Hydrocele: clinical features and mechanism

A hydrocele is fluid collecting in the tunica vaginalis, the thin, double-layered sac that surrounds the testis. It forms when the production and reabsorption (the sac's drainage of fluid back into the body) of that fluid fall out of balance. It can appear slowly or suddenly. In babies, the sac can also fill with fluid through a processus vaginalis, a connection between the abdomen and the scrotum that has stayed open.

Fluid around the testis explains the examination. Clear fluid makes the swelling typically painless and fluctuant, meaning soft and fluid-filled, so the fluid can be felt to move under the fingers. Clear fluid also lets light pass, so a pen torch held against the back of the scrotum makes the swelling glow. This is called transillumination. The fluid also surrounds the testis, so the testis usually cannot be felt separately from the swelling.

The examiner can also get above the swelling, meaning you can feel its upper edge within the scrotum. A mass that the examiner cannot get above is presumed to be a descended inguinal hernia (abdominal contents passing down the inguinal canal into the scrotum), and it should be referred to general surgeons.

Other scrotal lumps differ from a hydrocele in at least one of these findings. An epididymal cyst, a fluid-filled sac of the epididymis (the coiled tube behind the testis), also glows but sits apart from the testis. A varicocele, a tangle of dilated veins, feels like a bag of worms and is worse on standing. A testicular tumour is solid, so light does not pass through it.

In a young adult, a new hydrocele is investigated with an urgent scrotal ultrasound scan to look for an underlying testicular cause. A hydrocele is usually only operated on when it causes symptoms or for cosmetic reasons.