CIM Case: Fibrous Dysplasia - McCune-Albright Syndrome

Clinical Scenario

Patient: 15-year-old female Presentation: Bilateral hip pain and progressive waddling gait over 2 years, worsening with activity Relevant history: Reached menarche at age 8 (precocious puberty), multiple café-au-lait spots noticed since infancy, previous pathological fracture of left femur at age 10, no family history Examination findings:

• Multiple café-au-lait spots with irregular "coast of Maine" borders
• Bilateral coxa vara deformity with limited hip internal rotation
• Shortened limbs (5cm LLD, left shorter)
• Waddling gait with positive Trendelenburg bilaterally
• Tenderness over proximal femora
• No lymphadenopathy or masses
• Thyroid not enlarged
• Tanner stage 5 (early development)

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Investigations Provided

Laboratory Results

Imaging

Image 1: Pelvis and Bilateral Hip X-rays

Radiological features:

• Bilateral "shepherd's crook" deformity of proximal femora
• Coxa vara (neck-shaft angle 90° bilaterally)
• Ground-glass matrix throughout proximal femora
• Expansile lesions with smooth endosteal scalloping
• Previous healed fracture left femoral shaft with varus malunion
• Acetabula appear involved
• No aggressive periosteal reaction

Image 2: Whole Body Bone Scan

Findings:

• Increased uptake in bilateral proximal femora
• Increased uptake in skull base, left humerus, multiple ribs
• Pattern consistent with polyostotic fibrous dysplasia

Image 3: CT Skull

Findings:

• Extensive fibrous dysplasia of skull base
• Sphenoid bone involvement
• No optic canal encroachment currently
• Ground-glass appearance typical

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests McCune-Albright Syndrome:

• Polyostotic fibrous dysplasia
• Café-au-lait spots with "coast of Maine" borders
• Precocious puberty (especially in females)
• Endocrine abnormalities
• Ground-glass appearance on X-ray
• Shepherd's crook deformity

Distinguish from Other Conditions:

Critical Management Points

1. Coast of Maine (irregular) borders - distinguishes from NF1
2. Screen for multiple endocrinopathies - thyroid, GH, adrenal
3. Hypophosphataemia is FGF23-mediated - treat with phosphate + calcitriol
4. Bisphosphonates reduce pain - but don't prevent deformity
5. Use IM nails, not plates - plates create stress risers
6. Malignant transformation risk is 4-6% - radiation is contraindicated
7. Optic nerve surveillance - essential in craniofacial involvement

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Common Examiner Follow-ups

Q: "What is the role of burosumab in McCune-Albright syndrome?"

Burosumab is an anti-FGF23 monoclonal antibody:

• Approved for X-linked hypophosphataemia
• Being studied in FD/MAS-related hypophosphataemia
• Normalises phosphate without high-dose supplements
• May improve bone mineralisation
• Currently off-label for MAS
• Emerging evidence is promising

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Q: "What is the Mazabraud syndrome?"

Mazabraud syndrome is the association of:

• Fibrous dysplasia (usually polyostotic)
• Intramuscular myxomas (soft tissue masses)
• Rare, <100 cases reported
• Myxomas are benign but can be mistaken for malignancy
• Same GNAS mutation as McCune-Albright
• No endocrine abnormalities

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Q: "When would you biopsy a fibrous dysplasia lesion?"

Biopsy indications:

• Atypical imaging features
• Suspicion of malignant transformation
• Diagnostic uncertainty
• New pain in established lesion
• Aggressive periosteal reaction
• Soft tissue mass

Routine biopsy of classic-appearing FD is NOT required.

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Related Topics

• Monostotic Fibrous Dysplasia
• Bone Tumours in Children
• Pathological Fractures
• Hypophosphataemic Rickets
• Paediatric Endocrinology
• Craniofacial Surgery