The acute abdomen — structured approach, assessment and decision-making

A 68-year-old woman arrives at 2 a.m. with six hours of severe generalised abdominal pain, guarding, tachycardia and a lactate of 4.1. A 24-year-old man arrives ten minutes later with migratory right iliac fossa pain, anorexia and a low-grade fever. Both have an "acute abdomen", but one needs a resuscitation bay, broad-spectrum antibiotics and an operating theatre, while the other needs a score, a bedside ultrasound and a conditional CT. This page is the framework that tells those two apart — with the numbers that govern each decision.^[1][2]

Definition — what counts, and how often each cause appears

The acute abdomen presents with the sudden onset of maximally intense abdominal pain, sometimes with guarding, and often with impaired general well-being, ranging to manifestations of shock — and 15–20% of emergency patients with acute abdominal pain need interventional or surgical treatment.^[1] Time-based definitions in the literature converge: nontraumatic abdominal pain lasting fewer than seven days, or sudden spontaneous non-traumatic pain of less than a week, is the entry criterion most studies use.^[2][3]

The case-mix after the history is remarkably stable across continents. Aside from nonspecific abdominal pain (30–41%), the common specific causes are acute appendicitis (8–30%), cholecystitis (9–11%) and ileus (4–5%).^[1] A surgical cohort from Nepal found the same hierarchy at operation: appendicitis including its sequelae accounted for over half of cases, followed by perforation peritonitis and bowel obstruction.^[3] The American review literature orders the undifferentiated ED population slightly differently — gastroenteritis and nonspecific pain first, then cholelithiasis, urolithiasis, diverticulitis and appendicitis — but the message is identical: a handful of diagnoses dominate, and the framework must catch all of them.^[2]

Epidemiology and the hour rule

It carries a 2% to 12% mortality, with the figure rising for every elapsed hour until specific treatment is provided — the single sentence that justifies everything else on this page.^[1] Structured, quality-controlled, rapid and targeted diagnosis and treatment markedly lower that morbidity and mortality, which is why the workup is organised as a timed pathway rather than a leisurely differential.^[1]

The emergency-laparotomy audits put flesh on the range. In an Irish university hospital cohort the indications were peritonitis (32%), obstruction (22%), complicated hernia (19%) and mesenteric ischaemia (15%), with 30- and 90-day mortality of 6.7% and 11.6% — and 30-day mortality of 20% in octogenarians.^[39] An Australian centre benchmarking against the UK National Emergency Laparotomy Audit reported 9.7% 30-day mortality against NELA's 11.1%, with age, P-POSSUM and admission source driving death.^[41] The fellowship moral: the acute abdomen is common, usually non-urgent, but its urgent minority carries laparotomy-level mortality concentrated in the old.

Pathophysiology — three pains and two cascades

Teach the pain in three layers, because the layers localise the lesion. Visceral pain is dull, midline and poorly localised — foregut to epigastrium, midgut to periumbilical, hindgut to suprapubic — carried by autonomic afferents. Parietal (somatic) pain is sharp, precisely localised and worsened by movement or coughing once inflamed parietal peritoneum is involved. Referred pain follows shared segmental innervation (shoulder-tip diaphragmatic irritation, groin ureteric colic). The classic migration of appendiceal pain from periumbilical to right iliac fossa is simply the transition from visceral to parietal as inflammation reaches the parietal peritoneum — informative when present, but present in only a minority (migration plus direct RLQ tenderness in 28% of suspected cases).^[20]

Two cascades then compete for the patient. The peritonitis cascade: hollow-viscus contamination or bacterial translocation drives SIRS and sepsis, third-spacing and hypovolaemia, then multi-organ dysfunction — which is why generalised peritonitis mandates source control rather than further deliberation, and why intra-abdominal infections are counted among the major contributors to non-trauma deaths in emergency departments worldwide.^[35] The ischaemic cascade behaves differently: mucosa dies first while serosa still looks viable, producing pain out of proportion to examination (the mesenteric ischaemia signature covered in its own topic), and reperfusion itself can precipitate collapse. Both cascades explain why early observations can falsely reassure: compensation holds vitals while tissue dies, which is exactly when lactate kinetics and re-examination earn their keep.^[38]

