31st July 2023

Pathophysiology

There are different mechanisms underlying the pain associated with an acute abdomen.

Visceral pain relates to the organs of the abdominal cavity (i.e. the viscera). Autonomic nerve fibres respond to distension and muscle contraction to produce a pain, which is characteristically poorly localised, dull in nature, and often associated with nausea. The location of the pain roughly corresponds to the embryonic origin of the affected structure, i.e:

• Foregut structures (stomach, duodenum, liver and pancreas) cause upper abdominal pain
• Midgut structures (small bowel, proximal colon and appendix) cause periumbilical pain
• Hindgut structures (distal colon and genitourinary tract) cause lower abdominal pain.

Somatic pain is due to irritation of the parietal lining of the peritoneum, which is innervated by somatic nerve fibres. Irritation is caused by infectious, chemical or inflammatory processes. The associated pain is sharp in nature and well localised.

Referred pain is pain that is perceived distant from its source, and is due to the way nerve fibres converge in the spinal cord. For example, groin pain due to renal colic, shoulder tip pain due to diaphragmatic irritation, and scapular pain due to biliary colic. You should therefore be vigilant that pain in these areas may correspond to a different location of pathology.