Acute abdominal pain and the surgical abdomen

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

Acute abdominal pain is an anatomical and physiological problem before it is a named diagnosis. Establish onset, speed to peak, migration, site, character, radiation and relation to movement, meals, vomiting, stool, urine and menstruation. Visceral pain is often diffuse and poorly localised; parietal irritation becomes sharper and movement-sensitive; referred pain appears away from the affected organ. These patterns guide but do not prove a cause. Appendicitis need not start centrally, myocardial infarction can present epigastrically, lower-lobe pneumonia can mimic an acute abdomen, and diabetic ketoacidosis can produce pain without primary intra-abdominal disease.

Examination begins at the bedside rather than with palpation. Note distress, stillness versus writhing, colour, hydration, respiratory pattern, scars, distension, visible hernia and monitoring trends. Auscultation has limited discriminatory value and should not postpone more important assessment. Palpate gently away from pain, seeking focal tenderness, involuntary guarding, masses and organ enlargement; percussion or cough can reveal peritoneal irritation without repeated rebound testing. Examine groins and external genitalia when obstruction, torsion or hernia is possible. Rectal or pelvic examination should answer a specific question, be consented and use a chaperone rather than being performed ceremonially.

Investigations test the leading mechanisms and dangerous alternatives. Blood results describe inflammation, anaemia, organ injury, pancreatitis and physiological compromise but rarely replace imaging. A pregnancy test changes radiation, ectopic and medication decisions. Bedside ultrasound rapidly answers selected questions yet does not survey bowel or retroperitoneum comprehensively. CT abdomen and pelvis with intravenous contrast is central for many stable adults with peritonism, obstruction, diverticulitis or uncertain severe pain; CT angiography is required for suspected mesenteric ischaemia and often for stable suspected aortic catastrophe. Reassess after analgesia and resuscitation, communicate directly with the relevant surgeon and name the trigger for escalation if observation is chosen.

Key points

Decide whether the patient is unstable before refining the pain history: record respiratory rate, oxygen saturation, pulse, blood pressure, temperature, consciousness, perfusion and urine output.
Treat pain and nausea early while repeating examination; analgesia supports humane care and does not prevent competent recognition of peritonism or deterioration.
Pain beginning diffusely then localising can reflect evolving parietal irritation, while abrupt maximal-at-onset pain raises vascular rupture, ischaemia, perforation, torsion or renal colic.
Always ask about previous abdominal operations, hernias, anticoagulants, NSAIDs, alcohol, gallstones, vascular disease, pregnancy possibility and relevant gynaecological or testicular symptoms.
Peritonism is a syndrome of involuntary guarding, percussion or cough tenderness and rigidity; an older, immunosuppressed or steroid-treated patient may have dangerous disease with muted signs.
A normal lactate does not exclude early mesenteric ischaemia, and a raised lactate is not specific; clinical probability determines urgent CT angiography and surgical discussion.
Use ultrasound for focused biliary, pelvic and aortic questions, contrast CT for many undifferentiated acute abdomens, and arterial-phase imaging when vascular pathology is suspected.
Keep the patient fasting when surgery or procedural sedation is plausible, establish venous access, correct fluid and electrolyte deficits, and give antimicrobial treatment promptly when sepsis or perforation is suspected.

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

Inflammatory and infective

Appendicitis, cholecystitis, pancreatitis, diverticulitis and intra-abdominal infection commonly generate acute pain through local inflammation, oedema and peritoneal irritation.

Obstructive or perforating

Bowel obstruction, volvulus, strangulated hernia and perforated viscus cause distension, traction, ischaemia or contamination and may deteriorate rapidly.

Vascular and extra-abdominal

Mesenteric ischaemia, aneurysmal rupture, myocardial ischaemia, basal pneumonia, urinary disease and gynaecological emergencies can present primarily as abdominal pain.

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

1
Visceral nociception
Distension, ischaemia or inflammation activates poorly localised visceral afferents, often producing midline pain with autonomic nausea, sweating or pallor.
2
Parietal irritation
Extension to the parietal peritoneum produces sharper, localised pain, guarding and pain on movement or percussion.
3
Systemic deterioration
Ongoing bleeding, contamination, necrosis or inflammation drives hypovolaemia, sepsis and organ hypoperfusion, so physiology may worsen before diagnostic certainty is achieved.

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

Ruptured abdominal aortic aneurysmRed flag

Sudden abdominal, flank or back pain with syncope, shock or a known aneurysm should trigger immediate vascular consideration. The classic pulsatile mass may be absent, and transient response to fluid does not remove rupture risk.

Mesenteric ischaemiaRed flag

Severe pain disproportionate to early abdominal findings, atrial fibrillation, atherosclerosis, low-flow state or postprandial chronic symptoms raises bowel ischaemia. Peritonism indicates advanced injury; normal early lactate cannot safely exclude it.

Perforated viscusRed flag

Abrupt severe pain, generalised guarding or rigidity, sepsis and free intraperitoneal gas support perforation. Absence of visible free gas on plain radiography does not exclude a contained or early perforation.

