Osteomalacia

Osteomalacia is defective mineralisation of adult bone osteoid, with accumulation of unmineralised matrix.

• Causes: vitamin D deficiency (inadequate sun, malabsorption, coeliac, gastric bypass, liver/renal disease, anticonvulsants), phosphate wasting (X linked hypophosphataemia, tumour induced osteomalacia via FGF23, Fanconi syndrome, drugs such as tenofovir and iron infusions), hypophosphatasia, chronic acidosis, aluminium toxicity.
• Prevalence rising with low vitamin D levels (25 hydroxyvitamin D below 20 ng/mL).
• Complications: fractures, pseudofractures (Looser zones), proximal myopathy.

• Diffuse bone pain (ribs, spine, pelvis, legs), tenderness on pressure, proximal muscle weakness with waddling gait.
• Fractures with minimal trauma, tetany or paraesthesia (hypocalcaemia).
• Features of malabsorption or renal disease.

• Low 25 hydroxyvitamin D, low or normal calcium, low phosphate, raised alkaline phosphatase, raised PTH (secondary hyperparathyroidism), low urinary calcium.
• Phosphate wasting: low TmP/GFR, FGF23 level; tumour localisation with octreotide scan, DOTATATE PET, MRI.
• Radiographs: Looser zones (pseudofractures), biconcave vertebrae; DEXA shows low density; bone biopsy rarely required.
• Assess renal function, bicarbonate, coeliac serology.

• Vitamin D deficiency: cholecalciferol 50,000 IU weekly for 8 to 12 weeks (or 6000 IU daily), then 800 to 2000 IU daily maintenance; calcium 1000 to 1200 mg daily; higher doses for malabsorption; calcifediol in liver disease.
• Hypophosphataemic forms: phosphate 20 to 40 mg/kg/day elemental in 4 divided doses plus calcitriol 0.5 to 1.5 micrograms daily; burosumab 1 mg/kg every 4 weeks in XLH; resect or ablate tumour in tumour induced osteomalacia (burosumab if unresectable).
• Treat renal tubular acidosis with alkali (sodium bicarbonate or citrate).
• Avoid bisphosphonates until deficiency is corrected.
• Monitor calcium, phosphate, ALP, PTH every 3 months; symptoms improve in weeks, bone healing in months.