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Acute abdomen refers to a constellation of sudden, severe abdominal pain requiring urgent evaluation and often surgical intervention. The broad differential includes surgical emergencies such as perforation, ischemia, obstruction, and hemorrhage, as well as medical causes requiring timely medical management.
The pathophysiology depends on the underlying etiology. Peritoneal irritation from perforation or ischemia activates nociceptors in the parietal peritoneum, producing sharp, well-localized pain. Hollow viscus obstruction causes colicky pain from smooth muscle spasm. Referred pain patterns reflect shared dermatomal innervation — diaphragmatic irritation refers to the shoulder, appendiceal inflammation may begin periumbilically before localizing to the right lower quadrant.
Clinical reasoning requires integration of pain character (colicky vs. constant), location, onset (sudden vs. gradual), associated symptoms, and risk factors. Peritoneal signs (guarding, rigidity, rebound tenderness) suggest surgical emergency. The differential is guided by quadrant: RUQ (cholecystitis, hepatitis), epigastric (peptic ulcer, pancreatitis, ACS), RLQ (appendicitis, Crohn's), LLQ (diverticulitis, sigmoid volvulus). Imaging strategy — ultrasound first vs. CT — depends on clinical suspicion.
Key imaging focus: Free air (perforation), free fluid, inflammatory changes, vascular compromise