Atrial septal defect overview

# Overview
Atrial septal defects refer to a group of congenital heart diseases that involve the inter-atrial septum.  The inter-atrial septum is the tissue that separates the right and left atria from each other. This tissue prevents arterial and venous blood from mixing with each other. If there is a defect in this septum, a direct communication between the atria can occur, which allows shunting, resulting in mixing of arterial and venous blood. It is possible for blood to travel from the left side of the heart to the right side of the heart, or vice versa. Shunting can result in cyanosis. The excess flow to the right atrium and the right ventricle causes enlargement of these organs, and the increased flow through the pulmonary artery eventually causes pulmonary hypertension.

# Anatomy
The embryological development of human fetal heart takes place as early as 4th week of intrauterine life. The normal septal development requires a proper alignment and fusion of the two interatrial septums i.e. septum primum and septum secundum. Failure to do so may lead to the development of atrial septal defects.

# Classification
The normal septal development requires a proper alignment and fusion of the two interatrial septums i.e. septum primum and septum secundum. Failure to do so may lead to a patent fossa ovalis and the development of atrial septal defects (ASDs). Atrial septal defects are classified into various types based on their location and the nature of the embryological defect. The types of atrial septal defects that can occur are:  ostium primum,  ostium secundum,  sinus venosus, common or single atrium and coronary sinus defects. Patent foramen ovale, commonly associated with atrial septal defects, is a sister condition involving communication between the two atria.  Patent foramen ovale is not a true atrial septal defect as flap like tissue that functions usually like a one way valve is present in a PFO.

# Pathophysiology
In the case of a large ASD (>9 mm), which may result in a clinically significant left-to-right shunt, blood will shunt from the left atrium to the right atrium causing excessive mixing of the blood between the two atria.  In a hemodynamically significant ASD, Qp is the pulmonary flow and Qs is the systemic flow.  If the Qp:Qs is > 1.5:1, then the patient is often symptomatic, and a repair of the ASD may be indicated.  This extra blood from the left atrium may cause a volume overload of both the right atrium and the right ventricle, which if left untreated, can result in enlargement of the right side of the heart and ultimately right-sided heart failure.
If left uncorrected, the pressure in the right side of the heart will be greater than the left side of the heart. This will cause the pressure in the right atrium to be higher than the pressure in the left atrium. This will reverse the pressure gradient across the ASD, and the shunt will reverse; a right-to-left shunt will exist. This phenomenon is known as Eisenmenger's syndrome. Once right-to-left shunting occurs, a portion of the oxygen-poor blood will get shunted to the left side of the heart and ejected to the peripheral vascular system. This will cause signs of cyanosis.  Once Eisenmenger's syndrome develops, the patient can no longer benefit from surgery to correct the defect.

# Causes
Often an ASD occurs sporadically. However, ASD has been associated with genetic disorders such as the Holt-Oram syndrome (heart-hand syndrome) , Down syndrome, Noonan syndrome,Treacher Collins syndrome, and the thrombocytopenia-absent radii (TAR) syndrome.

# Epidemiology and Demographics
Atrial septal defects are the most common form of congenital heart disease, accounting for 20-40% of all congenital heart disease cases in adults. Atrial septal defects can be classified into : ostium secundum, ostium primum, sinus venosus, coronary sinus ASDs and common or single atrium defects. Collectively, atrial septal defects account for 10% of all congenital heart disease. Infants and adolescent patients may be asymptomatic until later in life. By 40 years of age, approximately 90% of untreated atrial septal defect patients will experience an onset of symptoms.

# Risk Factors
As is common with most congenital heart conditions, the exact cause of atrial septal defect is not known. Research suggests a potential link between genetic conditions such as Down syndrome and the development of an atrial septal defect. Other potential causes include exposure to environmental contaminants such as rubella as well as alcohol consumption. A clinician should encourage families with a history of congenital heart defect to consider genetic counseling to identify potential risks.

