How does a varicocele cause infertility?
We are certain that varicoceles decrease fertility but we have not yet categorically determined why this is so. There are several theories:
A) Increased temperature of the testicles
The testicles are located in the scrotum, which effectively regulates their temperature. They are maintained at a temperature slightly below body temperature. (This is probably why they are located outside the body rather than inside the body where they clearly would be better protected.) In cold weather you may notice that a man's testicles move close into his body as the cremasteric muscles, the muscles in the scrotum wall, tighten. In warm weather the cremasteric muscles relax and lengthen allowing the testicles to hang away from a man's body and cool down.
Some babies are born without their testicles having descended into their scrotum. They are trapped somewhere in their bodies and constantly exposed to body temperature. This is so harmful for the testicles that if they remain there past puberty they will stop producing sperm altogether and have a higher chance of developing cancer. Therefore if a boy's testicles do not descend into the scrotum by the time he is 12 months old, they should be surgically brought down and placed into the scrotum.
Varicoceles are a group of dilated veins filled with blood, which surround the testicles. The blood is at body temperature and if the testes are near these veins they will be kept at a higher temperature than is beneficial for them. Even if a man has a varicocele only on one side, the whole scrotum is warmed by the blood and both testicles can be negatively affected.
In general, larger testicles make more sperm than smaller testicles. Often however, you see men who have a large one-sided varicocele that has damaged the testis on one side making it smaller. The small teste makes significantly less sperm than the normal one. However even in the "normal" one the sperm quality is often very low. The varicocele is not only damaging the teste on the side where it is found but also suppressing the sperm production on the opposite (better) side.
When a varicocele is repaired the blood is no longer able to flow back into the scrotum. This affects not only the testes on that side, but also the opposite side with this normalization of temperature, there may be some dramatic improvement in sperm production. This improvement is most likely mostly coming from improved production in the larger better testicle.
B) Increased waste products back-flowing into the testicle
The veins draining the testicles connect into larger veins. On the left
side, they drain into the kidney vein, which is draining blood from the
kidney. The blood from the kidney carries waste products, which may then
drain backwards into the scrotum and collect there. This may negatively
effect sperm production.