Mechanical obstruction and strangulationRed flag

Colicky pain, vomiting, distension and obstipation suggest obstruction; constant worsening pain, focal tenderness, fever, tachycardia, acidosis or a closed-loop configuration raises compromised blood supply and demands urgent surgery.

Acute pancreatitis

Persistent upper abdominal pain radiating to the back with compatible lipase or imaging supports pancreatitis. Organ failure, systemic inflammation, hypoxaemia, kidney injury or hypotension determines severity more than enzyme height.

Localised inflammatory abdomen

Right iliac fossa migration supports appendicitis, left lower-quadrant pain with fever supports diverticulitis, and right upper-quadrant pain with Murphy-type tenderness supports gallbladder inflammation; atypical anatomy and age alter presentation.

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
Serial observations and NEWS2First step
Why
Detect shock, sepsis, respiratory compromise and trajectory while diagnostic work continues.
Interpretation and limitations
A rising respiratory rate, falling blood pressure, increasing oxygen need, oliguria or altered mentation overrides a superficially reassuring abdomen and requires senior escalation and resuscitation.
02
Venous blood gas and lactate
Why
Assess acid-base state, perfusion, glucose and severity in a sick patient.
Interpretation and limitations
Raised lactate supports physiological stress or ischaemia but is non-specific; a normal value does not rule out early mesenteric ischaemia. Trend alongside perfusion and imaging.
03
Full blood count, renal, liver and inflammatory profiles
Why
Identify anaemia, infection, dehydration, kidney injury and hepatobiliary patterns that shape imaging and treatment.
Interpretation and limitations
Leucocytosis and CRP support inflammation but may lag. Cholestatic tests redirect towards biliary obstruction, while anaemia can indicate bleeding or chronic disease; no single panel excludes surgery.
04
Serum lipase
Why
Test for acute pancreatitis when upper abdominal pain, vomiting or gallstone and alcohol risk makes it plausible.
Interpretation and limitations
A compatible clinical picture plus a sufficiently elevated result supports diagnosis; modest elevation occurs in renal and other abdominal disease. Enzyme magnitude does not grade severity.
05
Urinalysis and pregnancy test
Why
Identify urinary clues and ensure ectopic pregnancy and pregnancy-sensitive imaging or medicines are addressed.
Interpretation and limitations
Haematuria can accompany stones but does not exclude aneurysm, and pyuria can be reactive. A positive pregnancy test with pain requires urgent location and viability assessment through the local pathway.
06
Abdominal and pelvic ultrasound
Why
Answer focused questions about gallstones, biliary dilatation, pelvic pathology, free fluid or aortic calibre without ionising radiation.
Interpretation and limitations
Operator, body habitus and bowel gas limit sensitivity. A negative scan does not close appendicitis, obstruction or vascular disease when clinical concern remains.
07
Contrast CT abdomen and pelvis
Why
Define obstruction, perforation, inflammation, abscess, malignancy and alternative diagnoses in a stable patient.
Interpretation and limitations
Protocol must match the question; discuss arterial-phase CT angiography for vascular concern. Review renal and contrast factors without allowing routine delay to harm a time-critical patient.

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

Mesenteric or aortic emergency

Abrupt severe pain, vascular risk, pain out of proportion, pulse deficit or shock favours ischaemia or rupture and requires urgent vascular imaging.

Inflammatory surgical disease

Migration or localisation of pain, fever, focal tenderness and rising inflammatory markers support appendicitis, cholecystitis, diverticulitis or another local inflammatory process.

Extra-abdominal disease

Chest symptoms, hypoxia, electrocardiographic change, urinary findings, pregnancy possibility or pelvic symptoms redirect assessment towards cardiopulmonary, renal or gynaecological causes.

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

01Unstable abdomenResuscitate while summoning definitive careFirst stepDefinitiveAbdominal pain accompanies shock, peritonism or another immediate physiological threat.

1Begin ABCDE, obtain large-bore venous access, send urgent bloods and group-and-save or crossmatch, monitor urine output and provide appropriate oxygen and warmed fluid or blood support.
2Call the relevant senior surgeon immediately and involve vascular, gynaecology, urology, anaesthesia or critical care according to the leading threat.
3Give analgesia and antiemetic treatment, start locally recommended antimicrobials when sepsis or perforation is suspected, and keep the patient fasting.
4DefinitiveUse only imaging that changes immediate definitive management; do not transfer an unstable patient to an unsuitable scanner without senior planning and resuscitation capability.

02Stable severe painLocalise and discriminateThe patient is physiologically stable but pain is severe, focal or unexplained.

1Take a time-ordered history including operations, vascular, drug, alcohol, pregnancy and extra-abdominal factors, then perform targeted abdominal, groin and cardiorespiratory examination.
2Obtain focused bloods, urinalysis and pregnancy testing, provide analgesia, fasting and fluid correction, then repeat examination for evolving localisation.
3Choose ultrasound for a defined biliary or pelvic question, contrast CT for broad acute pathology, or CT angiography when ischaemia or vascular catastrophe remains plausible.
4Discuss the imaging request and patient directly with surgery or radiology when delay, contrast risk or uncertain protocol could change outcome.

