CIM Case: X-Linked Hypophosphataemic Rickets

Clinical Scenario

Patient: 14-month-old male Presentation: Bowed legs, delayed walking, stunted growth, family history of "bone problems" in mother and maternal grandmother Relevant history: Full-term delivery, normal birth, delayed motor milestones (not yet walking independently), no dietary deficiency suspected (breast-fed with adequate weaning), mother is short stature (150cm) with bow legs Examination findings:

• Height: 3rd percentile (below expected for mid-parental height)
• Weight: 25th percentile
• Bilateral genu varum with intercondylar distance 4cm
• Widened wrists (metaphyseal flaring)
• Frontal bossing
• Dental: Delayed eruption (no teeth at 14 months)
• No muscle weakness (unlike nutritional rickets)
• No hypotonia
• Normal neurological examination
• Mother has similar leg bowing and required childhood osteotomies

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Investigations Provided

Laboratory Results

Urine Studies

Imaging

Image 1: AP Radiographs of Knees and Wrists

Radiological features:

• Bilateral genu varum (tibial bowing)
• Metaphyseal widening and cupping at distal femur and proximal tibia
• Fraying of metaphyseal margins
• No periosteal reaction
• Wrist: Widening of distal radial and ulnar physes
• Rachitic rosary not prominent (unlike nutritional rickets)
• No fractures or Looser zones

Image 2: Standing Full-Length Lower Limb Alignment

Findings:

• Mechanical axis deviation: 25mm lateral (bilateral)
• Anatomical tibiofemoral angle: 20° varus
• Metaphyseal-diaphyseal angle: 15°
• No angular deformity at ankle

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Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests X-Linked Hypophosphataemic Rickets:

• Isolated hypophosphataemia with normal calcium
• Normal PTH (not secondary hyperparathyroidism)
• Normal vitamin D levels
• Renal phosphate wasting (high TmP/GFR)
• Family history of short stature/bowing (especially maternal line)
• Absence of muscle weakness

Critical Management Points

1. Differentiate from nutritional rickets - XLH has normal calcium, normal PTH, no muscle weakness
2. Treatment requires BOTH phosphate AND calcitriol - phosphate alone causes secondary hyperparathyroidism
3. Burosumab (anti-FGF23) is now available and may be first-line
4. Nephrocalcinosis is a treatment complication requiring monitoring
5. Surgery should wait until rickets healed and metabolically controlled
6. Dental surveillance is essential - abscesses occur in normal-looking teeth

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Common Examiner Follow-ups

Q: "What would you do if this child develops tertiary hyperparathyroidism?"

This occurs when long-standing secondary hyperparathyroidism becomes autonomous:

• PTH remains elevated despite normalised calcium
• Causes hypercalcaemia
• May develop parathyroid adenoma
• Requires parathyroidectomy
• Prevention: careful monitoring and calcitriol dose adjustment

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Q: "The mother asks about having another child - what do you tell her?"

Genetic counselling:

• XLH is X-linked dominant
• If the mother is affected:
  • 50% of sons will be affected
  • 50% of daughters will be affected
• Prenatal diagnosis is possible with known PHEX mutation
• Severity can vary even within families
• Early treatment improves outcomes

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Q: "At age 4, the deformity is worsening despite treatment. What now?"

Reassess:

1. Confirm compliance with medications
2. Check serum phosphate, ALP, PTH
3. Renal ultrasound for nephrocalcinosis
4. Consider burosumab if on conventional therapy
5. If metabolically controlled but still progressing:
   • Consider guided growth (8-plates) if mild
   • Osteotomy if severe
   • Ensure at least 6 months of optimised medical treatment first

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Related Topics

• Nutritional Rickets
• Vitamin D-Dependent Rickets
• Renal Osteodystrophy
• Paediatric Lower Limb Deformity
• Genu Varum/Valgum
• Blount's Disease
• Metabolic Bone Disease