Acute abdominal pain

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Abdominal pain can arise from the gastrointestinal tract, urinary system, reproductive organs, major vessels or structures outside the abdomen. Pain location is useful but imperfect because visceral pain may be diffuse and referred pain may be felt away from the lesion. The pattern over time, associated symptoms and physiological response often matter more than one description.

An acutely ill patient needs resuscitation and senior assessment alongside diagnostic work. Analgesia is part of that care and should not be withheld to preserve examination findings. Repeated examination after treatment can reveal progression or localisation. A safe disposition depends on what remains plausible, not merely whether one common diagnosis has been excluded.

Key points

Assess physiological stability and red flags before organising a location-based differential.
Consider ruptured aneurysm, ectopic pregnancy, bowel ischaemia, perforation and obstructed or strangulated bowel early.
Offer pregnancy testing when pregnancy is possible, including with apparently nonspecific gastrointestinal or urinary symptoms.
Give adequate analgesia while assessment continues; relief should be followed by repeat examination.
Choose ultrasound, contrast CT or other imaging to answer the most important suspected diagnosis.
Use serial clinical review when uncertainty persists; normal early blood tests cannot exclude important surgical disease.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Inflammatory and obstructive causes

Inflammation, perforation and luminal obstruction affect different abdominal structures and can progress from local symptoms to systemic illness or a surgical emergency.

Vascular and reproductive causes

Aneurysm rupture, mesenteric ischaemia, ectopic pregnancy and adnexal torsion threaten tissue or circulation and require early consideration in the appropriate clinical context.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Visceral pain
Distension and ischaemia activate visceral afferents, often causing poorly localised discomfort before inflammation reaches the more precisely localised parietal peritoneum.
2
Peritoneal inflammation
Irritation of the parietal peritoneum makes movement painful and can produce guarding or rigidity, although the strength of these signs varies between patients.
3
Systemic deterioration
Bleeding, fluid sequestration and infection can reduce perfusion or trigger systemic inflammation, so abdominal disease may first present with collapse or altered consciousness.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Peritoneal irritation

Guarding, rigidity and pain with movement or cough suggest peritoneal inflammation. These signs may be muted in frailty, immunosuppression or early disease, so their absence is not universally reassuring.

Vascular or ischaemic pattern

Abrupt pain, shock, atrial fibrillation or pain disproportionate to initial tenderness raises vascular concerns. Mesenteric ischaemia can have little early peritonism despite a dangerous underlying process.

Obstruction pattern

Colicky pain, vomiting, distension and failure to pass stool or flatus suggest obstruction. Constant worsening pain, systemic illness or peritonism raises concern about strangulation or perforation.

Pregnancy and pelvic causes

Ectopic pregnancy may present with abdominal pain, bleeding, shoulder-tip pain or collapse, sometimes without a recognised pregnancy. Sudden unilateral pelvic pain also requires consideration of torsion.

Red flags requiring action

Abdominal or back pain with circulatory collapse requires urgent consideration of a ruptured aneurysm, including when no pulsatile mass is palpable.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Pregnancy testing and pelvic assessmentFirst step
Why
Identify a pregnancy-related pathway that changes urgency and imaging.
Interpretation and limitations
Offer testing when relevant even if symptoms are nonspecific. A positive result with pain needs appropriate assessment, and instability demands emergency management rather than waiting for serial hormone measurements.
02
Blood count, renal function and lactate
Why
Assess consequences and prepare safe investigation and treatment.
Interpretation and limitations
Inflammatory markers may be normal early; a normal lactate does not reliably exclude early bowel ischaemia. Renal function informs treatment but should not create avoidable delays in life-saving investigation.
03
Lipase or amylase with clinical context
Why
Support a suspected acute pancreatitis diagnosis.
Interpretation and limitations
Enzyme elevation can also occur in other conditions. Discordant symptoms or severe deterioration should prompt reconsideration and appropriate imaging rather than automatic attribution to pancreatitis.
04
Targeted ultrasound or CT
Why
Define the suspected lesion and guide intervention.
Interpretation and limitations
Immediate aortic ultrasound is indicated when symptomatic or ruptured AAA is considered. Ultrasound identifying an aneurysm does not by itself demonstrate whether it has ruptured; vascular discussion remains urgent.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Extra-abdominal disease

Lower chest infection, myocardial ischaemia and metabolic acidosis can present with abdominal discomfort, particularly when the abdominal findings are insufficient to explain severity.

Urinary tract pathology

Renal colic, infection and retention can cause abdominal or flank pain; concurrent fever, renal impairment or obstruction changes the urgency and treatment.

