The Acute Abdomen

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Last updated: October 16, 2020
Revisions: 26

Last updated: October 16, 2020
Revisions: 26

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The ‘acute abdomen’ is defined as a sudden onset of severe abdominal pain developing over a short time period. It has a large number of possible causes and so a structured approach is required.

The initial assessment should attempt to determine if the patient has an acute surgical problem that requires immediate and prompt surgical intervention, or urgent medical therapy.

The first decision when you first see any patient is “Are they critically unwell?”. A 10-second assessment of their clinical state can be made by a general look (the “end-of-bed-o-gram”) and their observations.

If they are critically unwell, start initial management steps promptly, and call for help early before going into the detail with their history and examination.

Presentations Requiring Urgent Intervention


The most serious cause of intra-abdominal bleeding is often the ruptured abdominal aortic aneurysm, which requires swift referral to the vascular team and immediate surgical intervention.

Other common causes usually involve a slower rate of bleeding, but with urgent surgery still required, include ruptured ectopic pregnancy, bleeding gastric ulcer, and trauma.

These patients will typically go into hypovolemic shock. Clinical features include tachycardia and hypotension, pale and clammy on inspection, and cool to touch (as discussed here).

Figure 1 – Endoscopic image of a bleeding gastric ulcer, which requires urgent endoscopic intervention

Perforated Viscus

Peritonitis is the inflammation of the peritoneum, and a generalised peritonitis is most commonly caused by perforation of an abdominal viscus.

The causes of perforation are broad but include peptic ulceration, small or large bowel obstruction, diverticular disease, and inflammatory bowel disease.

Patients with a generalised peritonitis present will often lay completely still (not to move their abdomen) and look unwell; this is especially important when compared to a renal colic, whereby patients are constantly moving and cannot get comfortable.

On examination, they will show signs of:

  • Tachycardia and potential hypotension
  • A completely rigid abdomen with percussion tenderness
  • Involuntary guarding – the patient involuntarily tenses their abdominal muscles when you palpate the abdomen
  • Reduced or absent bowel sounds, suggesting the presence of a paralytic ileus

Ischaemic Bowel

Any patient who has severe pain out of proportion to the clinical signs has ischaemic bowel until proven otherwise.  They are often acidaemic with a raised lactate and physiologically compromised.

Patients will often complain of a diffuse and constant pain, however the examination can often otherwise be unremarkable. Definitive diagnosis is via a CT scan with IV contrast, with early surgical involvement.

Presentations That Are Less Acute


Colic is an abdominal pain that crescendos to become very severe and then goes away completely. The most common types of colic are seen in biliary colic, ureteric colic, and bowel obstruction.


Peritonism (not peritonitis) refers to the localised inflammation of the peritoneum, usually due to inflammation of a viscus that then irritates the visceral (and subsequently, parietal) peritoneum.

This leads to patients stating that their abdominal pain starts in one place (irritation of the visceral peritoneum) before localising to another area* (irritation of the parietal peritoneum) or becoming generalised.

The classic example of this is acute appendicitis, with the pain migrating from the umbilical region to the right iliac fossa

Differential Diagnosis

The location of abdominal pain is one useful feature that helps narrow the differential. These can be classified based upon quadrant or region affected, as shown in Fig. 2.

It must be remembered to always consider extra-abdominal organs as the cause for abdominal pain, including cardiac, respiratory, and gynaecological or testicular conditions.

Fig 1 - Differential diagnoses for pain felt in the different regions of the abdomen.

Figure 2 – Differential diagnoses for pain felt in the different regions of the abdomen.

Importantly, there are non-general surgical causes of abdominal pain that must not be missed, including testicular torsion, ruptured ectopic pregnancy, diabetic ketoacidosis, and myocardial infarction


Laboratory Tests

The investigations in all cases of the acute abdomen share the same generic outline:

  • Urine dipstick – for signs of infection or haematuria ±MC&S. Include a pregnancy test for all women of reproductive age
  • Arterial Blood Gas useful in bleeding or septic patients, especially for the pH, pO2, pCO2, and lactate for signs of tissue hypoperfusion, as well as a rapid haemoglobin level
  • Routine bloods – FBC, U&Es, LFTs, CRP, amylase*
    • Do not forget a group & save (G&S) if the patient is likely to need surgery soon
  • Blood cultures – if considering infection as a potential diagnosis

*Any amylase 3x greater than the upper limit is diagnostic of pancreatitis. Any raised value lower than this may also be due to another pathology, such as perforated bowel, ectopic pregnancy, or diabetic ketoacidosis (DKA)


Following assessment, initial imaging may help to further help focus the diagnosis if still unclear:

  • An erect chest plain film radiograph (eCXR) – for evidence of free abdominal air (Fig. 3) or lower lobe lung pathology
  • Ultrasound
    • Kidneys, ureters, and bladder (‘KUB’) –  can check for hydronephrosis and cortico-medullary differentiation
    • Biliary tree and liver –  can check for the presence of gallstones, gallbladder thickening, or duct dilatation
    • Transvaginal – for suspected tubo-ovarian pathology
  • CT imaging of the abdomen, often best discussed with a senior depending on the suspected underlying diagnosis if required

In the emergency setting, every patient with abdominal pain should also have an electrocardiogram to exclude cardiac pathology, as referred pain

Figure 3 – An erect chest radiograph, showing free air under the right diaphragm (pneumoperitoneum); this strongly suggests bowel perforation, but can also be seen in post-laparotomy patients


The definitive management of acute abdomen depends largely on the cause. However, a good initial management plan includes core points, regardless of the underlying aetiology.

These include intravenous access, nil-by-mouth (NBM) status set, analgesia +/- antiemetics, initial imaging (as discussed above), VTE prophylaxis, urine dip, bloods (as discussed above). If the patient is unwell, consider a urinary catheter and/or nasogastric tube if necessary, and start intravenous fluids and monitor fluid balance.

Key Points

  • The first decision when you first see any patient is “Are they critically unwell?”
  • Wide array of pathologies that cause an acute abdomen, important to differentiate the urgent from non-urgent
  • Routine bloods and initial imaging can help with aiding the diagnosis
  • Management depends on the underlying cause, however ensure to start adequate resuscitation and inform your seniors if at all concerned