Appendix — Positions, Blood Supply & Surface Marking

The vermiform appendix is a narrow, worm-like blind diverticulum arising from the posteromedial wall of the caecum. Its clinical importance vastly outweighs its physiological role: appendicitis is the commonest surgical abdominal emergency, and its presentation is dictated almost entirely by the position of the appendix and the end-artery nature of its blood supply. This topic is a recurring NEET PG anatomy–surgery crossover area.

Basic anatomy & development

• The appendix arises from the posteromedial wall of the caecum, about 2 cm below the ileocaecal valve, at the point where the three taeniae coli converge. This convergence is the single most reliable intra-operative landmark to locate a difficult appendix — trace any taenia downward and it leads to the base.
• It is a midgut derivative (supplied by the superior mesenteric artery via its branches). Embryologically the caecum and appendix appear in the 6th week; differential growth of the caecal wall later shifts the appendix to its variable adult positions.
• Length: average 9 cm (range 2–20 cm); lumen is narrow and may be partly obliterated in the elderly.
• It is a true intraperitoneal organ with its own small mesentery, the mesoappendix, derived from the posterior layer of the mesentery of the terminal ileum. The mesoappendix carries the appendicular artery and lymphatics.
• The appendix has the highest concentration of lymphoid (GALT) tissue in the gut per unit area — abundant submucosal lymphoid follicles, maximal between ages 12–20 (hence peak incidence of appendicitis in 2nd–3rd decades, when lymphoid hyperplasia readily obstructs the lumen).

High-yield: The base of the appendix lies at the confluence of the three taeniae coli on the caecum and is constant; the tip is highly variable. Therefore the base, not the tip, defines the surface marking.

Positions of the appendix

While the base is fixed, the tip wanders, giving several classic positions. The accepted frequency order is a perennial MCQ.

Position	Direction of tip	Approx. frequency	Clinical note
Retrocaecal / retrocolic	Up & behind caecum/colon	~64–75% (commonest)	May be silent; psoas irritation; rigidity may be absent as caecum shields it
Pelvic	Down into pelvis over pelvic brim	~20%	2nd commonest; irritates bladder/rectum → diarrhoea, dysuria; +ve obturator test
Subcaecal	Below caecum	~2%	—
Pre-ileal	In front of terminal ileum	~1%	—
Post-ileal	Behind terminal ileum	~0.5%	Diagnosis often delayed; atypical signs
Paracaecal	Alongside caecum	small %	—

Mnemonic for positions (clock-face on the caecum): picture the appendix base as the centre of a clock — retrocaecal = 11–12 o'clock, pelvic = 4–5 o'clock, subcaecal = 6 o'clock, pre-/post-ileal = 1–2 o'clock.

High-yield: The retrocaecal position is the single most frequently asked answer for "commonest position of appendix." Pelvic is the second commonest.

Why position dictates symptoms

The classic shifting of pain — initial periumbilical (T10 referred, visceral) pain that later localises to the right iliac fossa (somatic, parietal peritoneum) — only occurs when the inflamed appendix touches the anterior parietal peritoneum. Position changes this localisation:

1. Retrocaecal → appendix shielded by caecum; anterior tenderness/rigidity may be minimal; psoas sign positive (pain on extension of right hip) because the inflamed appendix lies on psoas major.
2. Pelvic → appendix lies near the bladder, rectum and (in females) the adnexa; gives suprapubic pain, dysuria, tenesmus, diarrhoea; obturator sign positive (pain on internal rotation of flexed right thigh); tenderness elicited only on per-rectal examination.
3. Post-ileal → inflammation walled off behind ileum; signs atypical and diagnosis frequently late, raising perforation risk.

High-yield: A positive psoas sign suggests a retrocaecal appendix; a positive obturator sign suggests a pelvic appendix.

Blood supply

• Arterial supply: appendicular artery, a branch of the lower division of the ileocolic artery (the ileocolic itself is a branch of the superior mesenteric artery, SMA).
• The appendicular artery descends behind the terminal ileum and then runs in the free border of the mesoappendix.
• It is a functional end-artery (no effective anastomoses). This is the crux of its clinical importance.

High-yield: The appendicular artery is an END-ARTERY → in appendicitis, thrombosis/compression causes early ischaemia, gangrene and perforation, especially at the tip (the part furthest from the supply). This is why appendicitis can progress to perforation rapidly (often within 24–48 h).

• An accessory appendicular artery may arise from the posterior caecal artery and supply the base; hence at appendicectomy the base may bleed even after the main mesoappendiceal vessel is ligated.
• Venous drainage: appendicular vein → ileocolic vein → superior mesenteric vein → portal vein. (Portal drainage explains how appendicular sepsis can seed the liver → portal pyaemia / pylephlebitis → liver abscess.)

