Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a topic covered in the 5-Minute Emergency Consult.

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Basics

Description

  • Most common cause of hyponatremia in hospitalized patients (and doubles inpatient mortality in some studies)
  • A water balance problem more than 1 of sodium (Na) balance
  • Normal regulation of water balance:
    • Antidiuretic hormone (ADH):
      • Integral controller of water balance
      • Increases water permeability of the collecting tubules, resulting in free water reabsorption
      • Synthesized by hypothalamus but secreted by posterior pituitary
    • Water deprivation (increased plasma osmolality) stimulates secretion as sensed by:
      • Osmoreceptors/atrial stretch receptors
      • Carotid baroreceptors
      • Aortic arch/pulmonary veins
  • Hyponatremia:
    • Mild: Serum sodium <135 mEq/L
    • Moderate: Serum sodium <130 mEq/L
    • Severe: Serum sodium <125 mEq/L
    • Excess extracellular water relative to Na
    • Depletional hyponatremia:
      • Sodium depletion can be caused by diet, GI losses, diuretic use, and renal or adrenal disease.
      • Often accompanied by extracellular fluid volume depletion
      • Hyponatremia associated with clinical signs of hypovolemia
      • Increased Hct, BUN, Cr
      • Urinary sodium excretion <20 mEq/L
    • Dilutional hyponatremia:
      • Increased extracellular water in presence of normal or increased total body sodium
      • Can be caused by increased fluid intake (oral, IV), drugs, or medical conditions
      • Euvolemia with edema
      • Normal or decreased Hct, BUN, Cr
      • Urinary sodium excretion >20 mEq/L
      • Inappropriate ADH secretion is a form of dilutional hyponatremia.
  • Definition of SIADH:
    • ADH secretion in absence of hyperosmolality or hypovolemia
  • Criteria for definition:
    • Essential features:
      • Hyponatremia—despite correction for hyperglycemia, hyperproteinemia, or hyperlipidemia
      • Euvolemia—no clinical signs of volume depletion (orthostasis, tachycardia) or volume overload (edema, ascites)
      • Hyposmolality of the plasma—<275 mOsm/kg of water
      • Normal renal, adrenal, and thyroid function
      • No recent diuretic use
      • Urine Osm >100 mOsm/kg of water
    • Supplemental features:
      • Plasma uric acid <4 mg/dL
      • BUN <10 mg/dL
      • FENa >1%
      • Failure to correct hyponatremia after an infusion of normal saline (NS) 0.9%
      • Abnormal water load test (inability to excrete ≥90% of a 20 mL/kg water load in 4 hr)

Etiology

  • Malignant disorders:
    • ADH-producing tumors
    • Cancer (Small-cell lung, pancreatic, prostate)
    • Pituitary tumors
    • Thymoma
    • Lymphoma
  • Pulmonary disorders:
    • Pneumonia
    • TB
    • Lung abscess
    • COPD
  • CNS disorders:
    • Meningitis/encephalitis
    • CVA
    • Head injury
  • Medications:
    • Thiazides
    • Chlorpropamide
    • Vincristine
    • Anticonvulsants (carbamazepine)
    • Antidepressants (tricyclics, SSRIs)
    • Antipsychotics
    • NSAIDs
    • Ecstasy (MDMA)
    • Vasopressin analogs (DDAVP, oxytocin, vasopressin)
  • Transient:
    • Endurance exercise
    • General anesthesia
    • Pain
    • Stress
  • Other:
    • Hereditary
    • Positive-pressure ventilation
    • HIV/AIDS
    • Idiopathic

ALERT
Cerebral salt-wasting syndrome (CSWS) can mimic SIADH.
  • Seen in patients with cerebral tumors or subarachnoid hemorrhage and in neurosurgical patients
  • Etiology unclear
  • Represents appropriate water resorption in the face of salt wasting (urine Na >30–40 mmol/L)
  • Fluid restriction can help differentiate the 2:
    • In SIADH: Hypouricemia will correct
    • In CSWS: Hypouricemia will persist
  • Treatment of CSWS may differ from that of SIADH:
    • Infusion of NS
    • May benefit from fludrocortisones therapy

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Citation

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TY - ELEC T1 - Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) ID - 307273 Y1 - 2016 PB - 5-Minute Emergency Consult UR - https://emergency.unboundmedicine.com/emergency/view/5-Minute_Emergency_Consult/307273/all/Syndrome_of_Inappropriate_Antidiuretic_Hormone_Secretion__SIADH_ ER -