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Syndrome of Inappropriate Antidiuretic Hormone Secretion


Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FACP, FASN more...
Updated: Oct 28, 2015
Overview
Practice Essentials
The syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH) is defined by the hyponatremia and hypo-osmolality resulting from inappropriate, continued secretion or action of the hormone despite normal or increased plasma volume, which
results in impaired water excretion. 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

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

In general, slowly progressive hyponatremia is associated with fewer symptoms than is a rapid drop of serum sodium to the same value
Signs and symptoms of acute hyponatremia do not precisely correlate with the severity or the acuity of the hyponatremia
Patients may have symptoms that suggest increased secretion of ADH, such as chronic pain, symptoms from central nervous system or pulmonary tumors or head injury, or drug use
Sources of excessive fluid intake should be evaluated
The chronicity of the condition should be considered
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:

Confusion, disorientation, delirium
Generalized muscle weakness, myoclonus, tremor, asterixis, hyporeflexia, ataxia, dysarthria, Cheyne-Stokes respiration, pathologic reflexes
Generalized seizures, coma
See Presentation for more detail.

Diagnosis

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[1] :

Hyponatremia with corresponding hypo-osmolality
Continued renal excretion of sodium
Urine less than maximally dilute
Absence of clinical evidence of volume depletion
Absence of other causes of hyponatremia
Correction of hyponatremia by fluid restriction
The following laboratory tests may be helpful in the diagnosis of SIADH:

Serum sodium, potassium, chloride, and bicarbonate
Plasma osmolality
Serum creatinine
Blood urea nitrogen
Blood glucose
Urine osmolality
Serum uric acid
Serum cortisol
Thyroid-stimulating hormone
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:

Chest radiography (for detection of an underlying pulmonary cause of SIADH) Computed tomography or magnetic resonance imaging of the head (for detection of cerebral edema occurring as a complication of SIADH, for identification of a CNS disorder responsible for SIADH, or for helping to rule out other potential causes of a change
in neurologic status)
See Workup for more detail.

Management

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

Degree of hyponatremia
Whether the patient is symptomatic
Whether the syndrome is acute (< 48 hours) or chronic
Urine osmolality and creatinine clearance
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 (CMP). Such treatment is warranted as follows:

Indicated in patients who have severe symptoms (eg, seizures, stupor, coma, and respiratory arrest), regardless of the degree of hyponatremia
Strongly considered for those who have moderate-to-severe hyponatremia with a documented duration of less than 48 hours
The goal is to correct hyponatremia at a rate that does not cause neurologic complications, as follows:

Raise serum sodium by 0.5-1 mEq/hr, and not more than 10-12 mEq in the first 24 hours
Aim at maximum serum sodium of 125-130 mEq/L
In an acute setting (< 48 hours since onset) where moderate symptoms are noted, treatment options for hyponatremia include the following:

3% hypertonic saline (513 mEq/L)
Loop diuretics with saline
Vasopressin-2 receptor antagonists (aquaretics, such as conivaptan)
Water restriction
In a chronic asymptomatic setting, the principal options are as follows:

Fluid restriction
Vassopressin-2 receptor antagonists
If vasopressin-2 receptor antagonists are unavailable or if local experience with them is limited, other agents to be considered include loop diuretics with increased salt intake, urea, mannitol, and demeclocycline
See Treatment and Medication for more detail.

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 (ADH) despite normal or increased plasma volume, which results in impaired water excretion. The antidiuretic hormone (ADH) 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 ADH is to cause arteriolar vasoconstriction and a rise in arterial blood pressure, the pressor effect.[2]

Physiology of ADH

Arginine vasopressin (AVP), the naturally occurring ADH in humans, is an octapeptide similar in structure to oxytocin. AVP is synthesized in the cell bodies of neurons in the supraoptic and paraventricular nuclei of the anterior hypothalamus and travels
along the supraopticohypophyseal tract into the posterior pituitary. Here, it is stored in secretory granules in association with a carrier protein, neurophysin, in the terminal dilatations of secretory neurons that rest against blood vessels.

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 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.[3] 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.[3] V2 receptors are also found in vascular
endothelial cells and stimulate the release of von Willebrand factor.[3]

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.

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.[4]

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.[4]

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).

