A hydrocele forms when fluid collects in the thin sac that surrounds a testicle, causing painless swelling in the scrotum. The underlying cause depends largely on age: in newborns, it’s almost always a leftover connection from fetal development, while in older children and adults, it typically results from injury, infection, or impaired fluid drainage. Understanding the specific mechanism behind yours helps predict whether it will resolve on its own or need treatment.
How Fluid Builds Up Around the Testicle
Each testicle sits inside a two-layered membrane called the tunica vaginalis. A small amount of fluid between these layers is normal and helps the testicle move freely. A hydrocele develops when the balance between fluid production and reabsorption tips in the wrong direction, and more fluid enters the space than the body can clear.
There are four basic ways this imbalance happens:
- An open channel to the abdomen that allows abdominal fluid to flow down into the scrotum (the congenital cause)
- Excess fluid production triggered by inflammation or injury near the testicle
- Poor fluid reabsorption by the surrounding tissue
- Blocked lymphatic drainage, most commonly from parasitic infection in tropical regions
The Congenital Cause in Newborns
During fetal development, each testicle descends from the abdomen into the scrotum through a small tunnel called the processus vaginalis. This tunnel normally seals shut before or shortly after birth. When it doesn’t close completely, abdominal fluid trickles down into the sac around the testicle, creating what’s called a communicating hydrocele.
This is the most common type in infants. The telltale sign is scrotal swelling that changes size throughout the day, often larger in the evening and smaller in the morning, because gravity pulls fluid down when the baby is upright and it drains back when the baby lies flat. Most of these hydroceles resolve on their own within the first year of life as the tunnel gradually closes. Surgery referral is typically considered only if the swelling persists beyond 12 to 18 months, or sooner if it’s large enough to cause discomfort or if an inguinal hernia develops alongside it.
A communicating hydrocele is considered more serious than a non-communicating one because the open channel can eventually allow a loop of intestine to push through, creating an inguinal hernia.
Non-Communicating Hydroceles
In a non-communicating hydrocele, the tunnel from the abdomen has closed, but fluid is still trapped around the testicle. This can happen in infants when fluid gets sealed in during development, or in adults when the tissue around the testicle produces more fluid than it absorbs. Without that open channel, the swelling stays roughly the same size day to day, and growth is slow. Many non-communicating hydroceles in babies also resolve on their own as the body gradually reabsorbs the trapped fluid.
Causes in Adults
When a hydrocele appears for the first time in an adult, the cause is usually something that irritates or damages the tissue around the testicle. The most common triggers are:
Infection. An infection of the testicle (orchitis) or the coiled tube behind the testicle where sperm mature (epididymitis) causes local inflammation. That inflammation increases fluid production in the surrounding membrane. Sexually transmitted infections are one recognized risk factor, though bacterial infections unrelated to STIs can have the same effect.
Injury. A direct blow or other trauma to the scrotum can damage the delicate tissue lining, leading to excess fluid production as part of the body’s healing response.
Surgery. Hydroceles can develop after operations in the groin area, particularly hernia repair. When surgery disrupts the lymphatic channels that normally drain fluid from the scrotum, fluid accumulates. The risk of a large, symptomatic hydrocele forming after pediatric inguinal hernia repair is low, around 0.06%, but it does happen and may appear years after the original procedure.
In many adult cases, no obvious trigger is ever identified. These are called idiopathic hydroceles, and they likely result from a gradual, age-related decline in the tissue’s ability to reabsorb fluid efficiently.
Parasitic Infection and Lymphatic Blockage
In tropical and subtropical regions, the most common cause of hydrocele is lymphatic filariasis, an infection spread by mosquitoes. The parasites responsible are microscopic roundworms, with one species (Wuchereria bancrofti) accounting for about 90% of cases worldwide. These worms nest inside lymphatic vessels and disrupt normal drainage. When the lymphatic system in the scrotum is blocked, fluid has no way to exit and steadily accumulates.
This mechanism is distinct from the inflammatory causes seen in Western countries. The lymphatic damage can be permanent, which is why filarial hydroceles tend to be large and progressive rather than self-limiting.
How a Hydrocele Is Identified
The classic test is transillumination: a light held against the swollen scrotum will glow through clearly if the swelling is fluid (a hydrocele), but won’t transmit light if the mass is solid. This simple check helps distinguish a hydrocele from other causes of scrotal swelling, like a tumor or a hernia containing bowel. An ultrasound is often used to confirm the diagnosis and rule out any underlying condition that may have triggered the fluid buildup.
Because an adult hydrocele can occasionally develop as a reaction to a testicular tumor or infection, the underlying cause matters as much as the hydrocele itself. When a hydrocele appears suddenly or grows quickly in an adult, imaging helps ensure nothing more serious is going on behind the fluid.
What Happens if a Hydrocele Doesn’t Resolve
A hydrocele that persists past infancy or develops in adulthood won’t typically go away without intervention. Small, painless hydroceles that aren’t growing may not need treatment at all. For those that become large or uncomfortable, surgical repair involves draining the fluid and closing or removing the sac to prevent recurrence. Recovery generally takes a few weeks, with some swelling and soreness in the days following the procedure.
Needle drainage is sometimes used as a temporary measure, particularly in people who aren’t good candidates for surgery. The drawback is that the fluid almost always returns because the underlying membrane is still producing more than it can reabsorb. The hydrocele itself doesn’t affect fertility or testosterone production, but persistent swelling can be uncomfortable and heavy enough to interfere with daily activities.