Clinical presentation and examination

Expect the canonical cluster — abdominal pain with nausea or vomiting, anorexia, constipation or obstipation, and fever; tenderness with rebound, guarding or rigidity, tachycardia and distension — the exact symptom-sign set reported in operative acute-abdomen cohorts.^[3] Two examination findings do disproportionate work at triage: patients judged urgent look unwell and have peritonism far more often than non-urgent patients (39.8% vs 17.4% and 56.6% vs 14.7% respectively).^[42] The "end-of-the-bed-o-gram" — the gestalt of a sick, still, tachycardic patient who does not want to move — is therefore data, not laziness.

Beware the textbook picture precisely because it is rare. Individual clinical features and laboratory results discriminate weakly in suspected appendicitis (AUC 0.50–0.65 each), and the "classical" combination of migratory pain, RLQ tenderness and rigidity occurs in only 6% of suspects — perfectly accurate in males but little better than a coin toss in females (46%).^[20] Combinations with high accuracy exist but are infrequent, which is why scores and conditional imaging exist: the examination sorts, it does not settle.^[20]

Differential diagnosis — the complete sort

Build the differential anatomically first, then add the systems that refer into the abdomen:

• Right upper quadrant: biliary colic, acute cholecystitis, cholangitis, hepatic abscess, perforated duodenal ulcer mimicking biliary pain, pneumonia (right lower lobe), myocardial ischaemia.
• Epigastric: perforated peptic ulcer, acute pancreatitis, myocardial infarction, gastritis, ruptured aortic aneurysm.
• Left upper quadrant: splenic injury or infarct, gastritis, pancreatitis tail, left lower-lobe pneumonia.
• Periumbilical / diffuse: small-bowel obstruction, mesenteric ischaemia, ruptured AAA, gastroenteritis, diabetic ketoacidosis, peritonitis of any cause.
• Right iliac fossa: appendicitis, mesenteric adenitis, Crohn's ileitis, ureteric colic, testicular torsion (referred), ectopic pregnancy or ruptured corpus luteum in women.
• Left iliac fossa: diverticulitis, ureteric colic, ectopic pregnancy, adnexal torsion, pelvic inflammatory disease, inflammatory bowel disease.
• Suprapubic / pelvic: urinary retention, cystitis/pyelonephritis, ectopic pregnancy, torsion, PID, ruptured ovarian cyst.
• Gynaecologic urgencies (any lower-quadrant pain in a woman): ectopic pregnancy, ruptured ovarian cyst, adnexal torsion and pelvic inflammatory disease.^[19]
• Medical mimics that must not reach theatre: gastroenteritis and nonspecific pain (the commonest ED diagnoses), urolithiasis, myocardial infarction, pneumonia, DKA, porphyria and abdominal-wall pain.^[2]

Two rules govern the gynae differential. First, a urine pregnancy or beta-hCG test is the mandatory first step in sexually active premenopausal patients — around 40% of ectopic pregnancies are misdiagnosed at the presenting visit.^[19] Second, know which diagnoses the examination can never clinch: cholecystitis, appendicitis and mesenteric ischaemia cannot be confirmed clinically and typically require imaging, while urolithiasis and some diverticulitis may be diagnosed clinically in selected cases.^[2]

Bedside assessment and triage — how good is the first look?

Honest numbers keep examiners happy. Surgical registrars distinguishing urgent from non-urgent abdominal pain at first assessment achieve roughly 75% sensitivity and 90% specificity, with seniors outperforming juniors — and the features that separate the groups are looking unwell and peritonism.^[42] Consultant seniority, reassuringly or not, adds certainty without accuracy: trainees make the correct diagnosis in 44.4% and surgeons in 42.9%, with only diagnostic certainty higher in the consultant group.^[43] The fellowship reading: triage by the sickest-looking features first, and have the senior re-examine rather than assume the senior's first glance was better.

