Hyponatremia and bone: an emerging relationship

Hyponatremia is the most common electrolyte disorder and is mainly known for its neurological complications. New studies suggest previously unrecognized complications of hyponatremia, including falls, osteoporosis and fractures. Because these novel associations are mainly derived from epidemiological studies, it remains unclear whether hyponatremia has a direct effect on bone or whether it is a surrogate marker of another etiology. However, one animal and one in vitro study now show that hyponatremia can have direct effects on bone, mainly via activation of osteoclasts. The association between hyponatremia and fractures appears to be independent of osteoporosis (defined as low BMD). Also, data suggest that this association cannot be fully explained by the possibility that hyponatremia predisposes to falls. Hyponatremia, therefore, also has an effect on bone quality that is not captured by BMD. Here, the emerging relationship between hyponatremia and bone is reviewed, with special emphasis on possible mechanisms, unanswered questions and clinical implications.