A hydrocele is a fluid-filled sac that surrounds a testicle. It causes swelling, and is common (1 to 2 percent) among newborns, often disappearing without treatment within 12 months. It can, however, form in teens or adults as a result of an injury to or inflammation of the the scrotum. Babies born prematurely are more likely to have the condition.

In babies, a hydrocele develops in utero. Under normal circumstances, testicles descend from a baby's abdominal cavity into his scrotum. Accompanying each testicle is a sac which allows fluid to surround the testicles. That sac then closes and the fluid is absorbed. If the fluid remains after the hydrocele closes, it will usually be slowly absorbed within the baby's first year. This condition is described as a “non-communicating hydrocele.” If the hydrocele stays open, however, it may chance size or, if compressed, fluid from it may flow back into the abdomen. That's a “communicating hydrocele.”

Hydroceles form in men over the age of 40 more commonly than younger men. They are borne of scrotal injuries or infection. The infection may be the result of sexually transmitted disease.

You may first suspect you are having hydrocele issues if you or your child experience swelling of the scrotum. Act quickly if this occurs shortly after receiving an injury to the scrotum, as the symptom may also be indicative of blocked blood flow in a twisted testicle. This condition – known clinically as testicular torsion – must be treated within hours of the beginning of symptoms to save the testicle.

Note that in most cases a hydrocele is not a cause for alarm, as it does not affect fertility. It may, however, be a symptom of a bigger problem and should be checked out. 

Your doctor will likely begin his diagnosis with a physical examination, checking for tenderness in the enlarged scrotum, shining a light through the scrotum, and applying pressure to the abdomen and scrotum to check for inguinal hernia. Blood and urine samples will likely be taken and analyzed to gauge for infection. An ultrasound image may be taken, in search of a testicular tumor or some other source of the swelling.

In most circumstances, a hydrocele will go away on its own. If, however, it persists past 12 months in a newborn or is very uncomfortable in an adult, it may be surgically removed. The procedure may be performed as an outpatient under general or local anesthesia.

Sources: The Mayo Clinic