Anterior abdominal wall

Embryology

• The abdominal wall begins to develop in the early stages of embryonic differentiation into ectoderm, endoderm and mesoderm
• Mesoderm splits into splanchnic and somatic layers, and the abdominal wall develops from the somatic layer
• Initially this is a U-shaped structure in the embryonic abdominal wall, which allows free communication with the extra-embryonic coelom. It takes until the end of the third month for this to close over (except at umbilical ring).
• Umbilical cord also develops from mesoderm - containing the omphalomesenteric duct, allantois, and fetal blood vessels; passing to and from the placenta. The developing gut also protrudes through the umbilical ring (since it grows faster than can be accommodated). However as the coelomic cavity enlarges, the intestine returns to the peritoneal cavity.
  • At birth, this closes over - no blood through umbilical vessels, and the omphalomesenteric duct has reduced to a fibrous cord

Boundaries:

• Inguinal ligaments, pelvic bones, mid-axillary lines, costal cartilages of ribs 7-10, xiphoid process

Anterolateral layers (9)

• Skin
• Subcutaneous fat
• Superficial fascia
  • Camper's fascia
    • Superficial fatty layer - contains most of the subcutaneous fat
  • Scarpa's fascia
    • Deeper, denser membranous layer
    • Particularly well-defined in children - may be mistaken for external oblique
    • Continuous with fascia lata of the thigh
    • Note that there is no fusion with the fascia of penis and scrotum or vulva, so blood/fluid from abdominal wall can track down to those structures
    • This layer is worth closing in lower abdominal incisions
• External oblique
  • Attaches ribs 5-12, with upper four slips interdigitating with serratus anterior, and lower four slips with latissimus dorsi; passes inferomedially as a wide, flat muscle; posterior fibres insert to anterior half of iliac crest, and anterior and middle fibres insert to external oblique aponeurosis
  • Free edge posteriorly between 12th rib and iliac crest
  • Aponeurotic line falls vertically from ninth costal cartilage to form linea semilunaris
  • Aponeurosis passes in front of the rectus muscle to insert into the linea alba
  • Entirely aponeurotic inferior to a line between ASIS and umbilicus
  • Medial half of aponeurosis rolls on itself at ASIS (heading posteriorly and superiorly), forming inguinal ligament, which attaches to pubic tubercle
  • Covered by a thin fascial layer - external oblique fascia (innominate fascia of Gallaudet) - which forms the external spermatic fascia
  • Supplied segmentally by the lower 6 intercostal nerves
• Internal oblique
  • Attaches thoracolumbar fascia, anterior two-thirds of the iliac crest, and the iliopsoas fascia beneath the lateral half of the inguinal ligament.
    • Superior fibres pass inferolateral to superomedial, and insert lower 5 ribs and costal cartilages.
    • Central fibres join the aponeurosis of the rectus sheath at the semilunar line
      • Above the arcuate line, splits in half to surround the rectus sheath
      • Below arcuate line, all passes anteriorly to rectus sheath
    • Inferior fibres pass inferomedially, inserting between the symphysis pubis and pubic tubercle. Some of the lower muscle fascicles form the cremasteric muscle.
  • Aponeurosis also forms about the level of ASIS, and fuses with transversus abdominis aponeurosis to form conjoint tendon (inguinal aponeurotic falx)
  • Supplied by the ventral rami of the lower 6 intercostal nerves and the first lumbar nerve (nerves enter the muscle more anteriorly than those to EO, which come from the main trunks)
• Transversus abdominis
  • Similar to IO: arises from inner surface of lower six costal cartilages, spines of the lumbar vertebrae, the thoracolumbar fascia, the iliac crest, and the iliopsoas fascia (beneath the lateral third of the inguinal ligament)
  • Upper fibres interdigitate with diaphragm
  • Mostly transverse fibres which pass medially and form an aponeurosis, passing posterior to rectus sheath above the arcuate line and anterior below
  • Inferior-most fibres (from the iliopsoas fascia) form the conjoint tendon along with fibres from the internal oblique
  • Supplied by the ventral rami of the lower five or six intercostal nerves and first lumbar nerve
• Transversalis fascia
  • The layer of fibro-fatty tissue which lies between the inner surface of transversus and the peritoneum
  • Covers most of abdo wall, as the deep surface of transversus abdominis muscle
  • Continues as internal spermatic fascia at deep inguinal ring
  • Helps form a complete fascial envelope around the abdominal cavity
    • Continuous with inferior diaphragmatic fascia
    • Continuous with iliopsoas and pelvic fascia
    • Fuses posteriorly anterior lamina of thoracolumbar fascia
  • Therefore, by definition, an abdominal hernia is a defect in the transversalis fascia
• Pre-peritoneal fat/areolar tissue
  • Contains inferior epigastric artery and vein, medial umbilical ligaments, median umbilical ligament, falciform ligament and ligamentum teres.
• Peritoneum
  • Thin layer of dense, irregular connective tissue
  • Inner surface covered by a single layer of squamous mesothelium
  • See separate entry under 'peritoneal cavity'

