Inguinal Hernia
Inguinal hernias can be classified as direct or indirect based on their relationship to the inferior epigastric vessels (Fig 1, 2). Direct inguinal hernias are medial to and indirect inguinal hernias are lateral to the inferior epigastric vessels. They can occur in children and adults but are more common in males, regardless of age. Indirect inguinal hernias are the most common and are caused by the protrusion of peritoneal content through a patent internal inguinal ring. In males, the hernia extends along the spermatic cord into the scrotum while in women it follows the course of the round ligament. Direct hernias, mainly diagnosed in adults, protrude through an acquired weakness in the musculature of the posterior wall of the inguinal canal.
On CT, contents which are continuous with intraabdominal structures are observed in the hernias. Bowel and fat are the most common hernial contents, but parts of any viscera found in the lower abdomen could be contained in the sac. Hernias with specific contents have eponymous names e.g. amyand hernia contains an incarcerated appendix.
Femoral Hernia
Femoral hernias are relatively uncommon, with a prevalence of less than one-tenth that of inguinal hernias. They are more likely to incarcerate and strangulate than inguinal hernias. They are more common in females, secondary to the dilatation of the femoral ring connective tissues seen with hormonal changes of pregnancy.
On CT, the sac of a femoral hernia passes through the femoral ring into the femoral canal, medial to the femoral vein. The femoral vein often appears compressed by the hernia (Fig 3-5). Femoral hernias typically have a funnel-shape neck.
Obturator Hernia
Obturator hernias are a rare pelvic floor hernia that occur through the obturator foramen. Obturator hernias are characterised by bowel herniating between the obturator and pectineus muscles (Fig 6, 7).
Ventral Hernia
The term 'Ventral hernia' encompasses several types of herniations in the anterior and lateral abdominal wall (Fig 8, 9). These are much less common than the groin hernias mentioned above. Most occur in the midline and emerge through the aponeurosis forming the linea alba. Midline defects include umbilical, paraumbilical, epigastric and hypogastric hernias. Umbilical hernias are by far the most common type of ventral hernia. They are usually small and particularly common in women with a history of multiple pregnancies and patients with obesity or increased abdominal pressure.
Hiatus Hernia
Hiatus hernias occur when there is herniation of abdominal contents through the oesophageal hiatus of the diaphragm into the thoracic cavity (Fig 10, 11).
Complications
Abdominal wall hernias may be complicated by bowel obstruction, incarceration and strangulation. These commonly present with vomiting, abdominal pain and distension.
On axial CT images, strangulated or incarcerated hernias demonstrate significantly engorged distal collateral veins, and signs of oedema and inflammation within the hernia sac and neck (Fig 13, 14). The risk of strangulation is inversely proportional to neck size and is suggested at imaging if there is fluid in the sac, bowel-thickening , or luminal dilatation. Thin-walled veins initially become engorged, causing mucosal hyperenhancement. This is followed by the development of arterial compromise which progresses to ischemia with mucosal hypoenhancement. Infarction causes pneumatosis and free perforation.
Abdominal wall hernias are the second leading cause of bowel obstruction, after adhesions. Femoral hernias are more likely to obstruct than inguinal hernias. The classic CT finding is of dilated bowel loops proximal to the hernia with collapse distally (Fig 14).