Osteomalacia

Osteomalacia is a disorder of incomplete bone mineralisation that causes loss of bone mineral density (BMD) and increased bone fragility in adults. The symptoms are often minor and nonspecific (eg bone pain, muscle weakness), and can be absent altogether.

In developed countries, vitamin D deficiency accounts for most cases of osteomalacia in patients with normal or near-normal kidney function. It typically only occurs if the serum 25-hydroxyvitamin D concentration is lower than 20 nanomol/L. Specialist referral is recommended if osteomalacia is suspected.

Osteomalacia can also be caused by severe kidney disease (chronic kidney disease–mineral and bone disorder [CKD-MBD]) and hypophosphataemia (eg tumour-induced osteomalacia). More complex treatment regimens and specialist management are required for these patients.

Transiliac bone biopsy with double tetracycline is the gold-standard test for diagnosis of osteomalacia, but this test is invasive. A diagnosis of osteomalacia caused by vitamin D deficiency can be made based on the combination of:

• typical symptoms (bone and muscle pain)
• characteristic biochemical changes, including:
  • reduced serum 25-hydroxyvitamin D concentration
  • reduced serum calcium concentration
  • reduced serum phosphate concentration
  • increased serum parathyroid hormone concentration
  • increased serum alkaline phosphatase concentration
• reduced BMD.

A history of limited exposure to sunlight supports the diagnosis.

The aim of treating osteomalacia is to:

• relieve symptoms
• prevent complications and recurrence
• reverse the underlying causes (if possible)
• correct vitamin D deficiency, hypocalcaemia, hypophosphataemia and secondary hyperparathyroidism, and normalise serum alkaline phosphatase.

For osteomalacia caused by vitamin D deficiency, supplementation with colecalciferol reduces bone pain and increases muscle strength within weeks. For an adult with normal kidney function, use:

Continue treatment for at least 6 to 12 months, with biochemical testing (serum 25-hydroxyvitamin D, calcium, phosphate, parathyroid hormone and alkaline phosphatase concentrations) every 3 months, until all biochemical abnormalities are corrected. If the risk for vitamin D deficiency is ongoing, continue treatment indefinitely.

Higher oral colecalciferol doses are usually required in patients with gastrectomy or fat malabsorption, obese patients, and patients taking a drug (eg rifampicin, an antiepileptic) that alters metabolism and storage of 25-hydroxyvitamin D in the liver. Patients with intestinal malabsorption may need parenteral administration of vitamin D for maintenance of normal serum calcium concentration. Higher colecalciferol doses taken less frequently can be considered to improve adherence—these regimens may increase the risk of dosing errors, so are not routinely recommended first line.

Ensure all patients with osteomalacia maintain a total daily calcium intake of 1300 mg, since the combination of calcium and vitamin D prevents fractures (see Calcium for advice on achieving adequate calcium intake).