Varicocele is a mass of enlarged veins that develops in the spermatic cord, which leads from the testicles (testes) up through a passageway in the lower abdominal wall to the circulatory system. The spermatic cord is made up of blood vessels, lymphatic vessels, nerves, and the duct that carries sperm from the body (vas deferens). If the valves that regulate bloodflow from these veins become defective, blood does not circulate out of the testicles efficiently, which causes swelling in the veins above and behind the testicles.

A varicocele can develop in one testicle or both, but in about 85% of cases it develops in the left testicle. The left spermatic vein drains into the renal vein between the superior mesenteric artery and the aorta; these two arteries can compress the renal vein and thus impede bloodflow from the spermatic vein. The right spermatic vein drains into the vein that returns blood to the heart (vena cava) and develops varicocele less often. A one-sided (unilateral) varicocele can affect either testicle.

Because of the impaired circulation of blood created by a varicocele, the blood does not cool as it does normally. The increased temperature of the blood raises the temperature of the testes, which is believed to contribute to infertility, as heat can damage or destroy sperm. The increased temperature may also impede production of new, healthy sperm.

Incidence and Prevalence 
Incidence of varicocele is 10-20% and is highest in men between the ages of 15 and 25. The sudden appearance of varicocele in an older man may indicate a renal tumor blocking the spermatic vein.

Approximately 40% of infertile men have a varicocele and among men with secondary infertility—those who have fathered a child but are no longer able to do so—prevalence may be as high as 80%.