Lecture 29: Retroperitoneum & Posterior Abdominal Wall

• Parietal peritoneum — lines the abdominal wall (“outer” layer). Somatic innervation → pain is sharp and localized.
• Visceral peritoneum — covers the organs (“inner” layer). Autonomic innervation → pain is dull, poorly localized, referred to midline dermatomes.
• Peritoneal cavity — potential space between them. Normally holds only a small amount of serous fluid.

• Intraperitoneal (IP) — organ is suspended within the peritoneal cavity by a mesentery; almost completely covered by visceral peritoneum.
• Retroperitoneal (RP) — organ lies posterior to the parietal peritoneum against the posterior abdominal wall; only its anterior surface is covered.

• Perinephric (perirenal) space — between the renal capsule and Gerota’s (renal) fascia. Contains the kidney, adrenal, renal vessels, and perinephric fat.
• Gerota’s fascia (renal fascia) — the tough connective-tissue envelope; fused superiorly with the diaphragmatic fascia, open inferiorly (why perinephric collections can track down).
• Paranephric space — external to Gerota’s fascia, contains paranephric fat; bounded anteriorly by the transversalis fascia.

• Duodenum — all but the 1st part; the duodenal bulb is IP.
• Pancreas — head, neck, body are RP; the tail is IP (it lies in the splenorenal ligament).
• Ascending & descending colon (the transverse and sigmoid colon are IP on their own mesenteries).
• Rectum — middle & inferior thirds are RP/subperitoneal (the upper third is IP).

• Stomach, 1st part of duodenum, jejunum, ileum
• Transverse colon, sigmoid colon
• Cecum (variable — usually IP but can be RP)
• Appendix, liver, gallbladder, spleen, tail of pancreas

• Never had a mesentery during development.
• Examples: kidneys, adrenals, ureters, aorta, IVC, lower esophagus, rectum.

• Began development intraperitoneally with a mesentery, then rotated and fused with the posterior body wall; mesentery regressed.
• Examples: most of the duodenum, pancreas (head, neck, body), ascending & descending colon.

• RP bleeds (AAA rupture, pelvic fracture, kidney laceration) can cause life-threatening hemorrhage with minimal peritoneal signs.
• Buzzword bruising: Grey-Turner sign (flank), Cullen sign (periumbilical) — indicate RP hemorrhage tracking along fascial planes.

• Psoas abscess — from vertebral osteomyelitis or Crohn’s tracking into psoas sheath; presents with back pain, hip flexion position, positive psoas sign.
• Appendicitis in a retrocecal appendix can present atypically with flank pain and less peritoneal signs.

• Pancreatitis behind the stomach is classic “pain radiating to the back” because the pancreas is RP.
• Duodenal perforation of the posterior wall (2nd part) may wall off retroperitoneally rather than spill freely.

Understanding kidney layering explains why retroperitoneal bleeds stay contained within fascial planes:

1. Paranephric (pararenal) fat — outermost, external to renal fascia. Thickest posteriorly.
2. Renal fascia (Gerota's fascia) — a dense connective tissue envelope that encloses the kidney, adrenal, and perinephric fat. Fuses inferomedially with the ureter sheath.
3. Perinephric fat — between the fibrous capsule and Gerota's fascia. "Cushion" around the kidney.
4. Fibrous capsule — tightly adherent to renal parenchyma.

From outside in:

Each kidney spans T12–L3. The right kidney is ~1–2 cm lower than the left (pushed down by the liver).

Inflammation or mass effect in the kidney can irritate overlying organs, and vice versa — knowing anterior relationships guides differential diagnoses and surgical planning.

• Enters the abdomen through the aortic hiatus at T12.
• Bifurcates into common iliac arteries at L4 (roughly at the umbilicus in thin patients).
• Lies just to the left of midline, anterior to the vertebral bodies.

