What are the differential diagnoses for a 12-year-old with right iliac fossa pain?

Differential Diagnosis of Right Iliac Fossa Pain in a 12-Year-Old

Acute appendicitis is the most common surgical cause and must be ruled out first, but non-specific abdominal pain, gynecological conditions, mesenteric adenitis, constipation, urinary tract infections, and inflammatory bowel disease are all important alternative diagnoses that require systematic evaluation. 1, 2, 3

Most Common Diagnoses by Frequency

Acute Appendicitis (Most Critical to Exclude)

• Accounts for approximately 39-50% of emergency presentations with right iliac fossa pain in this age group 4, 3
• Classic presentation: periumbilical pain migrating to right lower quadrant, anorexia, nausea/vomiting, fever, and leukocytosis 2, 5
• Critical pitfall: The complete classic triad is frequently absent even in confirmed cases, so its absence should NOT exclude appendicitis 2
• Atypical presentations are common, particularly in younger children 5
• Look specifically for: anorexia (key symptom), tachycardia, rebound tenderness, and guarding in the right iliac fossa 4, 2

Non-Specific Abdominal Pain

• Represents approximately 26% of admissions for right iliac fossa pain 4
• Diagnosis of exclusion after ruling out surgical and serious medical causes 4

Gynecological Causes (Critical in Adolescent Females)

• Account for approximately 22% of right iliac fossa pain presentations 4
• Ovarian torsion must be considered in ANY adolescent female with lower quadrant pain - this is a surgical emergency 1
• Ruptured ovarian cyst can cause acute pain if large 1
• Always obtain a pregnancy test in all adolescent females 1

Constipation

• Very common cause that can localize to the right lower quadrant 1, 2
• Often overlooked but easily treatable 1

Infectious/Inflammatory Causes

• Mesenteric adenitis: Can mimic appendicitis, often follows viral infection 1
• Urinary tract infection/pyelonephritis: Can present as abdominal pain 1
  • Urinalysis is systematically required to exclude this diagnosis 1, 2
• Lower lobe pneumonia: Can manifest as referred abdominal pain 1
• Inflammatory bowel disease (Crohn's disease, ulcerative colitis): Should always be considered in young patients with atypical presentations 1, 2

Rare but Important Diagnoses

• Giant hydronephrosis: Can present as right iliac fossa pain with palpable mass 6
• Bowel obstruction: From adhesions or hernias (check all hernia orifices) 3
• Mesenteric ischemia: Rare in adolescents but has 30-90% mortality if missed; presents with pain out of proportion to examination 3

Diagnostic Approach Algorithm

Step 1: Initial Clinical Assessment

• Check vital signs immediately for hemodynamic instability, tachycardia, or fever 3, 4
• Examine for: rebound tenderness, guarding, abdominal distension, palpable mass 2, 3
• Examine all hernia orifices and surgical scars 3
• Perform digital rectal examination to detect blood or masses 3

Step 2: Laboratory Evaluation

• Complete blood count with absolute neutrophil count 1, 2
  • WBC <10,000/mm³ strongly argues against appendicitis (LR 0.18-0.22) 2
  • Absolute neutrophil count ≤6,750-7,500/mm³ strongly argues against appendicitis (LR 0.06-0.35) 2
• C-reactive protein: Sensitivity 51.8%, specificity 85% for appendicitis 7
• Serum bilirubin: Elevated in appendicitis (mean 1.37 mg/dL vs 0.89 mg/dL in non-appendicitis) 7
• Urinalysis with microscopy - mandatory in all cases 1, 2
• Pregnancy test in all adolescent females 1
• Serum lactate if concerned about mesenteric ischemia 3

Step 3: Risk Stratification

• Use Pediatric Appendicitis Score (PAS) to categorize as low, intermediate, or high risk 2
• Score ≥9 most strongly associated with appendicitis 2
• Critical caveat: Clinical scores alone cannot eliminate the need for imaging in children 2

Step 4: Imaging Strategy

First-Line Imaging:

• Ultrasound of the right lower quadrant/abdomen is the initial imaging modality of choice 8, 1, 2
• Provides no radiation exposure with sensitivity 82-99% and specificity 94-96% 1
• Point-of-care ultrasound (POCUS) by experienced operator is recommended 8

Second-Line Imaging (if ultrasound equivocal or non-diagnostic):

• MRI abdomen without contrast: Sensitivity 86-98%, specificity 94-97% 1
  • Preferred over CT in children due to no radiation exposure 8
  • Pooled sensitivity/specificity for second-line MRI: 97.4%/97.1% 8
• Contrast-enhanced low-dose CT: Use if MRI unavailable 8
  • Sensitivity 90-94%, specificity 94-98% 2
  • Avoid routine CT as first-line in children 8

When to Proceed Directly to Surgery:

• High-risk patients with classic presentation may proceed without imaging, though imaging can aid surgical planning 2
• Do NOT delay surgical consultation for imaging when classic clinical picture is present - delay increases perforation risk to 17-32% 2

Step 5: Management Based on Findings

If appendicitis confirmed:

• Immediate surgical consultation 2
• Fluid resuscitation, analgesia, IV antibiotics 5

If imaging negative but pain persists:

• Cross-sectional imaging before any surgery for non-resolving right iliac fossa pain 8
• Exploratory laparoscopy recommended if progressive or persistent pain after negative imaging to establish alternative diagnoses 8

Critical Pitfalls to Avoid

1. Do not dismiss appendicitis based on absence of classic triad - many confirmed cases lack complete features 2
2. Do not rely solely on clinical scores - imaging remains essential in pediatric patients 2
3. Do not forget pregnancy test in adolescent females 1
4. Do not miss ovarian torsion - must be considered in any female with lower quadrant pain 1
5. Do not skip urinalysis - UTI can mimic appendicitis 1, 2
6. Longer pre-hospital symptom duration (>24-50 hours) increases perforation risk 4