Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Practice Essentials, Background, Pathophysiology (original) (raw)

Overview

Practice Essentials

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) involves the continued secretion or action of arginine vasopressin (AVP) despite normal or increased plasma volume. The resulting impairment of water secretion and consequent water retention produces the hyponatremia (ie, serum Na+ < 135 mmol/L) with concomitant hypo-osmolality (serum osmolality < 280 mOsm/kg) and high urine osmolality that are the hallmark of SIADH. [1] The key to understanding the pathophysiology, signs, symptoms, and treatment of SIADH is the awareness that the hyponatremia results from an excess of water rather than a deficiency of sodium.

Signs and symptoms of SIADH

Depending on the magnitude and rate of development, hyponatremia may or may not cause symptoms. The history should take into account the following considerations:

After the identification of hyponatremia, the approach to the patient depends on the clinically assessed volume status. Prominent physical findings may be seen only in severe or rapid-onset hyponatremia and can include the following:

See Presentation for more detail.

Diagnosis of SIADH

In the absence of a single laboratory test to confirm the diagnosis, SIADH is best defined by the classic Bartter-Schwartz criteria, which can be summarized as follows [2, 3] :

The following laboratory tests may be helpful in the diagnosis of SIADH:

The patient’s volume should be assessed clinically to help rule out the presence of hypovolemia.

Imaging studies that may be considered include the following:

See Workup for more detail.

Management of SIADH

Treatment of SIADH and the rapidity of correction of hyponatremia depend on the following:

If the duration of hyponatremia is unknown and the patient is asymptomatic, it is reasonable to presume chronic SIADH. Diagnosis and treatment of the underlying cause of SIADH are also important.

In an emergency setting, aggressive treatment of hyponatremia should always be weighed against the risk of inducing central pontine myelinolysis (CPM). Such treatment is warranted as follows:

The goal is to correct hyponatremia at a rate that does not cause neurologic complications, as follows:

In an acute setting (< 48 hours since onset) where moderate symptoms are noted, treatment options for hyponatremia include the following:

In a chronic asymptomatic setting, the principal options are as follows:

See Treatment and Medication for more detail.

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Background

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most common cause of euvolemic hyponatremia in hospitalized patients. The syndrome is defined by the hyponatremia and hypo-osmolality that results from inappropriate, continued secretion and/or action of antidiuretic hormone despite normal or increased plasma volume, which results in impaired water excretion.

The antidiuretic hormone in humans and most mammals is arginine vasopressin (AVP). AVP promotes the reabsorption of water from the tubular fluid in the collecting duct, the hydro-osmotic effect, and it does not exert a significant effect on the rate of Na+ reabsorption. A second action of AVP is to cause arteriolar vasoconstriction and a rise in arterial blood pressure, the pressor effect. [4]

Physiology of AVP

AVP is an nonapeptide similar in structure to oxytocin. AVP is synthesized in the cell bodies of neurons in the supraoptic and paraventricular nuclei of the anterior hypothalamus as a precursor protein, the vasopressin-neurophysin 2–copeptin preprotein, which travels along the supraopticohypophyseal tract into the posterior pituitary. There, the preprotein is cleaved to AVP, neurophysin 2 and co-peptin and stored in secretory granules in association with a carrier protein, neurophysin, in the terminal dilatations of secretory neurons that rest against blood vessels. [5]

The major stimuli for AVP secretion are hyperosmolality and effective circulating volume depletion, which are sensed by osmoreceptors and baroreceptors, respectively. Osmoreceptors are specialized cells in the hypothalamus that perceive changes in the extracellular fluid (ECF) osmolality. Baroreceptors are located in the carotid sinus, aortic arch, and left atrium; these receptors participate in the nonosmolar control of AVP release by responding to a change in effective circulating volume.

Three known receptors bind AVP at the cell membrane of target tissues: V1a, V1b (also known as V3), and V2; these mediate AVP’s various effects.

