Define Osteomalacia. Describe its causes, clinical features, investigations and management.

Definition

Osteomalacia is the adult equivalent of rickets: defective mineralisation of the mature bone matrix (osteoid) after the growth plates have closed. There is an excess of unmineralised osteoid, so the bone is soft and weak but not reduced in quantity — the problem is the quality (mineralisation) of bone, in contrast to osteoporosis, where the quantity is reduced but mineralisation is normal.

Causes

As with rickets, the dominant cause is vitamin D deficiency (poor sunlight, poor diet, malabsorption — e.g. Coeliac disease, gastric surgery). Other causes include chronic renal failure (impaired activation of vitamin D and phosphate handling), hypophosphataemia (including tumour-induced osteomalacia), certain anticonvulsants, and rare enzyme defects.

Clinical Features

The presentation is often insidious: diffuse bone pain and tenderness (especially in the spine, pelvis and legs), proximal muscle weakness causing a waddling gait and difficulty rising from a chair or climbing stairs, and fractures/pseudofractures. Hypocalcaemia may produce paraesthesiae or tetany.

Investigations

Biochemistry mirrors rickets: low/low-normal calcium, low phosphate, raised alkaline phosphatase, low vitamin D and raised PTH. Radiographs show generalised osteopenia and the characteristic Looser’s zones. A bone biopsy (rarely needed) shows widened osteoid seams.

Management

Treatment is vitamin D and calcium replacement and correction of the underlying cause (e.g. Treating malabsorption, phosphate supplementation, managing renal disease). Symptoms and biochemistry improve, and pseudofractures heal, with appropriate replacement. Established deformity is uncommon in adults but may require orthopaedic management.

Complications

Untreated osteomalacia leads to persistent bone pain, disabling proximal myopathy, and recurrent fractures and pseudofractures; hypocalcaemia can cause tetany and, rarely, seizures. In the elderly it contributes to falls and fractures and is easily overlooked as ‘osteoporosis’ unless the biochemistry is checked. With adequate vitamin D and calcium replacement the prognosis is excellent — pain resolves, muscle strength returns and pseudofractures heal.

Distinguishing Osteomalacia from Osteoporosis

Although both weaken bone and cause fractures in the elderly, the two are fundamentally different and are separated by simple biochemistry. In osteoporosis the calcium, phosphate and alkaline phosphatase are normal and the problem is a reduced quantity of normally mineralised bone; in osteomalacia the calcium and phosphate are low, the alkaline phosphatase and PTH are raised, and the vitamin D is low, reflecting defective mineralisation. Clinically, osteomalacia is distinguished by its diffuse bone pain and proximal myopathy, and radiologically by Looser’s zones — features absent in uncomplicated osteoporosis. Making this distinction matters because osteomalacia is corrected with vitamin D and calcium, not anti-resorptive drugs.

Osteoid is normal in amount but poorly mineralised — opposite of osteoporosis.

• Adult equivalent of rickets: defective mineralisation of mature osteoid → soft bone.
• Usually vitamin D deficiency; also renal failure, malabsorption, hypophosphataemia.
• Bone pain, proximal myopathy (waddling gait), fractures; ↓Ca, ↓PO₄, ↑ALP, ↑PTH.
• Looser’s zones (pseudofractures) are pathognomonic.
• Treat with vitamin D + calcium and the underlying cause; distinguish from osteoporosis biochemically.