Middle abdominal wall

• Rectus abdominis
  • Long, flat, triangular ribbons. Composed of long parallel fascicles interrupted by 3-5 tendinous intersections, which attach the muscle to the anterior rectus sheath. No attachment to posterior rectus sheath.
    • The intersections are nearly always more marked in the upper half, with one at xiphoid level, one at umbilicus, and one about halfway between those two
  • Attaches to costal cartilages 5, 6 and 7, and the xiphoid process. Inserts on pubic crest, pubic symphysis and superior ramus of pubis.
  • Bounded by linea alba medially and linea semilunaris laterally.
  • No attachments to posterior rectus sheath
  • Supplied by ventral rami of the lower six or seven intercostal nerves, entering the lateral posterior aspect of the muscle
• Pyrimidalis
  • Frequently absent
  • Small muscle lying anterior to inferior rectus muscle, within rectus sheath
  • Arises from linea alba and attaches to pubis and anterior ligamentous fibres of pubic symphysis.
  • Needs to be divided for lower midline incision
  • Supplied by ventral ramus of subcostal nerve
• Rectus sheath
  • Superior to costal margin:
    • No posterior sheath (since there are no aponeuroses of internal oblique or transversus)
    • Only an anterior sheath which is formed by external oblique aponeurosis
    • Only close anterior sheath at this level
  • Between costal margin and arcuate line:
    • Anterior sheath formed by fused aponeuroses of external oblique and anterior lamina of internal oblique aponeurosis
    • Posterior sheath formed by posterior aponeurosis of internal oblique and transversus aponeurosis, along with transversalis fascia
    • Need to close both anterior and posterior rectus sheaths at this level
  • Below arcuate line
    • All three aponeuroses blend together to form and anterior rectus sheath
    • Rectus abdominis lies directly anterior to transversalis fascia, which is thickened at this level
    • Only need to close anterior sheath at this level

• Linea alba
  • White fibro-tendinous raphe running vertically for entire length of anterior abdo wall, from xiphisternum to pubic symphysis
  • Formed by interlacing fibres from external and internal oblique and transversus abdominis
  • Thinner immediately inferior to umbilicus, with rectus muscles basically contiguous with one another
  • Wider above the umbilicus
• Umbilicus
  • Fibrous cicatrix passing through linea alba
  • Represents area of fusion between two medial umbilical ligaments and the median umbilical ligament
  • Round ligament of liver also traverses umbilical ring, at the inferior margin (can be recanalised as umbilical vein)

Important features

• Arcuate line (aka semicircular line of Douglas)
  • The free inferior edge of the posterior sheath
  • Between umbilicus and pubic crest - variable exact position
  • Below this line, all layers of anterior abdo wall pass anteriorly to rectus sheath (transversus fuses with internal oblique aponeurosis)
• Linea semilunaris
• Spigelius' fascia
  • The fascia at the lateral edge of the rectus muscle in the area of the arcuate line

Blood supply

• Three zones:
  • Zone I is upper anterior midline - supplied by superior and deep inferior epigastric vessels
    • Superior epigastric - terminal branch of internal mammary artery. Passes through costo-xiphoid space of diaphragm, stays anterior to TA muscle, and then descends within the rectus sheath. Anastomoses with branches of the inferior epigastric. Stays between 4-8cm from midline (table below).
  • Zone II is anterior midline inferior to zone I, supplied by superficial epigastric, superficial external pudendal, inferior epigastric, superficial circumflex iliac
    • Deep inferior epigastric - from external iliac, arising just above the midpoint of the inguinal ligament - courses through pre-peritoneal tissue, then enters the lateral rectus sheath at the arcuate line. Ascends between rectus muscle and posterior sheath (look for lateral umbilical fold), before anastomosing with superior epigastric. Stays between 4-8cm from midline (table below).
    • Deep circumflex iliac - from external iliac, originating about the same place DIEA - runs on the deep aspect of the anterior abdominal wall, parallel to the inguinal ligament; has an ascending branch that penetrates into abdominal wall near ASIS
  • Zone III is lateral to linea semilunaris and superior to zone II - supplied by musculophrenic, lower 6 intercostal, subcostal and 4 lumbar arteries
• Most vessels course in neurovascular plane between IO and TA. They perforate about 1cm medial to linea semilunaris.

Venous drainage

• Superficial veins above the umbilicus empty into SVC via internal mammary, intercostal and long thoracic veins
• Veins inferior to umbilicus (superficial epigastric, circumflex iliac and pudendal) converge to the saphenous opening in the groin to enter the GSV
• There are numerous collateral pathways between inferior and superior veins
  • Paraumbilical vein provides a collateral pathway between portal and systemic circulations - from the left branch of PV along ligamentum teres to the umbilicus. Dilated superficial paraumbilical veins are termed caput medusae in this setting.

Lymphatic drainage

• Superficial
  • Superior to umbilicus: axilla
  • Inferior: superficial inguinal nodes
• Deep
  • Lumbar, common and external iliac lymph nodes
• Lymphatic vessels from the liver follow ligamentum teres to communicate with superficial nodes at the umbilicus (Sister Mary Joseph nodule).

Nerves

• Muscles and skin supplied by thoraco-abdominal and subcostal nerves (derived from ventral rami of 7-12 intercostal nerves) and by first lumbar nerve (iliohypogastric and ilioinguinal)
  • T7 and T8 course slightly upwards to reach the epigastrium, and the lower nerves head more caudally
  • T10 to umbilicus
  • T12 to skin of hypogastrium
  • Iliohypogastric runs parallel to T12. It enters the neurovascular plane near the iliac crest, the pierces IO and travels between IO and EO to the external ring. It then provides sensory innervation to the anterior abdominal wall in the hypogastrium.
  • Ilioinguinal nerve runs parallel to iliohypogastric, but closer to the inguinal ligament. It travels alongside the spermatic cord to emerge from the external ring. It provides sensory innervation to the skin of the inguinal region and scrotum or labium.
• Run inferomedial in the neurovascular plane between internal oblique and transversus abdominis
  • This is the plane to do a TAP block in (see 'anaesthetic')
  • Medially, the nerves perforate rectus sheath to supply medial skin