Abdominal wall and peritoneum
Anterior abdominal wall, hernias and peritoneal spaces on imaging
The abdominal wall is a stack of layers that changes at the arcuate line, and the peritoneal spaces guide fluid, pus, gas and tumour along set routes. CPSP asks about the rectus sheath, the umbilical folds, the dependent spaces seen on FAST and CT, and the plain-film signs of pneumoperitoneum.
Layers of the anterior abdominal wall
- From outside in: skin, Camper fascia (fatty), Scarpa fascia (membranous), external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fat, parietal peritoneum.
- At laparotomy, the layer met immediately deep to transversus abdominis is the transversalis fascia, then extraperitoneal fat, then peritoneum.
- Scarpa fascia continues into the perineum as Colles fascia and fuses with the fascia lata below the inguinal ligament, so blood or urine deep to it spreads up the abdominal wall but not into the thigh.
- Linea alba: almost bloodless midline raphe, wide above the umbilicus, very narrow below it.
- Linea semilunaris: lateral edge of rectus. A Spigelian hernia passes through the aponeurosis here, usually near the arcuate line, and lies deep to the external oblique aponeurosis, so it may be impalpable and is shown on CT or ultrasound.
- Aponeuroses: the lower free edge of the external oblique aponeurosis forms the inguinal ligament (ASIS to pubic tubercle); external oblique interdigitates with serratus anterior and latissimus dorsi. The internal oblique aponeurosis splits around rectus above the arcuate line and, with transversus, forms the conjoint tendon.
- Umbilicus: about the L3–L4 disc; the aortic bifurcation (L4) lies close behind it in a slim patient (Veress needle injury). Dermatome T10.
Rectus sheath
| Level | Anterior wall | Posterior wall |
|---|---|---|
| Above the costal margin | External oblique aponeurosis | Deficient (rectus lies on costal cartilages) |
| Costal margin to arcuate line | External oblique + anterior half of internal oblique | Posterior half of internal oblique + transversus |
| Below the arcuate line | All three aponeuroses | Transversalis fascia only |
- The arcuate line lies about midway between umbilicus and pubis; the inferior epigastric artery enters the sheath here.
- Tendinous intersections fuse with the anterior sheath only, boxing in an abscess in front of rectus.
- Rectus sheath haematoma (anticoagulated patient, violent cough): on CT a spindle-shaped mass within the muscle above the arcuate line; below it the blood can track behind the muscle into the retropubic space and across the midline.
- Rectus is supplied by T7–T12 (thoracoabdominal and subcostal nerves). These nerves enter the sheath from its lateral side; a long incision along the lateral border of rectus denervates the muscle (weak bulge without a fascial defect).
- Superior epigastric (internal thoracic) and inferior epigastric (external iliac) arteries anastomose within rectus (TRAM flap; collateral round an aortic occlusion). Paracentesis and lateral ports go lateral to rectus to avoid the inferior epigastric vessels. A vertical stab wound just lateral to the linea alba, or a paramedian incision, can divide an epigastric artery inside the sheath, causing shock. The superficial epigastric artery is a branch of the femoral artery and runs in the superficial fascia.
Incisions and hernias of the wall
- Pfannenstiel incision: a low transverse suprapubic skin incision with transverse division of the anterior rectus sheath, then separation of the rectus muscles in the midline (not cutting them); standard for caesarean section.
- Incisional hernia: a swelling with a cough impulse at a previous scar, such as after repeated caesarean sections; obesity, infection and multiple operations weaken the repair.
- Exomphalos (omphalocele): bowel through the umbilical ring inside a translucent sac with the cord attached to it; the midgut failed to return after normal physiological herniation (until about 12 weeks). It is often associated with other anomalies (trisomy 18, Beckwith–Wiedemann, heart defects), so look for them; gastroschisis has no sac and is usually isolated.
- Patent urachus: the allantois (urachus) fails to obliterate, so urine leaks from the umbilicus.
Peritoneal folds on the inner surface
| Fold | Contains | Hernia relation |
|---|---|---|
| Median umbilical | Urachus remnant | Patent urachus: urine at umbilicus |
| Medial umbilical | Obliterated umbilical arteries | Direct hernia arises lateral to it, in the medial inguinal fossa |
| Lateral umbilical | Inferior epigastric vessels | Indirect hernia lateral, direct hernia medial (Hesselbach triangle) |
Venous patterns on the abdominal wall
- Portal hypertension: recanalised paraumbilical vein in the ligamentum teres; dilated veins radiate from the umbilicus (caput medusae), flow away from it.
