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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Acute abdomen = sudden severe abdominal pain requiring urgent evaluation. Not a diagnosis — it is a clinical presentation
- Surgical causes: appendicitis, perforated viscus, bowel obstruction, strangulated hernia, mesenteric ischaemia, ruptured AAA, ectopic pregnancy
- Medical causes: DKA, pneumonia (lower lobe), MI (inferior), renal colic, acute pancreatitis, pyelonephritis
- Peritonism (guarding, rigidity, rebound tenderness) = likely peritonitis = likely surgical. NBM + IV fluids + urgent senior surgical review
- CT abdomen/pelvis with IV contrast: investigation of choice for most acute abdominal presentations (replaces erect CXR for perforation in many centres)
- Ruptured ectopic pregnancy: ALWAYS do pregnancy test in women of reproductive age with abdominal pain ± haemodynamic instability
Overview
The acute abdomen describes sudden-onset abdominal pain of sufficient severity to warrant urgent evaluation and potential surgical intervention. It encompasses a wide differential diagnosis spanning surgical emergencies (appendicitis, perforated viscus, bowel obstruction, strangulated hernia, mesenteric ischaemia, ruptured AAA), gynaecological emergencies (ectopic pregnancy, ovarian torsion), urological emergencies (renal colic, testicular torsion), and medical mimics (DKA, basal pneumonia, inferior MI, porphyria). The pathophysiology of abdominal pain can be visceral (dull, poorly localised — from hollow organ distension or ischaemia), somatic/parietal (sharp, well-localised — from peritoneal inflammation), or referred (perceived at a distant site sharing the same dermatome). Systematic assessment using history, examination, and targeted investigations is essential to differentiate surgical from medical causes and identify time-critical diagnoses.
Epidemiology
Abdominal pain accounts for approximately 5–10% of all ED attendances — one of the most common presenting complaints. Appendicitis is the most common surgical emergency (lifetime risk ~7%). Non-specific abdominal pain (resolving without clear diagnosis) accounts for approximately 25–40% of presentations. The incidence of mesenteric ischaemia and ruptured AAA increases with age and cardiovascular risk factors. Ectopic pregnancy affects approximately 1 in 90 pregnancies in the UK and is the leading cause of first-trimester maternal death. Elderly and immunosuppressed patients may present atypically with minimal pain or peritonism despite significant intra-abdominal pathology.
Clinical Features
Symptoms
Sudden-onset severe abdominal pain — character, location, radiation, timing, and aggravating/relieving factors guide differential
RIF pain with migration from periumbilical = classic appendicitis (visceral → somatic as peritoneum involved)
Sudden-onset generalised pain (like a "thunderclap") = perforated viscus or mesenteric ischaemia or ruptured AAA
Colicky pain with vomiting and distension = bowel obstruction
Loin-to-groin colicky pain = renal colic
Epigastric pain radiating to back = pancreatitis or posterior duodenal ulcer perforation
Amenorrhoea + PV bleeding + abdominal pain = ectopic pregnancy until proven otherwise
Pain out of proportion to examination findings = mesenteric ischaemia (classic)
Signs
Peritonism: guarding (involuntary), rigidity (board-like), rebound tenderness = peritoneal inflammation
Abdominal distension (obstruction, ascites, ileus)
Absent bowel sounds (peritonitis/ileus). Tinkling/high-pitched (mechanical obstruction)
Haemodynamic instability: tachycardia, hypotension = ruptured AAA, ectopic, GI bleed, mesenteric ischaemia
Irreducible, tender hernia with signs of obstruction = strangulated hernia
Grey Turner sign (flank bruising) / Cullen sign (periumbilical bruising) = retroperitoneal haemorrhage (pancreatitis, AAA)