V1a receptor is the vascular smooth muscle cell receptor but is also found on a number of other cells, such as hepatocytes, cardiac myocytes, platelets, brain, and testis. The V1a receptors signal by activation of phospholipase C and elevation in intracellular calcium, which, in turn, stimulates vasoconstriction. V1b (V3) receptors are found predominantly in the anterior pituitary, where they stimulate adrenocorticotropic hormone (ACTH) secretion.

V2 receptors are coupled to adenylate cyclase, causing a rise in intracellular cyclic adenosine monophosphate (cAMP), which serves as the second messenger. V2 receptors are found predominantly on the basolateral membrane of the principal cells of the connecting tubule and collecting duct of the distal nephron. [6]

Activation of the V2 receptor results in insertion of the water channel aquaporin-2 in the luminal membrane of the collecting duct, thus making it more permeable to water. Activation of the V2 receptors also increases urea and Na+ chloride reabsorption by the ascending limb of loop of Henle, thus increasing medullary tonicity and providing the osmotic gradient for maximal water absorption. [6] V2 receptors are also found in vascular endothelial cells and stimulate the release of von Willebrand factor. [6]

Normally, AVP secretion ceases when plasma osmolality falls below 275 mOsm/kg. This decrease causes increased water excretion, which leads to a dilute urine with an osmolality of 40-100 mOsm/kg. When plasma osmolality rises, AVP is secreted, which results in an increase in water reabsorption and an increase in urine osmolality to as much as 1400 mOsm/kg. An 8-10% reduction in circulating volume also significantly increases AVP release.

In most physiologic states, the volume receptors and osmoreceptors act in concert to increase or decrease AVP release. However, a reduction in actual or effective circulating volume is an overriding stimulus for secretion of AVP and takes precedence over extracellular osmolality when osmolality is normal or reduced. Finally, AVP is also released in response to stressful stimuli, such as pain or anxiety, and by various drugs. The released AVP is rapidly metabolized in the liver and kidneys and has a half-life of 15-20 minutes.

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Pathophysiology

The key to understanding the pathophysiology, signs, symptoms, and treatment of SIADH is the awareness that the hyponatremia in this syndrome is a result of an excess of water and not a deficiency of Na+.

SIADH consists of hyponatremia, inappropriately elevated urine osmolality (>100 mOsm/kg), and decreased serum osmolality in a euvolemic patient. SIADH should be diagnosed when these findings occur in the setting of otherwise normal cardiac, renal, adrenal, hepatic, and thyroid function; in the absence of diuretic therapy; and in absence of other factors known to stimulate ADH secretion, such as hypotension, severe pain, nausea, and stress.

In general, the plasma Na+ concentration is the primary osmotic determinant of AVP release. In persons with SIADH, the nonphysiological secretion of AVP results in enhanced water reabsorption, leading to dilutional hyponatremia. While a large fraction of this water is intracellular, the extracellular fraction causes volume expansion. Volume receptors are activated and natriuretic peptides are secreted, which causes natriuresis and some degree of accompanying potassium excretion (kaliuresis). Eventually, a steady state is reached and the amount of Na+ excreted in the urine matches Na intake.

Ingestion of water is an essential prerequisite to the development of the syndrome. Regardless of cause, hyponatremia does not occur if water intake is severely restricted.

In addition to the inappropriate AVP secretion, persons with this syndrome may also have an inappropriate thirst sensation, which leads to an intake of water that is in excess of free water excreted. This increase in water ingested may contribute to the maintenance of hyponatremia.

Neurologic manifestations

Neurologic complications in SIADH occur as a result of the brain's response to changes in osmolality. Hyponatremia and hypo-osmolality lead to acute edema of the brain cells. The rigid calvaria prevent expansion of brain volume beyond a certain point, after which the brain cells must adapt to persistent hypo-osmolality. However, a rapid increase in brain water content of more than 5-10% leads to severe cerebral edema and herniation and is fatal.