Resuscitation precedes diagnosis whenever shock, generalised peritonitis or unstable vitals are present: wide-bore access, balanced fluids, blood if bleeding, broad-spectrum antimicrobials where sepsis is suspected, oxygen, urinary output monitoring and a nasogastric tube when obstructed — the early-recognition and prompt-resuscitation cornerstones that underpin all intra-abdominal infection care.^[35] Bedside adjuncts in every assessment are glucose, urinalysis, ECG (inferior MI mimics epigastric catastrophe), venous blood gas with lactate, and the pregnancy test above.^[2]

Laboratory investigations — weak alone, useful together

Order routinely: full blood count, CRP, urea and electrolytes, liver panel, lipase, lactate with venous gas, and urinalysis — with hepatobiliary markers, creatinine, glucose and pregnancy testing guided by the presentation.^[2] Then interpret with the pooled numbers, not hope:

• CRP beats WCC beats procalcitonin for uncomplicated appendicitis: pooled AUC 0.75 vs 0.72 vs 0.65, with sensitivities/specificities of 57%/87%, 62%/75% and 33%/89% respectively.^[28] Procalcitonin's niche is complicated disease (sensitivity 62%, specificity 94% for gangrene/perforation).^[28]
• A normal CRP argues against rupture, never against appendicitis: in one tertiary cohort WCC and CRP showed sensitivity 81%/100% but specificity only 46%/19%, no ruptured appendix had a normal CRP, and CRP tracked rupture severity.^[29]
• Combinations outperform singles: combined WCC/CRP cut-offs reached 98.7% sensitivity with 99.5% negative predictive value for complicated appendicitis in a paediatric cohort, and CRP carried the highest diagnostic accuracy for complicated disease.^[30] The exact cut-offs are cohort-specific — borrow the principle (combined, trend-aware interpretation), not the numbers.
• Lactate is kinetics, not a snapshot: in perforation peritonitis, pre-operative, immediate post-operative and 24-hour lactates (cut-offs 2.75, 2.8 and 2.45 mmol/L) each independently predicted 28-day mortality, and MPI plus 24-hour lactate reached AUC 0.99.^[38] But a single index lactate means little on its own — in the Irish laparotomy cohort it did not correlate with mortality at all.^[39] Trend it, combine it, never worship one value.

^{[28] [30] [38] [7]}

Clinical scores — Alvarado, AIR, and the rest

Alvarado (1986). Built from eight predictive factors, ordered by diagnostic weight: localised RLQ tenderness, leukocytosis, migration of pain, left shift, temperature elevation, nausea-vomiting, anorexia and rebound pain.^[21] Pooled performance is moderate: sensitivity 69%, specificity 77% in one meta-analysis (67%/74% in another).^[22][23] In children accuracy is likewise moderate (sensitivity 76%, specificity 71%) and relying on the score alone is not recommended — it is an auxiliary tool, not a diagnosis.^[24]

AIR (Appendicitis Inflammatory Response, 2008). Constructed from eight variables chosen for independent diagnostic value — RLQ pain, rebound tenderness, muscular defence, WCC, neutrophil proportion, CRP, temperature and vomiting — and designed from the start to separate advanced appendicitis, where it achieved ROC 0.97 (0.93 for all appendicitis), outperforming Alvarado's 0.92/0.88 in the derivation study.^[25] The three-zone design is the point: 63% of patients fall into low- or high-probability groups at 97.2% accuracy, leaving only 37% indeterminate for imaging or laparoscopy.^[25] Prospective Swedish validation (3,878 patients) confirmed ROC 0.89 for complicated and 0.83 for any appendicitis, with AIR below 4 carrying 99% negative predictive value for complicated disease and AIR above 8 reaching up to 96% positive predictive value in the young.^[26]

Pregnancy caveat. In a Swedish nested case-control study AIR reached AUC 0.88 overall and 0.90 for complicated appendicitis, with 100% sensitivity for complicated disease at scores of 4 or higher and 97% specificity at 9 or higher — against imaging sensitivity/specificity of only 45%/42% in the same cohort — but the authors require further validation in pregnancy before routine use.^[27]

