Understanding Abdominal Pain from an Anatomy and Physiology Perspective
Anatomic Framework for Pain Localization
Begin your evaluation by using pain location as the primary anatomic guide to narrow your differential diagnosis, as this is the single most valuable clinical starting point. 1, 2, 3
Pain Location and Corresponding Anatomic Structures
Right upper quadrant pain indicates hepatobiliary disease (gallbladder, liver, bile ducts) due to visceral innervation from T7-T9 dermatomes 1, 2
Right lower quadrant pain strongly suggests appendicitis, as the appendix typically lies in this region when inflamed, causing localized peritoneal irritation 4, 1, 2
Left lower quadrant pain points to diverticulitis, reflecting sigmoid colon pathology in the descending colon 1, 2
Diffuse or nonlocalized pain requires broader evaluation of multiple organ systems including bowel, vasculature, and peritoneum 4, 2
Periumbilical pain migrating to right lower quadrant represents the classic appendicitis pattern—initial visceral pain from appendiceal distension (referred to T10 dermatome) followed by parietal peritoneal irritation causing localized somatic pain 4
Physiologic Pain Mechanisms
Three Types of Abdominal Pain Based on Nerve Pathways
Visceral pain arises from hollow organ distension, inflammation, or ischemia; transmitted via autonomic nerves; poorly localized, dull, and cramping 5, 3
Parietal (somatic) pain results from peritoneal irritation; transmitted via spinal nerves; sharp, well-localized, and worsened by movement 5, 3
Referred pain occurs when visceral afferents converge with somatic nerves at the spinal cord level, causing pain perception at distant sites sharing the same dermatome 5
Physiologic Clues from Pain Quality
Colicky pain (intermittent cramping) indicates bowel attempting to overcome obstruction through peristaltic waves 2
Ripping, tearing, or stabbing pain suggests vascular catastrophe like aortic dissection from sudden vessel wall disruption 2
Pain out of proportion to examination is the hallmark of mesenteric ischemia, reflecting visceral hypoxia before peritoneal signs develop 2, 6
Systematic Anatomic-Physiologic Approach
Step 1: Assess Hemodynamic Stability (Physiologic Priority)
Hypotension, tachycardia, or peritoneal signs indicate potential vascular compromise, perforation, or sepsis requiring immediate surgical consultation before extended workup 2
Tachycardia alone is the most sensitive early warning sign of surgical complications and should trigger urgent investigation 2
Step 2: Obtain Mandatory Pregnancy Testing (Anatomic Consideration)
- β-hCG testing is mandatory in all women of reproductive age before any imaging to rule out ectopic pregnancy, as this fundamentally alters the anatomic structures being evaluated 1, 2, 7
Step 3: Location-Specific Anatomic Evaluation
Right Upper Quadrant (Hepatobiliary System)
Order liver function tests (ALT, AST, alkaline phosphatase, bilirubin) to assess hepatocellular injury versus biliary obstruction 1, 7
Obtain ultrasonography as initial imaging to visualize gallbladder wall thickening, stones, and biliary dilation 2, 7, 3
Right Lower Quadrant (Appendix and Terminal Ileum)
Check complete blood count for leukocytosis indicating appendiceal inflammation 1, 7
Proceed to CT abdomen/pelvis with IV contrast as the preferred imaging, which has 85.7-100% sensitivity for appendicitis 4, 2
Left Lower Quadrant (Sigmoid Colon)
- Obtain CT abdomen/pelvis with contrast to evaluate for diverticulitis, showing colonic wall thickening and pericolonic fat stranding 2
Diffuse or Nonlocalized Pain (Multiple Organ Systems)
- CT abdomen/pelvis with IV contrast is the preferred initial study, as it changes the leading diagnosis in 51% of patients and alters admission decisions in 25% of cases 4, 2
Step 4: Physiologic Assessment Through Laboratory Testing
Complete blood count assesses leukocytosis (infection/inflammation) and anemia (bleeding) 1, 7
Comprehensive metabolic panel evaluates organ function and electrolyte disturbances from vomiting or third-spacing 1, 7
Serum lipase is more specific than amylase for pancreatic inflammation 1, 7
Urinalysis detects urinary tract infection or hematuria from nephrolithiasis 1, 7
Lactate and D-dimer may help if mesenteric ischemia is suspected, though they lack specificity 2, 7
Critical Anatomic-Physiologic Pitfalls
Never proceed with CT imaging without pregnancy testing in reproductive-age women, as this delays diagnosis of ectopic pregnancy and causes unnecessary fetal radiation exposure 1, 2, 7
Do not order conventional radiography routinely, as it has limited diagnostic value and rarely changes management 1, 2, 7
In elderly patients, normal laboratory values do not exclude serious pathology because the inflammatory response is often blunted despite significant infection 4, 2
Immunocompromised patients may lack typical signs of peritonitis due to impaired inflammatory response, requiring lower threshold for imaging 4, 2
Consider abdominal wall pain when pain is localized to a circumscribed site, worsens with muscle contraction, and has no intra-abdominal source on imaging 8
In post-bariatric surgery patients, tachycardia is the most critical warning sign of complications like anastomotic leak, even before other symptoms develop 2
Special Anatomic Considerations
Always consider gynecologic structures (ovaries, fallopian tubes, uterus) in women with lower abdominal pain, requiring pelvic examination 1, 2
Evaluate cardiovascular system if considering mesenteric ischemia, as this reflects superior mesenteric artery occlusion 1
Assess for extra-abdominal sources including thoracic pathology (pneumonia causing referred pain), abdominal wall pathology, and metabolic causes 5, 6