Surface and Applied Abdominal Anatomy

Four quadrants:

• Divided by a vertical (midline, through the umbilicus) and horizontal (through the umbilicus) plane into right upper (RUQ), left upper (LUQ), right lower (RLQ), left lower (LLQ) quadrants: the simpler, more commonly used clinical system for rapid localisation.

Nine regions:

• Divided by two vertical midclavicular lines and two horizontal planes (subcostal and transtubercular) into: right/left hypochondrium, epigastric, right/left lumbar (flank), umbilical, right/left iliac (inguinal), hypogastric (suprapubic), a more precise system used for detailed anatomical description and organ localisation.

Transpyloric plane:

• Horizontal plane midway between the jugular notch and pubic symphysis, at approximately L1: correlates with the pylorus, pancreatic neck, renal hila, SMA origin, portal vein formation, and gallbladder fundus (see Overview section for the full list).

Subcostal plane:

• Connects the lowest points of the costal margins (10th costal cartilage), at approximately L3.

Transtubercular plane:

• Connects the iliac tubercles, at approximately L5: used with the midclavicular lines to define the nine-region grid.

Midclavicular lines:

• Vertical lines through the midpoint of each clavicle, used as the lateral boundaries defining the nine-region grid, and as reference points for organ surface projections (e.g., gallbladder fundus, splenic dullness).

Surface landmarks:

• Xiphoid process: inferior sternal landmark, approximately T9 level, superior boundary of the epigastric region.
• Costal margin: formed by the cartilages of ribs 7-10 (direct/indirect attachments) and the free ends of ribs 11-12; palpable boundary of the upper abdomen.
• Umbilicus: approximately L3/L4 level, T10 dermatome.
• ASIS: palpable anterior pelvic landmark, reference point for the transtubercular plane and McBurney's point.
• Pubic symphysis: inferior midline landmark, reference point for the transpyloric plane's midpoint calculation and for the lower boundary of the nine-region grid.
• Inguinal ligament: ASIS to pubic tubercle, palpable, boundary of the lower abdomen/upper thigh.

Surface projection of the liver:

• Upper border follows a curved line from the 5th right intercostal space (midclavicular line) to the 5th left intercostal space (near the apex beat region), roughly paralleling the diaphragmatic dome; lower border follows the costal margin on the right, crossing the midline near the xiphisternal junction, extending to approximately the 5th left intercostal space at the left border.

Surface projection of the gallbladder:

• Fundus: at the tip of the 9th costal cartilage, where the lateral border of rectus abdominis crosses the costal margin (essentially the intersection of the right midclavicular line/rectus border and the transpyloric plane/subcostal region), the classic site of Murphy's sign elicitation (palpation here during inspiration, causing pain and inspiratory arrest in acute cholecystitis).

Surface projection of the appendix:

• Base at McBurney's point: one-third of the way along a line from the ASIS to the umbilicus, the classic site of maximal tenderness in appendicitis (with the caveat of significant variability given the appendiceal tip's variable position).

Surface projection of the stomach:

• Roughly occupies the left hypochondrium and epigastric regions, with the pylorus typically lying near the transpyloric plane, slightly to the right of the midline; considerable positional variability depending on distension and body habitus.

Surface projection of the kidneys:

• Extend from approximately T12 to L3 posteriorly, straddling the subcostal plane roughly at their midpoints (right kidney slightly lower than left); the hilum of each kidney lies approximately at the level of the transpyloric plane (L1), a few centimetres from the midline.

Surface projection of the abdominal aorta:

• Begins at the transpyloric plane (T12/L1, aortic hiatus level) in the midline, descends slightly to the left of the midline, bifurcating at approximately the level of the umbilicus/L4 (intercristal plane) into the common iliac arteries, a useful landmark for palpating the aortic pulse and for surface-marking the site of aneurysmal disease.

Inguinal Hernia

• Indirect: through the deep ring, lateral to the inferior epigastric vessels, congenital (persistent processus vaginalis); direct: through Hesselbach's triangle, medial to the inferior epigastric vessels, acquired (posterior wall weakness).

Umbilical Hernia

• Through the natural umbilical fascial defect; common in infants (often resolves spontaneously) and adults with raised intra-abdominal pressure (obesity, pregnancy, ascites).
• The umbilicus's relatively avascular, weak fascial covering makes it a natural point of herniation.

