In a 14‑year‑old girl with one‑month left anterior superior iliac spine pain, no recollectable trauma, normal C‑reactive protein and erythrocyte sedimentation rate, marginal vitamin D3 deficiency with normal serum calcium, inorganic phosphate and alkaline phosphatase, mild anemia and negative antinuclear antibody, what are the possible differential diagnoses?

Differential Diagnoses for Left ASIS Pain in a 14-Year-Old Girl

The most likely diagnosis is an avulsion fracture of the anterior superior iliac spine (ASIS), even without recalled trauma, and this should be ruled out first with plain radiographs of the pelvis. 1, 2

Primary Differential Diagnoses

1. ASIS Avulsion Fracture (Most Likely)

• This is the leading diagnosis despite no recalled injury, as avulsion fractures at the ASIS are well-documented in adolescents and can occur with minimal or unrecognized trauma 1, 2
• Occurs at the sartorius muscle attachment during sudden hip flexion or extension
• The normal inflammatory markers (CRP, ESR) support a mechanical rather than inflammatory etiology
• Initial imaging: Plain pelvic radiographs to visualize the ASIS region
• If radiographs are equivocal, CT can confirm the diagnosis 2

2. Apophysitis of the ASIS

• Overuse injury from repetitive traction at the growth plate
• Common in adolescent athletes, but can occur with normal daily activities
• Normal inflammatory markers are consistent with this mechanical diagnosis
• Clinical presentation overlaps with avulsion fracture but represents chronic rather than acute injury

3. Vitamin D Deficiency-Related Bone Pain

• While the patient has marginal vitamin D deficiency, the normal alkaline phosphatase argues strongly against active rickets or osteomalacia 3
• Vitamin D deficiency can cause isolated bone pain, but typically presents with more generalized symptoms 3
• The localized, unilateral nature of pain makes this less likely as the primary diagnosis
• However, vitamin D deficiency may contribute to bone fragility and should be corrected

4. Early Axial Spondyloarthritis (Less Likely)

• The normal CRP and ESR make active inflammatory arthritis unlikely 4, 5, 6
• Negative ANA further reduces probability of inflammatory arthropathy
• AxSpA typically presents with inflammatory back pain characteristics: onset <40 years, worsening with rest, night pain, morning stiffness, improvement with exercise 4
• The ASIS is an atypical location for initial axSpA presentation
• If inflammatory back pain features are present clinically, MRI of sacroiliac joints would be appropriate 5

5. Infectious Sacroiliitis (Unlikely but Important)

• Normal CRP and ESR make this diagnosis very unlikely 7
• Infectious sacroiliitis typically presents with fever, elevated inflammatory markers, and inability to bear weight 7
• Consider only if clinical deterioration occurs or systemic symptoms develop

Clinical Approach Algorithm

Step 1: Obtain plain pelvic radiographs immediately

• Focus on bilateral ASIS regions for comparison
• Look for avulsion fractures, apophyseal irregularities, or other bony abnormalities

Step 2: If radiographs are negative or equivocal:

• Consider CT for better bony detail of ASIS region
• Assess for stress-related changes at the apophysis

Step 3: Address the vitamin D deficiency regardless:

• Supplement to achieve levels >30 nmol/L
• Recheck calcium and phosphate if supplementing high doses

Step 4: Reassess if pain persists beyond 4-6 weeks:

• Consider MRI of pelvis and sacroiliac joints if inflammatory features emerge
• Re-evaluate inflammatory markers

Important Clinical Pitfalls

• Do not dismiss the diagnosis of avulsion fracture based on absent trauma history – adolescents may not recall minor precipitating events 1, 2
• Do not over-attribute symptoms to marginal vitamin D deficiency when normal alkaline phosphatase suggests no active bone disease 3
• Do not pursue extensive inflammatory workup when CRP and ESR are normal and ANA is negative 6
• The mild anemia requires separate evaluation but is unlikely related to the localized ASIS pain

Management Implications

For avulsion fracture or apophysitis:

• Conservative management with rest, non-weight bearing initially
• Pain typically resolves within weeks 1, 2
• Return to activities at 6 months for complete fractures

The normal inflammatory markers effectively exclude active inflammatory or infectious processes at this time, making mechanical etiologies the primary focus of investigation.