Disorders of plasma sodium.

To the Editor: The review by Sterns (Jan. 1 issue)1 on disorders of plasma sodium concentration, which are among the most frequently encountered (and mismanaged) problems in medicine, was enhanced by the use of illustrative cases (which appear in the Supplementary Appendix, available with the full text of the article at NEJM.org). After the second patient had a subarachnoid hemorrhage, plasma hyponatremia associated with excessive urinary sodium loss developed. Did this patient really have the syndrome of inappropriate antidiuretic hormone secretion (SIADH), or did he have cerebral salt wasting, which is also seen in the clinical situation described? Information on the water balance (hypovolemic, euvolemic, or hypervolemic) would be helpful here, because cerebral salt wasting is associated with hypovolemia. The differential diagnosis of SIADH versus cerebral salt wasting is clinically relevant because the treatments are diametrically opposed.2 Axel Petzold M.D., Ph.D.