RIPASA and Tzanakis. RIPASA trades specificity for sensitivity against Alvarado (94% vs 69% sensitive; 55% vs 77% specific), suiting rule-out use where imaging is scarce.^[22] Tzanakis (which incorporates ultrasound) is more sensitive with better overall discrimination (sensitivity 0.86, AUC 0.93 vs Alvarado 0.67/0.74), while Alvarado keeps a marginal specificity edge for exclusion.^[23] Where CT is unavailable — developing countries and rural hospitals — the wide and safe use of both Alvarado-type scores is explicitly recommended.^[22]

Laparotomy risk scores. For the patient heading to theatre, NELA and P-POSSUM stratify emergency-laparotomy mortality (age and P-POSSUM dominate), and the Hajibandeh Index performs comparably to NELA overall (30-day AUC 0.86 vs 0.87) with an edge in the over-80s.^[40][41]

Imaging — the conditional strategy and its numbers

The OPTIMA programme (1,021 Dutch ED patients, expert-panel reference standard, two-thirds ultimately urgent) is the evidence spine. Clinical diagnosis alone generates many false-positive urgent diagnoses, cut down by either ultrasound or CT.^[4] Head-to-head, CT sensitivity for urgent conditions is 89% against ultrasound's 70% — but the conditional strategy (CT only after negative or inconclusive ultrasound) yields the highest sensitivity of all, missing just 6% of urgent cases while sparing half of patients the CT (only 49% scanned) and lowering radiation exposure.^[4] No BMI-, age- or pain-location-driven alternative preserves that sensitivity.^[4]

Disease-specific numbers sharpen the choice: for appendicitis CT 94% vs US 76%, for diverticulitis 81% vs 61%, for cholecystitis 73% each — and ultrasound sensitivity is not materially degraded by patient characteristics or reader experience.^[5] CT interpretation itself is reproducible: overall inter-observer kappa 0.66, rising to 0.84–0.90 for appendicitis, diverticulitis and obstruction.^[14] Imaging matters because clinical evaluation alone is inaccurate — the explicit premise of the whole field.^[13]

For suspected appendicitis specifically, conditional CT identifies as many cases as immediate CT while halving the scan count (at the cost of more false positives).^[6] Early CT in undifferentiated pain misses fewer serious diagnoses, corrects the admission diagnosis (only 50% right at admission, 76% by 24 hours) and may reduce mortality and stay — though the randomised signal is modest and inconsistently significant.^[7] Two meta-analyses frame the routine-vs-selective debate honestly: routine CT does not improve correct-diagnosis proportions or mortality over selective CT in pooled RCTs (722 patients), while a newer synthesis finds early routine CT improves 24-hour diagnostic correctness and 6-month mortality — small trials, heterogeneous populations, and the elderly explicitly excluded from the reassurance.^[8][9]

The selective-CT appendicitis data favour scanning the uncertain middle: sensitivity/specificity/accuracy 91%/92%/91%, negative appendicectomy down from 24% on clinical grounds alone to 16%, management plans altered in one patient in six — with routine CT in women and selective CT in men the authors' recommendation given the 24% negative rate in women on clinical suspicion alone.^[10] When scanning, demand the feature profiles: on US, thickened appendix plus transducer tenderness plus fat infiltration (95% appendicitis when all three present); on CT, complete visualisation with thickening, fat infiltration and enhancement — two or more essential features gives 92% sensitivity on US and 96% on CT.^[12]

Plain radiography — the modality with no place

State it plainly for the viva: numerous studies show low sensitivity and accuracy for plain abdominal radiography across acute abdominal pain including perforated viscus, obstruction, foreign body and ureteric stones, and there is no place for it in the workup of adult ED patients with undifferentiated pain.^[11] CT after negative ultrasound is simply the better workup — better decisions, better operative planning, fewer negative laparotomies.^[11]

One nuance examiners reward: in a selected operative bowel-obstruction cohort, plain X-ray was the most sensitive test (88%) with ultrasound the most specific (95%) — a reminder that test performance follows the population, and that the no-plain-film rule governs undifferentiated ED assessment, not the monitored obstructed inpatient.^[3] For perforation specifically, generalised peritonism on examination matches X-ray sensitivity — examine first, and do not let a normal film overrule a surgical abdomen.^[3]

