Diagnostic evaluation of adults with hyponatremia
- Richard H Sterns, MD
Richard H Sterns, MD
- Editor-in-Chief — Nephrology
- Section Editor — Fluid and Electrolytes
- Professor Emeritus
- University of Rochester School of Medicine and Dentistry
Hyponatremia, defined as a serum sodium concentration below 135 meq/L, is usually caused by a failure to excrete water normally [1,2]. In healthy individuals, the ingestion of water does not lead to hyponatremia because suppressed release of antidiuretic hormone (ADH), also called vasopressin, allows excess water to be excreted in a dilute urine (figure 1).
Renal water excretion is impaired in most patients who develop hyponatremia, usually due to an inability to suppress ADH secretion. An uncommon exception occurs in psychotic patients with primary polydipsia who drink such large quantities of fluid that, despite appropriately suppressed ADH release, the excretory capacity of the kidney is overwhelmed.
The diagnostic approach to the patient with hyponatremia will be reviewed here. Many patients with hyponatremia have a single cause, but multiple factors sometimes contribute to the fall in plasma sodium. As an example, when a patient infected with the human immunodeficiency virus (HIV) becomes hyponatremic, volume depletion, the syndrome of inappropriate ADH secretion (SIADH), and adrenal insufficiency all may be present. (See "Electrolyte disturbances with HIV infection".)
The causes and treatment of hyponatremia are discussed separately. (See "Causes of hyponatremia in adults" and "Overview of the treatment of hyponatremia in adults" and "Treatment of hyponatremia: Syndrome of inappropriate antidiuretic hormone secretion (SIADH) and reset osmostat".)
THE INITIAL EVALUATION
The initial diagnostic approach to the adult patient with hyponatremia consists of a directed history and physical examination as well as selected laboratory tests (see 'History and physical examination' below and 'Tests that are often initially available' below). When hyponatremia is first discovered, some elements of the history, key features of the physical exam, and the results of several helpful laboratory tests are usually already available, and these guide the subsequent diagnostic approach (algorithm 1).
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- THE INITIAL EVALUATION
- History and physical examination
- Tests that are often initially available
- When to measure the serum osmolality
- - Ineffective osmoles
- PATIENTS WHO MIGHT HAVE PSEUDOHYPONATREMIA
- Patients with lipemic serum
- Patients with obstructive jaundice
- Patients with a plasma cell dyscrasia
- PATIENTS WHO MIGHT HAVE ISOTONIC OR HYPERTONIC HYPONATREMIA
- Hyperglycemic patients
- Patients with recent prostate or uterine surgery
- Patients given mannitol or intravenous immune globulin
- PATIENTS WITH HYPOTONIC HYPONATREMIA
- Patients with severely reduced GFR
- Patients taking thiazides
- Other patients
- - Patients with edema and/or ascites
- - Nonedematous patients
- Apparent hypovolemia
- Apparent euvolemia
- - Low urine sodium and osmolality
- - High urine sodium and osmolality
- - Low urine sodium with high urine osmolality
- - Abnormal serum potassium and bicarbonate
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