• Celiac trunk — T12 — foregut (stomach, liver, spleen, upper duodenum, pancreas head).
• Superior mesenteric artery (SMA) — L1 — midgut (distal duodenum through proximal 2/3 transverse colon).
• Inferior mesenteric artery (IMA) — L3 — hindgut (distal 1/3 transverse colon through superior rectum).

• Middle suprarenal arteries — L1
• Renal arteries — L1/L2 (right is longer, passes behind IVC)
• Gonadal arteries — L2 (testicular / ovarian)

• Inferior phrenic and lumbar arteries — body wall.
• Inferior phrenic gives off the superior suprarenal artery.

• Formed at L5 by junction of common iliac veins, runs to the right of midline.
• Passes through the caval hiatus at T8 and drains into the right atrium.
• Anterior: crosses posterior to the epiploic foramen of Winslow (entrance to lesser sac), head of pancreas, 3rd part of duodenum, root of mesentery, and right liver.

• Common iliac veins
• Lumbar veins
• Right gonadal vein (the left gonadal drains into the left renal, not the IVC)
• Renal veins
• Right suprarenal (left suprarenal drains into left renal)
• Inferior phrenic veins
• Hepatic veins

• Paired, arise at L1–L2. Each divides into 5 segmental arteries — anatomic end arteries: no meaningful collaterals, so segmental occlusion causes infarction.
• The right renal artery is longer and passes posterior to the IVC.
• Kidneys receive ~25% of cardiac output despite being ~0.5% of body mass.

• Left renal vein is longer; crosses anterior to the aorta and posterior to the SMA — this is the “nutcracker angle.”
• Receives: left gonadal, left suprarenal, and left 2nd lumbar veins.
• Right renal vein is short, drains directly into IVC.

• Compression of the left renal vein between the aorta and SMA.
• Produces left-sided flank pain, hematuria, and left-sided varicocele (in males) or pelvic congestion (in females) from elevated left renal venous pressure.
• Diagnosis: Doppler US, CT angiography demonstrating narrowing.

• Superior suprarenal from the inferior phrenic artery
• Middle suprarenal from the aorta directly (L1)
• Inferior suprarenal from the renal artery

• Each adrenal has one vein.
• Right adrenal vein → directly into IVC (short, surgically delicate).
• Left adrenal vein → left renal vein.

• Drains abdominal GI tract (distal esophagus through upper rectum), spleen, pancreas, gallbladder → liver.
• Formed by splenic vein + superior mesenteric vein; IMV joins splenic.

• IVC drains abdominal wall, kidneys, adrenals, gonads, liver via hepatic veins.

• Posterior body wall veins (lumbar, ascending lumbar) that communicate with the azygos/hemiazygos system in the thorax via the aortic hiatus (T12).
• Critical collateral pathway if IVC is obstructed.

• Distal esophagus: left gastric ↔ esophageal (azygos) → esophageal varices
• Umbilicus: paraumbilical ↔ superficial epigastric → caput medusae
• Rectum: superior rectal (portal) ↔ middle/inferior rectal (caval) → internal hemorrhoids
• Retroperitoneal bare areas of liver and colon → silent but important in portal hypertension.

The abdominal aorta gives off 10 named branches before bifurcating at L4. Group them by pattern:

The celiac trunk is only ~½ inch long and projects directly anteriorly just below the diaphragm. It immediately trifurcates:

Runs along the lesser curvature and gives the esophageal branches to the distal 1/3 of the esophagus — site of esophageal varices in portal hypertension (left gastric v. ↔ esophageal vv.).

Key relationships (anterior → posterior):

1. Inferior pancreaticoduodenal a. — first branch; anastomoses with superior pancreaticoduodenal (from gastroduodenal)
2. Jejunal & ileal branches — many, arising from left side
3. Middle colic a. — first right-side branch; supplies proximal 2/3 transverse colon
4. Right colic a. — second right-side branch; ascending colon/hepatic flexure
5. Ileocolic a. — third right-side branch; gives the appendicular a. (sole blood supply to appendix)

The marginal artery of Drummond runs along the mesenteric border of the colon and links SMA branches to IMA branches. The arc of Riolan (meandering mesenteric artery) is a more central but less consistent collateral. If the IMA is occluded (e.g., during AAA repair) and these collaterals are inadequate → sigmoid ischemia.

