Anterior abdominal wall

The anterior abdominal wall extends from the xiphoid process and costal margins cranially to the pubic and iliac bones inferiorly and to the mid-axillary lines on either side. The abdomen is divided into regions or quadrants to more precisely describe abdominal symptoms and signs and help identify underlying organs. The muscles of the anterior abdominal wall are flat muscles and include the rectus abdominis, the external and internal obliques, the transversus abdominis, and the pyramidalis. These muscles allow the trunk to bend either forward or laterally, maintain abdominal tone, and help increase intra-abdominal pressure. The main arteries of the anterior abdominal wall are the inferior and superior epigastric arteries. The abdominal wall is innervated by the intercostal nerves, the subcostal nerves, and, to a lesser extent, the iliohypogastric and ilioinguinal nerves.

Boundaries of the anterior abdominal wall

• Cranial: xiphoid process and the costal margins
• Caudal: pubic and iliac bones
• Lateral: midaxillary line

Functions

• Protects abdominal viscera
• Assists in forceful expiration
• Decreases risk of herniation
• Enables the trunk to bend forward or laterally

Planes of the anterior abdominal wall

• Transverse abdominal planes
  • Subcostal plane
    • Passes through the 10^th costal cartilage (i.e., the inferior limit of the costal margins)
    • Lies at the level of the inferior border of the L3 vertebral body
  • Transtubercular plane
    • Lies midway between the transpyloric plane (see below) and the pubic symphysis
    • Passes through the iliac tubercles and the L5 vertebral body
  • Transpyloric plane
    • Passes through the 9^th costal cartilage
    • Lies at the level of the inferior border of L1 vertebral body.
• Vertical abdominal lines: the right and left midclavicular lines that extend from the midpoint of the clavicle cranially to the midinguinal point caudally

Divisions of the abdomen

The regions or quadrants of the abdomen are used in clinical contexts to describe abdominal symptoms and signs and help identify underlying organs.

Regions

• There are 9 abdominal regions
  • Upper regions: right hypochondriac (1), epigastric (2), left hypochondriac (3)
  • Middle regions: right lumbar (4), umbilical (5), left lumbar (6)
  • Lower regions: right inguinal (7), hypogastric or suprapubic (8), left inguinal (9)
• They are divided by:
  • Two transverse planes
    • Subcostal plane
    • Transtubercular plane
  • Two vertical lines: right and left midclavicular lines

Quadrants

• There are 4 abdominal quadrants.
  • Upper quadrants: 1. right upper quadrant (RUQ), 2. left upper quadrant (LUQ)
  • Lower quadrants: 3. right lower quadrant (RLQ), 4. left lower quadrant (LLQ)
• They are divided by horizontal and vertical imaginary lines that intersect at the umbilicus.
  • Umbilicus
    • Point where the umbilical cord entered the fetus
    • Located midway between the xiphoid process and the pubis symphysis

Components of the anterior abdominal wall

• Layers (from superficial to deep)
  1. Skin and subcutaneous tissue
  2. Superficial fascia
     • Superficial fatty layer (Camper fascia)
     • Deep membranous layer (Scarpa fascia)
  3. External oblique muscle
  4. Internal oblique muscle
  5. Transversus abdominis muscle
  6. Deep fascia (transversalis fascia): fuses with the deep fascia of the thigh (fascia lata)
  7. Preperitoneal adipose tissue
  8. Parietal peritoneum
• Rectus sheath: a sheath formed by the fusion of aponeuroses (broad tendons) of the flat muscles (external oblique, internal oblique, and transversus abdominis muscles), which encloses the vertical muscles (rectus abdominis and pyramidalis muscles)
  • Above the arcuate line
    • Anterior layer: composed of the external oblique aponeurosis and the anterior lamina of the internal oblique aponeurosis
    • Posterior layer: composed of the transversus abdominis aponeurosis and the posterior lamina of the internal oblique aponeurosis
  • Below the arcuate line, only the anterior layer of the rectus sheath is present, composed of the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles
  • At the arcuate line, the transversus abdominis aponeurosis and the posterior lamina of the internal oblique aponeurosis pass anterior to the rectus muscle. Thus, the lower 1/3^rd of the rectus muscle is in direct contact with the peritoneum.
• Arcuate line of the rectus sheath (linea semicircularis):
  • The bow-shaped inferior limit of the posterior rectus sheath, which lies approximately 1/3^rd of the distance between the umbilicus and the pubic symphysis
  • The inferior epigastric vessels perforate the rectus abdominis here.
• Linea alba: a tendinous median line that extends from the xiphoid process to the pubic symphysis
  • Formed by the fusion of the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles.
• Linea semilunaris: a curved vertical line on the anterior abdominal wall at the lateral edge of each rectus abdominis muscle