Structure	Appendix
Artery	Appendicular artery (branch of ileocolic → SMA)
Vessel location	Free edge of mesoappendix
Type	End-artery (no anastomosis)
Vein	Appendicular vein → SMV → portal vein
Lymphatics	→ ileocolic nodes → superior mesenteric nodes
Nerve (visceral)	Sympathetic + parasympathetic from SMA plexus; afferents enter cord at T10 → periumbilical referred pain

Surface marking & incisions

McBurney's point

• Lies at the junction of the lateral one-third and medial two-thirds of an imaginary line (spino-umbilical line / Monro's line) drawn from the right anterior superior iliac spine (ASIS) to the umbilicus.
• Marks the base of the appendix on the surface and the point of maximal tenderness in acute appendicitis (McBurney's sign).

High-yield: Remember the ratio — lateral 1/3 : medial 2/3 from ASIS to umbilicus. A common distractor swaps these; the point is closer to the ASIS.

Incisions for appendicectomy

• McBurney's (grid-iron) incision: an oblique incision centred on and at right angles to the spino-umbilical line, through McBurney's point. Muscles are split along the line of their fibres (muscle-splitting), preserving them.
• Lanz incision: a transverse / horizontal incision in the right iliac fossa at the level of the ASIS, giving a better cosmetic scar (runs in Langer's lines). Preferred in young patients/females.
• Rutherford Morrison (extended/“hockey-stick”) incision for difficult appendix mass.
• Modern standard: laparoscopic appendicectomy (three ports).

Layers crossed in a grid-iron incision (superficial → deep): Skin → superficial fascia (Camper's + Scarpa's) → external oblique aponeurosis → internal oblique → transversus abdominis → transversalis fascia → extraperitoneal fat → parietal peritoneum. The three flat muscles are split, not cut, in the line of their fibres.

High-yield: McBurney = grid-iron = oblique, muscle-splitting; Lanz = transverse, better cosmesis. The structure most at risk during these incisions is the iliohypogastric nerve (and ilioinguinal) — injury → weakened conjoint tendon → direct inguinal hernia.

Appendicitis — applied clinical correlation

Pathophysiology flow

Luminal obstruction (faecolith in adults / lymphoid hyperplasia in young) → continued mucus secretion → rising intraluminal pressure → venous & lymphatic congestion → mucosal ischaemia + bacterial invasion (E. coli, Bacteroides) → wall inflammation (parietal peritoneum irritation → RIF pain) → arterial (end-artery) compromise → gangrene → perforation → localised abscess or generalised peritonitis.

Diagnosis & investigation of choice

• Appendicitis is primarily a clinical diagnosis.
• Alvarado (MANTRELS) score (out of 10) guides decision:

Feature	Points
M – Migration of pain to RIF	1
A – Anorexia	1
N – Nausea/vomiting	1
T – Tenderness in RIF	2
R – Rebound tenderness	1
E – Elevated temperature	1
L – Leucocytosis	2
S – Shift of WBC to left	1

Score ≥7 → appendicitis likely (operate); 5–6 → observe/image; ≤4 → unlikely.

• Investigation of choice:
  • Adults / first-line broadly: contrast-enhanced CT abdomen — most accurate (sensitivity & specificity >90%); shows diameter >6 mm, wall thickening, fat stranding, appendicolith.
  • Children & pregnant women: ultrasound is investigation of choice (avoids radiation); non-compressible blind-ended tubular structure >6 mm, target sign.
  • Pregnancy with equivocal USG: MRI (no ionising radiation).

High-yield: USG is preferred first in children and pregnancy; CT is the most accurate overall in adults. The "6 mm" outer diameter cut-off is a favourite value.

Eponymous signs

• McBurney's sign – tenderness at McBurney's point.
• Rovsing's sign – RIF pain on palpating the left iliac fossa.
• Blumberg's sign – rebound tenderness.
• Psoas sign – pain on right hip extension (retrocaecal).
• Obturator sign – pain on internal rotation of flexed thigh (pelvic).
• Dunphy's sign – increased RIF pain on coughing.
• Markle (heel-drop) sign – jarring elicits RIF pain.

