Acute Abdomen

The "acute abdomen" is any sudden, severe abdominal pain of less than one week's duration that may require emergency surgical intervention. It is one of the most heavily tested surgical themes in NEET PG because it weaves together history-taking, classic eponymous signs, erect X-ray interpretation, and the decision to operate.

Definition & approach

An acute abdomen denotes the rapid onset of abdominal symptoms — pain being the cardinal feature — where a delay in diagnosis and treatment increases morbidity and mortality. The clinician's job is not always to make a precise diagnosis but to answer one question: does this patient need an emergency laparotomy?

The pain is broadly classified by mechanism:

Type of pain	Nerve fibres	Character	Localisation	Example
Visceral	Autonomic (C fibres)	Dull, cramping, colicky, poorly localised	Midline (foregut → epigastrium, midgut → umbilicus, hindgut → hypogastrium)	Early appendicitis (periumbilical)
Somatic / parietal	Somatic spinal nerves	Sharp, constant, well-localised	Over the inflamed peritoneum	Migration of pain to RIF in appendicitis
Referred	Shared dermatome	Felt away from source	Distant site	Diaphragmatic irritation → shoulder tip (Kehr sign)

High-yield: The shift of pain from the umbilicus to the right iliac fossa (RIF) in appendicitis reflects the transition from visceral (midgut, T10) pain to somatic (parietal peritoneum) pain — the single most characteristic feature of acute appendicitis.

A pragmatic diagnostic flow at the bedside:

Resuscitate (ABC, IV fluids, analgesia, NBM) → focused history (onset, site, radiation, aggravating factors) → examine for peritonism/guarding → erect chest & abdominal X-ray + bloods → decide: operate now vs. observe vs. image (USG/CT).

Etiology — the major surgical causes

The four classically examined causes (and the focus of this topic) are acute appendicitis, perforated peptic ulcer, intestinal obstruction, and mesenteric ischaemia. A broader list:

• Inflammatory: appendicitis, cholecystitis, diverticulitis, pancreatitis, Meckel diverticulitis.
• Perforation: perforated duodenal/gastric ulcer, perforated appendix, typhoid ileal perforation (commonest cause of small bowel perforation in India), perforated diverticulum.
• Obstruction: adhesions (commonest cause of small bowel obstruction overall), obstructed hernia, malignancy (commonest cause of large bowel obstruction), volvulus, intussusception.
• Ischaemic/vascular: mesenteric ischaemia, ruptured AAA, ovarian torsion, testicular torsion.
• Haemorrhage: ruptured ectopic pregnancy, ruptured spleen, ruptured AAA.
• Medical mimics: inferior MI, diabetic ketoacidosis, lower lobe pneumonia, acute intermittent porphyria, lead poisoning, sickle cell crisis.

High-yield: Always exclude a medical cause before laparotomy. Inferior wall MI, DKA, and basal pneumonia are classic mimics that can be ruined by an unnecessary operation.

Acute appendicitis

The commonest surgical cause of acute abdomen worldwide, peaking in the second–third decades.

Pathophysiology

Luminal obstruction (faecolith in adults, lymphoid hyperplasia in children) → increased intraluminal pressure → venous congestion → mucosal ischaemia → bacterial invasion (E. coli, Bacteroides fragilis) → transmural inflammation → gangrene → perforation (usually within 24–72 h).

Clinical features & signs

• Periumbilical pain shifting to RIF, anorexia (very sensitive — appendicitis is unlikely in a hungry patient), low-grade fever, nausea.
• Maximal tenderness at McBurney's point (junction of lateral 1/3 and medial 2/3 of a line from ASIS to umbilicus).

Sign	How elicited	Indicates
Rovsing sign	Pressure in LIF → pain in RIF	Peritoneal irritation in RIF
Psoas sign	Pain on right hip extension	Retrocaecal/retroperitoneal appendix
Obturator sign	Pain on internal rotation of flexed right hip	Pelvic appendix
Blumberg sign	Rebound tenderness	Parietal peritoneal inflammation
Dunphy sign	Increased RIF pain on coughing	Localised peritonitis

Investigation & scoring

• Alvarado (MANTRELS) score /10 — Migratory pain, Anorexia, Nausea, Tenderness RIF (2), Rebound, Elevated temperature, Leucocytosis (2), Shift of WBC to left. Score ≥7 → high probability.
• USG first-line in children/pregnancy/women (non-compressible blind-ended tubular structure >6 mm; target sign). Contrast CT is the most accurate (highest sensitivity/specificity) in adults.

High-yield: Investigation of choice in a pregnant woman with suspected appendicitis is USG; if inconclusive, MRI (avoids radiation). CT is reserved for non-pregnant adults.

