Overview of the abdomen

Anterior abdominal wall

Nine layers make up the anterior abdominal wall: skin, subcutaneous tissue, superficial fascia (Camper's anteriorly and Scarpa's posteriorly – their approximation aids with skin alignment during closure of surgical incisions in the lower abdomen), external oblique, internal oblique, transversus abdominis, transversalis fascia, preperitoneal fat and peritoneum ( Fig. 54.1 ). Of the three aforementioned flat abdominal wall muscles, external oblique is the largest and transversus abdominis is the smallest. In the midline, the anterior abdominal wall is composed of the rectus abdominis and pyramidalis. Formed by the bilaminar aponeuroses of the three flat muscles, the rectus sheath envelops the rectus abdominis anteriorly and posteriorly superior to the arcuate line (linea semicircularis of Douglas). Inferiorly, the aponeuroses of internal oblique and transversus abdominis pass anterior to rectus abdominis, which is therefore bordered posteriorly only by the transversalis fascia. Spigelian hernias and arcuate line hernias may develop at the intersection of these fascial layers, which is a potential weak spot in the abdominal wall. The decussation of the rectus sheath in the midline forms the linea alba, where the most common surgical incision, the midline incision, is created. When present, pyramidalis may serve as an accurate intraoperative landmark for the linea alba (the muscle is absent in 10–20% of people on one or both sides). In patients with complex ventral hernias who have insufficient abdominal wall for adequate abdominal closure, incision of the external oblique fascia parallel and just lateral to the edge of rectus abdominis (component separation) facilitates repair.

The arterial supply of the anterolateral abdominal wall arises from the last six intercostal and four lumbar arteries, superior and inferior epigastric arteries, and deep circumflex iliac arteries. Branches of the superior epigastric artery penetrate the anterior rectus sheath to supply the overlying skin. They are located in close proximity to the lateral borders of the rectus abdominis and may bleed if incisions are placed off the midline. When possible, old incisions should be reused in order to avoid necrosis from ischaemia to specific abdominal wall segments. The abdominal wall venous drainage follows its arterial supply.

Inguinal canal

The lower portion of the external oblique aponeurosis rolls posteriorly and superiorly to form a groove that extends from the anterior superior iliac spine to the pubic tubercle, and is referred to as Poupart's ligament or the inguinal ligament. The femoral artery, vein and nerve pass posterior to the inguinal ligament ( Fig. 54.2 ). Femoral hernias are located inferior to the inguinal ligament, and inguinal hernias are located superiorly. The external oblique also serves as the anterior boundary of the inguinal canal, which is about 4 cm in length and extends between the deep (internal) and superficial (external) inguinal rings. Superiorly, the inguinal canal is bounded by the musculo-aponeuroses of the internal oblique and transversus abdominis, and inferiorly by the inguinal and lacunar ligaments. Occasionally, the obturator artery arises from the inferior epigastric artery and travels along the posterior aspect of the lacunar ligament; care must be taken during femoral hernia repair to avoid division of the lacunar ligament to facilitate reduction, as haemorrhage will result. The floor of the canal (often considered the most anatomically and clinically important) is composed of the aponeurosis of transversus abdominis and the transversalis fascia. The region bounded between the rectus sheath medially, the inferior epigastric vessels superolaterally and the inguinal ligament inferiorly is referred to as Hesselbach's triangle, within which direct inguinal hernias occur. Indirect inguinal hernias arise lateral to this triangle. The inguinal canal contains the spermatic cord in men and the round ligament of the uterus in women. Important sensory nerves in the inguinal region include the ilioinguinal and iliohypogastric nerves, and the genital branch of the genitofemoral nerve. Entrapment or damage to these nerves during hernia repair may contribute to the development of postoperative chronic pain. For patients with intract­able chronic pain after hernia repair, triple neurectomy, or excision of all three major nerves, has been reported to have a success rate of 80% to 95%.

Posterior abdominal wall

The posterior abdominal wall is muscular and supports the retro­peritoneal organs, as well as the abdominal aorta and the inferior vena cava (IVC). Its principal muscles include the psoas (major and minor) medially, quadratus lumborum laterally, the diaphragm superiorly and iliacus inferiorly ( Fig. 54.3 ). Lumbar hernias may occur in areas where the lumbodorsal fascia is weak: they are not prone to strangulation. The posterior abdominal wall contains the lumbar plexus, including the iliohypogastric, ilioinguinal, genitofemoral, femoral and obturator nerves, the lateral cutaneous nerve of the thigh, and the lumbosacral trunk.

Retroperitoneum

The retroperitoneal space lies between the peritoneum and the posterior parietal wall of the abdominal cavity. Structures that lie behind the peritoneum are described as being retroperitoneal and include the adrenal glands, kidneys, ascending and descending colon, duodenum (except for the proximal portion, which is intraperitoneal) and pancreas. The retroperitoneum is also traversed by the ureter, renal and gonadal vessels, aorta and IVC. The abdominal aorta emerges from an opening in the diaphragm in the midline at the level of T12 and it divides into the right and left common iliac arteries at the level of L4. Its branches include the unpaired, anterior midline coeliac trunk, superior mesenteric artery (SMA) and inferior mesentery artery (IMA), and the paired middle adrenal arteries, renal arteries and the testicular or ovarian arteries, which branch laterally from the aorta. Posteriorly, it also gives off the inferior phrenic, lumbar and median sacral arteries. Bowel ischaemia is a rare but serious complication that may occur after abdominal aortic aneurysm repair and can be caused by both occlusive and non-occlusive mechanisms. The duodenum passes between the SMA anteriorly and the aorta posteriorly, a relationship that may lead to abdominal pain, as seen in SMA syndrome. The IVC is longer and larger in calibre than the abdominal aorta. Its main tributaries include the common iliac veins, lumbar veins, right testicular or ovarian vein (versus the left-sided equivalent veins that drain into the left renal vein), renal veins, right adrenal vein, inferior phrenic veins and hepatic veins. The left renal vein is located in close proximity to the splenic vein and may be used to construct a splenorenal shunt for treatment of portal hypertension.