Indirect inguinal hernias usually occur because of a persistent processus vaginalis. This leaves an empty peritoneal sac lying in the inguinal canal. The hernia becomes clinically evident when bowel or other abdominal content fills and enlarges the empty sac, creating a visible bulge. The hernia sac follows the tract of the spermatic cord down into the scrotal sac in men, or follows the round ligament in women to the pubic tubercle. Indirect hernias may be congenital (closely adherent to the vas deferens) or acquired (anatomically separate from the vas). Most hernias in women are indirect.
Direct hernias are always acquired and therefore unusual under the age of 25. A direct inguinal hernia occurs because of degeneration and fatty changes in the aponeurosis of the transversalis fascia that constitutes the inguinal floor or posterior wall in the Hesselbach triangle area. The Hesselbach triangle is defined inferiorly by the inguinal ligament, laterally by the inferior epigastric artery and vein, and medially by the lateral border of rectus abdominus. Most direct hernias do not have a true peritoneal lining and do not contain bowel, but mainly preperitoneal fat, and occasionally bladder. A large, long-standing direct hernia can extend into the scrotum. A large, longstanding, direct hernia can extend into the scrotum and can harbour bowel or abdominal content.
Because the structural defect in a direct hernia is most often a diffuse weakness and stretching of the inguinal floor tissues, rather than a discrete, sharply defined ring, it rarely strangulates. Strangulation is more common with indirect hernia, which has a narrow neck. As segments of the intestine prolapse through the defect in the anterior abdominal wall, they cause sequestration of fluid within the lumen of the herniated bowel. This initially impairs the lymphatic and venous drainage, which further compounds the swelling, and over time the arterial supply also becomes impaired. Gangrene ensues and, if left untreated, perforation occurs. Peritonitis occurs initially within the sac and then spreads to the peritoneal cavity.