Posterior abdominal wall
Posterior Abdominal Wall: Aorta, IVC, Lumbar Triangles and Lymph Drainage
Posterior abdominal wall for FCPS Part 1: psoas and quadratus lumborum, aortic branches and levels, IVC tributaries, collateral veins and para-aortic lymph. Covers AAA, ureteric injury, retroperitoneal fibrosis and lumbar hernia.
Muscles and fascia
| Muscle | Attachments | Nerve | Clinical notes |
|---|---|---|---|
| Psoas major | T12–L5 bodies, discs and transverse processes to lesser trochanter (with iliacus) | L1–L3 ventral rami | Lumbar plexus forms within it; psoas sign in retrocaecal appendicitis |
| Iliacus | Iliac fossa to lesser trochanter | Femoral | Flexes hip with psoas |
| Quadratus lumborum | Iliac crest to 12th rib and lumbar transverse processes | T12–L4 | Fixes the 12th rib in inspiration |
- The diaphragm arches over these muscles: the medial arcuate ligament is thickened psoas fascia; the lateral arcuate ligament is thickened quadratus lumborum fascia; the median arcuate ligament joins the crura in front of the aorta.
- Psoas abscess (classically tuberculosis of the spine) tracks down inside the psoas sheath, beneath the inguinal ligament, to point in the upper medial thigh or groin, where it may mimic a hernia or femoral aneurysm.
Lumbar triangles and lumbar hernia
| Triangle | Boundaries | Floor |
|---|---|---|
| Inferior lumbar (Petit) | Iliac crest below, external oblique in front, latissimus dorsi behind | Internal oblique |
| Superior lumbar (Grynfeltt) | 12th rib above, quadratus lumborum (erector spinae) behind, internal oblique in front | Transversalis fascia (aponeurosis of transversus) |
- A soft, reducible flank swelling with a cough impulse just above the iliac crest is an inferior lumbar (Petit) hernia. The triangle of auscultation lies on the back of the chest and is not a hernia site.
- A penetrating wound in the lumbar region, from outside in: skin, superficial fascia, latissimus dorsi or external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fat, then retroperitoneal organs.
Abdominal aorta
Enters through the aortic hiatus at T12 (behind the median arcuate ligament, with the thoracic duct and azygos vein); bifurcates at L4, near the umbilicus.
| Group | Branches (level) |
|---|---|
| Unpaired, anterior | Coeliac (T12), SMA (L1), IMA (L3) |
| Paired, lateral | Middle suprarenal (L1), renal (L1–L2), gonadal (L2) |
| Paired, posterior (parietal) | Inferior phrenic (T12, the first branch), four lumbar arteries (L1–L4) |
| Unpaired, posterior | Median sacral, arising from the back of the bifurcation |
- The right renal artery is longer and passes behind the IVC.
- An aneurysm at the aortic hiatus may compress the thoracic duct, azygos vein or the crura.
- The infrarenal abdominal aorta is the commonest site of severe atherosclerosis.
Abdominal aortic aneurysm
- Usually infrarenal; elderly male hypertensive smokers; screening ultrasound in men at 65; elective repair at about 5.5 cm or rapid growth.
- Rupture: sudden back or flank pain, hypotension and a pulsatile, expansile mass above the umbilicus; usually into the left retroperitoneum.
- During aorto-iliac repair, the ureter crosses in front of the common iliac bifurcation at the pelvic brim and must be protected; the gonadal vessels in turn cross in front of the ureter.
Inferior vena cava
Formed at L5 by the common iliac veins, ascends to the right of the aorta, passes through the central tendon at T8.
- Directly into the IVC: common iliacs, lumbar veins (3rd and 4th), right gonadal, both renal veins, right suprarenal, inferior phrenic, hepatic veins.
- Left gonadal and left suprarenal drain into the left renal vein, which crosses in front of the aorta just below the SMA origin (nutcracker compression). A new left varicocele in an older man suggests a left renal tumour invading the renal vein.
- In front of the IVC, from below up: right common iliac artery, root of the mesentery, right gonadal artery, third part of the duodenum, head of the pancreas and CBD, portal vein, first part of the duodenum, epiploic foramen, liver.
Caval obstruction and collateral veins
| Block | Collateral route | Sign |
|---|---|---|
| IVC (tumour thrombus, compression) | Thoracoepigastric veins (superficial epigastric to lateral thoracic), lumbar to azygos, vertebral plexus | Tortuous veins running up the flanks from groin to axilla, flow upwards; bilateral leg oedema |
| IVC above the azygos entry | Lumbar and ascending lumbar to azygos and hemiazygos | Dilated azygos system |
| Portal hypertension | Paraumbilical to epigastric | Caput medusae radiating from the umbilicus |
- Caput medusae is portosystemic; flank veins in IVC block are caval–caval.
