Anatomy, Abdomen and Pelvis: Anterolateral Abdominal Wall

Structure and Function

The main functions of the anterolateral abdominal muscles include stabilizing the vertebral column, moving the trunk, and tensing the abdominal wall. The function of the anterolateral wall as a whole includes protection of the abdominal viscera, maintenance of anatomical position, assistance in forceful expiration, and involvement in any activity that increases intra-abdominal pressure.

Embryology

Embryonic differentiation is based on 3 germ layers: the outermost protective layer, termed the ectoderm; the middle layer, termed the mesoderm; and the innermost layer, termed the endoderm. The mesoderm is further divided into 2 layers: the splanchnic layer, which gives rise to the abdominal viscera, and the somatic layer, which gives rise to the abdominal wall.

Blood Supply and Lymphatics

Arterial Supply

The arterial supply to the abdominal wall is derived from the following:

Six most inferior intercostal arteries and lumbar arteries: These vessels course from lateral to medial between the transversus abdominis and internal oblique muscles, along with the intercostal, iliohypogastric, and ilioinguinal nerves. The branches pierce the lateral border of the rectus sheath and freely communicate with the epigastric arteries.

Superior epigastric arteries: These arteries are terminal branches of the internal thoracic artery, also known as the internal mammary artery,bilaterally. They descend within the rectus sheath, posterior to the rectus muscle but anterior to the posterior rectus sheath, to form an anastomosis with the inferior epigastric artery.

Inferior epigastric arteries: These arteries are branches of the external iliac artery just before the artery crosses the inguinal ligament. They course superiorly in the preperitoneal space, between the transversalis fascia and parietal peritoneum, to meet the superior epigastric vessels.

Deep circumflex iliac arteries: These arteries arise from the external iliac artery laterally, just distal to the inferior epigastric artery branching, and contribute blood to the abdominal wall via an ascending branch.

Venous Drainage

The venous drainage of the abdominal wall superior to the umbilicus is through the internal thoracic, intercostal, and long thoracic veins. These veins ultimately drain into the superior vena cava. Inferior to the umbilicus, the venous drainage is through the superficial epigastric, circumflex iliac, and pudendal veins. These veins drain to the groin at the saphenofemoral junction and ultimately reach the inferior vena cava. An important embryological consideration of the venous drainage is the ligamentum teres. The ligamentum teres contains the remnant of the umbilical vein, which is a connection between the abdominal wall veins and the left portal vein branch. In the setting of portal hypertension, this vein may recanalize, leading to dilation of the abdominal wall veins, termed caput medusae.

Lymphatic Drainage

The lymphatic drainage of the abdominal wall parallels the venous drainage. The abdominal wall lymphatics superior to the umbilicus drain to the axillary lymph node basin, and those inferior to the umbilicus drain to the inguinal lymph node basin. Additionally, the embryological origin of the ligamentum teres allows gastrointestinal tract cancers to metastasize to the periumbilical region of the abdominal wall, forming Sister Mary Joseph nodules.

Nerves

The sensory and motor innervation to the abdominal wall is primarily derived from the anterior rami of T7 through T12. These nerves course in an inferior and medial direction between the transversus abdominis and internal oblique muscles. The anterior ramus of T12, along with the first lumbar nerve, contributes to the common trunk of the ilioinguinal and iliohypogastric nerves. The ilioinguinal nerve is found within the inguinal canal and provides sensation to the ipsilateral medial thigh and scrotum. The iliohypogastric nerves provide sensation to the anterior abdominal wall in the suprapubic region.

Muscles

Layers of the Anterolateral Abdominal Wall

Subcutaneous tissues: Camper fascia can be thought of as superficial subcutaneous fat; this layer contains little collagen and therefore provides little strength. Scarpa fascia is contiguous with the tensor fascia lata of the thigh. The Scarpa fascia lies deeper than the fatty layer and contains more collagen; many surgeons approximate this layer during abdominal wall closures.

