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**Textbook:** Harrison’s Principles of Internal Medicine (21st Edition)  
**Chapter Number & Name:** Chapter 15 – Abdominal Pain  
**Topics Covered:**  
- Mechanisms & Origins of Abdominal Pain  
- Parietal Peritoneal Inflammation & Visceral Obstruction  
- Vascular Disturbances & Referred Pain  
- Metabolic & Neurogenic Causes  
- Approach to Diagnosis: History, Exam, Imaging, Labs  
- Common Causes by Location  
**Page Numbers:** 1036–1050 (Approx.)

Abdominal Pain

OVERVIEW

  • Abdominal pain is a frequent but challenging complaint; accurate diagnosis is crucial.

  • Both subtle and severe pathologies may present similarly, so thorough evaluation is vital.

  • Clinicians must differentiate between urgent surgical causes and those managed non-operatively.

🟡 High-Yield: History and physical exam remain the cornerstones of diagnosing acute abdominal pain.

PAIN MECHANISMS

1️⃣ Inflammation of the Parietal Peritoneum

  • Quality: Steady, aching pain localized over the inflamed area; transmitted by somatic nerves.

  • Key Features:

    • Aggravated by movement (e.g., coughing, sneezing) or palpation.

    • Patients often lie still to minimize pain.

    • Abdominal muscle guarding/rigidity is common with acute processes (peritonitis).

2️⃣ Obstruction of Hollow Viscera

  • Colicky or cramping pain (though can be more constant if distension is severe).

  • Small-bowel obstruction: Peri- or supraumbilical intermittent pain; can lose colicky character over time.

  • Biliary tree “colic”: Typically a steady, severe ache (misnomer “colic”) in the RUQ or epigastrium.

  • Ureteral colic: Severe flank pain radiating to groin/genitalia; associated with nephrolithiasis.

🔵 Clinical Pearl: Gradual duct obstruction (e.g., by tumor) often produces milder, vague, or no pain compared to acute obstruction.

3️⃣ Vascular Disturbances

  • Occlusion of mesenteric vessels (embolism, thrombosis):

    • “Pain out of proportion” to exam → minimal findings despite severe, diffuse pain.

    • Over time → peritoneal signs if infarction or perforation occurs.

  • Ruptured AAA (abdominal aortic aneurysm):

    • Sudden severe pain with possible radiation to back, flank, or groin.

    • May have hemodynamic collapse if rupture is major.

4️⃣ Abdominal Wall

  • Constant, aching pain worsened by movement.

  • Pain often reproducible by tensing abdominal muscles or palpation.

  • Example: Rectus sheath hematoma – watch for swelling/tenderness in the lower quadrants in anticoagulated patients.

5️⃣ Referred Pain

  • Disease outside the abdomen can mimic an intraabdominal process.

  • Thoracic causes: Lower lobe pneumonia, MI, PE, pericarditis → can present as upper abdominal pain.

  • Spinal nerve compression: May cause “pseudoabdominal” pain with normal abdominal exam.

  • Testicular pathology: Can present with lower abdominal/pelvic pain.

METABOLIC & NEUROGENIC CAUSES

Metabolic

  • Porphyria, lead colic, hyperlipidemia, diabetic ketoacidosis can present with abdominal pain.

  • Familial Mediterranean fever: Recurrent episodes of abdominal pain + fever; can mimic acute abdomen.

  • Uremia: Non-specific pain, variable location.

Neurogenic

  • Causalgia (burning nerve pain): Pain is disproportionate to exam, can be triggered by light touch.

  • Herpes zoster: Radicular pain in dermatomal distribution, can precede rash.

🔴 Exam Alert: Always consider metabolic and neurogenic etiologies for “atypical” abdominal pain, especially with normal imaging.

SPECIAL POPULATIONS: IMMUNOCOMPROMISED

  • Blunted inflammatory response → minimal abdominal findings despite serious pathology.

  • Acalculous cholecystitis, CMV colitis, typhlitis (neutropenic enterocolitis) are common in immunosuppressed.

  • Keep a high index of suspicion; imaging and serial exams are crucial.

APPROACH TO DIAGNOSIS

🟡 High-Yield: A careful, systematic approach—history, exam, labs, imaging—often yields a correct diagnosis without delay.

1️⃣ History

  • Onset (acute vs. insidious), character (sharp, dull, crampy), location & radiation.

  • Chronological sequence of symptoms: e.g., epigastric → RLQ (appendicitis).

  • Associated factors: Nausea/vomiting, bowel changes, fever, urinary symptoms.

  • Menstrual history in women (ectopic pregnancy, ovarian torsion).

  • Past medical history: GI diseases, surgeries, risk factors.

2️⃣ Physical Examination

  • Inspection: Posture, facial expression (pain vs. stable), distension, scars, bruising (Grey-Turner sign, Cullen sign).

  • Auscultation: Bowel sounds (hyperactive, absent, tinkling) → not always definitive.

  • Palpation (gentle first!): Identify tenderness, rebound, guarding, masses, organomegaly.

    • Percussion can detect peritoneal irritation.

  • Pelvic & Rectal exams are critical in both sexes → detect localized tenderness or masses.

