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Rickets (in children) and osteomalacia (in adults) result from inadequate mineralization of bone matrix (osteoid). They are two clinical manifestations of the same disease which is usually caused by a defect in vitamin D availability or metabolism.
Cause of Rickets
Vitamin D deficiency is usually due to inadequate sunlight exposure, particularly in Asian women in Western countries whose clothing covers their skin, and whose diet may contain only small amounts of vitamin D. The elderly who are immobile and housebound similarly are not exposed to sunlight. Anticonvulsant therapy may affect vitamin D metabolism; these drugs are also toxic to osteoblasts. Malabsorption of vitamin D rarely occurs in gastrointestinal disease.
Pathology
In children the growth plate is elongated with distortion of the arrangement of chondrocytes. Calcification is delayed and vascularization impaired. In adults, osteomalacia is characterized by increased osteoid width (> 15 µm) and delayed mineralization assessed by double-tetracycline labelling. Unmineralized osteoid may cover up to 100% of trabecular bone surface (normal < 27%).
Symptoms and clinical features of Rickets
At birth, neonatal rickets may present as craniotabes (thin deformed skull). In the first few years of life there may be widened epiphyses at the wrists and beading at the costochondral junctions, producing the 'rickety rosary', or a groove in the rib cage (Harrison's sulcus). In older children, lower limb deformities are seen. A myopathy also occurs.
Investigation
- Increased serum alkaline phosphatase , indicating increased osteoblast activity, is the most common abnormality (note: alkaline phosphatase is elevated during skeletal growth).
- Plasma calcium is low or normal, in association with secondary hyperparathyroidism and a raised PTH.
- Serum phosphate may be low, owing to increased PTH-dependent phosphaturia, though this is variable.
- Serum 25-hydroxyvitamin D 3 is usually low (exceptions being vitamin-D-resistant rickets).
- X-rays are often normal in adults, but may show defective mineralization, especially in the pelvis, long bones and ribs, with 'Looser's zones' - linear areas of low density surrounded by sclerotic borders.
- Iliac crest biopsy with double tetracycline labelling (see above) is occasionally necessary if biochemical tests are equivocal.
Treatment of Rickets
Multiple formulations of vitamin D and its metabolites are available. When deficiency is nutritional, 'replacement' doses are needed (400-800 IU daily). Much higher 'pharmacological' doses (up to 40 000-100 000 IU) may be needed in patients with gastrectomy, malabsorption, liver disease or hypoparathyroidism. However, some patients respond to more conventional doses. Experts should initiate such treatment and all patients receiving pharmacological doses of vitamin D should have their serum calcium measured regularly; excessive dosage presents with the features of hypercalcaemia, often with nausea and vomiting.
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