03ObstructionIdentify strangulation earlyVomiting, distension, colicky pain and reduced stool or flatus suggest mechanical obstruction.

1Assess dehydration, hernias, scars, tenderness and peritonism; correct electrolytes, obtain venous access, institute fasting and consider nasogastric decompression for significant vomiting under surgical direction.
2Arrange early surgical review and contrast CT to identify level, cause, closed loop, free fluid, perforation or ischaemia unless immediate operation is already required.
3EscalationEscalate constant pain, fever, tachycardia, acidosis, localised tenderness or worsening lactate as possible strangulation rather than waiting for complete obstipation.

04ObservationMake reassessment an active testInitial evaluation is non-diagnostic and immediate intervention is not indicated.

1Document the leading diagnoses, dangerous alternatives and why observation is currently safe, including baseline pain, examination and physiology.
2Specify timing for repeated observations, abdominal examination, urine output and selected blood tests, and define the change that triggers imaging or senior review.
3Do not discharge until oral intake, mobility, analgesia needs and safety-netting are considered; arrange explicit follow-up for unresolved imaging or symptoms.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Relieves suffering, permits safer examination and supports resuscitation while the cause of acute abdominal pain is established.

Titrated opioid or non-opioid analgesia

Give incremental analgesia using the current local acute-pain and BNF regimen, adjusted for frailty, kidney function, respiratory risk and previous exposure.

Monitor sedation, respiration and blood pressure; avoid NSAIDs when bleeding, kidney injury or perforation is plausible, and do not let symptom improvement cancel reassessment.

Treats suspected perforation, cholangitis, complicated diverticulitis or abdominal sepsis while source control is arranged.

Empirical intravenous antimicrobials

Use the locally approved severe intra-abdominal infection regimen promptly after appropriate cultures, with allergy, renal function and likely source considered.

Antibiotics cannot replace drainage or surgery. Record indication and review against cultures, imaging and source control; local resistance and stewardship policy determine selection and duration.

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

Sepsis and shock

Perforation, infection, haemorrhage or bowel necrosis can cause circulatory collapse, acute kidney injury and multiorgan failure without timely resuscitation and source control.

Bowel ischaemia and perforation

Unrelieved obstruction or vascular compromise progresses from venous congestion to infarction, contamination and peritonitis, substantially increasing operative risk.

Diagnostic delay

False reassurance from early normal tests, analgesic response or atypical examination can postpone definitive imaging or surgery until physiological reserve is lost.

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

Trend respiratory rate, oxygen requirement, pulse, blood pressure, temperature, mental state, capillary refill and urine output at an interval matched to severity.
Repeat abdominal examination after analgesia, fluid resuscitation and any change in pain, documenting localisation, distension and peritoneal signs rather than writing unchanged generically.
Review haemoglobin, renal function, acid-base status and lactate trajectory when bleeding, sepsis or ischaemia is possible, while recognising that laboratory stability can lag clinical deterioration.
Track time to senior surgical decision, definitive imaging, antimicrobial delivery and source control in suspected emergency surgical disease.
Ensure every provisional discharge has written return triggers for worsening pain, persistent vomiting, fever, syncope, bleeding, distension or inability to hydrate.

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

Pain relief preserves diagnosis

Appropriate analgesia does not erase involuntary guarding or physiological deterioration. Withholding it harms patients and can make examination less reliable through fear and voluntary muscle tension.

Lactate is a late witness

Mesenteric ischaemia can precede systemic lactate elevation, particularly before extensive infarction. Strong clinical probability should lead to urgent arterial imaging and surgical discussion despite an early normal value.

Obstruction changes character

Intermittent colic can become constant when bowel perfusion is compromised. That transition, especially with focal tenderness or systemic change, is more concerning than absolute pain score alone.

Older adults underperform signs

Frailty, immunosuppression, neuropathy and corticosteroids can blunt fever, guarding and leucocytosis. New functional decline, delirium or unexplained acidosis may be the dominant presentation of abdominal catastrophe.

Extra-abdominal mimics matter

Inferior myocardial infarction, pneumonia, ketoacidosis, porphyria and testicular torsion can present as abdominal pain. A focused chest, cardiac, metabolic and genital assessment prevents an anatomically narrow error.

11Common pitfallsFrequent interpretation and management errors.

01
Waiting for a complete classic triad before considering ruptured abdominal aortic aneurysm.
02
Using a normal early lactate to exclude mesenteric ischaemia despite disproportionate pain and vascular risk.
03
Withholding analgesia in the belief that pain relief makes surgical examination impossible.
04
Relying on absent bowel sounds or a plain abdominal radiograph to exclude obstruction or perforation.
05
Omitting pregnancy testing or groin examination from an apparently gastrointestinal presentation.
06
Choosing routine portal-venous CT when an arterial vascular question requires CT angiography.