The Acute Abdomen

The ‘acute abdomen’ is defined as a sudden onset of severe abdominal pain of less than 24 hours duration.

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/prompt surgical intervention, or urgent medical therapy.


Presentations Requiring Urgent Surgery

Bleeding

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

Fig 1 - Endoscopic image of a bleeding gastric ulcer. This requires urgent surgical intervention.

Fig 1 – Endoscopic image of a bleeding gastric ulcer. This requires urgent surgical intervention.

Other common causes usually involve a slower rate of bleeding, but with urgent surgery still required:

These patients will typically go into hypovolemic shock. Clinical features include tachycardia and hypotension, pale and clammy on inspection, and cool to touch with a thread pulse.

Peritonitis and Perforation

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, bowel ischaemia, and inflammatory bowel disease, including toxic megacolon.

Patients with a generalised peritonitis present with some characteristic features:

  • Tachycardia and potential hypotension.
  • Patients often lay completely still, not to move their abdomen
    • This is especially important when compared to a renal colic, whereby patients are constantly moving and cannot get comfortable.
  • A completely rigid ‘washboard’ abdomen with percussion tenderness.
  • Rebound tenderness – abdominal pain is worse when the hand is removed from the abdomen, rather than when pressing in.
  • Involuntary guarding – the patient involuntarily tenses their abdominal muscles when you touch the abdomen.
  • Reduced or absent bowel sounds – suggesting the presence of a paralytic ileus.

Presentations That Are Less Acute

Colic

Colic is an abdominal pain that crescendos to become very severe and then goes away completely. This is most typically seen in either ureteric obstruction or bowel obstruction.

Biliary ‘colic’ is not a true colic as the pain does not go away completely – instead, it gets periodically better and worse (colloquially termed ‘waxes and wanes’).

Peritonism (Not Peritonitis)

Peritonism 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 or becoming generalised (irritation of the parietal peritoneum). The classic example of this is acute appendicitis.

Differential Diagnosis

The location of abdominal pain is perhaps the most useful initial feature to help narrow your 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.

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

Investigations

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.
  • ABG 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.
  • Routine bloods – FBC, U&Es, Liver Function, CRP, amylase.
    • Consider measuring serum calcium in suspected pancreatitis.
    • Do not forget a group & save (G&S) if the patient is likely to need surgery soon.
  • Blood cultures – if considering sepsis as a diagnosis

Note: 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).

Imaging

In the emergency setting, every patient with abdominal pain should have an ECG to exclude myocardial infarction. Other imaging modalities that may be initially requested include:

  • Non-radiological (ultrasound):
    • Kidneys, ureters and bladder (‘KUB’) –  for suspected renal tract pathology
    • Biliary tree and liver –  for suspected gallstone disease
    • Ovaries, fallopian tubes and uterus – for suspected tubo-ovarian pathology
  • Radiological:
    • Abdominal x-ray (AXR) – for evidence of bowel obstruction
    • An erect chest x-ray (CXR) – for evidence of bowel perforation
Fig 3 - An erect chest x-ray, showing free air under the right diaphragm - pneumoperitoneum. This strongly suggests bowel perforation, but can also be seen in post-laparotomy patients.

Fig 3 – An erect chest x-ray, showing free air under the right diaphragm – pneumoperitoneum. This strongly suggests bowel perforation, but can also be seen in post-laparotomy patients.


Management

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

These include admission, IV access, NBM, analgesia +/- antiemetics, imaging (as discussed above), VTE prophylaxis, urine dip, bloods (as discussed above). Consider a urinary catheter and/or nasogastric tube. Start IV fluids and monitor fluid balance.

Further Reading

Do opiates affect the clinical evaluation of patients with acute abdominal pain?
Ranji SR et al., JAMA

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