Lymph drainage
- Para-aortic (lumbar) nodes: kidneys, adrenals, testes and ovaries, uterine fundus and tubes, posterior wall; receive common iliac nodes.
- Preaortic nodes (coeliac, superior and inferior mesenteric): gut.
- Lumbar and intestinal trunks join the cisterna chyli (L1–L2, right of the aorta, behind the right crus), which continues as the thoracic duct.
- Testicular cancer spreads first to para-aortic nodes at L1–L2, not to inguinal nodes (unless the scrotal skin is involved).
Retroperitoneal fibrosis and the ureters
- Dense fibrous plaque around an infrarenal aorta of normal calibre, often with raised ESR and dull back pain. It encases and pulls the ureters medially, causing bilateral hydronephrosis and renal failure. Lymph node masses usually push the ureters laterally.
- The ureter is injured in hysterectomy where the uterine artery crosses above it near the ischial spine, or at the pelvic brim when the ovarian vessels are tied. Ligation causes loin pain within 48 hours.
- Pelviureteric junction stone pain is felt in the loin and flank (T11–L1), moving towards the groin as the stone descends.
Nerves on the posterior wall
| Nerve | Emerges from psoas |
|---|---|
| Iliohypogastric, ilioinguinal, lateral cutaneous nerve of thigh | Lateral border |
| Femoral | Lateral border, in the groove between psoas and iliacus |
| Genitofemoral | Anterior surface |
| Obturator, lumbosacral trunk | Medial border |
- The sympathetic trunk runs on the vertebral bodies along the medial edge of psoas; the coeliac plexus surrounds the coeliac trunk.
Retroperitoneal compartments
- Anterior pararenal (pancreas, duodenum, ascending and descending colon), perirenal (kidney, adrenal, within Gerota's fascia), posterior pararenal (fat).
- Retroperitoneal haemorrhage: trauma, AAA rupture; Grey Turner sign in haemorrhagic pancreatitis.
Remember Coeliac 12, SMA and renals 1, gonadals 2, IMA 3, bifurcation 4, IVC forms at 5.
Exam trap The gonadal vessels cross in front of the ureter; the ureter crosses in front of the iliac bifurcation.
Exam trap Medially drawn ureters point to retroperitoneal fibrosis, not lymphadenopathy.
How FCPS asks about this
- First branch of the abdominal aorta: inferior phrenic; lumbar arteries arise from the back of the aorta.
- Left gonadal vein drains into: left renal vein; right into the IVC.
- Ruptured pulsatile mass with back pain and shock: ruptured AAA, usually infrarenal.
- Structure crossing the iliac bifurcation at the pelvic brim: ureter.
- Flank veins from groin to axilla with IVC thrombus: thoracoepigastric (caval–caval) collaterals.
- Hernia above the iliac crest in the flank: inferior lumbar triangle of Petit.
- Testicular tumour spread: para-aortic nodes at L1–L2.
Test yourself on this topic
5 of the 47 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
On the second day after open repair of an infrarenal aortic aneurysm, a 70-year-old man passes bloody diarrhoea and has left-sided abdominal tenderness. His lactate is raised. Sigmoidoscopy shows dusky, oedematous mucosa. Which of the following best explains this?
Answer: Ligation of the inferior mesenteric artery
The inferior mesenteric artery arises from the front of the infrarenal aorta at L3 and is often ligated or covered during aneurysm repair. The sigmoid colon then relies on collaterals from the SMA (via the marginal artery and middle colic) and from the internal iliac arteries (via the rectal arteries). If these are poor, especially when an internal iliac is also excluded, the left colon becomes ischaemic, giving bloody diarrhoea within days. Colonoscopy or sigmoidoscopy confirms it.
Colon ischaemia after AAA
- IMA from aorta at L3
- Often ligated in AAA repair
- Sigmoid depends on marginal artery and internal iliacs
- Bloody diarrhoea day 1–3
Remember Bloody stool after AAA repair: think IMA.
Exam trap Bleeding per rectum after AAA repair is colonic ischaemia until proved otherwise.
Why the other options are wrong
- Clamping above the renal arteries Would threaten the kidneys rather than the colon.
- Injury to the superior mesenteric vein Would cause small bowel venous congestion.
- Damage to the pelvic splanchnic nerves Cause erectile and bladder problems, not ischaemia.
- Embolism to the coeliac trunk Supplies the foregut, not the sigmoid.
A 22-year-old man has right iliac fossa and flank pain with fever but little anterior tenderness. Pain increases when the right hip is passively extended while he lies on his left side. Where is the appendix most likely to lie?
Answer: Retrocaecal
A retrocaecal appendix lies behind the caecum on the posterior abdominal wall, in contact with the iliopsoas. Inflammation irritates psoas, so stretching it by extending the hip causes pain (psoas sign), and the patient may hold the hip slightly flexed. Because the caecum lies in front, anterior tenderness and guarding can be mild, and pain may be felt in the flank. A pelvic appendix instead irritates obturator internus (obturator sign) and the rectum or bladder.
