🫀 When TOF Mimics DORV

Why Severe Override Can Confuse the Diagnosis

In fetal echocardiography,

one of the most challenging scenarios is:

Severe TOF that looks like DORV.

When the aorta overrides extensively,

the distinction becomes subtle.

1️⃣ Why Does TOF Mimic DORV?

In classic TOF:

• Aorta overrides the septum
• Pulmonary stenosis present
• LV still contributes to aortic outflow

But in severe override:

• The aorta may appear almost entirely above the RV
• LV contribution looks minimal
• Great vessels may seem misaligned

At this stage, it can resemble DORV.

2️⃣ The Critical Difference

Even in severe TOF:

✔ There is still fibrous continuity between LV and aorta

✔ LVOT tracing will show connection

In DORV:

❗ Both great vessels originate predominantly from RV

❗ LV has no direct aortic continuity

3️⃣ Practical Ultrasound Tips

When unsure:

Step 1 — Trace LVOT carefully

Follow the flow from LV to aorta in multiple planes.

Step 2 — Assess override percentage

• <50% override → more consistent with TOF
• 50% override → consider DORV

(Note: percentage alone is not diagnostic.)

Step 3 — Evaluate vessel relationship

Are they crossing normally?

Or appearing parallel?

4️⃣ 3VT Pitfall

On 3VT:

• Both severe TOF and DORV may show Ao dominance
• V-shape may be distorted

👉 3VT alone cannot differentiate them.

Outflow tract imaging is essential.

5️⃣ Why This Distinction Matters

TOF:

• Standard repair pathway
• Pulmonary obstruction dominant issue

DORV:

• Surgical approach depends on VSD position
• May require complex rerouting

Prenatal counseling differs.

🔥 Bottom Line

Severe override does not automatically mean DORV.

The presence or absence of true LV–aorta continuity defines the diagnosis.