- IVC obstruction: flank veins carry blood upwards from below the umbilicus towards the axilla (superficial epigastric to lateral thoracic), with bilateral leg oedema. A large liver tumour may suggest portal hypertension, but upward flow with leg oedema means the IVC is compressed.
- SVC obstruction: arm-vein contrast fills the internal thoracic and epigastric veins behind rectus.
- Lymph: skin above the umbilicus drains to axillary nodes, below it to superficial inguinal nodes.
Peritoneal spaces
- Falciform ligament attaches the liver to the anterior abdominal wall; its free edge carries the ligamentum teres.
- Lesser sac: behind the stomach and lesser omentum, in front of the pancreas; entered through the epiploic foramen (front: hepatoduodenal ligament with bile duct right, hepatic artery left, portal vein behind; back: IVC; above: caudate lobe; below: first part of duodenum). Its superior recess wraps the caudate lobe. Posterior gastric perforation and fluid from a pancreatic injury or pancreatitis collect here, forming a pseudocyst.
- Supramesocolic spaces: right and left subphrenic, right subhepatic (its posterior part is the hepatorenal pouch of Morison), lesser sac.
- Infracolic spaces: divided by the root of the small-bowel mesentery (DJ flexure at L2 left, to the right sacroiliac joint). The right infracolic space is closed below by the root, so fluid stays there; the left opens into the pelvis.
- Paracolic gutters: the right gutter connects Morison pouch with the pelvis; the left is closed above by the phrenicocolic ligament.
- Dependent sites: supine, Morison pouch is the lowest space above the pelvic brim (right upper FAST view); the rectouterine pouch (Douglas) is the lowest point of the female cavity (rectovesical pouch in men). On a supine film, pelvic fluid either side of the bladder fills the paravesical recesses ("dog ears").
- Circulation: diaphragmatic movement draws fluid up the right gutter to the right subphrenic space (ovarian cancer deposits on the right hemidiaphragm, perihepatitis after pelvic infection); leaking duodenal ulcer fluid runs down the same gutter to the right iliac fossa.
- Retroperitoneum: anterior pararenal (pancreas, duodenum, ascending and descending colon), perirenal (inside Gerota fascia), posterior pararenal. Pancreatitis fluid fills the anterior pararenal space and the transverse mesocolon; retroperitoneal blood reaches the umbilicus along the falciform ligament.
Gas in the wrong place
| Finding | Meaning |
|---|---|
| Gas under the diaphragm on erect chest film | Pneumoperitoneum: perforation, or residual gas after open surgery, which may persist for a week or more in a slim patient; a well, eating patient needs no action, while rising gas on serial films suggests a leak |
| Gas over the liver on left lateral decubitus film | Pneumoperitoneum in a patient who cannot stand; not confused with stomach gas |
| Both sides of the bowel wall visible (Rigler sign) | Large pneumoperitoneum on a supine film |
| Gas outlining kidney and psoas, none under the diaphragm | Retroperitoneal perforation (duodenum D2–D3) |
| Gas under the right dome crossed by haustral folds, not shifting | Chilaiditi: colon between liver and diaphragm |
Fat and peritoneal lesions on CT
| Lesion | CT clue | Point |
|---|---|---|
| Epiploic appendagitis | Oval fat-density lesion (2–3 cm) beside the colon with a thin dense rim and central dot (thrombosed vein); colon wall normal | Torsion of an epiploic appendage; self-limiting; mimics diverticulitis |
| Segmental omental infarction | Larger ill-defined fat stranding between the anterior wall and ascending colon, no dense rim | Mimics appendicitis in obese adults; conservative care |
| Sclerosing mesenteritis | Misty fat at the small-bowel mesenteric root with small nodes; fat ring of normal fat round each vessel and a thin capsule | Follow up, as lymphoma can mimic it |
| Encapsulating peritoneal sclerosis | Small bowel clumped centrally in a thick, partly calcified membrane with loculated fluid | Long-term peritoneal dialysis; can appear after dialysis stops |
| CSF pseudocyst | Thin-walled loculated collection with a ventriculoperitoneal shunt tip inside | Shunt malfunction; culture CSF, as many are infected |
| TB peritonitis | Ascites, thick peritoneum, nodes with low-density centres and enhancing rims | Low density is caseous necrosis |
Other imaging points
- Laparoscopy uses carbon dioxide because it is very soluble in blood and non-flammable, so any gas embolus dissolves quickly and diathermy is safe; absorbed CO2 can cause hypercapnia in severe lung disease.
