How do you differentiate between ostium primum and ostium secundum (secondum) atrial septal defects (ASDs) in a patient?

Differentiating Ostium Primum from Ostium Secundum Atrial Septal Defects

The key to distinguishing ostium primum from ostium secundum ASDs lies in their anatomical location and associated features: secundum defects are centrally located in the fossa ovalis region, while primum defects sit inferiorly at the crux of the heart adjacent to the atrioventricular valves with loss of normal AV valve offset and nearly always accompanied by a cleft anterior mitral valve leaflet. 1, 2, 3

Anatomical Location

Ostium Secundum ASD:

• Located centrally in the middle of the interatrial septum in the region of the fossa ovalis 1, 2, 3
• Represents 75% of all ASD cases 1
• Results from deficiency in the septum primum, excessive resorption of the septum primum, or deficiency in the septum secundum 2
• Can occur in the superior part of the interatrial septum as a variant 1

Ostium Primum ASD:

• Positioned inferiorly near the crux of the heart, in close proximity to the atrioventricular valves 1, 3
• Represents 15-20% of all ASD cases 1
• Characterized as a variant of atrioventricular septal defect (AVSD) 2

Critical Echocardiographic Features

For Ostium Secundum:

• The defect appears centrally in the fossa ovalis on subcostal and parasternal views 3
• Normal offset between tricuspid and mitral valves is preserved 1
• 3D transesophageal echocardiography shows the defect in the middle of the interatrial septum on en-face views from the right atrial perspective 1

For Ostium Primum:

• Absence of the usual offset between the atrioventricular valves is pathognomonic 1, 3
• The defect is located just above the atrioventricular valve plane 2
• A cleft in the anterior mitral valve leaflet is nearly always present 1
• No interventricular communication is present below the atrioventricular valve (distinguishing it from complete AVSD) 2
• Separate right and left atrioventricular valvular orifices with varying degrees of malformation of the left-sided component 2

Imaging Approach Algorithm

1. Start with transthoracic echocardiography using subcostal views with deep inspiration and high right parasternal views 3

2. Assess the entire atrial septum from superior vena cava orifice to inferior vena cava orifice 3

3. Identify defect location:

   • Central/fossa ovalis region → secundum ASD 3
   • Inferior/adjacent to AV valves → primum ASD 3

4. Evaluate AV valve relationship:

   • Normal offset preserved → secundum ASD 1
   • Loss of offset → primum ASD 1, 3

5. Examine the mitral valve:

   • Normal mitral valve (though prolapse may occur) → secundum ASD 1
   • Cleft anterior mitral leaflet → primum ASD 1

6. Use 3D transesophageal echocardiography when transthoracic images are suboptimal or for definitive characterization and intervention planning 1, 3

Associated Features to Assess

Ostium Secundum may be associated with:

• Mitral valve prolapse (frequently) 1
• Partial anomalous pulmonary venous drainage (small number of cases) 1
• Valvular pulmonic stenosis or flow-related RV outflow gradient 1

Ostium Primum is associated with:

• Cleft anterior mitral valve leaflet (nearly always) 1
• Discrete subaortic stenosis (may develop postoperatively) 1
• Part of the spectrum of atrioventricular septal defects 2

Common Diagnostic Pitfalls

• Sinus venosus defects are often missed because they are located superiorly near the SVC entry and may be mistaken for secundum defects on suboptimal imaging 1, 3, 4
• Apparent septal dropout on 2D images may mimic an ASD; use contrast echocardiography or TEE to confirm true defects 3
• Patients with unexplained RV volume overload on transthoracic echo should undergo TEE to exclude missed primum or sinus venosus defects 3

Clinical Implications for Management

Both defect types require assessment of:

• Right ventricular volume overload and enlargement (hallmark of hemodynamically significant shunting) 3
• Qp/Qs ratio ≥1.5:1 indicating need for closure 5
• Pulmonary vascular resistance and pressures 5

Treatment differs significantly:

• Secundum ASDs are amenable to percutaneous device closure in the cardiac catheterization laboratory 6, 7
• Primum ASDs require surgical correction due to their location and associated mitral valve abnormalities 6, 7

The distinction between these two defect types is critical because it fundamentally determines the treatment approach and has implications for long-term outcomes and associated complications.