13.3 Abdominal Wall, GI Viscera, Pelvis & Retroperitoneum

13.3 Abdominal Wall, GI Viscera, Pelvis & Retroperitoneum

Independent study guide by OpenExamPrep.

Core Examination Pearl: Board examinations place heavy emphasis on the aponeurotic transitions of the rectus sheath above and below the arcuate line, the differential diagnosis of indirect versus direct versus femoral hernias (including Hesselbach's triangle boundaries), the embryonic gut vascular supplies (celiac, SMA, IMA), the retroperitoneal organs (SAD PUCKER), and the course of the pudendal nerve around the ischial spine.

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1. Anterolateral Abdominal Wall & Rectus Sheath

The anterolateral abdominal wall consists of skin, subcutaneous fascial layers, three flat muscular sheets, paired vertical muscles, transversalis fascia, and parietal peritoneum.

Fascial & Muscular Layers (Superficial to Deep)

1. Skin
2. Camper's Fascia: Fatty, superficial layer of subcutaneous tissue.
3. Scarpa's Fascia: Membranous, deep layer of subcutaneous tissue. Extends into the perineum as Colles' fascia and into the scrotum as the dartos tunic; straddle injuries with urethral rupture allow urine to extravasate into the scrotum and anterior abdominal wall beneath Scarpa's fascia, but urine cannot track into the thighs due to Scarpa's attachment to the fascia lata.
4. External Oblique Muscle: Fibers course inferomedially ('hands in pockets'); free inferior aponeurotic border folds back on itself to form the inguinal (Poupart's) ligament.
5. Internal Oblique Muscle: Fibers course superomedially, perpendicular to external oblique.
6. Transversus Abdominis Muscle: Deepest flat muscle; fibers course horizontally.
7. Transversalis Fascia: Continuous endoabdominal fascial lining.
8. Extraperitoneal Fat
9. Parietal Peritoneum

The Rectus Sheath & The Arcuate Line

The rectus abdominis muscles are paired vertical strap muscles separated down the anterior midline by the linea alba. They are encased within a fibrous envelope—the rectus sheath—formed by the interweaving aponeuroses of the three flat abdominal wall muscles. The architecture of the rectus sheath changes fundamentally at the arcuate line (linea semicircularis of Douglas), located roughly one-third of the distance from the umbilicus to the pubic crest.

                      RECTUS SHEATH ARCHITECTURE
  
  ABOVE ARCUATE LINE:
       Anterior Wall:  External Oblique Aponeurosis + Anterior Leaf of Internal Oblique
       [ RECTUS ABDOMINIS MUSCLE ]
       Posterior Wall: Posterior Leaf of Internal Oblique + Transversus Abdominis Aponeurosis
                       + Transversalis Fascia + Parietal Peritoneum
  
  =================================== [ ARCUATE LINE ] ===================================
  
  BELOW ARCUATE LINE:
       Anterior Wall:  External Oblique + Internal Oblique + Transversus Abdominis Aponeuroses
                       (ALL THREE APONEUROSES PASS ANTERIOR)
       [ RECTUS ABDOMINIS MUSCLE ]
       Posterior Wall: NO APONEUROSIS! Formed ONLY by Transversalis Fascia
                       + Parietal Peritoneum

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2. Inguinal Canal & Hernia Pathology

The inguinal canal is an oblique intermuscular tunnel approximately 4 cm long oriented inferomedially, positioned directly superior and parallel to the medial half of the inguinal ligament.

Inguinal Canal Boundaries

• Floor (Inferior): Inguinal ligament (curved shelf of external oblique aponeurosis) and lacunar ligament medially.
• Roof (Superior): Arching inferior fibers of the internal oblique and transversus abdominis muscles.
• Anterior Wall: External oblique aponeurosis throughout, reinforced laterally by originating fibers of internal oblique. Features the Superficial Inguinal Ring (a triangular aperture in the external oblique aponeurosis).
• Posterior Wall: Transversalis fascia throughout, reinforced medially by the Conjoint Tendon (Falx Inguinalis) (fused aponeurotic insertion of internal oblique and transversus abdominis onto pubic crest and pectineal line). Features the Deep Inguinal Ring (an oval evagination of transversalis fascia located lateral to the inferior epigastric vessels).

Inguinal Canal Contents & Spermatic Cord Architecture

• Males: Transmits the spermatic cord and the ilioinguinal nerve (which enters between muscle layers, travels on the surface of the cord, and exits the superficial ring to supply sensation to the anterior scrotum and medial thigh).
• Females: Transmits the round ligament of the uterus and the ilioinguinal nerve.
• Spermatic Cord Coverings ('Rule of Three'):
  1. External Spermatic Fascia: Derived from the external oblique aponeurosis.
  2. Cremasteric Muscle and Fascia: Derived from the internal oblique muscle and aponeurosis (innervated by the genital branch of the genitofemoral nerve, mediating the L1–L2 cremasteric reflex).
  3. Internal Spermatic Fascia: Derived from the transversalis fascia. (Note: The transversus abdominis muscle does not contribute a covering to the spermatic cord!)
• Spermatic Cord Vessels and Nerves:
  • Three Arteries: Testicular artery (from aorta), Cremasteric artery (from inferior epigastric), Artery to vas deferens (from inferior vesical).
  • Three Nerves: Genital branch of genitofemoral nerve, autonomic sympathetics, ilioinguinal nerve (on outside of cord).
  • Other structures: Vas (ductus) deferens, pampiniform venous plexus, lymphatics.

