Atrial Septal Defect

Overview

Definition

Atrial septal defect (ASD) is a communication between the right atrium (RA) and the left atrium (LA) secondary to a defect in the development of the atrial septum during embryogenesis. Atrial septal defect is usually a benign condition and is classified as an acyanotic congenital heart disease (CHD).

Classification

Atrial septal defects are classified based on the anatomic location of the defect:

• Ostium secundum (75%–90% of cases): 
  • The defect is usually in the middle of the interatrial septum, in the fossa ovalis.
  • Caused by early termination in the growth of the septum secundum or excessive absorption of the septum primum 
  • Associated with mitral valve prolapse and anomalous pulmonary venous return
• Ostium primum (15%):
  • Located on the lower part of the interatrial septum
  • Develops if the septum primum does not fuse with the endocardial cushion 
• Sinus venosus (10%): 
  • Located in the upper and/or part of the interatrial septum 
  • Caused by malposition of the insertion of the superior or inferior vena cavae 
• Coronary sinus (1%): 
  • Not a true ASD
  • The wall between the coronary sinus and the LA is missing.

Epidemiology

• Account for 10%–15% of all cases of CHD
• Incidence: 0.67–2.1 per 1,000 live births
• Girls > boys, with a girl-to-boy ratio of 2:1
• The mortality rate is low, at < 1%.

Etiology

• Ostium secundum usually occurs as an isolated defect but may be associated with a syndrome (genetic component):
  • Holt-Oram syndrome: autosomal dominant, associated with upper limb deformity and heart block
  • Noonan’s syndrome: autosomal dominant, associated with facial defects, deformity of the sternum, and clotting deficiencies
  • Treacher Collins syndrome: autosomal dominant, associated with craniofacial deformities and conductive hearing loss
• Ostium primum is mainly seen in patients with Down’s syndrome.

Pathophysiology

• Because the pressure in the LA is higher, the blood flows from the LA to the RA through the patent interatrial septum at the end of late systole and early diastole, forming a left-to-right shunt (L–R).
• The L–R shunt leads to an increase in volume in the right side of the heart, resulting in:
  • Pulmonary overcirculation → pulmonary hypertension → hypertrophy of the right ventricle (RV)
  • Dilatation of the RA → supraventricular arrhythmias (e.g., atrial flutter and fibrillation) 
• Eventually, the RV hypertrophy causes an increase in pressure in the right side → the shunt is reversed → Eisenmenger’s syndrome occurs
  • Eisenmenger’s syndrome: a condition in which an initially acyanotic heart defect with an L–R shunt is reversed by the right-sided hypertrophy; an R–L shunt is created, leading to cyanosis
• The L–R shunt also results in decreased blood volume being pumped into the systemic circulation → heart failure
• The larger the diameter of the defect, the bigger the blood flow and the more severe the symptoms will be.

Clinical Presentation

Signs and symptoms

The signs and symptoms depend on the size of the defect and the severity of the shunt:

• Small defects are usually asymptomatic.
• Infants and children with smaller defects may be diagnosed incidentally:
  • Due to a murmur upon physical examination
  • During an echocardiogram being performed for another reason
• Larger defects present in infancy or young children with:
  • Failure to thrive
  • Recurrent lung infections
  • Symptoms of heart failure:
    • Dyspnea during physical exertion
    • Tiring/fatigue when feeding (mostly in infants)
    • Difficulty breathing when lying down
    • Swelling of the legs
  • Paradoxical embolism can occur with ASDs: An embolus can enter the systemic circulation through the shunt, resulting in a stroke or acute limb ischemia.

Findings upon physical examination

Upon physical examination, the following can be seen/detected:

• General appearance: The patient may have dyspnea or lethargy.
• Precordial bulge due to atrial enlargement
• Murmur:
  • Increased flow at the pulmonary valve → mid-systolic ejection murmur (over the 2nd intercostal space)
  • Increased flow at the tricuspid valve → soft mid-diastolic murmur (over the lower left sternum)
  • Associated mitral regurgitation → holosystolic murmur (over the apex)
• Fixed, wide split S2
• Signs of pulmonary hypertension:
  • Increased jugular venous pressure and prominent ‘a’ wave
  • Accentuated pulmonary S2
  • Right-sided S4
  • Pleural effusion
• Signs and symptoms of heart failure:
  • S3 and S4 heart sounds
  • Displaced cardiac apex
  • Edematous lower limb
  • Hepatic congestion → ascites
  • Hepatomegaly

Diagnosis and Management

Diagnosis

• Echocardiogram confirms the diagnosis by detecting:
  • Size of the defect
  • Volume
  • Shunt ratio
  • Pulmonary arterial pressures
• Other supportive tests include:
  • Electrocardiogram (ECG) (changes are only seen in a large defect):
    • Incomplete right bundle branch block
    • Supraventricular tachycardia (atrial fibrillation or atrial flutter)
  • Chest X-ray:
    • Prominent pulmonary vasculature
    • Cardiomegaly

Management

• Yearly follow-up for cases of ostium secundum sub-type < 3 mm that close spontaneously by 2 years of age (most cases)
• Medical management for symptomatic ASDs:
  • Nutritional support
  • Manage heart failure.
  • Manage pulmonary hypertension.
  • Antibiotic prophylaxis until 6 months post-surgical repair 
• Surgical patching percutaneous repair: if the patient is symptomatic despite medical management
• Lung and heart transplant: if Eisenmenger’s syndrome has occurred

Prognosis

• Percutaneous closure has a success rate of 88%–98%. 
• Participation in sports depends on the state of the ASD and if there is pulmonary hypertension.

Clinical Relevance

The following conditions are associated with ASDs and can cause or modify the clinical features of ASDs: 

• Failure to thrive: suboptimal weight gain and growth in children. As with the majority of CHDs, the 1st presentation of ASD may be of an infant or child who is failing to meet the necessary caloric requirement for growth due to the increased demand of a failing heart and decreased blood flow as seen in ASDs.
• Pulmonary hypertension: characterized by elevated pulmonary arterial pressure due to pulmonary overcirculation. Pulmonary hypertension is a common complication of large ASDs. Hypertension causes the majority of the symptoms of ASD and is the main pathology behind Eisenmenger’s syndrome.
• Heart failure: the inability of the heart to supply the body with normal cardiac output to meet metabolic needs. The decreased volume into the left ventricle and, therefore, reduced stroke volume result in ventricular remodeling to maintain cardiac output. With time, the cardiac pump fails to keep up, and patients may present with heart failure in adulthood.
• Ebstein’s anomaly: a cyanotic CHD characterized by the downward displacement of the septal and posterior leaflets of the tricuspid valve. One of the associated features of Ebstein’s anomaly is an interatrial defect. In cases where an ASD presents with cyanosis, an echocardiogram must be done to rule out Ebstein’s anomaly.