Anterior abdominal wall

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CHAPTER 61 Anterior abdominal wall

The anterior abdominal wall extends from the costal margins and xiphoid process superiorly to the iliac crests, pubis and pubic symphysis inferiorly. It overlaps and is connected to both the posterior abdominal wall and paravertebral tissues. It forms a continuous but flexible sheet of tissue across the anterior and lateral aspects of the abdomen. The anterior abdominal wall is composed of the integument, muscles and connective tissue lining the peritoneal cavity (Figs 61.161.5). It has an important role in maintaining the form of the abdomen and is involved in many physiological activities. Anterior abdominal wall tissues form the inguinal canal that connects the abdominal cavity to the scrotum in men or labia majora in women, and also form the umbilicus; both of these sites are of considerable clinical importance.

SKIN AND SOFT TISSUE

The integument of the anterior abdominal wall comprises skin, soft tissues, lymphatic and vascular structures, and segmental nerves. The outer layer is formed from the skin and subcutaneous fat. The skin is non-specialized and variably hirsute, depending on the sex and race. All post-pubertal individuals have some extension of the pubic hair onto the anterior abdominal wall skin, although this is commonly most pronounced in males, in whom the hair may extend almost up to the umbilicus in a triangular pattern. The subcutaneous fat of the abdominal wall is highly variable in thickness and is one of the areas where excess fat is stored during periods of obesity, particularly in males.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

The anterior abdominal wall receives its blood supply from paired superior and inferior epigastric arteries running vertically through the tissues, and from paired posterior intercostal, subcostal and lumbar vessels running obliquely around the anterolateral aspects of the abdomen.

Superior epigastric artery and veins

The superior epigastric artery is a terminal branch of the internal thoracic artery. It descends between the costal and xiphoid slips of the diaphragm, accompanied by two or more veins (Fig. 61.4). The vessels pass anterior to the lower fibres of transversus thoracis and the upper fibres of transversus abdominis. The artery enters the rectus sheath behind rectus abdominis and runs down to anastomose with the inferior epigastric artery usually above the level of the umbilicus. Branches supply rectus abdominis and perforate the sheath to supply the abdominal skin. A branch given off in the upper rectus sheath passes anterior to the xiphoid process of the sternum and anastomoses with the same contralateral branch. This vessel may give rise to troublesome bleeding during surgical incisions that extend up to and alongside the xiphoid process. The superior epigastric artery supplies small branches to the anterior part of the diaphragm. On the right, small branches reach the falciform ligament, where they anastomose with branches arising from the hepatic artery.

Inferior epigastric artery and veins

The inferior epigastric artery originates from the external iliac artery posterior to the inguinal ligament (Fig. 61.6). Its accompanying veins, usually two, drain into the external iliac vein. It curves forwards in the anterior extraperitoneal tissue and ascends obliquely along the medial margin of the deep inguinal ring. It lies posterior to the spermatic cord, but is separated from it by the transversalis fascia. It pierces the transversalis fascia which forms the flimsy posterior support of rectus abdominis, and ascends between the muscle and the fascia and overlying pre-peritoneal connective tissue. In this part of its course, it raises the parietal peritoneum of the anterior abdominal wall as the lateral umbilical fold but has little supporting tissue posteriorly. Disruption of the artery, by surgical incisions e.g. for laparoscopic ports or drains, is not uncommon and the resulting haematoma may expand to considerable size because of the lack of tissue against which the bleeding is effectively compressed. The artery divides into numerous branches. Those which anastomose with branches of the superior epigastric artery do so posterior to rectus abdominis at a variable height above the umbilicus. The inferior epigastric vessels are usually significantly larger than the superior vessels and provide the ‘dominant’ supply to rectus abdominis. Preparatory ligation of the inferior epigastric artery is often performed for myo(cutaneous) flaps using the mid or lower rectus abdominis based on the superior epigastric artery to allow expansion of the superior arterial flow to improve viability of the flap. Branches anastomose with terminal branches of the lower six posterior intercostal arteries posterior to rectus abdominis at the lateral border close to the sheath. The artery is an important inferomedial relation of the deep inguinal ring, and may be damaged during extensive medial dissection of the deep ring during hernia repair, particularly when this is performed in the preperitoneal plane. The vas deferens in the male, or round ligament in the female, wind laterally round it. It has the following branches: the cremasteric artery, a pubic branch, and muscular and cutaneous branches.

The cremasteric artery accompanies the spermatic cord in males and supplies the cremaster and other coverings of the cord. It anastomoses with the testicular artery. In females it is small and accompanies the round ligament. A pubic branch, near the femoral ring, descends posterior to the pubis and anastomoses with the pubic branch of the obturator artery. Occasionally, the pubic branch of the inferior epigastric artery is larger than the main obturator artery origin, and it supplies the majority of flow into the obturator artery in the thigh. It is then referred to as the aberrant obturator artery. It lies close to the medial border of the femoral ring and may be damaged in medial dissection of the ring during femoral hernia repair. Muscular branches supply the abdominal muscles and peritoneum, and anastomose with the circumflex iliac and lumbar arteries. Cutaneous branches perforate the aponeurosis of external oblique, supply the skin and anastomose with branches of the superficial epigastric artery.

Sometimes the inferior epigastric artery arises from the femoral artery. It then ascends anterior to the femoral vein into the abdomen to follow its course as above. It occasionally arises from the external iliac artery, in common with an aberrant obturator artery and, rarely, from the obturator artery.

