Vermiform appendix

Appendix vermiformis

Also known as: appendix

The vermiform appendix is a narrow, blind, worm-like tube about 6–10 cm long arising from the caecum. Its wall is packed with lymphoid tissue, and acute appendicitis is the commonest abdominal surgical emergency.

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Overview

Length≈6–10 cm (average ≈8–9 cm)

Commonest positionRetrocaecal (≈60–65%), then pelvic

Surface markingMcBurney point (⅓ ASIS → umbilicus)

The base of the appendix is constant. It arises from the posteromedial wall of the caecum about 2 cm below the ileocaecal junction, where the three taeniae coli converge. Its surface marking is McBurney point, one-third of the way along the line from the anterior superior iliac spine to the umbilicus. The tip is very variable. It is most often retrocaecal (about 60–65%) or pelvic (about 25–35%), and less often subcaecal, pre-ileal or post-ileal.

The appendix has its own triangular mesentery, the mesoappendix, which hangs from the back of the mesentery of the terminal ileum. It carries the appendicular artery, a branch of the ileocolic artery. This is an end artery, so an inflamed, swollen appendix readily becomes gangrenous and perforates.

The mucosa and submucosa contain abundant lymphoid follicles (GALT). These peak in adolescence and regress with age, which partly explains the age distribution of appendicitis. The appendix may also act as a "safe house" for commensal bacteria, allowing the colon to be recolonised after diarrhoeal illness.

Location: Right iliac fossa. The base lies deep to McBurney point (one-third of the way from the ASIS to the umbilicus).

Parts & subdivisions

Base
Constant position, at the convergence of the taeniae coli on the caecum.
Mesoappendix
Peritoneal fold carrying the appendicular artery and vein and lymphatics.
Lymphoid follicles
Dense GALT in the mucosa and submucosa.

Anatomy

Innervation: Sympathetic fibres from T10 (lesser splanchnic nerve) and vagal fibres via the superior mesenteric plexus. Visceral afferents enter at T10, which is why early appendicitis pain is felt around the umbilicus.
Arterial supply: Appendicular artery, an end artery from the ileocolic artery (SMA) running in the mesoappendix.
Venous drainage: Appendicular vein → ileocolic vein → superior mesenteric vein → portal vein. Infection can spread this way to the liver (pylephlebitis or liver abscess).
Lymphatic drainage: Appendicular and ileocolic nodes → superior mesenteric nodes.
Histology: Colon-like mucosa with crypts and goblet cells, and a continuous ring of lymphoid follicles. The outer longitudinal muscle forms a complete layer because the taeniae fuse.

Function

Mucosal immunity: IgA induction and lymphocyte education.
Possible reservoir of commensal microbiota.

Clinical significance

Acute appendicitis

The lumen becomes obstructed by a faecolith or lymphoid hyperplasia. Distension, bacterial overgrowth, venous congestion, ischaemia and finally perforation follow. Peak incidence is at 10–30 years.

Pain begins as vague periumbilical visceral pain (T10). It then migrates to the right iliac fossa once the parietal peritoneum is irritated (somatic pain), with anorexia, nausea, fever and leucocytosis. Signs include tenderness at McBurney point, Rovsing sign (RIF pain when the LIF is pressed), the psoas sign (retrocaecal appendix) and the obturator sign (pelvic appendix). Scoring systems such as Alvarado help. Treatment is appendicectomy; antibiotics alone are an option in selected uncomplicated cases.

Appendiceal tumours

Neuroendocrine (carcinoid) tumours are the most common, often found incidentally at the tip. Mucinous neoplasms can rupture and cause pseudomyxoma peritonei.

Test yourself

The early periumbilical pain of acute appendicitis occurs because:

Visceral afferents from the appendix enter the spinal cord at T10, the umbilical dermatome
The appendix lies directly behind the umbilicus
The parietal peritoneum at the umbilicus is inflamed first
The obturator nerve is irritated

Answer: Visceral afferents from the appendix enter the spinal cord at T10, the umbilical dermatome

The appendix is midgut. Its visceral afferents travel with sympathetic fibres to T10, so early pain is referred to the umbilical region. Pain shifts to the right iliac fossa once the somatically innervated parietal peritoneum is inflamed.