Special situations — pregnancy, children, and the bedside probe

Pregnancy. Appendiceal ultrasound performs poorly in pregnancy (sensitivity ~45% in the Swedish cohort), so inconclusive US should prompt MRI rather than CT where available.^[27][2] The Brussels data justify that step: appendix visualised in essentially all patients, MRI sensitivity 100% and specificity 98.7% against clinical/US/laboratory specificity of 62%.^[17] Cochrane pools MRI at 0.95/0.96 overall, holding at 0.96/0.97 in 2,282 pregnant women across 21 studies, and calls MRI highly accurate for confirming and excluding appendicitis regardless of protocol.^[16] The OPTIMAP rationale generalises the point: ultrasound is suboptimal, CT's accuracy costs ionising radiation and contrast risk, and MRI replaces both without either.^[15]

Children. Alvarado is moderate and never standalone in children.^[24] Combined inflammatory markers perform better for complicated disease (near-perfect sensitivity and NPV in the paediatric cohort), and the US-first, MRI-over-CT hierarchy applies with even greater force given radiation sensitivity.^[30][16]

POCUS rules. Bedside ultrasound reduces CT use for appendicitis but a negative scan never rules it out; conversely it accurately diagnoses and rules out gallbladder pathology and diagnoses urolithiasis — so use POCUS to confirm RUQ and renal colic presentations and to accelerate, never to exclude, appendicitis assessment.^[18]

Resuscitation and analgesia — treat pain, then re-examine

The WSES cornerstones for intra-abdominal infection admit no delay: early recognition, adequate source control, appropriate antimicrobial therapy — with prompt resuscitation of ongoing sepsis understated nowhere.^[35] Fluids, electrolytes, NG or long-tube decompression when obstructed, nil by mouth, urinary monitoring and broad-spectrum antibiotics for the septic or peritonitic patient are the resuscitation bundle; the operation or the drain is the source control.^[33][35]

On analgesia the evidence is liberating and consistent. Cochrane finds opioid analgesia does not increase the risk of diagnostic error or wrong management decisions in acute abdominal pain.^[31] A morphine-versus-placebo trial in suspected appendicitis confirmed it experimentally: identical negative appendicectomy and perforation rates, and diagnostic accuracy curves superimposed (AUC 0.63 vs 0.61).^[32] Give the opioid early, document the pre-analgesia examination, and re-examine after — the second look is the point, not the drug's supposed unmasking.

Definitive management — operate, drain, or watch with rules

Generalised peritonitis means operation, not further imaging — clinical signs match X-ray for perforation detection, and delay feeds the 2–12% mortality.^[3][1] For confirmed intra-abdominal infection the global standard is source control plus appropriate antimicrobials with sepsis resuscitation running in parallel.^[35]

Adhesive small-bowel obstruction without peritonism is usually non-operative: nil by mouth, nasogastric or long-tube decompression, IV fluids and electrolytes — effective in most patients.^[33] The hard contraindications to watchful management are peritonitis, strangulation and ischaemia; CT is the technique of choice whenever the adhesive aetiology is uncertain or those contraindications might be present.^[33] Bologna-aligned care delivers shorter stay and more optimal outcomes in the multinational SnapSBO data, which also exposes how rarely full compliance is achieved — the gap is implementation, not knowledge.^[34]

Laparoscopy first in the stable patient. The WSES Cesena consensus suggests laparoscopy as the first approach for stable emergency general surgery and trauma patients: safe, feasible and effective as therapy or as triage to further management, with improved outcomes regardless of conversion — provided selection is rigorous and surgeon experience and training are adequate.^[36] Unstable, grossly contaminated or frankly peritonitic patients remain laparotomy cases; judgement, not dogma, selects the access.