Management

• Appendicectomy (laparoscopic preferred) is the definitive treatment.
• Appendicular lump/mass presenting after 3–5 days → conservative Ochsner–Sherren regimen (IV fluids, antibiotics, observation) followed by interval appendicectomy after 6 weeks. Operating on a mass risks injuring matted bowel.

Perforated peptic ulcer

Clinical features

• Sudden, severe, generalised abdominal pain ("like a knife"), often in a known ulcer patient or NSAID user.
• Board-like rigidity of the abdomen (generalised peritonitis), obliteration of liver dullness (due to free gas), absent bowel sounds, shock.
• Duodenal (anterior wall) perforations are commonest.

Investigation of choice

• Erect chest X-ray showing free gas under the diaphragm (pneumoperitoneum) — present in ~70–80% of cases.

High-yield: Absence of gas under the diaphragm on erect chest X-ray does NOT exclude perforation. CT abdomen is far more sensitive and is the investigation of choice when X-ray is negative but suspicion is high.

Management

• Resuscitation, NG decompression, IV PPI and antibiotics.
• Graham's omental (omentopexy) patch repair — the classic operation for perforated duodenal ulcer. Followed by H. pylori eradication.

Intestinal obstruction

Classification & causes

Feature	Small bowel obstruction (SBO)	Large bowel obstruction (LBO)
Commonest cause	Adhesions (in developed countries); obstructed hernia historically	Colorectal carcinoma
Vomiting	Early, bilious, profuse	Late, faeculent
Distension	Less, central	Marked, peripheral
Constipation	Late	Early (absolute)
X-ray	Central, valvulae conniventes (cross full width)	Peripheral, haustra (do not cross full width)
Air-fluid levels	Multiple, central, "step-ladder"	Few, peripheral

Cardinal features

The four classic features: colicky pain, vomiting, distension, and absolute constipation (no passage of faeces or flatus). Proximal obstruction → vomiting predominates; distal → distension predominates.

Investigation

• Erect & supine abdominal X-ray (dilated loops, air-fluid levels). Small bowel dilatation >3 cm, large bowel >6 cm, caecum >9 cm are significant.
• CT identifies the transition point, cause, and signs of strangulation/ischaemia.

High-yield: Features suggesting strangulation (a surgical emergency): continuous (rather than colicky) pain, tachycardia, fever, localised tenderness, raised lactate, and a "closed loop" on CT. This mandates urgent operation.

Management

• Uncomplicated adhesive SBO → conservative ("drip and suck": IV fluids + NG tube) with serial review.
• Closed-loop, strangulation, obstructed hernia, or failure of conservative management → laparotomy.

High-yield: Sigmoid volvulus classically shows the "coffee-bean" / omega sign on X-ray; first-line treatment is endoscopic (sigmoidoscopic) decompression with flatus tube, NOT immediate surgery, unless gangrene is suspected.

Mesenteric ischaemia

Etiology

• Acute mesenteric ischaemia (AMI) — superior mesenteric artery (SMA) territory.
  • Embolic (commonest, ~50%) — usually from AF or recent MI; lodges typically just distal to the middle colic artery.
  • Thrombotic — pre-existing atherosclerosis.
  • Non-occlusive (NOMI) — low-flow states (shock, vasopressors).
  • Venous thrombosis — hypercoagulable states.

Clinical hallmark

High-yield: Pain out of proportion to physical findings is the classic sign of acute mesenteric ischaemia. Early on, the abdomen is soft with minimal tenderness despite agonising pain. Peritonism appears late, signifying infarction.

Investigation

• CT angiography is the investigation of choice. Lactate is raised; metabolic acidosis is a late sign.

Management

• Resuscitation, anticoagulation, and revascularisation (embolectomy/thrombolysis/stenting). Resection of frankly gangrenous bowel at laparotomy; a planned second-look laparotomy at 24–48 h assesses viability of borderline bowel.

Erect X-ray interpretation — exam pearls

• Gas under the right hemidiaphragm → pneumoperitoneum (perforation). Best seen on erect chest X-ray with the patient sitting upright for 5–10 minutes.
• Rigler's sign (gas on both sides of the bowel wall), football sign, and falciform ligament sign → large pneumoperitoneum on supine film.
• Multiple air-fluid levels → obstruction or paralytic ileus.
• Sentinel loop → localised ileus adjacent to inflammation (e.g., pancreatitis).
• "Coffee-bean" → sigmoid volvulus; "bird-beak" on contrast → volvulus.
• Thumbprinting → mucosal oedema in ischaemic/inflammatory colitis.
• Pneumobilia + SBO + ectopic gallstone (Rigler triad) → gallstone ileus.

Indications for emergency laparotomy

Operate without delay when there is:

1. Generalised peritonitis (board-like rigidity, diffuse rebound).
2. Pneumoperitoneum from a hollow viscus perforation.
3. Strangulated/obstructed bowel or closed-loop obstruction.
4. Uncontrolled intra-abdominal haemorrhage (e.g., ruptured AAA, ruptured ectopic, splenic rupture with instability).
5. Bowel ischaemia / infarction.
6. Failure of non-operative management with clinical deterioration.

