[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Gastrointestinal tract](https://durgical.com/en/general-pathology/areas/gi/) › Acute appendicitis

# Acute appendicitis

- Lesion

When the appendix gets blocked and its pressure rises, bacteria and lack of blood inflame it; if untreated, it can burst.

Updated Nov 12, 2026

## Key points

- Diagnosis requires neutrophils in the muscularis propria; mucosal neutrophils alone are not enough.
- Stages: early (congestion, perivascular neutrophils, dull granular serosa), suppurative (abscesses in the wall, fibrinopurulent serositis), gangrenous (necrosis to the serosa), perforation.
- Obstruction found in half to four fifths: faecolith most often; also lymphoid hyperplasia after viral infection, pinworms (Enterobius), tumour.
- Periumbilical pain moving to the right lower quadrant, nausea, low fever, mild leukocytosis; McBurney point tenderness.
- Mimics: mesenteric lymphadenitis (Yersinia, viruses), salpingitis, ectopic pregnancy, mittelschmerz, Meckel diverticulitis.
- Complications: perforation, peritonitis, abscess, pylephlebitis, portal vein thrombosis, liver abscess.

## 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.

## Memory aid

Block, build pressure, bacteria, burst: and the muscle must be full of neutrophils to call it appendicitis.

## 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.

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## Elsewhere in DurgiPedia

- [Large intestine, appendix and anal canal](https://durgical.com/en/histology/large-intestine-appendix-and-anal-canal/)

## Connections

## Related 6

- [Tumours of the appendix](https://durgical.com/en/general-pathology/tumours-of-the-appendix/)
- [Diseases of the peritoneum](https://durgical.com/en/general-pathology/diseases-of-the-peritoneum/)
- [Invasive bacterial enterocolitis](https://durgical.com/en/general-pathology/invasive-bacterial-enterocolitis/)
- [Intestinal helminths](https://durgical.com/en/general-pathology/intestinal-helminths/)
- [Patterns of acute inflammation: serous, fibrinous, purulent, abscess, ulcer](https://durgical.com/en/general-pathology/patterns-of-acute-inflammation-serous-fibrinous-purulent-abscess-ulcer/)
- [Outcomes of acute inflammation: resolution, abscess, scar or chronic inflammation](https://durgical.com/en/general-pathology/outcomes-of-acute-inflammation-resolution-abscess-scar-or-chronic-inflammation/)

## Related conditions 5

- [Acute appendicitis](https://durgical.com/en/conditions/acute-appendicitis/)
- [Peritonitis](https://durgical.com/en/conditions/peritonitis/)
- [Intra-abdominal abscess](https://durgical.com/en/conditions/intra-abdominal-abscess/)
- [Portal vein thrombosis](https://durgical.com/en/conditions/portal-vein-thrombosis/)
- [Pyogenic liver abscess](https://durgical.com/en/conditions/pyogenic-liver-abscess/)

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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](https://www.ncbi.nlm.nih.gov/books/NBK493193/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/acute-appendicitis/