A defect in the upper linea alba can cause an epigastric hernia.

Herniation of intra-abdominal contents through the linea semilunaris is known as a Spigelian hernia.

Inguinal canal

• The muscles and fascial layers of the anterior abdominal wall continue inferiorly to form the inguinal ligament and parts of the inguinal canal.
• Extends between the deep (internal) and superficial (external) ring
• Contains the ilioinguinal nerve and in:
  • Males: spermatic cord
  • Females: round ligament of the uterus
• Borders
  • Roof (superior): internal oblique and transversus abdominis muscles
  • Floor (inferior): inguinal ligament (shelving edge of external oblique) and lacunar ligament (medially)
  • Posterior wall: transversalis fascia laterally; conjoint tendon medially
  • Anterior wall: external oblique aponeurosis and internal oblique muscle laterally

References:^[1][2][3][4]

• Inguinal ligament
  • The lower free border of the external oblique aponeurosis folds upon itself to form the inguinal ligament.
  • Extends from the anterior superior iliac spine laterally to the pubic tubercle medially
  • The internal oblique and transversus abdominis muscles originate from its lateral end.
• Conjoint tendon: formed by the fusion of the medial fibers of the internal oblique muscle and the lower fibers of the transversus abdominis aponeurosis

The conjoint tendon forms the medial portion of the posterior inguinal canal. A weakness of the conjoint tendon can result in a direct inguinal hernia.

References:^[1][2]

Arteries

• Superficial arteries: branches of the femoral artery
  • Superficial epigastric artery: anastomoses with branches of the inferior epigastric artery
  • Superficial external pudendal arteries
  • Superficial circumflex iliac arteries
• Deep arteries
  • Upper abdominal wall: branches of the internal thoracic artery
    • Superior epigastric artery
    • Musculophrenic artery
  • Lower abdominal wall: branches of the external iliac artery
    • Inferior epigastric artery
      • Main artery of the anterior abdominal wall
      • Anastomoses with superior epigastric artery
      • Gives rise to the cremasteric artery (runs in spermatic cord)
    • Deep circumflex iliac arteries

The inferior epigastric artery forms the lateral boundary for the Hesselbach triangle, which helps distinguish between direct and indirect inguinal hernias intraoperatively.

Veins

• Thoracoepigastric veins
  • Connections between the lateral thoracic vein and the superficial epigastric vein
  • Provide an alternative route for venous drainage when the portal vein or the vena cava is obstructed, become engorged and tortuous, and form caput medusae.
  • Above the umbilicus: drain into subclavian veins
  • Below the umbilicus: drain into external iliac veins

Lymphatics

• Upper abdominal wall: axillary lymph nodes
• Lower abdominal wall: superficial inguinal nodes

Cutaneous innervation and dermatomes

• Cutaneous innervation: from T₇–T₁₂ (via the lower intercostal nerves and the subcostal nerve) and L₁ (via the iliohypogastric and ilioinguinal nerves)
• Each intercostal nerve supplies a band-shaped dermatome.
• Important abdominal dermatomes:
  • T₇: xiphoid process
  • T₁₀: umbilicus (site of referred pain for appendicitis)
  • L₁: area just above the inguinal ligament

• Abdominal examination
• Differential diagnoses of abdominal pain
• Differential diagnoses of acute abdomen
• Referred pain
• Inguinal hernia
• Umbilical hernia
• Epigastric hernia
• Rectus sheath hematoma