⚠️ Must-Not-Miss Diagnoses
Ruptured AAA (sudden severe back/flank pain + hypotension), mesenteric ischemia (pain out of proportion to exam), ectopic pregnancy (any woman of childbearing age), perforated viscus (rigid abdomen, free air), appendicitis (before perforation), aortic dissection.
Differential Diagnosis by Key Features
| Clinical Feature | Think Of | Priority |
|---|---|---|
| Severe flank/back pain + hypotension, pulsatile mass, elderly | Ruptured AAA | HIGH — surgical emergency |
| Pain out of proportion to exam, atrial fibrillation, post-prandial | Mesenteric ischemia | HIGH |
| Woman of childbearing age, LMP, vaginal bleeding, adnexal pain | Ectopic pregnancy | HIGH |
| RLQ pain, migration from periumbilical, anorexia, nausea | Appendicitis | High |
| RUQ pain, Murphy's sign, fever, fatty food trigger | Cholecystitis | Moderate |
| Epigastric radiation to back, nausea/vomiting, alcohol or gallstones | Pancreatitis | Moderate |
| LLQ pain, elderly, fever, change in bowel habits | Diverticulitis | Moderate |
Systematic Approach
Localize
Quadrant-based differential
RUQ: cholecystitis, hepatitis, hepatic abscess, Fitz-Hugh-Curtis. Epigastric: PUD, pancreatitis, ACS (referred). RLQ: appendicitis, Crohn's, ovarian pathology, psoas abscess. LLQ: diverticulitis, sigmoid volvulus, ovarian pathology. Diffuse: peritonitis, mesenteric ischemia, IBD.
Peritoneal Signs
Guarding, rigidity, rebound — surgical emergency
Involuntary guarding and rigidity indicate peritoneal irritation. Rebound tenderness confirms peritoneal involvement. These findings mandate urgent surgical consultation. A rigid, board-like abdomen = perforated viscus until proven otherwise.
Special Populations
Always consider ectopic in women, AAA in elderly men
In any woman of reproductive age: quantitative beta-hCG before any imaging. In elderly patients with vascular risk factors: keep AAA high on differential even for atypical presentations.
Imaging
Ultrasound vs CT based on diagnosis
RUQ pain → ultrasound first (cholecystitis, biliary). Suspected appendicitis → CT abdomen/pelvis (or ultrasound in children/pregnant). Suspected ectopic → pelvic ultrasound. Suspected AAA → bedside ultrasound (fastest), then CT if stable.
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