The IVC is formed by the union of the two common iliac veins at the L5 level (slightly right of midline, behind the beginning of the right common iliac artery). It ascends on the right of the aorta.

The hepatic portal vein forms posterior to the neck of the pancreas from the union of the splenic vein and the superior mesenteric vein (SMV). The inferior mesenteric vein (IMV) usually drains into the splenic vein (not directly into the portal v.).

The portal vein delivers nutrient-rich, poorly-oxygenated blood from the GI tract to the liver, where it is processed and then returned to the IVC via the hepatic veins. It is a low-pressure system with a capillary bed at each end (visceral organs ↔ liver sinusoids).

When the liver is diseased (cirrhosis, schistosomiasis, Budd-Chiari), portal pressure rises and blood reverses through normally-small anastomoses with the systemic (caval) system. Four classic sites:

Intestinal and lumbar lymph trunks converge at the cisterna chyli at ~L1–L2 (posterior to the right crus of the diaphragm), which becomes the thoracic duct as it passes through the aortic hiatus at T12. The thoracic duct ascends through the thorax and empties into the left subclavian/internal jugular venous junction.

The diaphragm is a dome-shaped musculotendinous sheet separating the thoracic and abdominal cavities. It develops from four components: the septum transversum (central tendon), pleuroperitoneal membranes, dorsal mesentery of the esophagus (crura), and body wall (muscular periphery).

Failure of pleuroperitoneal membrane fusion → congenital diaphragmatic hernia (most often left-sided Bochdalek), with abdominal viscera herniating into the thorax and pulmonary hypoplasia.

The phrenic nerve is also sensory to the central portion of the diaphragm (both pleural and peritoneal surfaces). The peripheral diaphragm is supplied by the lower intercostal nerves (T5–T11) and the subcostal nerve (T12).

The caval hiatus lies in the central tendon at the level of the T8 vertebra. Because it pierces the tendon (not muscle), the opening is held open during inspiration — actually aiding venous return as the diaphragm contracts.

Contents:

The left phrenic nerve pierces the muscular dome of the diaphragm separately (to the left of the pericardium). Only the right phrenic nerve accompanies the IVC through the caval hiatus before innervating its dome from below.

The esophageal hiatus is a sling in the right crus of the diaphragm at the level of T10. Unlike the caval hiatus, it is muscular, so crural contraction compresses the distal esophagus — contributing to the anti-reflux barrier at the gastro-esophageal junction.

Contents:

Widening of the esophageal hiatus → hiatal hernia:

The aortic hiatus is a posterior opening at T12, bounded by the median arcuate ligament (a fibrous arch connecting the right and left crura) anteriorly and the vertebral body posteriorly. Because the aorta lies behind the diaphragm rather than piercing muscle, it is not compressed during inspiration.

Contents:

Abnormally low or tight median arcuate ligament compresses the celiac trunk just below the aortic hiatus → post-prandial epigastric pain, weight loss, epigastric bruit. Treated surgically by ligament release.

The phrenic nerve (C3–C5) shares spinal cord levels with the supraclavicular nerves (C3–C4), which innervate the shoulder and neck skin. Central diaphragm irritation is therefore referred to the ipsilateral shoulder tip.

Injury (surgical, malignant invasion, viral neuritis) → hemidiaphragm paralysis, visible on chest X-ray as an elevated hemidiaphragm with paradoxical movement on sniff test.

Blunt abdominal trauma most commonly ruptures the left hemidiaphragm (the liver shields the right). Herniation of the stomach/colon into the left chest causes respiratory distress and a "bowel in chest" sign on imaging.

The posterior abdominal wall is built from four muscles. The diaphragm forms its superior boundary, and the pelvis/iliac fossa form its inferior boundary.