In response to a decrease in osmolality, brain ECF fluid moves into the CSF. The brain cells then lose potassium and intracellular organic osmolytes (amino acids, such as glutamate, glutamine, taurine, polyhydric alcohol, myoinositol, methylamine, and creatinine). This occurs concurrently to prevent excessive brain swelling. [7]

Following correction of hyponatremia, the adaptive process does not match the extrusion kinetics. Electrolytes rapidly reaccumulate in the brain ECF within 24 hours, resulting in a significant overshoot above normal brain contents within the first 48 hours after correction. Organic osmolytes return to normal brain content very slowly over 5-7 days. Electrolyte brain content returns to normal levels by the fifth day after correction, when organic osmolytes return to normal.

Irreversible neurologic damage and death may occur when the rate of correction of Na+ exceeds 0.5 mEq/L/h for patients with severe hyponatremia. At this rate of correction, osmolytes that have been lost in defense against brain edema during the development of hyponatremia cannot be restored as rapidly when hyponatremia is rapidly corrected. The brain cells are thus subject to osmotic injury, a condition termed osmotic demyelination. Certain factors such as hypokalemia, severe malnutrition, and advanced liver disease predispose patients to this devastating complication. [7]

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Etiology

SIADH is most often caused by either inappropriate hypersecretion of ADH from its normal hypothalamic source or by ectopic production. The causes of SIADH can be divided into four broad categories: nervous system disorders, neoplasia, pulmonary diseases, and drug induced (which include those that [1] stimulate AVP release, [2] potentiate effects of AVP action, or [3] have an uncertain mechanism).

Disorders causing SIADH

Nervous system disorders are as follows:

Neoplastic disorders are as follows:

Pulmonary disorders are as follows:

Miscellaneous causes are as follows:

Drugs causing SIADH

The list of drugs that can induce SIADH is long. However, a study of 146 cases of drug‐associated SIADH found that the following five drug classes were implicated in 82.3% of patients [11] :

Many chemotherapeutic drugs cause nausea, which is a powerful stimulus of vasopressin secretion. SIADH is also a leading cause of hyponatremia in children following chemotherapy or stem cell transplantation.

Drugs that stimulate AVP release are as follows:

Drugs that potentiate the effects of AVP action (primarily facilitate peripheral action of ADH) are as follows:

Drugs with an uncertain mechanism are as follows:

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Epidemiology

Occurrence in the United States

Hyponatremia is the most common electrolyte derangement occurring in hospitalized patients. When defined as plasma Na+ concentration of less than 135 mEq/L, the prevalence of hyponatremia in hospitalized patients has been reported in different studies as ranging from 2.5% to 30%. [13, 14, 15, 16] In the majority of cases, the hyponatremia was hospital acquired or aggravated by the hospitalization and may have been secondary to the administration of hypotonic intravenous (IV) fluids. [13] SIADH can also arise postoperatively from stress, pain, and medications used. However, not all hospital-acquired hyponatremia is SIADH and SIADH should be differentiated from the hyponatremia that occurs in patients with limited capacity to excrete free water, such as those with chronic kidney disease.

Sex- and age-related demographics

Increasing age (> 30 y) is a risk factor for hyponatremia in hospitalized patients. [16] Men appear to be more likely to develop mild or moderate, but not severe, hyponatremia. [16] Low body weight is also a risk factor for hyponatremia. Women appear to be more prone to drug-induced hyponatremia and to exercise-induced hyponatremia (marathon runners), although this may be an association with low body weight rather than sex. [6]

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Prognosis

The prognosis of SIADH correlates with the underlying cause and to the effects of severe hyponatremia and its overzealous correction. Rapid and complete recovery tends to be the rule with drug-induced SIADH when the offending agent is withdrawn. Successful treatment of pulmonary or CNS infection also can lead to correction of SIADH. However, patients who present with neurologic symptoms or have severe hyponatremia even without symptoms may develop permanent neurologic impairment. Patients whose serum Na+ is rapidly corrected, especially those who are asymptomatic, can also develop permanent neurologic impairment from central pontine myelinolysis (CPM).