The superior and inferior epigastric arteries are important sources for a potential collateral circulation between the internal thoracic artery and the external iliac artery in situations where flow in the thoracic or abdominal aorta is compromised. Small tributaries of the inferior epigastric vein drain the skin around the umbilicus and anastomose with the terminal branches of the umbilical vein, draining the inner surface of the umbilicus via the falciform ligament. These anastomoses may open widely in cases of portal hypertension, when portal venous blood may drain into the systemic circulation via the inferior epigastric vessels. The radiating dilated veins seen under the umbilical skin are referred to as the ‘caput medusae’.

SEGMENTAL NERVES

The seventh to the 12th lower thoracic ventral rami continue anteriorly from the intercostal spaces into the abdominal wall (Fig. 61.3). Approaching the anterior ends of their respective spaces, the seventh and eighth nerves curve superomedially across the deep surface of the costal cartilages between the digitations of transverse abdominis. They reach the deep aspect of the posterior layer of the aponeurosis of internal oblique. Both the seventh and eighth nerves then run through this aponeurosis, pass posterior to rectus abdominis and supply branches to the upper portion of the muscle. They pass through the muscle near its lateral edge and pierce the anterior rectus sheath to supply the skin of the epigastrium.

The ninth to 11th intercostal nerves pass from their intercostal spaces between digitations of the diaphragm and transversus abdominis. They enter the layer between transversus abdominis and internal oblique. Here, the ninth nerve runs forwards almost horizontally, whereas the tenth and 11th pass inferomedially. At the lateral edge of rectus abdominis, the nerves pierce the posterior layer of the aponeurosis of internal oblique and pass behind the muscle to end, like the seventh and eighth intercostal nerves, with cutaneous branches. The ninth nerve supplies skin above the umbilicus, the tenth supplies skin, which includes the umbilicus, and the 11th supplies skin below the umbilicus (see Fig. 15.12 and Chapters 42, 45 and 79). The 12th thoracic nerve (subcostal nerve) connects with the first lumbar ventral ramus (dorsolumbar nerve). It accompanies the subcostal vessels along the inferior border of the 12th rib, passing behind the lateral arcuate ligament and kidney and anterior to the upper part of the quadratus lumborum. It perforates the transversus abdominis fascia, running deep to the internal oblique, to be distributed like the lower intercostal nerves. It supplies the anterior gluteal skin reaching down to the greater trochanter.

The seventh to 12th intercostal nerves supply the intercostal, subcostal and abdominal muscles. The tenth, 11th and 12th supply serratus posterior inferior. All six nerves also provide sensory fibres to the costal parts of the diaphragm and related parietal pleura and peritoneum. Like the upper intercostal nerves, they give off collateral and lateral cutaneous branches before they reach the costal angles. The collateral branch may rejoin its parent nerve but, if it does, it leaves it again near the lateral border of rectus abdominis. It then runs forward, through the muscle and its anterior sheath near the linea alba to supply the overlying skin. The lateral cutaneous branches pierce the intercostal muscles and external oblique and divide into anterior and posterior branches. These branches supply the skin of the abdomen and back. The anterior branches supply external oblique. The posterior branches pass back to supply the skin over latissimus dorsi. Each lateral cutaneous nerve descends as it pierces external oblique and the superficial fascia and reaches the skin on a level with the anterior and posterior cutaneous nerves of the segment.

SOFT TISSUE

Superficial fascia

The superficial fascia of the abdominal wall consists mostly of a single layer that contains a variable amount of fat. It lies between the skin and muscles of the anterior abdominal wall. In the lower part, the fascia differentiates into superficial and deep layers between which lie superficial vessels and nerves and, in the region of the groin, superficial inguinal lymph nodes.

Transversalis fascia

The transversalis fascia is a thin layer of connective tissue lying between the inner surface of transversus abdominis and the extraperitoneal fat. It is part of the general layer of fascia between the peritoneum and the abdominal wall. Posteriorly, it is continuous with the anterior layer of the thoracolumbar fascia, and it forms a continuous sheet anteriorly. Inferiorly, it is continuous with the iliac and pelvic fasciae, and superiorly it blends with the fascial covering of the inferior surface of the diaphragm. It is attached to the entire length of the iliac crest between the origins of transversus abdominis and iliacus and to the posterior margin of the inguinal ligament between the anterior superior iliac spine and the femoral vessels. In the inguinal region it is thick and dense, and augmented by the aponeurosis of transversus abdominis. Medial to the femoral vessels it is thin and fused to the pubis behind the conjoint tendon. Anterior to the femoral vessels, fascia extends down from the fascia transversalis to form the anterior part of the femoral sheath. The fascia is strengthened here by fibres, which arch transversely. Some fibres spread laterally towards the anterior superior iliac spine, some fibres run medially behind rectus abdominis, and some descend to the pubis behind the conjoint tendon. These arched fibres constitute the deep crural arch. The curved fibres of the deep crural arch thicken the inferomedial part of the rim of the deep inguinal ring. The spermatic cord in the male, or the round ligament of the uterus in the female, pass through the transversalis fascia at the deep inguinal ring. This opening is not visible during dissection through the skin because the transversalis fascia is prolonged on these structures as the internal spermatic fascia, but it can be seen from within the abdomen once the peritoneum has been stripped off. The internal spermatic fascia surrounds the testis and blends with the areolar tissue on the parietal layer of the tunica vaginalis: it may contain smooth muscle fibres.