Incisional Hernia

• Through a previous surgical incision site, more common with midline (linea alba) incisions given the comparatively poor blood supply and tensile strength of the linea alba compared to muscle-splitting or transverse incisions.
• Risk factors include wound infection, obesity, and raised intra-abdominal pressure post-operatively.

Appendicitis

• Luminal obstruction (faecolith) → increased pressure → venous congestion → ischaemia, rapidly progressing given the appendix's single end-artery blood supply.
• Classic pain migration: periumbilical (visceral, midgut referred) → right iliac fossa/McBurney's point (somatic, parietal peritoneal irritation).

Peritonitis

• Localised (confined parietal peritoneal irritation, well-localised pain/guarding) or generalised (diffuse irritation, rigidity, surgical emergency).
• Spreads via paracolic gutters (right deeper than left) and subphrenic/subhepatic spaces.

Ascites

• Pathological peritoneal fluid accumulation, most commonly from portal hypertension; follows gravity-dependent distribution, detected by shifting dullness.
• Large volumes collect first in the pelvis and paracolic gutters before becoming generally distributed.

Portal Hypertension

• Raised pressure within the portal venous system, most commonly from liver cirrhosis obstructing hepatic sinusoidal flow.
• Forces blood through portosystemic anastomoses (oesophageal varices, haemorrhoids, caput medusae), and contributes to ascites and splenomegaly (splenic venous congestion, via the splenic vein's contribution to the portal vein).

Portosystemic Anastomoses

• Three classic sites: lower oesophagus (left gastric vein ↔ oesophageal plexus/azygos), anal canal (superior rectal vein ↔ middle/inferior rectal veins), umbilicus (paraumbilical veins ↔ superficial epigastric veins).

Splenic Rupture

• Most commonly injured organ in blunt abdominal trauma, given its fragile capsule and fixed position against the lower left ribs (9-11).
• Delayed rupture of a subcapsular haematoma is a recognised, classically tested pattern.

Gallstones

• Form from bile composition imbalance; classically impact in Hartmann's pouch (biliary colic/cholecystitis) or migrate into the CBD (choledocholithiasis, obstructive jaundice, risk of ascending cholangitis).
• Calot's triangle dissection is the critical safe-surgery landmark during cholecystectomy.

Biliary Obstruction

• Can occur at any level of the biliary tree; common causes include choledocholithiasis, pancreatic head carcinoma (compressing the paraduodenal CBD), and cholangiocarcinoma.
• The level of obstruction determines the imaging/clinical pattern (intrahepatic-only vs full biliary tree dilatation, including the gallbladder).

Pancreatitis Relations

• The pancreas's deep retroperitoneal position, directly against the vertebral column and behind the stomach, explains the classic epigastric pain radiating through to the back.
• Close relations to the splenic vessels (splenic vein thrombosis, pseudoaneurysm risk), the SMA/SMV/portal vein confluence (vascular complications), and the duodenum (obstruction risk) explain the range of serious complications seen in severe pancreatitis.

Abdominal Aortic Aneurysm

• Most commonly occurs in the infrarenal segment of the abdominal aorta (between the renal arteries and the bifurcation), a relative watershed zone of the aortic wall's vasa vasorum blood supply and a site of characteristic haemodynamic turbulence.
• Close proximity to the IVC, duodenum (third/fourth parts), and the origins of the renal, gonadal, and inferior mesenteric arteries explains the range of possible compressive/erosive complications and the technical considerations in surgical/endovascular repair.

Ureteric Obstruction

• Classically occurs at the three constrictions: pelviureteric junction, pelvic brim, vesicoureteric junction.
• Produces the characteristic colicky "loin to groin" pain pattern, reflecting the ureter's segmental visceral afferent innervation.

Renal Angle Tenderness

• Elicited over the costovertebral angle (between the 12th rib and erector spinae), overlying the kidney's posterior surface.
• A classic sign of renal pathology (pyelonephritis, renal stone, perinephric abscess).

Retroperitoneal Haemorrhage

• Can arise from retroperitoneal organ injury (kidney, pancreas, duodenum), major vessel rupture (aortic aneurysm, renal vessel injury), or anticoagulation-related bleeding (e.g., from the iliopsoas muscle).
• Given the retroperitoneal space's relatively unconfined, expansile nature compared to the rigid pericardial or intraperitoneal cavity, substantial blood volume can accumulate before becoming clinically apparent (hypotension, flank bruising, Grey Turner's sign) or radiologically obvious, making this a potentially insidious cause of haemodynamic compromise.