Disorders of Water Metabolism

Disorders of water balance and serum Na ( S Na ) are very common in hospitalized patients [ 1 ] . In health, water balance and plasma osmolality, and thereby S Na , are tightly regulated by the kidney, which makes minute-by-minute adjustments to the composition of urine in order to maintain a near-constant plasma osmolality. The development of hyponatremia or hypernatremia may therefore re fl ect serious underlying illness and both conditions often suggest a poor prognosis for the underlying causative disease. Hypoand hypernatremia are associated with increased morbidity and, equally vexing to the clinician, inappropriate correction may itself cause serious morbidity or mortality. A careful therapeutic approach as well as close attention to the patient’s response are therefore important for the clinician caring for patients with these conditions. Hypoand hypernatremia are each the common manifestation of several distinct underlying diseases. Effective therapy depends upon an accurate evaluation of the underlying process and different treatment strategies are often necessary, even when patients present with the same serum sodium level. A patient presenting with a S Na of 115, for example, may be bene fi ted or harmed by hydration with saline, depending upon whether the hyponatremia was caused by volume contraction, congestive heart failure, or the syndrome of inappropriate secretion of anti discretion hormone (SIADH). Furthermore, water handling by the kidney can change quickly as the patient’s condition changes. A patient who is volume contracted and is concentrating his or her urine at presentation may subsequently start excreting free water in response to rehydration. This patient’s urine osmolality would initially be high but would fall in response to volume resuscitation. Perhaps the most important aspect of therapy, then, is continuous reassessment of patients and their response to therapy.