Complications

The following complications are noted in SIADH:

Morbidity and mortality

Previously, mild hyponatremia was considered relatively asymptomatic. However, evidence suggests that even mild hyponatremia can cause significant impairment, such as unsteady gait, and lead to frequent falls. This effect may be greater in elderly persons, who are more sensitive to changes in serum Na+. [20] Hyponatremia may also be a risk factor for osteoporosis and bone fracture. [21]

The mortality of patients with hyponatremia (Na+< 130 mEq/L) is increased 60-fold compared with that of patients without documented hyponatremia, although this may be partly related to their comorbid conditions rather than to the hyponatremia itself. Predictors for higher morbidity and mortality rates include being hospitalized, acute onset, and severity of hyponatremia. [15] When the Na+ concentration drops below 105 mEq/L, life-threatening complications are much more likely to occur. [19]

In a retrospective case note review by Clayton and colleagues, patients with a multifactorial cause for hyponatremia in an inpatient setting had significantly higher mortality rates. [22] The etiology of hyponatremia was a more important prognostic indicator than the level of absolute serum Na+ in the patients. The outcome was least favorable in patients with normal sodium levels on admission who became hyponatremic during the course of their hospitalization.

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Author

Christie P Thomas, MBBS, FRCP, FASN, FAHA Professor, Department of Internal Medicine, Division of Nephrology, Departments of Pediatrics and Obstetrics and Gynecology, Medical Director, Kidney and Kidney/Pancreas Transplant Program, University of Iowa Hospitals and Clinics

Christie P Thomas, MBBS, FRCP, FASN, FAHA is a member of the following medical societies: American College of Physicians, American Heart Association, American Society of Nephrology, Royal College of Physicians

Disclosure: Nothing to disclose.

Specialty Editor Board

Eleanor Lederer, MD, FASN 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, FASN is a member of the following medical societies: American Association for the Advancement of Science, 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

Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: American Society of Nephrology
Received income in an amount equal to or greater than $250 from: Healthcare Quality Strategies, Inc.

Chief Editor

Vecihi Batuman, MD, FASN Professor of Medicine, Section of Nephrology-Hypertension, Deming Department of Medicine, Tulane University School of Medicine

Vecihi Batuman, MD, FASN is a member of the following medical societies: American College of Physicians, American Society of Hypertension, American Society of Nephrology, Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Additional Contributors

Mony Fraer, MD, MHCDS, FACP, FASN Associate Professor, Division of Nephrology, Department of Medicine, University of Iowa Hospitals and Clinics; Staff Physician, Iowa City Veterans Affairs Medical Center

Disclosure: Nothing to disclose.

Acknowledgements

Howard A Bessen, MD Professor of Medicine, Department of Emergency Medicine, UCLA School of Medicine; Program Director, Harbor-UCLA Medical Center

Howard A Bessen, MD is a member of the following medical societies: American College of Emergency Physicians

Disclosure: Nothing to disclose.

Keenan Bora, MD Fellow, Medical Toxicology, Detroit Medical Center; Attending Physician, Medical Center Emergency Services, Detroit

Keenan Bora, MD is a member of the following medical societies: American Academy of Clinical Toxicology, American Academy of Emergency Medicine, American College of Emergency Physicians, American College of Medical Toxicology, and American Medical Association

Disclosure: Nothing to disclose.

Meher Chaudhry, MD Chief Resident, Department of Emergency Medicine, Detroit Receiving Hospital, University Health Center

Disclosure: Nothing to disclose.

Sonali Deshmukh, MBBS Consulting Staff, Omaha Nephrology, Nebraska

Sonali Deshmukh, MBBS is a member of the following medical societies: American Society of Nephrology

Disclosure: Nothing to disclose.