Psoas sign
- Hip extension stretches psoas
- Positive in retrocaecal appendix
- Little anterior tenderness
- Pelvic appendix: obturator sign
Remember Pain on hip extension: appendix behind caecum.
Exam trap A retrocaecal appendix can give few anterior signs and be missed.
Why the other options are wrong
- Pelvic Gives an obturator sign and pelvic symptoms.
- Pre-ileal Lies in front of the ileum, away from psoas.
- Subcaecal Lies below the caecum, in the iliac fossa.
- Paracaecal (anterior) Gives marked anterior tenderness.
A 58-year-old man has dull low back pain, an ESR of 70 mm/h and a rising creatinine. CT shows bilateral hydronephrosis and a smooth plaque of soft tissue wrapped around a normal-calibre infrarenal aorta, drawing both ureters towards the midline. What is the most likely diagnosis?
Answer: Retroperitoneal fibrosis
Retroperitoneal fibrosis is an inflammatory fibrotic process that begins around the infrarenal aorta and iliac arteries and spreads over the posterior abdominal wall. It encases and pulls the ureters medially, obstructing them and causing hydronephrosis and renal failure, with a raised ESR and dull back pain. Many cases are idiopathic or part of IgG4-related disease; drugs such as methysergide are a classic cause. Bulky para-aortic lymphoma tends to push the ureters laterally and lift the aorta away from the spine.
Retroperitoneal fibrosis
- Fibrous plaque around infrarenal aorta
- Ureters pulled medially
- Hydronephrosis, raised ESR
- Causes: idiopathic, IgG4, methysergide
Remember Ureters drawn in: fibrosis; pushed out: nodes.
Exam trap Medial deviation of the ureters points to fibrosis, not lymph node enlargement.
Why the other options are wrong
- Para-aortic lymphoma Usually displaces ureters laterally and lifts the aorta.
- Inflammatory aortic aneurysm Needs a dilated aorta; this one is normal in size.
- Bilateral PUJ obstruction Would not show a periaortic mass.
- Bilateral psoas abscess Gives fever and a fluid collection in psoas, not a plaque.
An abscess behind the kidney tracks along a muscle and presents as a swelling in the groin with hip flexion contracture. Which muscle sheath is involved?
Answer: Psoas major
The psoas major arises from the T12–L5 vertebrae and passes beneath the inguinal ligament to insert into the lesser trochanter with iliacus. Its fascial sheath allows spread of infection from the spine (tuberculous spondylitis, Pott's disease—the classic 'cold abscess'), kidney or bowel (Crohn's, appendicitis) down into the groin below the inguinal ligament, mimicking a femoral hernia or lymph node. Irritation causes the hip to be held flexed (psoas sign). It lies posterior to the kidney with quadratus lumborum laterally.
Psoas abscess
- Tracks within psoas sheath to groin
- Causes: spinal TB (cold abscess), Crohn's, kidney
- Hip held flexed (psoas sign)
Remember Psoas carries pus from the spine to the groin.
Why the other options are wrong
- Quadratus lumborum Lies posterior and lateral; does not reach the groin.
- Transversus abdominis Anterolateral wall.
- Rectus abdominis Anterior wall.
- Gluteus maximus Buttock.
Two weeks after retroperitoneal lymph node dissection around the upper abdominal aorta for testicular cancer, a 30-year-old man has painless abdominal distension. An ascitic tap gives milky fluid rich in triglycerides. Which structure was most likely damaged?
Answer: Cisterna chyli
The cisterna chyli is a lymph sac lying on the bodies of L1–L2, to the right of the aorta and behind the right crus of the diaphragm. It receives the intestinal (chyle-carrying) and lumbar lymph trunks and continues upwards as the thoracic duct. Dissection around the upper para-aortic region can tear it or its tributaries, causing chylous ascites: milky fluid with a high triglyceride. Injury to the duct in the neck would cause a chyle leak there, not ascites.
Cisterna chyli
- At L1–L2, right of aorta
- Behind right crus
- Receives intestinal and lumbar trunks
- Injury: chylous ascites
Remember Milky ascites after para-aortic surgery: cisterna chyli.
Exam trap Chylous ascites comes from abdominal lymph injury; a chylothorax from the thoracic duct.
Why the other options are wrong
- Thoracic duct in the neck Would cause a neck or chest leak, not ascites.
- Inferior mesenteric vein Bleeds; does not carry chyle.
- Left renal vein A vein; injury causes bleeding.
- Right lumbar sympathetic trunk Injury causes warm, dry foot, not milky ascites.
Where this sits in the CPSP syllabus
- Paper 1: Abdomen, Posterior abdominal wall, aorta, IVC and lymph drainage
- Surgery & Allied, Paper 2: Abdomen, Posterior abdominal wall, aorta, IVC and lymph drainage