Management / drug of choice

• Acute uncomplicated appendicitis → appendicectomy (laparoscopic preferred), with perioperative antibiotics.
• Appendicular mass (presenting after ~3–5 days, palpable lump): Ochsner–Sherren conservative regimen — IV fluids, antibiotics, monitor; interval appendicectomy after 6–8 weeks. Surgery deferred because dense adhesions make immediate surgery hazardous.
• Appendicular abscess: drainage (often percutaneous/USG-guided) + antibiotics, then interval appendicectomy.
• Antibiotic of choice (empirical): cover Gram-negative + anaerobes — e.g. ceftriaxone + metronidazole, or piperacillin–tazobactam; tailor to local protocol.

High-yield: The Ochsner–Sherren regimen (conservative management of appendicular mass + interval appendicectomy) is repeatedly examined. Failure of conservative treatment (rising pulse, spreading tenderness, enlarging mass) mandates intervention.

Complications

• Perforation → generalised peritonitis (fastest in infants/elderly because the omentum cannot wall it off well; "omentum = abdominal policeman" is least effective at extremes of age).
• Appendicular mass / abscess.
• Pylephlebitis (portal vein septic thrombophlebitis) → liver abscess — via portal venous drainage.
• Faecal fistula, wound infection, adhesive obstruction post-operatively.
• Stump appendicitis (if a long stump is left).
• Mucocele of the appendix; rupture → pseudomyxoma peritonei.
• Carcinoid tumour — the appendix is the commonest site of GI carcinoid; usually at the tip, mostly benign if <2 cm.

Key differentials of right iliac fossa pain

Differential	Distinguishing pointer
Mesenteric adenitis	Child, preceding URTI, higher fever, shifting tenderness, self-limiting
Meckel's diverticulitis	Mimics appendicitis exactly; suspect if appendix normal at surgery; "rule of 2s"
Ruptured ectopic pregnancy	Female, amenorrhoea, +βhCG, shock
Ovarian torsion / ruptured follicle (Mittelschmerz)	Mid-cycle, adnexal mass, USG
Acute PID / tubo-ovarian abscess	Cervical motion tenderness, discharge, bilateral
Right ureteric colic	Loin-to-groin pain, haematuria, restless patient
Crohn's terminal ileitis	Chronic diarrhoea, weight loss, skip lesions
Right-sided diverticulitis / caecal pathology	Older patient, CT

High-yield: If at appendicectomy the appendix looks normal, always examine the terminal ileum for a Meckel's diverticulum (located ~2 ft/60 cm from the ileocaecal valve).

Recently asked / exam angle

• Commonest position of appendix → Retrocaecal (single most repeated MCQ); 2nd → pelvic.
• Appendicular artery is a branch of → ileocolic artery (→ SMA); and it is an end-artery.
• McBurney's point = junction of lateral 1/3 and medial 2/3 of the ASIS–umbilicus line.
• Sign for retrocaecal appendix → psoas sign; for pelvic → obturator sign.
• Investigation of choice in pregnancy/children → USG (CT in adults).
• Base of appendix located by → convergence of three taeniae coli.
• Lanz vs grid-iron incision orientation and cosmesis.
• Layers in grid-iron incision and nerve at risk (iliohypogastric).
• Venous drainage → portal vein → pylephlebitis/liver abscess.
• Commonest GI carcinoid site → appendix (tip).
• Management of appendicular mass → Ochsner–Sherren + interval appendicectomy.

Rapid revision

1. Appendix arises from posteromedial caecal wall where the three taeniae coli converge (= fixed base).
2. Commonest position = retrocaecal (~75%), then pelvic (~20%).
3. Appendicular artery = branch of ileocolic (→ SMA), runs in mesoappendix, is an END-ARTERY → early gangrene/perforation at the tip.
4. Venous drainage → SMV → portal vein → risk of pylephlebitis & liver abscess.
5. McBurney's point = junction of lateral 1/3 & medial 2/3 of ASIS-to-umbilicus line = base of appendix = max tenderness.
6. Grid-iron (McBurney) = oblique muscle-splitting; Lanz = transverse, better cosmesis; iliohypogastric nerve at risk.
7. Psoas sign = retrocaecal; obturator sign = pelvic; Rovsing = LIF press → RIF pain.
8. Pain shifts periumbilical (T10 visceral) → RIF (somatic) once parietal peritoneum is involved.
9. Alvarado/MANTRELS score; CT diameter >6 mm diagnostic; USG = IOC in children/pregnancy, CT most accurate in adults.
10. Young patients obstruct via lymphoid hyperplasia; adults via faecolith.
11. Appendicular mass → Ochsner–Sherren conservative regimen + interval appendicectomy at 6–8 weeks.
12. Appendix = commonest site of GI carcinoid (usually benign, at tip); normal appendix at surgery → look for Meckel's diverticulum.