Differential Diagnosis: Indirect vs. Direct vs. Femoral Hernias

                         HERNIA TOPOGRAPHICAL MAP
  
             Rectus Abdominis          Inferior Epigastric Vessels
                 |                                  |
                 |       Hesselbach's Triangle      |   Deep Inguinal Ring
                 |     [DIRECT INGUINAL HERNIA]     |  [INDIRECT INGUINAL]
                 |              (Medial)            |      (Lateral)
                 |                                  |
                 +==================================+
                         Inguinal Ligament
                 ------------------------------------
                 [FEMORAL CANAL / FEMORAL HERNIA]
                 (Inferior to Inguinal Ligament, Medial to Femoral Vein)

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3. Gastrointestinal Viscera, Mesenteric Blood Supply & Retroperitoneum

Embryonic Gut Partitioning & Arterial Supply

The primitive gut tube is divided into three distinct segments based on embryonic origin, blood supply, and autonomic innervation.

+-----------------------------------------------------------------------------------------+
|                        EMBRYONIC GUT SEGMENT VASCULAR PROFILE                           |
+-----------------------+-----------------------------+-----------------------------------+
| Gut Segment           | Arterial Supply             | Parasympathetic & Sympathetic     |
+-----------------------+-----------------------------+-----------------------------------+
| **Foregut**           | **Celiac Trunk**            | Parasympathetic: Vagus (CN X)     |
| (Pharynx to mid-      | Branches: Left Gastric,     | Sympathetic: Greater Splanchnic   |
| duodenum, liver,      | Splenic, Common Hepatic     | (T5–T9) -> Celiac Ganglion        |
| gallbladder, pancreas)|                             |                                   |
+-----------------------+-----------------------------+-----------------------------------+
| **Midgut**            | **Superior Mesenteric       | Parasympathetic: Vagus (CN X)     |
| (Distal duodenum to   | Artery (SMA)**              | Sympathetic: Lesser Splanchnic    |
| proximal 2/3 of       | Branches: Jejunal/Ileal,    | (T10–T11) -> SM Ganglion          |
| transverse colon)     | Ileocolic, Right/Middle Colic|                                  |
+-----------------------+-----------------------------+-----------------------------------+
| **Hindgut**           | **Inferior Mesenteric       | Parasympathetic: Pelvic Splanchnic|
| (Distal 1/3 of        | Artery (IMA)**              | (S2–S4 via inferior hypogastric)  |
| transverse colon to   | Branches: Left Colic,       | Sympathetic: Lumbar Splanchnic    |
| upper anal canal)     | Sigmoidal, Superior Rectal  | (L1–L2) -> IM Ganglion            |
+-----------------------+-----------------------------+-----------------------------------+

• Critical Ischemic Watershed Zones:
  • Griffith's Point (Splenic Flexure): Watershed zone between the terminal branches of the SMA (middle colic artery) and IMA (left colic artery).
  • Sudeck's Point (Rectosigmoid Junction): Watershed zone between the lowest sigmoidal branch of the IMA and the superior rectal artery.
  • These watershed areas are exquisitely vulnerable to systemic hypoperfusion, presenting clinically as ischemic colitis (crampy abdominal pain followed by hematochezia).

Retroperitoneal Organs: 'SAD PUCKER'

Organs that lie behind the posterior parietal peritoneum are designated as retroperitoneal. Primary retroperitoneal organs developed outside the peritoneal cavity, while secondary retroperitoneal organs were initially intraperitoneal during early development before fusing to the posterior abdominal wall.

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4. Pelvic Floor, Pelvic Diaphragm & Pudendal Pathways

The Bony Pelvis & Pelvic Diaphragm

• Bony Pelvis Architecture: The pelvic brim (sacral promontory, arcuate lines, pectineal lines, pubic crests) demarcates the Greater (False) Pelvis superiorly from the Lesser (True) Pelvis inferiorly.
• Pelvic Diaphragm (Pelvic Floor): A dynamic, funnel-shaped muscular hammock separating the pelvic cavity from the perineum. Composed of two principal muscles:
  1. Levator Ani: Subdivided into three distinct components:
     • Puborectalis: Forms a muscular U-shaped sling around the anorectal junction. Tonic contraction creates the 80-degree anorectal angle, maintaining fecal continence; relaxation straightens the angle, allowing defecation.
     • Pubococcygeus: Arises from the pubis, inserts into anococcygeal body.
     • Iliococcygeus: Originates from the tendinous arch of obturator internus.
  2. Coccygeus (Ischiococcygeus): Originates from the ischial spine and inserts into the lower sacrum and coccyx, overlying the sacrospinous ligament.
• Clinical Significance: Weakness or tears of the levator ani (frequently following prolonged vaginal delivery) lead to pelvic organ prolapse (cystocele, rectocele, uterine descent) and stress urinary incontinence.

Pudendal Nerve Anatomy (S2, S3, S4)

• Spinal Origin: Formed by the ventral rami of S2, S3, and S4 ('S2, 3, 4 keep shit/pee off the floor').
• Course:
  1. Leaves the pelvis through the Greater Sciatic Foramen inferior to the piriformis muscle.
  2. Hooks over the Ischial Spine and the Sacrospinous Ligament.
  3. Re-enters the perineum through the Lesser Sciatic Foramen.
  4. Travels through the Pudendal (Alcock's) Canal, a fascial tunnel in the lateral wall of the ischioanal fossa formed by the obturator internus fascia.
• Terminal Branches:
  • Inferior Rectal Nerve: Supplies external anal sphincter and perianal skin.
  • Perineal Nerve: Supplies urogenital triangle muscles (bulbospongiosus, ischiocavernosus, superficial transverse perineal) and sensory scrotal/labial branches.
  • Dorsal Nerve of the Penis / Clitoris: Primary sensory nerve for external genitalia.
• Pudendal Nerve Block: Executed by infiltrating local anesthetic transvaginally near the ischial spine, which is palpated through the vaginal wall to provide regional anesthesia for episiotomy or second-stage labor.