Abdominal fascial planes

Superficial Fascia - The Two-Faced Layer

• A bilaminar structure inferior to the umbilicus, composed of two distinct layers.
• Camper's Fascia: The superficial, fatty layer. Its thickness varies with body habitus.
• Scarpa's Fascia: The deeper, membranous layer, which is continuous with other fascial planes.
  • Inferiorly, it fuses with the fascia lata of the thigh.
  • In the perineum, it continues as Colles' fascia.
  • In the scrotum, it contributes to the dartos fascia.

⭐ In a urethral rupture, urine can extravasate superiorly into the potential space between Scarpa's fascia and the deep abdominal muscle fascia.

Muscles & Rectus Sheath - The Abdominal Corset

• Lateral Muscles (The "Corset"): Innervated by thoracoabdominal (T7-T11), subcostal (T12), & L1 nerves.

  • External Oblique: "Hands in pockets" fibers (inferomedial).
  • Internal Oblique: Fibers run superomedially.
  • Transversus Abdominis: Deepest; transverse fibers.

• Anterior Muscle:

  • Rectus Abdominis: Primary trunk flexor; enclosed by the rectus sheath.

• Rectus Sheath Composition: Varies above and below the arcuate line.

  • Above Arcuate Line: Anterior & posterior layers fully enclose the rectus muscle.
  • Below Arcuate Line: All three lateral muscle aponeuroses pass anterior to the rectus muscle. The posterior wall is deficient, leaving only the transversalis fascia.

⭐ The arcuate line is where the inferior epigastric vessels perforate the rectus abdominis. Below this line, the posterior rectus sheath is absent, creating a potential site for herniation.

Arcuate Line & Deep Layers - The Great Divide

• Arcuate Line: A horizontal line marking the lower limit of the posterior rectus sheath, found roughly one-third of the way from the umbilicus to the pubic crest.

• Composition Above Arcuate Line:

  • Anterior Wall: External oblique & anterior lamina of internal oblique.
  • Posterior Wall: Posterior lamina of internal oblique & transversus abdominis.

• Composition Below Arcuate Line:

  • Anterior Wall: Aponeuroses of all 3 flat muscles pass anterior to the rectus abdominis.
  • Posterior Wall: Deficient. Rectus muscle is in direct contact with transversalis fascia.

⭐ Below the arcuate line, the inferior epigastric vessels enter the rectus sheath and are a source of bleeding in rectus sheath hematomas.

📌 Mnemonic: PASS → Posterior Aponeurosis Stops Short.

Clinical Spaces & Correlates - Danger Zones & Incisions

• Rectus Sheath Hematoma: Accumulation of blood in the rectus sheath, often from epigastric artery rupture. Presents as a painful abdominal mass that does not cross the midline.
• Spigelian Hernia: A rare ventral hernia through the Spigelian fascia, lateral to the rectus abdominis muscle at the semilunar line.
• Surgical Incisions:
  • Midline: Through linea alba for rapid access.
  • Pfannenstiel: Transverse suprapubic incision for gynecologic/obstetric procedures; strong cosmetic results.

⭐ Cullen's & Grey Turner's Signs: Periumbilical (Cullen's) or flank (Grey Turner's) ecchymosis suggests retroperitoneal hemorrhage, as blood tracks along the transversalis and renal fascia.

High-Yield Points - ⚡ Biggest Takeaways

• Camper's fascia is the superficial fatty layer; Scarpa's fascia is the deep membranous layer that limits fluid/pus spread.
• The transversalis fascia is the primary layer deep to the abdominal muscles, forming the deep inguinal ring.
• The arcuate line marks the lower limit of the posterior rectus sheath, a key landmark for hernias and hematomas.
• The linea alba is a midline, avascular raphe, representing a common site for surgical incisions.
• A psoas abscess can track from the thoracic spine down the psoas sheath to present as a groin mass.
• Spigelian hernias occur along the semilunar line, lateral to the rectus abdominis muscle.