Atrial septal defect

Atrial septal defect (ASD) is the second most common congenital heart defect after ventricular septal defects (VSDs). Diagnosis is often delayed until adulthood when complications arise such as atrial arrhythmias or pulmonary hypertension. The radiologist may be the first to suggest the diagnosis by recognising the typical features on a chest radiograph.

Terminology

The term ASD is used loosely to describe any communication that allows blood to shunt between the atria, including defects in the endocardial cushion, sinus venosus and coronary sinus.

The true atrial septum is the small area around the fossa ovalis which could be removed without creating an epicardial defect. The small size of this area of atrial contiguity is a consequence of the method of formation of the septum secundum, - by infolding of the atrial wall with associated epicardial tissues.

Although defects in the true septum are commonly known as ‘secundum ASDs’ they would be more accurately described as oval fossa defects. The anatomical ostium secundum is a defecthole that develops in the septum primum which permits foetal systemic venous return to bypass the developing lungs and this lies closer to the atrial roof. At autopsy, up to 34% of adults have a probe-patent tunnel running from the fossa ovalis to the ostium secundum, - termed a patentand this has been labelled ‘patent foramen ovale’.

Embryology

(For a full description see development of the heart.)

The foetal circulation must bypass the lungs until birth when oxygenation relies on the lungs and the pulmonary vasculature is sufficiently developed to function in series with the systemic circulation.

Division of the atrial chambers begins as the thin septum primum grows from the atrial roof towards the endocardial cushions at the atrioventicular junction. The ostium primum is the gap between the septum and the cushions and before this closes, an area nearer the roof must open, becoming the ostium secundum must open. This lies near the atrial roof.

The septum secundum forms by incomplete invagination of the atrial wall along the right atrial aspect of the septum primum leaving a bare area of septum primum aroundat the fossa ovalis. Blood continues to shunt from right to left through the patent foramen ovale, exiting through the ostium secundum. After birth and the onset of respiration, the left atrial pressure rises. This pushes the septum primum against the fossa ovalis (like a flap valve) causing functional closure. In about 2 out of 3 people adherence closes the tunnel over the ensuing months.

Epidemiology

Atrial septal defects account for 10% of congenital heart disease 7 and are more common in women, M:F = 2:1.

Associations

Atrial septal defects are seen in association with the following:

• Down syndrome: secundum ASD more commonly than primum ASD 11-13

• Holt-Oram syndrome

• Ellis-van Creveld syndrome

• mitral valve prolapse

• Lutembacher syndrome

• anomalous pulmonary venous return (especially with sinus venosus defects)

  • total anomalous pulmonary venous return (TAPVR)

  • partial anomalous pulmonary venous return (PAPVR)

Clinical presentation

Most patients are asymptomatic but as cardiac failure develops they may present with shortness of breath, palpitations, and weakness 7. Chest auscultation classically reveals an ejection systolic murmur heard at the left upper sternal border, attributed to increased flow across the pulmonary valve rather than blood shunting across the defect itself 7. 

ECG

There are characteristic ECG findings in patients with an atrial septal defect.

• incomplete right bundle branch block (RBBB morphology with QRS duration between 110-120 ms) 

  • increased specificity with crochetage sign in the inferior (II, III, aVF) leads

• right precordial (V1-3) "defective T waves"

  • describes biphasic morphology, initial T wave flattening or inversion and sharp, positive upstroke to terminal positive deflection 9

  • peak of the T wave delayed when compared to lateral precordial leads

• notching of the terminal upstroke of the R wave (crochetage sign) correlates with the size of ASD and implies a greater degree of shunting 10

• left axis deviation

  • characteristic of ostium primum defects and anatomical distortion of the left bundle branch fascicles

  • associated first degree AV block

• right axis deviation

  • suggests ostium secundum defect

• low atrial ectopic rhythms

  • negative P wave polarity in lead II

  • found in sinus venosus ASDs

Pathology

Classification

An understanding of complex cardiac development is important in understanding the numerous possible abnormalities that can arise. There are four major types of atrial septal defect (ASD) 4, distinguished according to their location within the septum:

• secundum ASD

  • 60-90% of all ASDs

  • usually an isolated abnormality

• primum ASD

  • 5-20%

  • associated with cleft anterior mitral valve leaflet (partial atrioventricular septal defect)

• sinus venosus

  • 5%

  • associated with anomalous right pulmonary venous return to the superior vena cava or right atrium

• coronary sinus type ASD("unroofed coronary sinus")

  • <1%

A patent foramen ovale (PFO) is a form of atrial septal defect.

