Acute appendicitis
- Lesion
When the appendix gets blocked and its pressure rises, bacteria and lack of blood inflame it; if untreated, it can burst.
What it is
The appendix is a normal true diverticulum of the caecum. Acute appendicitis is most common in adolescents and young adults, with a lifetime risk of a few percent. It is thought to begin with rising pressure in the lumen that compromises venous outflow, most often from obstruction by a faecolith, and less often a gallstone, tumour, worms or swollen lymphoid tissue. Stasis lets bacteria proliferate, and ischaemia and inflammation follow.
Structure
- Early: congested subserosal vessels; serosa turns dull, granular and red.
- Suppurative: neutrophilic exudate, fibrinopurulent serosal coat, intramural abscesses.
- Gangrenous: haemorrhagic ulceration and green-black necrosis through the wall.
Step by step
- The lumen is blocked by a faecolith, lymphoid hyperplasia or a mass.
- Mucus secretion continues and intraluminal pressure rises.
- Venous outflow is compromised: congestion and oedema.
- Bacteria multiply in the stagnant lumen and invade the ischaemic wall.
- Neutrophils fill the muscularis; abscesses form; the wall becomes gangrenous and perforates.
Clinical notes
- Atypical presentations are common: a retrocaecal appendix gives flank or pelvic pain; the very young and very old present late.
- In older adults, an appendiceal tumour can present as appendicitis.
- Surgeons accept removing some normal appendices because perforation is dangerous.
Test yourself
Think of the answer, then tap the question to see it.
What histological finding is required to diagnose acute appendicitis?
Neutrophilic infiltration of the muscularis propria.
What is the most common cause of luminal obstruction in appendicitis?
A faecolith, a small hard mass of stool.
Which two conditions can mimic acute appendicitis?
Mesenteric lymphadenitis (for example from Yersinia) and ectopic pregnancy; also salpingitis, mittelschmerz and Meckel diverticulitis.
Elsewhere in DurgiPedia
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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Appendicitis (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.