Fibrous Dysplasia - McCune-Albright Syndrome
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)
Investigations Provided
Laboratory Results
| Test | Result | Normal Range | Interpretation |
|---|---|---|---|
| Haemoglobin | 128 g/L | 115-145 | Normal |
| WCC | 6.8 x 10âč/L | 4-11 | Normal |
| Calcium | 2.35 mmol/L | 2.15-2.55 | Normal |
| Phosphate | 0.65 mmol/L | 0.8-1.5 | LOW |
| ALP | 420 U/L | 40-150 | Elevated |
| PTH | 4.5 pmol/L | 1.5-6.5 | Normal |
| 25-OH Vitamin D | 75 nmol/L | >50 | Normal |
| TSH | 0.3 mU/L | 0.4-4.0 | Low-normal |
| Free T4 | 22 pmol/L | 10-20 | High-normal |
| IGF-1 | 450 ng/mL | 200-600 | Normal |
| FGF23 | 180 RU/mL | <100 | Elevated |
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
What is the diagnosis and what endocrine abnormalities may be present?
How do you manage the shepherd's crook deformity?
What is the risk of malignant transformation?
What are the imaging characteristics of fibrous dysplasia?
What is the histopathology and how does it differ from other lesions?
How would you manage skull base involvement and what are the ophthalmological concerns?
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:
| Feature | McCune-Albright | Neurofibromatosis | Monostotic FD |
|---|---|---|---|
| Café-au-lait borders | Irregular (Maine) | Smooth (California) | None |
| Bone lesions | Polyostotic | Rare osseous | Single |
| Endocrine | Common | No | No |
| Inheritance | Somatic mosaic | Autosomal dominant | Sporadic |
Critical Management Points
- Coast of Maine (irregular) borders - distinguishes from NF1
- Screen for multiple endocrinopathies - thyroid, GH, adrenal
- Hypophosphataemia is FGF23-mediated - treat with phosphate + calcitriol
- Bisphosphonates reduce pain - but don't prevent deformity
- Use IM nails, not plates - plates create stress risers
- Malignant transformation risk is 4-6% - radiation is contraindicated
- Optic nerve surveillance - essential in craniofacial involvement
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
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
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.
Related Topics
- Monostotic Fibrous Dysplasia
- Bone Tumours in Children
- Pathological Fractures
- Hypophosphataemic Rickets
- Paediatric Endocrinology
- Craniofacial Surgery