Performed when intra-abdominal inflammation (especially retrocecal appendicitis) is suspected:

1. Patient lies on unaffected side
2. Examiner passively extends the affected hip (or patient actively flexes hip against resistance)
3. Reproduction of pain = positive psoas sign

An inflamed retrocecal appendix or psoas abscess lies against the psoas muscle; stretching/contracting the muscle irritates the overlying peritoneum.

The lumbar plexus (L1–L4, with T12 subcostal contribution) is formed within the psoas major. Its terminal branches emerge at the medial or lateral border of psoas to supply the pelvic wall and lower limb.

The three parts converge on the central tendon (which is pierced by the IVC at T8).

The kidneys lie in the perinephric space between T12 and L3 vertebral levels, enclosed by Gerota's (renal) fascia and surrounded by perinephric fat. The right kidney sits ~1–2 cm lower than the left (displaced inferiorly by the liver).

The posterior relationships differ for the superior and inferior poles:

Urine flows from the collecting ducts into the minor calyces → major calyces → renal pelvis → ureter. Special smooth-muscle pacemaker cells in the minor calyces generate ~4 peristaltic waves/min that propel urine.

Stones most often form at the renal papilla (site of supersaturation and Randall's plaques) and may lodge anywhere along the urinary tract.

Each ureter is ~25 cm long. It descends anterior to the psoas major and is crossed anteriorly by the gonadal vessels ("water — ureter — under the bridge — gonadal vessels"). The ureters travel at roughly the level of the tips of the lumbar transverse processes.

At the pelvic brim, the ureter crosses over the bifurcation of the common iliac artery, then courses posteriorly along the lateral pelvic wall to the bladder.

The ureter receives multiple small arterial branches that approach from the medial side in the abdomen and from the lateral side in the pelvis — a key surgical principle for preserving ureteric viability.

Stones characteristically impact at three physiologic narrowings:

The ureter is innervated by sympathetic fibers traveling with segments T11–L2. Because these cord levels also receive somatic afferents from the flank, scrotum/labium, and medial thigh, ureteric colic is felt as a classic loin-to-groin pain pattern.

Classic rule: "water under the bridge" — the ureter ("water") passes under the overlying artery ("bridge"):

Stones <5 mm usually pass spontaneously with hydration, analgesia, and tamsulosin (medical expulsive therapy). Stones >10 mm or with obstruction/infection require ureteroscopy, shock wave lithotripsy, or percutaneous nephrolithotomy (staghorn stones).

The paired renal arteries arise from the abdominal aorta at the L1–L2 intervertebral disc level, just inferior to the origin of the SMA. Both arteries course horizontally/slightly inferolaterally to reach the renal hila.

Each kidney receives ~20–25% of cardiac output, reflecting the high perfusion demand of glomerular filtration.

Within the renal hilum, each renal artery divides into 5 segmental arteries supplying 5 renal segments (apical, upper, middle, lower, posterior). These are anatomic end arteries: they do not anastomose with one another.

Accessory (supernumerary) renal arteries arise directly from the aorta in ~25–30% of individuals and enter the kidney at the poles (most commonly lower pole) rather than the hilum. Because they are end arteries, they must be preserved during renal transplantation or partial nephrectomy.

Renal veins drain directly into the IVC at ~L1–L2. The left renal vein receives 3 extra tributaries that the right renal vein does not:

• Left suprarenal (adrenal) vein
• Left gonadal vein (testicular/ovarian)
• Left 2nd lumbar vein (inconstant)

• Non-contrast helical CT (stone protocol) — gold standard; detects all stones including radiolucent uric acid.
• Renal ultrasound — first-line in pregnancy and children; detects hydronephrosis and larger stones but misses small/ureteric stones.
• KUB plain film — limited; misses radiolucent uric acid and small stones. Useful for follow-up of known radiopaque stones.

