A varicocele forms when the small veins draining the testicle become swollen and enlarged, similar to varicose veins in the leg. The root cause is almost always a failure of the one-way valves inside these veins, which allows blood to pool and stretch the vessel walls instead of flowing back toward the heart. About 15% of all men have a varicocele, and the condition is far more common on the left side due to a quirk of anatomy.
How Venous Valves Fail
Blood leaving the testicle travels through a network of small veins called the pampiniform plexus. These veins contain tiny flap-like valves that open to let blood flow upward and close to prevent it from sliding back down under gravity. When those valves are absent from birth or stop functioning properly, blood reverses course and pools in the plexus. The sustained pressure gradually stretches and dilates the veins, producing the characteristic soft, worm-like swelling in the scrotum.
This valve dysfunction can be something you’re born with (some men simply have fewer valves in these veins) or it can develop over time as the valves weaken. Standing for long periods increases the gravitational load on these veins, which is why varicoceles tend to become more noticeable after prolonged upright activity and less prominent when lying down.
Why the Left Side Is Affected More Often
Varicoceles occur on the left side in roughly 80 to 90% of cases, and the reason is anatomical. The left testicular vein empties into the left renal vein at a right angle, creating a tall column of blood that must fight gravity to drain. The right testicular vein, by contrast, drains directly into the body’s largest vein (the inferior vena cava) at a gentler angle and a lower point, giving it a much easier path.
That right-angle junction on the left side generates higher hydrostatic pressure in the pampiniform plexus. Even with functioning valves, the left-side plumbing is under more strain. When the valves are weak or missing, the pressure difference makes left-sided reflux far more likely than right-sided reflux. A varicocele that appears only on the right side, or one that develops suddenly in an older man, can signal something else entirely and warrants imaging to rule out other causes.
Nutcracker Effect
In some men, the left renal vein gets physically compressed where it passes between two major blood vessels in the abdomen: the aorta and the superior mesenteric artery. This is sometimes called nutcracker syndrome, and it raises pressure inside the renal vein, making it harder for blood from the left testicular vein to drain into it.
When outflow from the renal vein is restricted, blood backs up into the testicular vein and the pampiniform plexus below it, causing venous congestion and dilation. The body may try to compensate by routing blood through collateral veins near the kidney and lumbar spine, but this often isn’t enough to fully relieve the pressure. The result is a persistent left-sided varicocele that may resist standard treatments if the underlying compression isn’t addressed.
What a Varicocele Does Inside the Scrotum
The pooling of warm venous blood raises scrotal temperature. In men with varicoceles, scrotal temperature is roughly 1.5°C higher than in men without them. That sounds minor, but testicular cells begin to undergo damage with increases of just 1 to 1.8°C. Sperm production is exquisitely temperature-sensitive, which is why the testicles sit outside the body in the first place.
The elevated temperature also triggers a cascade of oxidative stress. Cells in the testicle produce excess free radicals, which are unstable molecules that damage sperm membranes, fragment sperm DNA, and impair the proteins sperm need to function. The severity of this oxidative damage tends to track with the severity of the varicocele. Over time, both sperm count and sperm motility can decline, which is why varicoceles are the most common correctable cause of male infertility. They’re found in about 35% of men with primary infertility and up to 80% of men who previously fathered a child but can no longer conceive.
Secondary Causes to Be Aware Of
The vast majority of varicoceles have no sinister underlying cause. However, a varicocele that appears suddenly (especially in men over 40), occurs only on the right side, or stays swollen when lying flat can indicate a mass in the abdomen blocking venous drainage. Renal cell carcinoma is the most well-known culprit, because a kidney tumor can invade or compress the renal vein and obstruct outflow from the testicular vein. Retroperitoneal sarcoma, lymphoma, and a scarring condition called retroperitoneal fibrosis can produce the same effect.
These secondary varicoceles are uncommon, but their clinical pattern is distinctive: they don’t deflate in the supine position the way typical varicoceles do, and they tend to come on acutely rather than developing gradually during adolescence or early adulthood.
How Varicoceles Are Graded
Doctors classify varicoceles on a three-point scale based on physical examination:
- Grade 1: Only detectable when you bear down (a Valsalva maneuver) while standing. You can’t feel it at rest.
- Grade 2: Palpable at rest while standing, without needing to bear down.
- Grade 3: Visible through the scrotal skin without any special maneuvers.
Ultrasound can confirm the diagnosis and measure the degree of blood reflux, which helps guide treatment decisions. Grade 1 varicoceles that aren’t causing symptoms or fertility problems are typically monitored rather than treated.
When Repair Helps
Surgical repair (varicocelectomy) or a catheter-based procedure to block the faulty vein is considered when the varicocele is causing pain, testicular shrinkage, or abnormal semen parameters in a man trying to conceive. About 60 to 70% of men see measurable improvements in sperm quality after repair, including higher counts and better motility. Results tend to show up on semen analysis within three to six months, since a full cycle of sperm production takes roughly 72 days.
Not every varicocele needs treatment. Many men live with one their entire lives without pain or fertility issues. The decision to intervene depends on the grade, symptoms, and whether you’re actively trying to have children.