Summary
A varicocele is when the veins in your scrotum become enlarged and cease to function effectively due to inadequate valves. Effectively, it's a varicose vein but in your scrotum. Unfortunately, they are very common, occuring in approximately 15–20% adult male population.
The effect of varicocele is to prevent testosterone from leaving the scrotum and entering the bloodstream, it can also cause the testosterone and dihydrotestosterone to go the wrong way, backing up to your prostate causing concentrations in the order of 300% normal and causing benign prostatic hyperplasia (BPH). The discovery of this effect explains some of the paradoxical data related to testosterone, dihydrotestosterone and prostate growth - whereby 5-alpha-reductase (5AR) inhibitors that reduce men's production of dihydrotestosterone improved BPH, but giving men dihydrotestosterone shrunk the prostate. This all makes sense because injected dihydrotestosterone will reduce the body's own production in the testes, so it won't back up to the prostate.
Varicocele can be treated surgically, or by supplementing with aescin-escin. Carnitine may also help.
Prevalence
Varicocele are very common causes of male infertility
https://www.ncbi.nlm.nih.gov/pubmed/8458466/
"A varicocele was palpable in 35% (352/1,001) of men with primary infertility and 81% (79/98) of men with secondary infertility."
Prevalence estimated to be 15-20% of adult male population
https://www.ncbi.nlm.nih.gov/pubmed/11212076
Consequences
Varicocele Is Associated with Impaired Semen Quality and Reproductive Hormone Levels: A Study of 7035 Healthy Young Men from Six European Countries.
https://www.ncbi.nlm.nih.gov/pubmed/27423503
- " testosterone and free testosterone were not significantly different between men with and without varicocele"
- "Varicocele were associationed with higher serum levels of follicle-stimulating hormone, lower inhibin B, and higher levels of luteinising hormone"
- "Increasing varicocele grade was associated with poorer semen quality, even in grade 1 varicocele. In grade 3 varicocele, sperm concentration was less than half of that in men with no varicocele."
Impact of surgery
Review: Surgical varicocele repair can improve serum testosterone in men
https://www.ncbi.nlm.nih.gov/pubmed/28078218
Microsurgical varicocelectomy resulted in significant increases in the serum testosterone level, independent of the varicocele grade.
https://www.ncbi.nlm.nih.gov/pubmed/23561709
Microsurgical varicocelectomy results higher serum testosterone and improved erectile function
https://www.ncbi.nlm.nih.gov/pubmed/20722780
Microsurgical repair of varicocele improves testosterone, erectile and ejaculatory function.
https://www.ncbi.nlm.nih.gov/pubmed/27196029
- There are some recent studies that suggest that supplements like carnitine and aescin-escin can produce similar results to surgery.