Radiographic features

Plain radiograph

• can be normal in early stages when the atrial septal defect is small

• signs of increased pulmonary flow (pulmonary plethora or shunt vascularity)

  • enlarged pulmonary vessels

  • upper zone vascular prominence

  • vessels visible to the periphery of the film

  • eventual signs of pulmonary arterial hypertension

• chamber enlargement 

  • right atrium

  • right ventricle

  • note: left atrium is normal in size unlike VSD or PDA

  • note: aortic arch is small to normal

Treatment and prognosis

Atrial septal defects do not cause any impairment in cardiac function in utero and even most neonates are asymptomatic. The defect can be closed surgically or percutaneously (e.g. atrial septal occlusion device). However, careful evaluation has to be made to ensure lack of development of elevated right heart pressures or a right to left shunt before any intervention.  

Complications

In approximately 10% of untreated patients, pulmonary hypertension develops. In this situation, flow through the shunt eventually reverses and becomes right-to-left leading to cyanosis, known as Eisenmenger syndrome.

Other complications include:

• paradoxical emboli

• cardiac conduction defects, e.g. atrial fibrillation, atrial flutter

Terminology

The term ASD is used loosely to describe any communication that allows blood to shunt between the atria, including defects in the endocardial cushion, sinus venosus and coronary sinus.

The true atrial septum is the small area around the fossa ovalis which could be removed without creating an epicardial defect. The small size of this area of atrial contiguity is a consequence of the method of formation of the septum secundum, - by infolding of the atrial wall with associated epicardial tissues.

Although defects in the true septum are commonly known as ‘secundum ASDs’ they would be more accurately described as oval fossa defects. The anatomical ostium secundum is a defect that develops in the septum primum which permits foetal systemic venous return to bypass the developing lungs and this lies closer to the atrial roof. At autopsy, up to 34% of adults have a probe-patent tunnel running from the fossa ovalis to the ostium secundum, - termed a patent foramen ovale.

Embryology

(For a full description see development of the heart.)

The foetal circulation must bypass the lungs until birth when the pulmonary vasculature is sufficiently developed to function in series with the systemic circulation.

Division of the atrial chambers begins as the thin septum primum grows from the atrial roof towards the endocardial cushions at the atrioventicular junction. The ostium primum is the gap between the septum and the cushions and before this closes, an area nearer the roof becomes the ostium secundum, formed by coalescing perforations.

The septum secundum forms by incomplete invagination of the atrial wall along the right atrial aspect of the septum primum leaving a bare area at the fossa ovalis with a muscular rim. Blood continues to shunt from right to left through the patent foramen ovale, exiting through the ostium secundum. After birth and the onset of respiration, the left atrial pressure rises. This pushes the septum primum against the fossa ovalis (like a flap valve) causing functional closure. In about 2 out of 3 people adherence closes the tunnel over the ensuing months.

Epidemiology

Atrial septal defects account for 10% of congenital heart disease ⁷ and are more common in women, M:F = 2:1.

Associations

Atrial septal defects are seen in association with the following:

• Down syndrome: secundum ASD more commonly than primum ASD ^11-13

• Holt-Oram syndrome

• Ellis-van Creveld syndrome

• mitral valve prolapse

• Lutembacher syndrome

• anomalous pulmonary venous return (especially with sinus venosus defects)

  • total anomalous pulmonary venous return (TAPVR)

  • partial anomalous pulmonary venous return (PAPVR)