Essential to the understanding of hernias are the anatomic characteristics of the abdominal cavity and, in particular, its fascial and aponeurotic layers. Embryologic development produces localized areas of inherent weakness in the abdominal wall. These include areas where extra peritoneal structures penetrate (as in the inguinal, femoral, and obturator canals, the sciatic foramen, and the umbilical region) and areas devoid of strong multilayer structural support (as in the anterior abdominal wall's linea alba and semilunar line). In addition, surgical incision and trauma may produce areas of abdominal wall weakness.
Herniations may include preperitoneal fat, retroperitoneal organs, and a hernial sac composed of peritoneum containing intraperitoneal structures (e.g., omentum or organs). Clinically significant herniation without a peritoneal sac is uncommon. Hernias may be complicated by inclusion of a viscus forming one wall of the hernial sac. This involves a partially retroperitoneal organ and is called a sliding hernia. Sliding inguinal hernias most frequently involve the colon.
The entrapment of the content of a hernia is more likely when the hernia opening is narrow. When the content can be returned to its normal cavity by manipulation, the hernia is reducible; when it cannot, it is irreducible or incarcerated. Incarceration may be acute or chronic. Incarceration of a single wall of a hollow viscus is known as Richter hernia. Incarcerated hernias are subject to inflammatory and edematous changes and are at risk for strangulation. Strangulation of a hernia refers to vascular compromise of the incarcerated contents. When strangulation is not relieved in a timely fashion, gangrene develops.
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