Infantile hydroceles and inguinal hernias are congenital defects of the processus vaginalis. The processus vaginalis is derived from a layer of peritoneum that the testis passes through after 12 weeks' gestation on route through the inguinal canal to the scrotum.
The processus vaginalis normally becomes obliterated around 32 weeks gestation. However, if it persists then it extends as a hollow tube or sac of peritoneum that extends from the external ring towards the external genitalia. If the opening is narrow, it only allows peritoneal fluid to track down and manifest as a hydrocele. If the opening is large then larger organs can descend along this pathway, e.g. intestines, and this constitutes a hernia and manifests as a lump in the groin.
The factors causing obliteration of the sac are unknown.
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