Osteomalacia
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Direct answer
Osteomalacia is a disorder of adult bone in which newly formed osteoid fails to mineralise, so the bone is soft even though its total mass may be normal. Vitamin D deficiency is the commonest cause, although phosphate deficiency and renal tubular disorders also produce it. Its characteristic features are bone pain, proximal muscle weakness and pseudofractures known as Looser zones.
What you must remember
- Defect: osteoid accumulates because mineralisation of new matrix is impaired, with wide osteoid seams on bone histology.
- Commonest cause: vitamin D deficiency from poor sunlight exposure or intake, which remains common in India.
- Biochemistry in vitamin D deficiency: calcium low or low-normal, phosphate low, alkaline phosphatase raised, parathyroid hormone raised and 25-hydroxy vitamin D low.
- Looser zones: radiolucent bands perpendicular to the cortex, typically at the femoral neck, pubic rami and scapula, and they represent pseudofractures.
- Milkman syndrome: an eponym for osteomalacia presenting with diffuse bone pain and pseudofractures.
- Other causes: hypophosphataemia, renal tubular acidosis, Fanconi syndrome and enzyme-inducing anticonvulsants that accelerate vitamin D breakdown.
- Treatment: correction of vitamin D and calcium deficiency, with phosphate replacement where indicated, and treatment of the underlying cause.
Viva angles on biochemistry and pseudofractures
Examiners often ask which biochemical change points to osteomalacia and what the Looser zone represents.
- Trap: treating reduced density as osteoporosis without checking calcium, phosphate and alkaline phosphatase. Correct: raised alkaline phosphatase with low phosphate points to osteomalacia, and bisphosphonates alone would miss the cause.
- Trap: expecting normal alkaline phosphatase in vitamin D deficiency. Correct: it is raised because osteoblastic activity increases in response to defective mineralisation, which is a key clue.
- Trap: calling Looser zones fractures that need fixation. Correct: they are pseudofractures that heal once vitamin D and phosphate are replete, and most patients need no surgery.
Frequently asked questions
What is the commonest cause of osteomalacia in adults?
Vitamin D deficiency is the commonest cause of osteomalacia in adults, arising from poor sunlight exposure, low dietary intake or malabsorption. It remains common in India, so serum 25-hydroxy vitamin D should be measured in patients with bone pain and muscle weakness.
Which biochemical findings distinguish osteomalacia from osteoporosis?
Osteomalacia typically shows low or low-normal calcium, low phosphate and raised alkaline phosphatase, whereas osteoporosis has normal values for all three. Vitamin D status and parathyroid hormone add further information in osteomalacia.
What are Looser zones in osteomalacia?
Looser zones are radiolucent pseudofractures that run perpendicular to the cortex, most often at the femoral neck, pubic rami and scapula. They reflect areas of defective mineralisation and usually heal once vitamin D and phosphate levels are corrected.
Why is alkaline phosphatase raised in osteomalacia?
Alkaline phosphatase is raised because osteoblasts are stimulated by the defective mineralisation, increasing bone turnover and enzyme release. A raised value therefore reflects active bone formation on unmineralised osteoid, which is a useful clue in the diagnosis.
How is osteomalacia from vitamin D deficiency treated?
Vitamin D and calcium are replaced to correct the deficiency, and phosphate is added where hypophosphataemia is present. The underlying cause, such as malabsorption or an enzyme-inducing drug, is also treated, and biochemistry is monitored during therapy.