🔵 Clinical Pearl: Eliciting rebound tenderness can be done gently (e.g., with percussion) instead of abrupt release to avoid patient distress.

3️⃣ Laboratory Tests

  • CBC: Leukocytosis may indicate infection/inflammation but is not definitive.

  • Electrolytes, BUN, Creatinine, Glucose: Evaluate hydration, renal function, metabolic status.

  • Liver function tests & Bilirubin: Suggest hepatic or biliary pathology.

  • Amylase/Lipase: Elevated in pancreatitis, but also can be elevated in other disorders (e.g., perforated ulcer).

  • Urinalysis: Rule out infection or nephrolithiasis.

4️⃣ Imaging

  • Plain abdominal X-ray: Limited utility; may help in suspected perforation (free air) or obstruction (dilated loops).

  • Ultrasound: First-line for RUQ pain (gallbladder disease), pelvic pain (ovarian torsion, cysts), or AAA screening.

  • CT scan (often with IV/oral contrast): Gold standard for many acute abdominal conditions → appendicitis, diverticulitis, pancreatitis, etc.

  • MRI: Used selectively (e.g., pregnancy, complex biliary or liver lesions).

  • Diagnostic Laparoscopy: Both diagnostic & therapeutic, especially in ambiguous pelvic or right lower quadrant pathology.

DIFFERENTIAL DIAGNOSIS BY LOCATION

Region Common Causes
RUQ Cholecystitis, Cholangitis, Hepatitis, Pneumonia, Subdiaphragmatic Abscess
Epigastric PUD, Gastritis, Pancreatitis, MI, Pericarditis, Aortic aneurysm
LUQ Splenic infarct/rupture, Pancreatitis, Gastric ulcer, Abscess
RLQ Appendicitis, Salpingitis, Ectopic pregnancy, IBD, Nephrolithiasis
Periumbilical Early appendicitis, Gastroenteritis, Bowel obstruction, Aortic aneurysm
LLQ Diverticulitis, Salpingitis, Ectopic pregnancy, IBS, Nephrolithiasis
Diffuse Gastroenteritis, Mesenteric ischemia, Peritonitis, IBS, Malaria

MERMAID DIAGRAM: APPROACH TO ACUTE ABDOMINAL PAIN

flowchart LR
    A[Patient with Acute Abdominal Pain] --> B{Assess Stability}
    B -- Unstable / Shock? --> C[Consider ruptured AAA, major hemorrhage, emergent surgery]
    B -- Stable --> D[History + Physical Exam]
    D --> E{Location/Character of Pain?}
    E --> F[Order Labs: CBC, Chemistries, LFTs, Lipase, UA]
    F --> G[Obtain Imaging]
    G --> H{Likely Diagnosis?}
    H -- Positive Findings --> I[Treat or consult surgery if indicated]
    H -- Still Unclear --> J[Consider Laparoscopy or further imaging]

MANAGEMENT PEARLS

  1. Resuscitation & Stabilization: IV fluids, correct electrolyte imbalances.

  2. Pain Control: Adequate analgesics do not mask important signs; it’s humane and appropriate.

  3. Consultation: Surgical or specialty consult if suspicion of appendicitis, AAA, or other urgent surgical pathology.

  4. Serial Examinations: Reassess frequently; subtle changes can confirm or rule out evolving diagnoses.

🔴 Exam Alert: Never rely solely on a single lab or single exam. Combine history, repeated exams, labs, and imaging for the final disposition.

REFERENCES (APA Style, Annotated)

  • Bhangu, A., et al. (2015). Acute appendicitis: Modern understanding of pathogenesis, diagnosis, and management. The Lancet, 386(10000), 1278–1287.

    — Comprehensive overview of appendicitis, including pathophysiology and updated diagnostic/therapeutic guidelines.

  • Cartwright, S. L., & Knudson, M. P. (2015). Diagnostic imaging of acute abdominal pain in adults. American Family Physician, 91(7), 452–459.

    — Summarizes imaging approaches for common causes of acute abdominal pain.

  • Huckins, D. S., et al. (2017). Diagnostic performance of a biomarker panel as a negative predictor for acute appendicitis in acute emergency department patients with abdominal pain. The American Journal of Emergency Medicine, 35(3), 418–422.

    — Evaluates combined biomarkers for excluding appendicitis in ED patients.

  • Nayor, J., et al. (2016). Tracing the cause of abdominal pain. New England Journal of Medicine, 375(6), e8.

    — Illustrates a case-based approach to diagnosing abdominal pain.

  • Silen, W., & Cope, Z. (2010). Cope’s Early Diagnosis of the Acute Abdomen (22nd ed.). Oxford University Press.

    — Classic text on the clinical approach and operative findings in acute abdominal pain.

Bottom Line:

  • Abdominal pain demands a systematic approach: thorough history, physical exam, plus targeted labs and imaging.

  • Serial evaluations are often necessary if the diagnosis is unclear initially.

  • Always remain vigilant for life-threatening causes, especially in unstable or immunocompromised patients.

[▶️ Video Placeholder: “Bedside Ultrasound Techniques for RUQ and Pelvic Pain Evaluation”]

If you have further questions or need additional detail on specific pathologies, let me know!