Idiopathic hypercalciuria: Can we prevent stones and protect bones?
ABSTRACT
Idiopathic hypercalciuria increases the risk of urinary stones and osteoporosis. The aim of this review is to delineate our current understanding of idiopathic hypercalciuria in the context of bone health, specifically its definition, causes, epidemiology, laboratory evaluation, and potential treatments.
KEY POINTS
- Idiopathic hypercalciuria is common in patients with kidney stones and is also present in up to 20% of postmenopausal women with osteoporosis but no history of kidney stones.
- Idiopathic hypercalciuria has been directly implicated as a cause of loss of trabecular bone, especially in men. But reversing the hypercalciuria in this condition has not been definitively shown to diminish fracture incidence.
- Patients with kidney stones who have low bone mass and idiopathic hypercalciuria should increase their daily fluid intake, follow a diet low in salt and animal protein, and take thiazide diuretics to reduce the risk of further calcium stone formation. Whether this approach also improves bone mass and strength and reduces fracture risk in this patient group requires further study.
Potassium citrate
When prescribing thiazide diuretics, one should also consider prescribing potassium citrate, as this agent not only prevents hypokalemia but also increases urinary citrate excretion, which can help to inhibit crystallization of calcium salts.6
In a longitudinal study of 28 patients with hypercalciuria,53 combined therapy with a thiazide or indapamide and potassium citrate over a mean of 7 years increased bone density of the lumbar spine by 7.1% and of the femoral neck by 4.1%, compared with treatment in age- and sex-matched normocalcemic peers. In the same study, daily urinary calcium excretion decreased and urinary pH and citrate levels increased; urinary saturation of calcium oxalate decreased by 46%, and stone formation was decreased.
Another trial evaluated 120 patients with idiopathic calcium nephrolithiasis, half of whom were given potassium citrate. Those given potassium citrate experienced an increase in distal radius bone mineral density over 2 years.54 It is theorized that alkalinization may decrease bone turnover in these patients.
Bisphosphonates
As one of the proposed main mechanisms of bone loss in idiopathic hypercalciuria is direct bone resorption, a potential target for therapy is the osteoclast, which bisphosphonates inhibit.
Ruml et al55 studied the impact of alendronate vs placebo in 16 normal men undergoing 3 weeks of strict bedrest. Compared with the placebo group, those who received alendronate had significantly lower 24-hour urine calcium excretion and higher levels of PTH and 1,25-dihydroxyvitamin D.
Weisinger et al56 evaluated the effects of alendronate 10 mg daily in 10 patients who had stone disease with documented idiopathic hypercalciuria and also in 8 normocalciuric patients without stone disease. Alendronate resulted in a sustained reduction of calcium in the urine in the patients with idiopathic hypercalciuria but not in the normocalciuric patients.
Data are somewhat scant as to the effect of bisphosphonates on bone health in the setting of idiopathic hypercalciuria,57,58 and therapy with bisphosphonates is not recommended in patients with idiopathic hypercalciuria outside the realm of postmenopausal osteoporosis or other indications for bisphosphonates approved by the US Food and Drug Administration (FDA).
Calcimimetics
Calcium-sensing receptors are found not only in parathyroid tissue but also in the intestines and kidneys. Locally, elevated plasma calcium in the kidney causes activation of the calcium-sensing receptor, diminishing further calcium reabsorption.59 Agents that increase the sensitivity of the calcium-sensing receptors are classified as calcimimetics.
Cinacalcet is a calcimimetic approved by the FDA for treatment of secondary hyperparathyroidism in patients with chronic kidney disease on dialysis, for the treatment of hypercalcemia in patients with parathyroid carcinoma, and for patients with primary hyperparathyroidism who are unable to undergo parathyroidectomy. In an uncontrolled 5-year study of cinacalcet in patients with primary hyperparathyroidism, there was no significant change in bone density.60
Anti-inflammatory drugs
The role of cytokines in stimulating bone resorption in idiopathic hypercalciuria has led to the investigation of several anti-inflammatory drugs (eg, diclofenac, indomethacin) as potential treatments, but studies have been limited in number and scope.61,62
Omega-3 fatty acids
Omega-3 fatty acids are thought to alter prostaglandin metabolism and to potentially reduce stone formation.63
A retrospective study of 29 patients with stone disease found that, combined with dietary counseling, omega-3 fatty acids could potentially reduce urinary calcium and oxalate excretion and increase urinary citrate in hypercalciuric stone-formers.64
A review of published randomized controlled trials of omega-3 fatty acids in skeletal health discovered that 4 studies found positive effects on bone mineral density or bone turnover markers, whereas 5 studies reported no differences. All trials were small, and none evaluated fracture outcome.65