# Natural History, Complications and Prognosis
The natural history of atrial septal defect depends on the location, the size of the defect, the amount of shunting of blood and the presence of associated congenital anomalies. A small atrial septal defect may remain asymptomatic throughout life and/or may close spontaneously in infants.  Patients with isolated atrial septal defects generally survive to adulthood.  An ASD can also present in adulthood with the insidious development of symptoms.  Complications like pulmonary hypertension, atrial fibrillation, right heart failure and stroke can develop if large sized defects are left uncorrected.  The mortality rate associated with surgical repair is less than 1% for patients under the age of 45, who have no history of heart failure or pulmonary artery hypertension.  Virtually all ASD patients develop symptoms by the age of 60.  An ASD that is repaired early in life is associated with no reduction in life expectancy.
Pregnancy causes an increase in cardiac output and stroke volume. This can cause an increased left-to-right shunting of blood. Despite the increased workload on heart, females with isolated asymptomatic atrial septal defects tolerate the pregnancy well. Pregnant females with an atrial septal defect may have increased frequencies of some complications for instance arrhythmias, thromboembolism, and bleeding. Despite this, there are no studies suggesting that pregnancy requires different indications for closure in pregnant females with atrial septal defect compared to a non-pregnant female with atrial septal defect. The ACC/AHA guidelines, however, do dictate clear deviations in course of treatment in certain special circumstances.  Pregnancy in patients with ASD and severe PAH (Eisenmenger syndrome) is not recommended owing to excessive maternal and fetal mortality and should be strongly discouraged.

# Diagnosis

## History and Symptoms
The development of symptoms associated with atrial septal defect relates to the size and severity of intracardiac shunting of blood across the defect. A large atrial septal defect will result in the presentation of symptoms at a younger age. However, smaller, less severe defects may be asymptomatic until adulthood. Smaller defects cause less hemodynamic disruptions. Symptoms such as difficulty breathing,exercise intolerance, and fatigue may be seen. As a person ages, the potential for the development of symptoms increases. Adults, especially those over the age of 40, will become symptomatic. Nearly all adults with an atrial septal defect will present with symptom onset by the age of 60.

## Physical Examination
Volume overload of the right side of heart can lead to right heart failure that may present with symptoms of swelling of the extremities, difficulty breathing and signs such as hepatomegaly and an elevated jugular venous pulse. 
On cardiovascular examinations there is a fixed splitting of second heart sound. Also, a  systolic ejection murmur that is attributed to the increased flow of blood through the pulmonic valve can be heard.

## Electrocardiogram
The ECG findings may show a prolonged PR interval (first degree heart block). The prolongation of the PR interval is probably due to the enlargement of the atria that is common in ASD. Other findings include right bundle branch block (RBBB), right axis deviation, right ventricular hypertrophy (RVH), atrial fibrillation and atrial flutter.

## Chest X Ray
Chest x rays may detect an atrial septal defect.  Chest x rays can be limited in imaging quality and may only supplement other imaging modalities. The chest x-ray may demonstrate cardiomegaly (right ventricular and right atrial enlargement), a prominent pulmonary artery segment, and increased pulmonary vascular markings.

## CT
Computed tomography can be helpful as a diagnostic tool in conditions where the echocardiographic findings are inconclusive. It is not the technique of choice as it has limitations in defining shunt volume and pressure differences.

## MRI
Magnetic resonance imaging (MRI) can be used as a diagnostic tool in identifying an atrial septal defect. It also helps in visualization of the pulmonary veins, quantifying right ventricular volume and shunt size. Velocity-encoded, phase difference MRI can assess the magnitude of left-to-right shunting as reliably as is done by cardiac catheterization . Also, it has the advantage of being a non-invasive tool compared to the invasive cardiac catheterization. Additionally, phase-contrast cine MRI is more reliable than spin-echo images in determining the size of defect that in turn helps to make a better therapeutics decision i.e. trans-cutaneous or the surgical closure .However, its current utility is limited for smaller defects and in inconclusive echocardiography findings.