External oblique: The external oblique is the most superficial, largest, and thickest of the 3 anterolateral abdominal wall muscles. This muscle originates from the lower 7 ribs and runs obliquely from superolateral to inferomedial (hands-in-pockets orientation) to insert on the anterior half of the iliac crest. The most inferior extension folds posteriorly and superiorly to form the inguinal ligament. At the midclavicular line, the muscle belly ends, but the aponeurosis extends medially to the linea alba, contributing to the anterior rectus sheath.

Internal oblique: The internal oblique is the middle layer of the 3 anterolateral abdominal wall muscles, originating from the iliopsoas fascia, lateral iliac crest, and thoracolumbar fascia. Its fibers course opposite to the external oblique, from inferolateral to superomedial, to insert on the cartilage of the lower 5 ribs. Inferiorly, the fibers take a more transverse course and insert onto the pubic tubercle. At this inferior portion, the internal oblique aponeurosis joins the aponeurosis of the transversus abdominis muscle to create the conjoined tendon. Like the external oblique, this muscle gives off a medial extension of the aponeurosis to contribute to the rectus sheath. Above the semicircular line of Douglas, approximately just inferior to the level of the umbilicus, the internal oblique aponeurosis splits around the rectus abdominis muscle to contribute to the anterior and posterior rectus sheaths. Below this line, the aponeurosis contributes only to the anterior rectus sheath. Inferiorly, this muscle gives off fibers that encircle the spermatic cord, termed the cremasteric fibers.

Transversus abdominis: The transversus abdominis is the thinnest and deepest of the 3 anterolateral abdominal wall muscles. This muscle originates from the lower 6 costal cartilages, lumbar vertebrae, iliac crests, and iliopsoas fascia. The fibers are oriented transversely and contribute to the medial aspect of the rectus sheath. Above the semicircular line of Douglas, the aponeurosis courses posterior to the rectus muscles to contribute to the posterior rectus sheath. Inferiorly, the aponeurosis contributes only to the anterior rectus sheath.

Transversalis fascia: The endoabdominal fascia lines the entire abdominal cavity. The endoabdominal fascia has specific names by anatomic location, including the diaphragmatic, obturator, and iliopsoas fascias, among others. The transversalis fascia is just deep to the transversus abdominis and posterior rectus sheaths. The transversalis fascia is an important layer because its violation defines an abdominal wall hernia.

Parietal peritoneum: The parietal peritoneum is a thin layer of connective tissue deep to the transversalis fascia. The deep surface contains a single cell layer of squamous mesothelium.[1]

Physiologic Variants

Anatomical variations are common in the literature.[2] Classic anatomy is described in just over 50% of observed specimens. Common variations include different sensory distribution of nerves, presence or absence of accessory internal oblique or pyramidalis muscles, and different composition of the anterior and posterior rectus sheaths. Embryologic malformations may result in ventral abdominal wall defects at birth. These malformations include ectopia cordis, bladder exstrophy, omphalocele, and gastroschisis.[3]

Surgical Considerations

Detailed knowledge of the abdominal wall is necessary when performing abdominal wall reconstruction. Techniques such as component separation require meticulous dissection of the different abdominal wall layers and preservation of the vascular supply. Surgeons must also avoid injury to the innervation of the abdominal wall because denervation can contribute to loss of function and abdominal wall weakness.[4][5]

Clinical Significance

Knowledge of abdominal wall innervation and its anatomic location is important when considering postoperative analgesia. Clinicians use several techniques to deliver local anesthetic to the nerves of the abdominal wall to improve analgesia after abdominopelvic surgical procedures. A popular example is termed the transversus abdominis plane block (TAP block).[6] This established technique involves administering a local anesthetic to the sensory innervation of the abdominal wall, situated in a plane between the transversus abdominis and internal oblique muscles. Typically, 2 techniques are used: one involves ultrasonography guidance, and the other uses direct visualization. Both methods have yielded statistically significant improvements in postoperative pain control.

Other Issues

Expanding knowledge of the abdominal wall will likely lead to new techniques for abdominal wall reconstruction and postoperative analgesia. Every clinician, particularly surgeons, should have a thorough understanding of this complex anatomy.