Nervous system disorders are as follows:

Acute psychosis
Acute intermittent porphyria
Brain abscess
Cavernous sinus thrombosis
Cerebellar and cerebral atrophy
Cerebrovascular accident
CNS lupus
Delirium tremens
Encephalitis (viral or bacterial)
Epilepsy
Guillain-Barré syndrome
Head trauma
Herpes zoster (chest wall)
Hydrocephalus
Hypoxic ischemic encephalopathy
Meningitis (viral, bacterial, tuberculous, and fungal)
Midfacial hypoplasia
Multiple sclerosis
Perinatal hypoxia
Rocky Mountain spotted fever
Schizophrenia
Shy-Drager syndrome
Subarachnoid hemorrhage
Subdural hematoma
Ventriculoatrial shunt obstruction
Wernicke encephalopathy
Neoplasia disorders are as follows:

Pulmonary - Lung carcinoma and mesothelioma
Gastrointestinal - Carcinomas of the duodenum, pancreas, and colon Genitourinary - Adrenocortical carcinoma; carcinomas of cervix, ureter/bladder, and prostate; and ovarian tumors
Other - Brain tumors, carcinoid tumors, Ewing sarcoma, leukemia, lymphoma, nasopharyngeal carcinoma, neuroblastoma (olfactory), and thymoma
Pulmonary disorders are as follows:

Acute bronchitis/bronchiolitis
Acute respiratory failure
Aspergillosis (cavitary lesions)
Asthma
Atelectasis
Bacterial pneumonia
Chronic obstructive lung disease
Cystic fibrosis
Emphysema
Empyema
Pneumonia (viral, bacterial [mycoplasmal], fungal)
Pneumothorax
Positive pressure ventilation
Pulmonary abscess
Pulmonary fibrosis
Sarcoidosis
Tuberculosis
Viral pneumonia
Drugs that stimulate AVP release are as follows:

Acetylcholine
Antineoplastic agents - Adenine arabinoside, cyclophosphamide, ifosfamide, vincristine, vinblastine
Barbiturates
Bromocriptine
Carbachol
Chlorpropamide
Clofibrate
Cyclopropane
Dibenzazepines (eg, carbamazepine, oxcarbazepine
Halothane
Haloperidol
Histamine
Isoproterenol
Lorcainide
Opiates e.g. Morphine
Nicotine (inhaled tobacco)
Nitrous oxide
Phenothiazines (eg, thioridazine)
Thiopental
Monoamine oxidase inhibitors (eg, tranylcypromine)
Tricyclic antidepressants (eg, amitriptyline, desipramine)
Drugs that potentiate the effects of AVP action (primarily facilitates peripheral action of ADH) are as follows:

Clofibrate
Griseofulvin
Hypoglycemic agents - Metformin, phenformin, tolbutamide
Oxytocin (large doses)
Prostaglandin synthetase inhibitors (inhibit renal PGE 2 synthesis) - Indomethacin, aspirin, nonsteroidal anti-inflammatory drugs
Theophylline
Triiodothyronine
Vasopressin analogs (eg, AVP, DDAVP)
Drugs with an uncertain mechanism are as follows:

Antineoplastic agents - Cisplatin, melphalan, methotrexate, imatinib Ciprofloxacin
Clomipramine
Ecstasy
Phenoxybenzamine
Na + valproate
SSRIs (eg, sertraline, fluoxetine, paroxetine)
Thiothixene
The list of drugs that can induce SIADH is long. SIADH has been reported as an adverse effect of multiple psychotropic medications.[5] 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.

Miscellaneous causes are as follows:

Exercise-induced hyponatremia
Giant cell arteritis
HIV infection - Hyponatremia has been reported in as many as 40% of adult patients with HIV infection. Patients with acquired immunodeficiency syndrome (AIDS) can have many potential causes for increased ADH secretion, including volume depletion and
infection of the lungs and the CNS. [6] Although one third of the hyponatremic patients with AIDS are clinically hypovolemic, the remaining hyponatremic patients fulfill most of the criteria for SIADH.
Idiopathic
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
being between 2.5% and 30%.[7, 8, 9, 10] In the majority of cases, the hyponatremia was hospital acquired or aggravated by the hospitalization and may be secondary to the administration of hypotonic intravenous (IV) fluids.[7] 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 in
patients with chronic kidney disease.

Sex- and age-related demographics

Increasing age (>30 y) is a risk factor for hyponatremia in hospitalized patients.[10] Men appear to be more likely to develop mild or moderate, but not severe, hyponatremia.[10] 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.[3]

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:

Cerebral edema may be observed when plasma osmolality decreases faster than 10 mOsm/kg/h. This can lead to cerebral herniation.
Noncardiogenic pulmonary edema may develop, especially in marathon runners. [11]
CPM is the feared complication of excessive, overly rapid correction of hyponatremia. Typical features are disorders of upper motor neurons, including spastic quadriparesis and pseudobulbar palsy, as well as mental disorders ranging from confusion to
coma. [12] The risk is increased in persons with hepatic failure, potassium depletion, large burns, and malnutrition. [13] Premenopausal patients undergoing surgery, especially gynecologic or related procedures, and those with serum Na of less than 105
may also have an increased risk. Once CPM occurs as a complication, there is no proven treatment.
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+.[14] Hyponatremia may also be a risk factor for osteoporosis and bone fracture.[15]

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.[9] When the Na+ concentration drops below 105 mEq/L, life-threatening complications are much more likely to occur.[13]

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.[16] 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 who were normonatremic at admission and became hyponatremic during the course of their hospitalization.

Clinical Presentation



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http://emedicine.medscape.com/article/246650-overview#a5

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