Functional or self-limiting symptoms

Benign explanations remain possible after appropriate assessment, but severe progression or physiological disturbance requires reconsideration rather than anchoring on a previous functional diagnosis.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Unstable patientResuscitation and time-critical specialist reviewFirst stepAcute abdominal pain is accompanied by collapse, hypoperfusion, peritonism or rapidly worsening physiology.

1Assess ABCs, obtain IV access and relevant samples, provide monitored analgesia and start appropriate resuscitation while urgently contacting the surgical or other responsible specialist team.
2Consider internal bleeding, vascular rupture, perforation and ischaemia explicitly; organise blood components when haemorrhage is suspected rather than relying on routine maintenance fluids.
3Choose imaging jointly according to the patient’s stability and likely intervention, ensuring the diagnostic process does not delay necessary theatre, vascular or gynaecological care.

02Aneurysm concernPain and collapse in a patient at risk of AAANew abdominal or back pain with haemodynamic compromise makes a symptomatic or ruptured abdominal aneurysm plausible.

1Arrange immediate bedside aortic ultrasound if available while continuing resuscitation and preparing a vascular referral.
2Discuss immediately with the regional vascular service if an aneurysm is shown, or if ultrasound is unavailable or nondiagnostic and suspicion remains.
3Coordinate further arterial-phase CT and transfer with the vascular team when appropriate for repair planning; do not require a palpable mass before activating this pathway.

03Stable diagnostic pathwayFocused investigation with repeated reassessmentThe patient is haemodynamically stable, without an immediate operative indication, but the cause remains uncertain.

1Clarify onset, migration, vomiting, bowel and urinary symptoms, previous surgery, medicines and relevant reproductive history; examine hernial orifices and other systems when indicated.
2Provide suitable analgesia and hydration, select imaging for the leading differential and consider alternatives such as lower-lobe pneumonia, ACS or ketoacidosis when the abdominal findings do not explain the illness.
3Re-examine after treatment and before disposition, arranging observation or timely review when a dangerous evolving diagnosis remains possible despite initially reassuring results.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

Provide baseline analgesia for a stable patient or as part of a broader pain-management plan.

Paracetamol when the oral route is suitable

For an adult weighing at least 50 kg without relevant dose-reduction factors, give 500 mg–1 g orally per dose, at least 4 hours apart, up to 4 g in 24 hours; review ongoing need and stop when pain resolves.

Check all combination products and reduce the dose when low body weight, liver disease or other risk factors require it. Oral treatment is unsuitable with unsafe swallowing or significant vomiting, and severe pain needs additional monitored analgesia.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Sepsis and shock

Uncontrolled intra-abdominal infection or bleeding can lead to circulatory failure, requiring source control and resuscitation rather than symptom treatment alone.

Bowel necrosis

Strangulation or vascular occlusion can progress to non-viable bowel and perforation, making recognition of disproportionate pain and clinical deterioration particularly important.

Delayed diagnosis

Premature discharge or attribution to a benign cause can postpone definitive intervention; explicit safety-netting and planned reassessment reduce the risk from evolving disease.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat abdominal examination and observations after analgesia, documenting changing tenderness, guarding, distension and systemic findings.
Follow urine output, fluid balance and electrolytes when significant vomiting, sepsis or resuscitation makes these clinically relevant.
Check whether the patient can tolerate oral intake only when the suspected diagnosis and treatment plan make that assessment appropriate.
At discharge, specify urgent return for worsening pain, persistent vomiting, fever, bleeding, fainting or inability to maintain hydration, with a defined review plan.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Older and frail patients

Serious disease may produce little fever or guarding. Physiological change, loss of function and collateral history can justify investigation despite a quiet examination.

Previous abdominal surgery

Adhesions increase obstruction probability but should not obscure a new hernia, malignancy or another cause of the presentation.

Analgesia and reassessment

Pain relief improves comfort and can make examination more informative; the diagnosis should rely on the entire evolving picture rather than deliberately untreated pain.

Pregnancy of unknown location

An inconclusive early scan is a follow-up state, not proof that ectopic pregnancy is excluded. New symptoms and instability take priority over a previously planned serial test.

11Common pitfallsFrequent interpretation and management errors.

01
Do not diagnose constipation without checking for obstruction or a more dangerous explanation of severe new pain.
02
Do not exclude ectopic pregnancy because the patient has not missed a clearly recognised menstrual period.
03
Do not treat a raised pancreatic enzyme as definitive proof of pancreatitis regardless of the clinical presentation.
04
Do not dismiss persistent severe pain solely because inflammatory markers or the first examination appear reassuring.

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