Veins are thin-walled vessels that permit the flow of blood in one direction: back to the heart. Tiny valves inside the veins prevent blood flow from reversing direction. Damage to valves in the scrotal veins causes blood to go the wrong direction and increases pressure in the veins. This causes the scrotal veins to bulge and prevents more valves from working normally.

Causes of a varicocele include:

• Scrotal injury
• Birth defect: abnormal veins form in the scrotum