- Referred pain: early appendicitis pain is felt round the umbilicus because visceral afferents enter the cord at T10, the umbilical dermatome.
- Umbilical venous catheter: umbilical vein, left portal vein, ductus venosus, IVC; a tip curving into the liver has missed the ductus. A high umbilical arterial catheter tip lies at T6–T9.
- The midgut lies in the cord normally until about 12 weeks, so a small mass at the cord base at 9 weeks is not yet abnormal.
- Mesothelioma: sheet-like peritoneal thickening after asbestos exposure.
Remember Below the arcuate line everything goes in front of rectus, leaving only transversalis fascia behind.
Exam trap The lateral umbilical fold holds the inferior epigastric vessels, not an umbilical artery remnant.
How FCPS asks about this
- Rectus sheath below the arcuate line: all aponeuroses in front, only transversalis fascia behind.
- Layer deep to transversus abdominis: transversalis fascia.
- Pancreatic juice after upper abdominal trauma collects in: the lesser sac.
- Posterior boundary of the epiploic foramen: inferior vena cava.
- Abdominal wall veins filling from below upwards with leg oedema: IVC obstruction.
- Fat lesion with a dense rim and central dot beside the colon: epiploic appendagitis.
- Urine from the umbilicus: patent urachus (allantois).
Test yourself on this topic
5 of the 133 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 62-year-old man has vague abdominal pain from a mass at the root of the small-bowel mesentery. Biopsy shows fat necrosis with foamy macrophages, chronic inflammation and fibrosis of the mesenteric fat, with no malignant cells. What is the diagnosis?
Answer: Sclerosing mesenteritis
Sclerosing mesenteritis is a non-specific inflammatory and fibrotic disorder of the mesenteric fat, usually at the root of the small-bowel mesentery, with stages from fat necrosis (mesenteric panniculitis) to chronic inflammation and fibrosis (retractile mesenteritis). On CT it gives a 'misty mesentery' with a halo of normal fat around the vessels (fat ring sign). It can be associated with other malignancies, so follow-up is advised.
Sclerosing mesenteritis
- Inflammation of mesenteric fat
- Fat necrosis, inflammation, fibrosis
- At the mesenteric root
- CT: misty mesentery, fat ring
Remember Inflamed, fibrosed mesenteric fat: sclerosing mesenteritis.
Exam trap Misty mesentery can accompany lymphoma or other cancers, so follow-up imaging is often advised.
Why the other options are wrong
- Mesenteric lymphoma Sheets of malignant lymphoid cells.
- Carcinoid tumour of the mesentery Nests of neuroendocrine cells with desmoplasia.
- Mesenteric venous thrombosis Thrombus in the vein with bowel wall infarction.
- Acute pancreatitis Necrosis centred on the pancreas with raised amylase.
In severe pancreatitis, inflammatory fluid spreads forward from the pancreatic body between the layers of the peritoneal fold that carries the middle colic vessels, causing thickening and stricture of the transverse colon. Along which fold has it spread?
Answer: Transverse mesocolon
The transverse mesocolon attaches along the anterior surface of the pancreas and carries the middle colic vessels to the transverse colon. Because its root lies directly on the pancreas, inflammatory fluid and enzymes from pancreatitis can spread between its layers to the transverse colon, causing colitis, stricture or the 'colon cut-off' sign. The gastrocolic ligament links the stomach to the transverse colon, the small bowel mesentery carries the superior mesenteric vessels to the small bowel, and the hepatoduodenal ligament carries the portal triad.
Transverse mesocolon
- Root on the anterior pancreas
- Carries middle colic vessels
- Route for pancreatitis spread
- Colitis, colon cut-off sign
Remember Pancreas to colon via the transverse mesocolon.
Exam trap Colonic wall thickening in pancreatitis can be mistaken for primary colitis or tumour.
Why the other options are wrong
- Gastrocolic ligament Joins stomach to transverse colon; does not carry the middle colic vessels.
- Small bowel mesentery Carries the superior mesenteric vessels to the small bowel.
- Gastrosplenic ligament Carries the short gastric vessels to the spleen.
