1. Loss of support to the aortic cusp.

2. Venturi effect- the high pressure jet sucks the aortic cusp down.

3. Defective commissure- this mechanism works only in perimembranous VSD, not in outlet VSD. In perimembranous VSD, the RCC-NCC commissure is defective.

4. Infective endocarditis causing damage to the aortic cusps- very rare.

 

Note that aortic cusp prolapse in VSDs can occur in perimembranous and outlet VSDs. In outlet VSDs, the RCC prolapses while in perimembranous VSDs, the RCC or the NCC can prolapse.