Disposition, prognosis and audit — what happens next, and who dies

Most acute abdomens go home or to short-stay observation; the urgent minority goes to theatre, HDU or ICU by physiology, not diagnosis. Quote the benchmarks: NELA 30-day mortality ~11%, the benchmarked Australian unit 9.7%, the Irish unit 6.7% at 30 days and 11.6% at 90 days — with octogenarian mortality at 20% and mesenteric ischaemia the deadliest indication.^[41][39] Age and P-POSSUM (or NELA/HI equivalents) drive the number you quote the family.^[41][40]

Prognosticate with trends: serial lactate with the Mannheim index approaches perfect 28-day mortality discrimination after perforation peritonitis surgery, while a lone admission lactate may mean nothing.^[38][39] In older adults specifically, atypical presentation, diagnostic delay, comorbidity and lost reserve convert survivable disease into high morbidity and mortality — particularly in the frail undergoing emergency surgery — so the threshold to image, escalate and admit stays low, with source control and judicious antibiotics as the therapeutic pillars.^[37]

Special populations

• Elderly and frail: atypical presentations and diagnostic delays on top of comorbidity and decreased reserve; emergency surgery carries the cohort's highest mortality; NELA/P-POSSUM benchmarking, consultant presence and goal-concordant, time-limited decision-making are the exam answer.^[37][39][41]
• Pregnancy: appendix displacement and physiological leukocytosis degrade scores and US; AIR needs further pregnancy validation despite promising numbers; MRI after inconclusive US is the pathway; fetal radiation caution favours non-CT strategies throughout.^[27][17][16]
• Children: moderate Alvarado performance demands imaging backup; combined WCC/CRP interpretation and US-first/MRI-preferred pathways protect against both missed appendicitis and unnecessary radiation.^[24][30][16]
• Low-resource settings: where CT is unavailable, validated scores carry the workup — Alvarado/RIPASA-style instruments are explicitly recommended for health systems lacking electronic diagnostics — with transfer thresholds set low for peritonism, obstruction or suspected ischaemia.^[22]

Evidence, guidelines and controversies — the regional picture

• International (WSES): intra-abdominal infection cornerstones and Bologna obstruction rules are GRADE-appraised global standards; Cesena laparoscopy-first and SnapSBO compliance data are the current operative direction.^{[35][33][36][34]}
• UK/ANZ (NELA tradition): emergency-laparotomy mortality benchmarking with P-POSSUM/NELA/HI risk adjustment; the Logan experience shows a regional unit matching NELA figures while sitting at the upper end per risk band — audit, consultant presence and HDU access are the levers.^[41][40]
• Netherlands/EU (OPTIMA tradition): ultrasound-first conditional CT as the default ED pathway, with MRI positioned as the radiation-free CT replacement in appendicitis workup.^[4][15]
• US (AFP/ACR tradition): IV-contrast CT for generalised, LUQ and lower-abdominal pain; ultrasound first for RUQ pain; MRI after inconclusive US in pregnancy; POCUS to accelerate bedside decisions.^[2][18]
• Live controversies: routine versus selective CT (pooled RCTs neutral; newer synthesis favouring early routine scanning — small numbers, selected populations); score choice by setting (sensitive RIPASA vs specific Alvarado vs discriminating AIR); and how far MRI can displace CT outside pregnancy and childhood given cost and access.^{[8][9][22][16]}

Revision summary

Sudden maximal pain under a week is the entry ticket; 15–20% need intervention and 2–12% die, faster with delay.^[1] Sort urgent from non-urgent at the bedside (registrars ~75%/90%; seniors better; peritonism and looking unwell decide).^[42] Pregnancy-test every at-risk woman (40% of ectopics missed first visit).^[19] Labs: CRP > WCC > procalcitonin, combinations beat singles, lactate is a trend.^[28][38] Scores: Alvarado moderate (69%/77%), AIR discriminating (NPV 99% below 4, PPV to 96% above 8), RIPASA sensitive, Tzanakis sharp with US inside.^[22][26][23] Image conditionally: US first, CT for negative/inconclusive US (misses 6%, halves scans); no plain films in undifferentiated adults; MRI-first in pregnancy and children.^[4][11][16] Analgesia never masks (Cochrane).^[31] Peritonitis operates; stable obstruction usually watches per Bologna; stable emergencies go laparoscopic first.^[33][36] Benchmark mortality with NELA/P-POSSUM and respect the octogenarian's 20%.^[39]

^{[1] [39] [4] [26] [28] [10]}

In the paediatric cohort the WCC 13.1 plus CRP 1.17 mg/dL combination called appendicitis with sensitivity 98.7% and NPV 99.5%.^[30]

References43ShowHide

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