The ureter has a segmental, longitudinally anastomotic blood supply. The side of approach matters clinically:

• L2: Origin at renal pelvis / ureteropelvic junction (UPJ).
• Crosses psoas major anteriorly as it descends.
• Crosses sacroiliac joint / bifurcation of common iliac artery at pelvic brim — 2nd ureteric constriction.
• Passes posterior to ductus deferens / uterine artery ("water under the bridge") in pelvis.
• Ureterovesical junction at bladder trigone — 3rd constriction; most common lodging site.

The paired suprarenal (adrenal) glands cap the superior poles of the kidneys within Gerota's fascia, but in their own fascial compartment (separate from the kidney — so they don't descend with ptotic kidneys).

Each gland has two functionally and embryologically distinct parts:

The cortex has no direct autonomic innervation controlling its secretion. Steroid production is instead regulated by circulating hormones and local factors:

The medulla is unique: it is innervated directly by pre-ganglionic sympathetic fibers (thoracic splanchnic nerves, T5–T11, primarily via the greater splanchnic nerve) that synapse on chromaffin cells. These cells function as modified post-ganglionic sympathetic neurons without axons — they release catecholamines (~80% epinephrine, ~20% norepinephrine) directly into the bloodstream.

The dual origin explains the dual control system: the mesodermal cortex responds to circulating hormones, while the neural-crest medulla responds to direct neural input — the same logic that governs all sympathetic ganglia.

Each adrenal receives blood from three arteries:

Catecholamine-secreting tumor of chromaffin cells. Classic episodic triad: headache, palpitations, sweating, plus hypertension. Lab: elevated urinary/plasma metanephrines. Tx: α-blockade before β-blockade to avoid hypertensive crisis.

Loss of both cortex layers → hypotension, hyponatremia, hyperkalemia, hypoglycemia, hyperpigmentation (from ↑ ACTH/MSH). Because the defect is cortical, catecholamine release is preserved.

Hemorrhagic adrenal infarction (classically with Neisseria meningitidis septicemia) → acute adrenal failure → shock.

• Vagus nerve (CN X): supplies foregut and midgut → up to the splenic flexure (Cannon's point), also called the watershed between midgut and hindgut.
• Pelvic splanchnic nerves (S2–S4): supply hindgut from splenic flexure → upper anal canal, plus all pelvic viscera (bladder, internal genitalia). "S2, 3, 4 keeps the gut off the floor" — parasympathetic promotes defecation, micturition, erection.

The gut has an intrinsic neural network — the enteric nervous system — often called the "second brain" (~100 million neurons). Two plexuses:

• Myenteric (Auerbach's) plexus: between inner circular and outer longitudinal muscle layers — controls motility / peristalsis.
• Submucosal (Meissner's) plexus: in the submucosa — controls secretions and local blood flow.

• Origin: stretch, distension, ischemia, or inflammation of a hollow viscus or visceral peritoneum.
• Afferent pathway: visceral afferents travel alongside sympathetic fibers back to the spinal cord (DRG at T5–L2).
• Character: dull, crampy, poorly localized, midline. Referred to the dermatome of the spinal level of entry.
• Localization pattern:
  • Foregut (T5–T9) → epigastric
  • Midgut (T10–T11) → periumbilical
  • Hindgut (T12–L2) → hypogastric / suprapubic

• Origin: irritation of the parietal peritoneum (somatic structure lining the body wall).
• Afferent pathway: somatic afferents via segmental spinal nerves of the overlying body wall.
• Character: sharp, well-localized, lateralized to the site of inflammation; worsened by movement, cough, percussion (rebound tenderness).

• Cholecystitis → right shoulder / scapular tip (phrenic C3–C5 shares with supraclavicular nerves).
• Diaphragmatic irritation (any cause) → Kehr's sign (ipsilateral shoulder).
• Renal colic → loin to groin (T11–L2 via iliohypogastric, ilioinguinal, genitofemoral).
• Testicular pain → periumbilical (testis originates in abdomen at T10; retains embryologic innervation).
• Myocardial ischemia → left arm, jaw, epigastrium (T1–T4).
• Pancreatitis → epigastric with band-like radiation through to back (retroperitoneal + celiac plexus).