## Echocardiography
Echocardiography along with doppler ultrasound are the preferred imaging modalities to diagnose atrial septal defect. On transthoracic echocardiography, an atrial septal defect may be seen on color flow imaging as a jet of blood from the left atrium to the right atrium. Usually transesophageal echocardiography is a better tool for diagnosing atrial septal defects than transthoracic echo. Trials have demonstrated that transesophageal echocardiography is superior to transthoracic echocardiography in diagnosing sinus venosus ASD.

## Transcranial Doppler Ultrasound
Trans-cranial doppler ultrasound is a diagnostic tool that involves a simple intravenous injection of saline under minimal sedation. It is primarily utilized to further diagnostically evaluate a patent foramen ovale defect.

## Cardiac Catheterization
Cardiac catheterization is a lesser utilized imaging modality. Many of the observations/visualizations made on a cardiac catheterization can be visualized confidently in non-invasive measures such as echocardiography. When echocardiography falls short, cardiac catheterization can be an effective tool in detection of the atrial septal defect. It is especially effective in determining the coronary anatomy of the atrial septal defect and surrounding structures.

## Exercise Testing
Exercise testing can be used to document changes in oxygen saturation. However, it is contraindicated in severe pulmonary artery hypertension.

# Treatment
Atrial septal defects can remain asymptomatic until the fourth decade of life. The treatment can involve either a medical or surgical approach. Surgical closure further could be divided into percutaneous repair or open heart surgery involving either suture or patch closure.

## Medical Therapy
Medical therapy has a limited role in the definitive treatment of ASD. Medical therapy consists rate control of atrial fibrillation and anticoagulation to reduce the risk of embolization among patients with atrial fibrillation.

## Surgery

### Indications for Surgical Repair in Adults
The decision to surgically close an atrial septal defect depends upon many contributing factors including the type of defect, the size of defect, the amount of left-to-right shunting, the development or worsening of symptoms, the presence of pulmonary hypertension and the presence of any associated anomalies.

### Surgical Closure
Surgical closure is the most common treatment method for atrial septal defect patients and has been the gold standard for many years. It is still the most popular method for repair of defects like sinus venosus ASD, coronary sinus ASD, or primum ASD. However, surgery is contraindicated in patients with severe irreversible pulmonary artery hypertension, Eisenmenger's syndrome and no evidence of a left-to-right shunt.

### Minimally Invasive Repair
Minimally invasive repair of atrial septal defect has been shown to be as successful as the conventional sternotomy, with the added advantage of being less invasive, less post-surgical complications and decreased hospital stay.

### Robotic Repair
Robotic repair is done through the 'da Vinci Surgical System' (Intuitive Surgical) is the most recent and advanced approach for the repair of atrial septal defect . Due to this technique a surgeon can perform operations from a remote distance. The surgeon gets an excellent three dimensional view of the heart through the fiberoptic stereoscopic camera. The surgeon's hand motions are relayed to a computer processor, which digitizes and relays them to the fine instrument placed into the chest through small port incisions.

### Percutaneous Closure
Percutaneous closure is commonly performed for ostium secundum ASDs.  This procedure is still not FDA approved for the treatment of other types of atrial septal defects like sinus venosus ASD, coronary sinus ASD or primum ASD. With appropriate patient selection, percutaneous closure of an ostium secundum ASD has been demonstrated to be as successful, safe and effective as surgical closure.  Additionally, percutaneous closure has been associated with fewer complications and a reduced average length of hospital stay compared to surgical care.

### Post Surgical Follow Up
Due to the development of new minimally invasive techniques, percutaneous closure and improvement in  surgical closure, most patients with atrial septal defect can start eating and ambulating within the first or second postoperative days. Also, most patients with  surgical closure are discharged by the third or fourth postoperative days and patients with percutaneous closure, are generally discharged the next day. Surgical follow-up care is mostly for 1-2 months. Ideally, at least 1 follow-up echocardiogram to confirm complete closure of the atrial septal defect should be obtained. A cardiologist with good experience with heart defects should continue patient care. An yearly follow up to monitor development of complications like arrhythmias should be arranged. Six months of aspirin with or without clopidogrel is recommended for prevention of thrombus formation.