Osteomalacia

Osteomalacia means 'soft bones'. Bone is built as a protein scaffold that is then hardened by depositing calcium and phosphate — mineralisation. In osteomalacia this hardening step fails, so newly made bone stays soft and weak. The commonest cause by far is a lack of vitamin D, which the body needs to absorb calcium and phosphate from food; this can come from too little sunlight, poor diet, darker skin, covering the skin, malabsorption (as in coeliac disease or after gut surgery), or kidney and liver problems that stop vitamin D being activated. Less commonly it comes from low phosphate due to phosphate-wasting disorders (where FGF-23 is often involved), or from certain drugs. The same process in a growing child is called rickets. Symptoms build slowly: aching bone pain (often in the hips, legs, ribs and spine), muscle weakness that can make climbing stairs or rising from a chair hard and cause a waddling walk, and bones that fracture with little force. Blood tests typically show low or borderline vitamin D, low or low-normal calcium and phosphate, a raised alkaline phosphatase, and a raised PTH. Because the cause is usually a correctable deficiency, osteomalacia often responds well to treatment — replacing vitamin D (and calcium or phosphate as needed) — with real improvement in pain and strength.

• Vitamin D — usually low; the commonest underlying cause.
• Calcium — often low or low-normal.
• Phosphate — often low; markedly low in phosphate-wasting forms.
• Alkaline phosphatase (and bone-specific ALP) — typically raised.
• PTH — often raised (secondary hyperparathyroidism from low vitamin D/calcium).
• FGF-23 — high in phosphate-wasting forms.
• Kidney function — to identify a kidney cause.

• Aching bone pain, often in the hips, legs, ribs and lower back
• Muscle weakness, especially in the thighs and hips
• Difficulty climbing stairs or rising from a chair; a waddling walk
• Bones that fracture with minor force
• Sometimes muscle cramps or tingling if calcium is very low
• Tiredness