- Hepatoduodenal ligament Carries the portal vein, hepatic artery and bile duct.
A 50-year-old man has a perforated posterior gastric ulcer. Fluid collects around the caudate lobe of the liver, between it and the diaphragm, close to the IVC. Which peritoneal space contains this fluid?
Answer: Superior recess of the lesser sac
The lesser sac (omental bursa) lies behind the stomach and lesser omentum. Its superior recess extends upward behind the lesser omentum and around the caudate lobe of the liver, between the caudate lobe, the diaphragm and the IVC. Fluid entering the lesser sac, for example from a posterior gastric perforation or pancreatitis, can therefore be seen surrounding the caudate lobe. The right subphrenic and hepatorenal spaces belong to the greater sac and lie lateral and inferior to the right lobe.
Superior recess of the lesser sac
- Extends behind the lesser omentum
- Wraps the caudate lobe
- Near IVC and diaphragm
- Fills with posterior gastric or pancreatic fluid
Remember Fluid around the caudate lobe: lesser sac.
Exam trap Fluid around the caudate lobe on imaging can be mistaken for a mass or nodes at the porta.
Why the other options are wrong
- Right anterior subphrenic space Lies over the right lobe beneath the diaphragm, part of the greater sac.
- Hepatorenal (Morison) pouch Lies between the right lobe and the right kidney.
- Left anterior subphrenic space Lies around the left lobe and spleen.
- Splenic recess of the lesser sac Lies at the left edge of the lesser sac near the spleen.
A 45-year-old man with severe necrotising pancreatitis develops bluish discoloration around the umbilicus on day 3, from retroperitoneal haemorrhage. Along which route does the blood most likely reach the umbilicus?
Answer: Along the falciform ligament and ligamentum teres
Periumbilical bruising (Cullen sign) results from blood-stained fluid tracking from the retroperitoneum. The fluid passes from the anterior pararenal space into the gastrohepatic ligament and then along the falciform ligament, whose free edge contains the ligamentum teres, which runs to the umbilicus. It therefore reaches the subcutaneous tissue around the umbilicus. Flank bruising (Grey Turner sign) results from spread through the posterior pararenal space to the flank. Both indicate retroperitoneal haemorrhage, often in severe pancreatitis.
Periumbilical bruising
- Retroperitoneal haemorrhage
- Tracks via falciform ligament
- Ligamentum teres to umbilicus
- Flank bruising: posterior pararenal route
Remember Blood reaches the umbilicus along the falciform ligament.
Exam trap These signs appear late and are present in only a few patients; their absence does not exclude severe pancreatitis.
Why the other options are wrong
- Through the inguinal canal Leads to the groin and scrotum, not the umbilicus.
- Along the psoas sheath Leads to the groin and thigh.
- Through the paraumbilical veins in the bloodstream Bruising is extravasated blood, not blood flowing in veins.
- Along the obliterated umbilical arteries These lie low on the inner wall towards the bladder.
A 32-year-old woman who had two laparotomies for a ruptured appendix and pelvic abscess now has pelvic discomfort. At laparoscopy, a multiloculated collection of clear fluid trapped by thin adhesions fills the pelvis and surrounds a normal right ovary. What is the diagnosis?
Answer: Peritoneal inclusion cyst
A peritoneal inclusion cyst (peritoneal pseudocyst) forms in premenopausal women with peritoneal adhesions from surgery, infection or endometriosis. The ovary keeps producing fluid, but the adhesions stop the peritoneum from absorbing it, so the fluid is trapped in loculated pockets that take the shape of the surrounding space. The key feature is a normal ovary lying within or at the edge of the collection. An ovarian tumour arises from the ovary and replaces normal tissue.
Peritoneal inclusion cyst
- Adhesions after surgery or infection
- Ovarian fluid trapped
- Shape follows the pelvic space
- Normal ovary inside it
Remember Fluid around a normal ovary after surgery: inclusion cyst.
Exam trap These cysts recur after drainage and can be mistaken for ovarian cancer, leading to radical surgery.
Why the other options are wrong
- Mucinous cystadenoma of the ovary Arises from the ovary itself and replaces it.
- Hydrosalpinx A distended fluid-filled tube separate from the ovary.
- Endometrioma A chocolate-coloured cyst within the ovary.
- Lymphocele Follows lymph node dissection and lies along the pelvic vessels.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Abdomen, Anterior abdominal wall and peritoneal spaces