Hyponatremia 

  • Author: Eric E Simon, MD; Chief Editor: Vecihi Batuman, MD, FACP, FASN   more...
 
Updated: Mar 6, 2012
 

Background

Hyponatremia is an important and common electrolyte abnormality that can be seen in isolation or, as most often is the case, as a complication of other medical illnesses.

Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis is vital to the normal physiological function of cells. The normal serum sodium level is 135-145 mEq/L. Hyponatremia is defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L.

This article reviews the epidemiology, pathophysiology, differential diagnosis, evaluation, and treatment of this disorder.[1, 2]

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Pathophysiology

Hypoosmolality (serum osmolality < 280 mOsm/kg) always indicates excess total body water relative to body solutes or excess water relative to solute in the extracellular fluid (ECF), as water moves freely between the intracellular and the extracellular compartments. This imbalance can be due to solute depletion, solute dilution, or a combination of both.

Under normal conditions, renal handling of water is sufficient to excrete as much as 15-20 L of free water per day. Further, the body's response to a decreased osmolality is decreased thirst. Thus, hyponatremia can occur only when some condition impairs normal free water excretion.[3] Generally, hyponatremia is of clinical significance only when it reflects a drop in the serum osmolality (ie, hypotonic hyponatremia), which is measured directly via osmometry or is calculated as 2(Na) mEq/L + serum glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Note that urea is not an effective osmole, so when the urea levels are very high, the measured osmolality should be corrected for the contribution of urea.

The recommendations for treatment of hyponatremia rely on the current understanding of CNS adaptation to an altered serum osmolality. In the setting of an acute drop in the serum osmolality, neuronal cell swelling occurs due to the water shift from the extracellular space to the intracellular space (ie, Starling forces). Swelling of the brain cells elicits the following 2 osmoregulatory responses:

  • It inhibits both arginine vasopressin secretion from neurons in the hypothalamus and hypothalamic thirst center. This leads to excess water elimination as dilute urine.
  • There is an immediate cellular adaptation with loss of electrolytes, and over the next few days, there is a more gradual loss of organic intracellular osmolytes.[4]

Therefore, correction of hyponatremia must take into account the chronicity of the condition. Acute hyponatremia (duration < 48 h) can be safely corrected more quickly than chronic hyponatremia. Correction of serum sodium that is too rapid can precipitate severe neurologic complications. Most individuals who present for diagnosis, versus individuals who develop it while in an inpatient setting, have had hyponatremia for some time, so the condition is chronic, and correction should proceed accordingly.

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Epidemiology

Frequency

United States

The incidence of hyponatremia depends largely on the patient population and the criteria used to establish the diagnosis. A hospital incidence is high (15-20% as defined as a serum sodium level of < 135 mEq/L), while only 1-4% of patients who are hospitalized have a serum sodium level of less than 130 mEq/L. Hyponatremia's prevalence is lower in the ambulatory setting.

Mortality/Morbidity

Severe hyponatremia (< 125 mEq/L) has a high mortality rate. In instances when the serum sodium level is less than 105 mEq/L, the mortality is over 50%, especially in alcoholics.[5] In patients with acute ST-elevation myocardial infarction, the presence of hyponatremia on admission or early development of hyponatremia is an independent predictor of 30-day mortality, and the prognosis worsens with the severity of hyponatremia.[6] Similarly, cirrhotic patients with persistent ascites and a low serum sodium level awaiting transplant have a high mortality risk despite low severity (MELD) scores. The independent predictors - ascites and hyponatremia - are findings indicative of hemodynamic decompensation.[7, 8]

Race

Hyponatremia affects all races.

Sex

No sexual predilection exists for hyponatremia. However, symptoms are more likely to occur in young women than in men.

Age

Hyponatremia is more common in elderly persons, because they have an increased incidence of comorbid conditions (eg, cardiac, hepatic, or renal failure) that can be complicated by it.

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Contributor Information and Disclosures
Author

Eric E Simon, MD  Professor of Medicine, Chief, Section of Nephrology and Hypertension, Tulane University School of Medicine; Director, Nephrology Training, Medical Director, Dialysis Clinic, Inc, Canal Street

Eric E Simon, MD is a member of the following medical societies: American Federation for Medical Research, American Heart Association, American Society for Cell Biology, American Society of Nephrology, Association for Psychological Science, Central Society for Clinical Research, International Society of Nephrology, National Kidney Foundation, Phi Beta Kappa, and Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Coauthor(s)

Seyed Mehrdad Hamrahian, MD  Assistant Professor of Medicine

Seyed Mehrdad Hamrahian, MD is a member of the following medical societies: American Society of Nephrology and National Kidney Foundation

Disclosure: Nothing to disclose.

Federico J Teran, MD  Instructor of Clinical Medicine, Section of Nephrology and Hypertension, Tulane University School of Medicine

Federico J Teran, MD is a member of the following medical societies: American College of Physicians, American Medical Association, American Society of Nephrology, Louisiana State Medical Society, National Kidney Foundation, and Renal Physicians Association

Disclosure: Nothing to disclose.

Specialty Editor Board

James H Sondheimer, MD, FACP  Associate Professor of Medicine, Wayne State University School of Medicine; Medical Director of Hemodialysis, Harper University Hospital at Detroit Medical Center; Medical Director, DaVita Greenview Dialysis (Southfield)

James H Sondheimer, MD, FACP is a member of the following medical societies: American College of Physicians and American Society of Nephrology

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Eleanor Lederer, MD  Professor of Medicine, Chief, Nephrology Division, Director, Nephrology Training Program, Director, Metabolic Stone Clinic, Kidney Disease Program, University of Louisville School of Medicine; Consulting Staff, Louisville Veterans Affairs Hospital

Eleanor Lederer, MD is a member of the following medical societies: American Association for the Advancement of Science, American Federation for Medical Research, American Society for Biochemistry and Molecular Biology, American Society for Bone and Mineral Research, American Society of Nephrology, American Society of Transplantation, International Society of Nephrology, Kentucky Medical Association, National Kidney Foundation, and Phi Beta Kappa

Disclosure: Dept of Veterans Affairs Grant/research funds Research

Rebecca J Schmidt, DO, FACP, FASN  Professor of Medicine, Section Chief, Department of Medicine, Section of Nephrology, West Virginia University School of Medicine

Rebecca J Schmidt, DO, FACP, FASN is a member of the following medical societies: American College of Physicians, American Medical Association, American Society of Nephrology, International Society of Nephrology, National Kidney Foundation, Renal Physicians Association, and West Virginia State Medical Association

Disclosure: Renal Ventures Ownership interest Other

Chief Editor

Vecihi Batuman, MD, FACP, FASN  Professor of Medicine, Section of Nephrology-Hypertension, Tulane University School of Medicine; Chief, Medicine Service, Southeast Louisiana Veterans Health Care System

Vecihi Batuman, MD, FACP, FASN is a member of the following medical societies: American College of Physicians, American Society of Hypertension, American Society of Nephrology, and International Society of Nephrology

Disclosure: Nothing to disclose.

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