High-yield: Spreading/generalised peritonitis is itself an indication for laparotomy — you do not need a precise diagnosis. Diagnostic laparoscopy is increasingly used in equivocal cases, especially in young women, to avoid a negative laparotomy.

Key differentials by region

Pain location	Surgical	Gynaecological/Urological	Medical
RIF	Appendicitis, Meckel, Crohn	Ectopic, ovarian torsion, PID	—
RUQ	Cholecystitis, biliary colic	—	Hepatitis, basal pneumonia
Epigastrium	Perforated ulcer, pancreatitis	—	Inferior MI
Loin/flank	—	Ureteric colic, pyelonephritis	—
Generalised	Perforation, ischaemia, obstruction	Ruptured ectopic	DKA, porphyria, sickle crisis

Murphy's sign (arrest of inspiration on palpating the RUQ) → acute cholecystitis. Cullen sign (periumbilical bruising) and Grey-Turner sign (flank bruising) → haemorrhagic pancreatitis/retroperitoneal bleed. Kehr sign (left shoulder tip pain) → splenic rupture/diaphragmatic irritation.

Complications

• Appendicitis: perforation, appendicular abscess, faecal fistula, pylephlebitis (portal vein septic thrombophlebitis → liver abscess).
• Perforated ulcer: generalised peritonitis, septic shock, subphrenic abscess.
• Obstruction: strangulation, perforation, fluid/electrolyte loss (hypokalaemic, hypochloraemic metabolic alkalosis with gastric outlet obstruction), short bowel syndrome after extensive resection.
• Mesenteric ischaemia: transmural infarction, gangrene, sepsis, short bowel syndrome, very high mortality (50–80%).

Mnemonics

• Appendicitis signs / Alvarado = MANTRELS: Migratory pain, Anorexia, Nausea, Tenderness RIF, Rebound, Elevated temperature, Leucocytosis, Shift to left.
• Causes of acute abdomen = "APPENDICITIS" — Appendicitis, Pancreatitis, Perforation, Ectopic, Necrosis (ischaemia), Diverticulitis, Inflammation (cholecystitis), Cancer, Intussusception, Trauma, Infarction, Stones.
• Pneumoperitoneum causes = the surgeon's rule: perforated viscus, recent surgery/laparoscopy, peritoneal dialysis, PV (per vaginal) entry.

Recently asked / exam angle

• "Pain out of proportion to signs" → single best answer: acute mesenteric ischaemia; next investigation CT angiography.
• Best initial investigation for suspected perforation → erect chest X-ray (gas under diaphragm). If negative with high suspicion → CT.
• Investigation of choice for appendicitis in pregnancy → USG, then MRI.
• Commonest cause of SBO → adhesions; commonest cause of LBO → carcinoma colon.
• Sigmoid volvulus management → endoscopic decompression first (coffee-bean sign).
• Appendicular mass treatment → conservative Ochsner–Sherren + interval appendicectomy.
• Operation for perforated duodenal ulcer → Graham's omental patch.
• Pain shifting umbilicus → RIF mechanism → visceral to somatic transition.
• Embolus in mesenteric ischaemia lodges distal to middle colic artery origin.
• Two-mark image-based questions on valvulae conniventes vs. haustra to distinguish SBO from LBO.

Rapid revision

1. Acute abdomen = sudden severe pain <1 week; key question is "does this need a laparotomy?"
2. Pain shifting from umbilicus to RIF = appendicitis (visceral → somatic).
3. McBurney's point = junction of lateral 1/3 and medial 2/3 of ASIS–umbilicus line.
4. Rovsing (LIF press→RIF pain), Psoas (retrocaecal), Obturator (pelvic), Dunphy (cough) signs.
5. Alvarado/MANTRELS ≥7 = high probability of appendicitis; CT most accurate in adults, USG/MRI in pregnancy.
6. Appendicular mass → Ochsner–Sherren conservative + interval appendicectomy at 6 weeks.
7. Perforated ulcer = board-like rigidity + gas under diaphragm; repair with Graham's omental patch.
8. SBO commonest cause = adhesions; LBO = colorectal cancer; valvulae conniventes (SBO) cross full width, haustra (LBO) do not.
9. Strangulation = continuous pain + tachycardia + raised lactate → emergency surgery.
10. Sigmoid volvulus = coffee-bean sign → endoscopic decompression first.
11. Mesenteric ischaemia = pain out of proportion to signs; CT angiography is investigation of choice; embolus from AF.
12. Generalised peritonitis, pneumoperitoneum, ischaemia, strangulation, and uncontrolled haemorrhage are absolute indications for emergency laparotomy.