome significantly increases the risk of episodes of TdP.

Atrial fibrillation increases the risk of heart failure by 11 per 1000, kidney problems by 6 per 1000, death by 4 per 1000, stroke by 3 per 1000, and coronary heart disease by 1 per 1000. Women have a worse outcome overall than men. Evidence increasingly suggests that atrial fibrillation is independently associated with a higher risk of developing dementia.

Several genetic causes of Loeys–Dietz syndrome have been identified. A "de novo" mutation in TGFB3, a ligand of the TGF ß pathway, was identified in an individual with a syndrome presenting partially overlapping symptoms with Marfan Syndrome and Loeys-Dietz Syndrome.

It is unknown if heart-hand syndromes are caused by shared or distinct genetic defects. It has been claimed that congenital heart diseases are caused by a limited number of shared genetic defects.

Holt–Oram syndrome, Brachydactyly-long thumb syndrome, Patent ductus arteriosus-bicuspid aortic valve syndrome and Heart-hand syndrome, Slovenian type are known to be autosomally dominant disorders.

Brachydactyly-long thumb syndrome is known to have been transmitted from male-to-male in a single instance.

22q11.2 deletion syndrome was estimated to affect between one in 2000 and one in 4000 live births. This estimate is based on major birth defects and may be an underestimate, because some individuals with the deletion have few symptoms and may not have been formally diagnosed. It is one of the most common causes of mental retardation due to a genetic deletion syndrome.

The prevalence of 22q11.2DS has been expected to rise because of multiple reasons: (1) Thanks to surgical and medical advances, an increasing number of people are surviving heart defects associated with the syndrome. These individuals are in turn having children. The chances of a 22q11.2DS patient having an affected child is 50% for each pregnancy; (2) Parents who have affected children, but who were unaware of their own genetic conditions, are now being diagnosed as genetic testing become available; (3) Molecular genetics techniques such as FISH (fluorescence in situ hybridization) have limitations and have not been able to detect all 22q11.2 deletions. Newer technologies have been able to detect these atypical deletions.

Recently, the syndrome has been estimated to affect up to one in 2000 live births. Testing for 22q11.2DS in over 9500 pregnancies revealed a prevalence rate of 1/992.

As there is no known cure, Loeys–Dietz syndrome is a lifelong condition. Due to the high risk of death from aortic aneurysm rupture, patients should be followed closely to monitor aneurysm formation, which can then be corrected with interventional radiology or vascular surgery.

Previous research in laboratory mice has suggested that the angiotensin II receptor antagonist losartan, which appears to block TGF-beta activity, can slow or halt the formation of aortic aneurysms in Marfan syndrome. A large clinical trial sponsored by the National Institutes of Health is currently underway to explore the use of losartan to prevent aneurysms in Marfan syndrome patients. Both Marfan syndrome and Loeys–Dietz syndrome are associated with increased TGF-beta signaling in the vessel wall. Therefore, losartan also holds promise for the treatment of Loeys–Dietz syndrome. In those patients in which losartan is not halting the growth of the aorta, irbesartan has been shown to work and is currently also being studied and prescribed for some patients with this condition.

If an increased heart rate is present, atenolol is sometimes prescribed to reduce the heart rate to prevent any extra pressure on the tissue of the aorta. Likewise, strenuous physical activity is discouraged in patients, especially weight lifting and contact sports.

Prior to modern cardiovascular surgical techniques and drugs such as losartan, and metoprolol, the prognosis of those with Marfan syndrome was not good: a range of untreatable cardiovascular issues was common. Lifespan was reduced by at least a third, and many died in their teens and twenties due to cardiovascular problems. Today, cardiovascular symptoms of Marfan syndrome are still the most significant issues in diagnosis and management of the disease, but adequate prophylactic monitoring and prophylactic therapy offers something approaching a normal lifespan, and more manifestations of the disease are being discovered as more patients live longer. Women with Marfan syndrome live longer than men.

A family history of AF may increase the risk of AF. A study of more than 2,200 people found an increased risk factor for AF of 1.85 for those that had at least one parent with AF. Various genetic mutations may be responsible.

Four types of genetic disorder are associated with atrial fibrillation:

- Familial AF as a monogenic disease

- Familial AF presenting in the setting of another inherited cardiac disease (hypertrophic cardiomyopathy, dilated cardiomyopathy, familial amyloidosis)

- Inherited arrhythmic syndromes (congenital long QT syndrome, short QT syndrome, Brugada syndrome)

- Non-familial AF associated with genetic backgrounds (polymorphism in the ACE gene) that may predispose to atrial fibrillation

During pregnancy, even in the absence of preconception cardiovascular abnormality, women with Marfan syndrome are at significant risk of aortic dissection, which is often fatal even when rapidly treated. Women with Marfan syndrome, then, should receive a thorough assessment prior to conception, and echocardiography should be performed every six to 10 weeks during pregnancy, to assess the aortic root diameter. For most women, safe vaginal delivery is possible.

Marfan syndrome is expressed dominantly. This means a child with one parent a bearer of the gene has a 50% probability of getting the syndrome. In 1996, the first preimplantation genetic testing (PGT) therapy for Marfan was conducted; in essence PGT means conducting a genetic test on early-stage IVF embryo cells and discarding those embryos affected by the Marfan mutation.

Males with pathogenic "MECP2" mutations usually die within the first 2 years from severe encephalopathy, unless they have an extra X chromosome (often described as Klinefelter syndrome), or have somatic mosaicism.

Male fetuses with the disorder rarely survive to term. Because the disease-causing gene is located on the X chromosome, a female born with an MECP2 mutation on her X chromosome has another X chromosome with an ostensibly normal copy of the same gene, while a male with the mutation on his X chromosome has no other X chromosome, only a Y chromosome; thus, he has no normal gene. Without a normal gene to provide normal proteins in addition to the abnormal proteins caused by a MECP2 mutation, the XY karyotype male fetus is unable to slow the development of the disease, hence the failure of many male fetuses with a MECP2 mutation to survive to term.

Females with a MECP2 mutation, however, have a non-mutant chromosome that provides them enough normal protein to survive longer. Research shows that males with Rett syndrome may result from Klinefelter's syndrome, in which the male has an XXY karyotype. Thus, a non-mutant "MECP2" gene is necessary for a Rett's-affected embryo to survive in most cases, and the embryo, male or female, must have another X chromosome.

There have, however, been several cases of 46,XY karyotype males with a MECP2 mutation (associated with classical Rett syndrome in females) carried to term, who were affected by neonatal encephalopathy and died before 2 years of age. The incidence of Rett syndrome in males is unknown, partly owing to the low survival of male fetuses with the Rett syndrome-associated MECP2 mutations, and partly to differences between signs caused by MECP2 mutations and those caused by Rett's.

Females can live up to 40 years or more. Laboratory studies on Rett syndrome may show abnormalities such as:

- EEG abnormalities from 2 years of age

- atypical brain glycolipids

- elevated CSF levels of "beta"-endorphin and glutamate

- reduction of substance P

- decreased levels of CSF nerve growth factors

A high proportion of deaths are abrupt, but most have no identifiable cause; in some instances death is the result most likely of:

- spontaneous brainstem dysfunction

- cardiac arrest, likely due to long QT syndrome, ventricular tachycardia or other arrhythmias

- seizures

- gastric perforation