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.
1๏ธโฃ 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.
2๏ธโฃ The Critical Difference
Even in severe TOF:
โ There is still fibrous continuity between LV and aorta
โ LVOT tracing will show connection
In DORV:
โ Both great vessels originate predominantly from RV
โ LV has no direct aortic continuity
3๏ธโฃ Practical Ultrasound Tips
When unsure:
Step 1 โ Trace LVOT carefully
Follow the flow from LV to aorta in multiple planes.
Step 2 โ Assess override percentage
- <50% override โ more consistent with TOF
- 50% override โ consider DORV
(Note: percentage alone is not diagnostic.)
Step 3 โ Evaluate vessel relationship
Are they crossing normally?
Or appearing parallel?
4๏ธโฃ 3VT Pitfall
On 3VT:
- Both severe TOF and DORV may show Ao dominance
- V-shape may be distorted
๐ 3VT alone cannot differentiate them.
Outflow tract imaging is essential.
5๏ธโฃ 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.
๐ฅ Bottom Line
Severe override does not automatically mean DORV.
The presence or absence of true LVโaorta continuity defines the diagnosis.