R obert J Ferry Jr, MD Chief, Division of Pediatric Endocrinology and Metabolism, Le Bonheur Children's Hospital; Professor, Department of Pediatrics, University of Tennessee Health Science Center at Memphis; St. Jude Children's Research Hospital, Memphis, TN; Brigade Surgeon, 36th Sustainment Brigade, U.S. Army; Adjunct Professor, Pediatric Surgery Department, King Saud University, Riyadh, Saudi Arabia

Robert J Ferry Jr, MD is a member of the following medical societies: American Academy of Pediatrics, American Diabetes Association, American Medical Association, Endocrine Society, Lawson-Wilkins Pediatric Endocrine Society, Society for Pediatric Research, and Texas Pediatric Society

Disclosure: Nutropin Speakers Bureau Honoraria Speaking and teaching; Genotropin Speakers Bureau Honoraria Speaking and teaching; Eli Lilly & Co. Grant/research funds Investigator; MacroGenics, Inc. Grant/research funds Investigator; Ipsen, S.A. (formerly Tercica, Inc.) Grant/research funds Investigator; NovoNordisk SA Grant/research funds Investigator; Diamyd Investigator

Stephen Kemp, MD, PhD Professor, Department of Pediatrics, Section of Pediatric Endocrinology, University of Arkansas College of Medicine and Arkansas Children's Hospital

Stephen Kemp, MD, PhD is a member of the following medical societies: American Academy of Pediatrics, American Association of Clinical Endocrinologists, American Pediatric Society, Endocrine Society, Phi Beta Kappa, Southern Medical Association, and Southern Society for Pediatric Research

Disclosure: Nothing to disclose.

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

Lynne Lipton Levitsky, MD Chief, Pediatric Endocrine Unit, Massachusetts General Hospital; Associate Professor of Pediatrics, Harvard Medical School

Lynne Lipton Levitsky, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics, American Diabetes Association, American Pediatric Society, Endocrine Society, Lawson-Wilkins Pediatric Endocrine Society, and Society for Pediatric Research

Disclosure: Pfizer Grant/research funds P.I.; Tercica Grant/research funds Other; Eli Lily Grant/research funds PI; NovoNordisk Grant/research funds PI

Chike Magnus Nzerue, MD Associate Dean for Clinical Affairs, Vice-Chairman of Internal Medicine, Meharry Medical College

Chike Magnus Nzerue, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Society of Nephrology, and National Kidney Foundation

Disclosure: Nothing to disclose.

Jose F Pascual-y-Baralt, MD Chief, Division of Pediatric Nephrology, San Antonio Military Pediatric Center; Clinical Professor, Department of Pediatrics, University of Texas Health Science Campus

Jose F Pascual-y-Baralt, MD is a member of the following medical societies: American Academy of Pediatrics, American Society of Nephrology, American Society of Pediatric Nephrology, Association of Military Surgeons of the US, and International Society of Nephrology

Disclosure: Nothing to disclose.

Alexandr Rafailov, MD Staff Physician, Department of Emergency Medicine, State University of New York Downstate/Kings County Hospital

Disclosure: Nothing to disclose.

Arlan L Rosenbloom, MD Adjunct Distinguished Service Professor Emeritus of Pediatrics, University of Florida; Fellow of the American Academy of Pediatrics; Fellow of the American College of Epidemiology

Arlan L Rosenbloom, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Epidemiology, American Pediatric Society, Endocrine Society, Florida Pediatric Society, Lawson-Wilkins Pediatric Endocrine Society, and Society for Pediatric Research

Disclosure: Nothing to disclose.

Erik D Schraga, MD Consulting Staff, Department of Emergency Medicine, Mills-Peninsula Emergency Medical Associates; Consulting Staff, Permanente Medical Group, Kaiser Permanente, Santa Clara Medical Center

Disclosure: Nothing to disclose.

Richard H Sinert, DO Associate Professor of Emergency Medicine, Clinical Assistant Professor of Medicine, Research Director, State University of New York College of Medicine; Consulting Staff, Department of Emergency Medicine, Kings County Hospital Center

Richard H Sinert, DO is a member of the following medical societies: American College of Physicians and Society for Academic Emergency Medicine

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

Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Pharmacy Editor, Medscape

Disclosure: Nothing to disclose.