What type of Ventricular Septal Defect (VSD) is typically seen in Tetralogy of Fallot (TOF)?

Type of Ventricular Septal Defect in Tetralogy of Fallot

The ventricular septal defect (VSD) in Tetralogy of Fallot is predominantly a malalignment type VSD, resulting from anterocephalad deviation of the developing conal septum. 1

Characteristics of the VSD in Tetralogy of Fallot

Primary Type

• Malalignment VSD: The most common and characteristic type in TOF
  • Caused by anterocephalad deviation of the conal (outlet) septum
  • Results in the classic "overriding aorta" component of TOF
  • Located in the subaortic position in most cases

Anatomical Classification

VSDs in TOF can be further classified into specific subtypes:

1. Perimembranous VSD (most common in Western populations)

   • Located in the membranous septum with extension into the outlet septum
   • Often extends posteriorly beneath the septal leaflet of the tricuspid valve

2. Subarterial/Doubly Committed Subarterial VSD

   • More common in Asian populations than Western populations 2, 3
   • Located directly beneath both great arteries
   • Associated with higher rates of transannular patch use during repair (74.6-100%) 2, 3
   • Higher incidence of late residual VSD after repair (33.3% vs 6.45% in perimembranous VSDs) 3

3. Muscular Outlet VSD

   • Less common variant
   • Completely surrounded by muscle tissue

4. Doubly Committed Subarterial with Perimembranous Extension

   • Combined features of both types 4

Clinical Implications of VSD Type

Surgical Considerations

• VSD type affects surgical approach and outcomes:
  • Subarterial VSDs require more frequent transannular patch placement 2, 3
  • Continuous suturing technique has higher residual VSD rates compared to interrupted suturing (particularly important in TOF patients with poor myocardial quality) 5
  • Precise identification of VSD type preoperatively helps in surgical planning 4

Associated Features

• The malalignment VSD directly contributes to the other components of TOF:
  • Right ventricular outflow tract obstruction
  • Overriding aorta (by less than 50% of its diameter)
  • Right ventricular hypertrophy

Diagnostic Evaluation

• Echocardiography is the primary diagnostic tool
• Cardiac MRI provides detailed assessment of VSD type, size, and relationship to surrounding structures
• Multi-detector CT can identify VSD type and associated anomalies, particularly when evaluating complex variants of TOF 6

Management Implications

• Understanding the specific VSD type is crucial for:
  • Surgical planning and approach
  • Anticipating potential complications
  • Determining the need for transannular patch
  • Predicting risk of residual VSD after repair

The malalignment VSD is the pathological cornerstone of TOF, with its anterocephalad deviation of the conal septum leading to the cascade of anatomical abnormalities that define this congenital heart defect.