Acute abdominal pain

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• Visceral vs. Somatic Pain Pathways: Abdominal pain pathophysiology hinges on understanding nerve fiber transmission. Visceral pain is mediated by stretch receptors in the walls of hollow organs or organ capsules; it is typically midline, poorly localized, and less easily described. As the underlying disease progresses to irritate the parietal peritoneum, the pain transitions to somatic pain, which is sharp, intense, and well-localized to the affected quadrant.
• The Ischemic Cascade: In acute mesenteric ischemia, embolic occlusion or thrombosis drastically reduces bowel perfusion. The mucosa sloughs, leading to profound third-spacing of fluids, bacterial translocation, lactic acidosis, and eventual bowel necrosis.
• The Hemorrhagic Void: In vascular catastrophes (e.g., ruptured Abdominal Aortic Aneurysm [AAA] or ruptured ectopic pregnancy), rapid intraperitoneal bleeding leads to hypovolemic shock. Crucially, the cardiovascular system compensates via massive sympathetic drive; therefore, systolic blood pressure does not drop until 30% to 40% of normal blood volume is lost.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: For critically ill patients, simultaneously resuscitate and evaluate. Place the patient on a continuous cardiac monitor, apply oxygen (2 to 4 L/min via nasal cannula or mask), establish large-bore IV access, and administer an isotonic fluid bolus adjusted for age and cardiovascular status.
• Analgesia (First-Line): Administer IV Morphine at 0.05 to 0.10 mg/kg (typically 2 to 5 mg in adults) every 15 to 20 minutes until pain is controlled (target numeric pain score <4).
• Renal Impairment Protocol: Morphine is not preferred in patients with renal dysfunction due to metabolite accumulation; use IV Fentanyl or Hydromorphone instead.
• Antiemetics: Administer prompt antiemetic therapy (e.g., ondansetron, prochlorperazine, promethazine, metoclopramide, or droperidol) to prevent aspiration and improve comfort.
• Essential Monitoring: Obtain baseline labs simultaneously with IV placement: electrolytes, BUN/creatinine, CBC, clotting studies, lipase, and a Type & Screen (or cross-match if hemorrhage is suspected).

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• "Can't-Miss" Differential Diagnoses:
• Ruptured Abdominal Aortic Aneurysm (AAA): Older patients with vascular risk factors; presents with abdominal/back pain and syncope.
• Acute Mesenteric Ischemia: Classically presents with "pain out of proportion" to a benign abdominal exam; highly associated with atrial fibrillation.
• Ruptured Ectopic Pregnancy: Must be ruled out in every female of childbearing age.
• Perforated Viscus: Usually a duodenal ulcer eroding through the serosa, spilling contents, and causing rigid peritonitis.
• Acute Myocardial Infarction (AMI): Atypical presentation (nausea, epigastric discomfort), especially in elderly females.
• Prioritized Diagnostic Workup:
• Urine or Serum $\beta$-hCG: Mandatory first test for all women of reproductive age (10–55 years).
• CT Angiography (CT-A): The gold standard for evaluating suspected mesenteric ischemia or a stable AAA leak.
• Non-Contrast or Contrast CT Abdomen/Pelvis: For suspected appendicitis, diverticulitis, or undifferentiated pain in older adults.
• Pelvic US: Critical first step for suspected ectopic pregnancy or ovarian torsion.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• The 12-Lead ECG: Look explicitly for ST-segment elevations or ischemic changes in the inferior leads (II, III, aVF). Inferior wall MI frequently mimics upper abdominal or epigastric pain.
• Point-of-Care Ultrasound (POCUS / eFAST): Evaluate the unstable patient immediately at the bedside. Look for free intraperitoneal fluid (black/anechoic) in Morison's pouch, the splenorenal recess, and the pelvis. Also, scan the abdominal aorta to measure the diameter; a pulsatile mass >3.0 cm indicates an aneurysm, and an aneurysm >5.0 cm carries a very high risk of rupture. Assess the IVC to gauge volume status.
• Upright Chest X-Ray (CXR): In patients with a rigid abdomen, look for a dark crescent of free intraperitoneal air under the right or left hemidiaphragm, diagnostic of a perforated viscus.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• Modified Alvarado Score for Acute Appendicitis: Aids in risk-stratifying suspected appendicitis.
• Symptoms: Migratory pain (1 pt), Anorexia (1 pt), Nausea/Vomiting (1 pt).
• Signs: Right lower quadrant tenderness (2 pts), Rebound tenderness (1 pt), Fever (1 pt).
• Labs: WBC > 10,000/mm³ (2 pts).
• Cutoffs: Low risk (1–4), Possible/Probable Appendicitis (5–9). Note: Clinical judgment outperforms the score; it is only 72% sensitive compared to 93% for experienced clinical judgment.
• High-Risk Patient Profiling: Always escalate imaging and consultation in populations at mathematically higher risk for catastrophic pathology: age >60 years, immunocompromised states (including low-dose steroid use), and patients with previous bariatric surgery (high risk for internal hernias).

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Cognitive Trap (Withholding Analgesia): Refusing to give opioids out of fear that it will "mask the surgical abdomen." Decades of evidence show early analgesia does not increase diagnostic error; it improves patient comfort and actually increases the reliability of the physical exam by reducing voluntary guarding.
• Cognitive Trap (Anchoring on Vitals): Assuming a patient with "stable" vital signs or an absence of fever does not have a surgical emergency. The elderly and immunocompromised frequently mount no tachycardic or febrile response to surgical sepsis.
• Critical Action (The AAA Bypass): If an older patient with a known AAA or a pulsatile mass is hemodynamically unstable, do not send them to the CT scanner. Assume rupture and move directly to emergent exploratory laparotomy with vascular surgery.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzwords:
• "Pain out of proportion to examination" + atrial fibrillation = Acute Mesenteric Ischemia.
• "Board-like, rigid abdomen" + "free air under the diaphragm" = Perforated Peptic Ulcer.
• "Elderly female" + "nausea/vomiting and epigastric discomfort" = Check an ECG for acute MI.
• Classic Distractor: A 70-year-old with a history of A-fib presents with severe, diffuse 10/10 abdominal pain but a perfectly soft, non-tender abdomen. The options will include "Plain radiograph" or "Right upper quadrant ultrasound." Correction: This is a classic presentation of acute mesenteric ischemia; the correct answer is Computed Tomography Angiography (CT-A).

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Resuscitation Hook: "This patient is critically ill. I will simultaneously resuscitate and evaluate. I am placing the patient on a cardiac monitor and oxygen, inserting two large-bore IVs, and administering a bolus of normal saline. I want a STAT bedside point-of-care ultrasound to evaluate for an abdominal aortic aneurysm and free intraperitoneal fluid."
• The Analgesia Pivot: "The patient is in severe pain. I will not withhold analgesia. I am ordering IV Morphine 4 mg immediately, which will relieve their suffering and allow me to perform a more reliable, focused abdominal examination without voluntary guarding."
• The High-Risk Disposition: "Given the patient's advanced age and the severe, sudden onset of their symptoms, their risk for vascular catastrophe is exceedingly high. I am ordering a CT Angiogram to rule out mesenteric ischemia and consulting General Surgery early for admission and serial abdominal examinations."