Investigations
First-line
Pregnancy test (urine β-hCG)MANDATORY in ALL women of reproductive age with abdominal pain — exclude ectopic pregnancy
BloodsFBC (WCC for infection), CRP, U&Es, LFTs, amylase/lipase (pancreatitis), lactate (ischaemia, sepsis), clotting, group and save
VBGLactate (mesenteric ischaemia/sepsis), pH (acidosis), glucose (DKA)
UrinalysisHaematuria (renal colic), leucocytes/nitrites (UTI/pyelonephritis)
Second-line
CT abdomen/pelvis with IV contrastGold standard for most acute abdominal presentations. Identifies: perforation (free air), obstruction, appendicitis, diverticulitis, pancreatitis, AAA, mesenteric ischaemia
Erect CXRFree air under diaphragm (pneumoperitoneum) = perforation. Increasingly replaced by CT but still useful
USS pelvis (transvaginal)If ectopic pregnancy suspected — locate gestational sac
USS abdomenGallstones, AAA screening, free fluid, urinary retention
Specialist
CT angiographyIf mesenteric ischaemia or ruptured AAA suspected
Diagnostic laparoscopyIf diagnosis uncertain despite imaging — both diagnostic and potentially therapeutic
1
Initial management (all acute abdomens)
- ABCDE approach. IV access, fluid resuscitation if shocked
- Analgesia: IV morphine titrated to effect — do NOT withhold analgesia (does not mask peritonism or delay diagnosis)
- NBM if surgical cause suspected
- Antiemetics: ondansetron or cyclizine
- IV antibiotics if sepsis suspected (e.g. co-amoxiclav + metronidazole for intra-abdominal sepsis)
- Pregnancy test in ALL women of reproductive age — non-negotiable
- Urgent surgical review if peritonism, haemodynamic instability, or suspected surgical emergency
2
Definitive management (by cause)
- Appendicitis: appendicectomy (laparoscopic preferred)
- Perforated viscus: emergency laparotomy + repair/resection + washout
- Bowel obstruction: conservative initially (drip and suck: NGT, IV fluids, NBM). Surgery if strangulation, perforation, or failure to resolve
- Ruptured AAA: emergency open or endovascular repair (EVAR). Cross-match 10 units
- Mesenteric ischaemia: CT angiography → embolectomy/bypass ± bowel resection. Very high mortality
- Ectopic pregnancy: methotrexate (if stable, <35 mm, no heartbeat) or salpingectomy/salpingotomy
- Strangulated hernia: emergency surgical repair
Complications
- Peritonitis: Generalised peritoneal inflammation — from perforation, ischaemia, or infection. Requires emergency surgery
- Sepsis: Intra-abdominal sepsis carries high mortality. Source control (surgery/drainage) is essential alongside antibiotics
- Bowel ischaemia and necrosis: From strangulation (hernia, volvulus) or mesenteric ischaemia. Delay → bowel death → perforation → sepsis
- Haemorrhagic shock: From ruptured AAA, ectopic pregnancy, splenic rupture. Immediately life-threatening
- Short bowel syndrome: After extensive bowel resection for ischaemia or obstruction with necrosis
UKMLA Exam Tips
- 1ALWAYS do a pregnancy test in women of reproductive age with abdominal pain. ALWAYS. This is exam gold
- 2Peritonism (guarding, rigidity, rebound) = peritoneal irritation = likely needs surgery. Absent bowel sounds = ileus/peritonitis
- 3Pain out of proportion to exam findings + raised lactate = mesenteric ischaemia until proven otherwise
- 4Erect CXR: free air under diaphragm = perforation (most commonly peptic ulcer). But up to 20% of perforations show no free air
- 5Appendicitis: periumbilical pain migrating to RIF + anorexia + nausea + RIF tenderness + raised inflammatory markers
- 6Grey Turner (flanks) and Cullen (umbilicus) signs = retroperitoneal haemorrhage — think severe pancreatitis or ruptured AAA
- 7Analgesia: morphine IV titrated to effect. NEVER withhold because "it might mask signs" — this is outdated and unethical
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