SIADH

Syndrome of Inappropriate Antidiuresis (SIADH)

Definition and Core Pathophysiology

• The syndrome of inappropriate antidiuresis (SIADH) is characterized by hyponatremia resulting from a primary elevation in vasopressin secretion or inappropriate constitutive activation of the vasopressin V2 receptor.
• Vasopressin (antidiuretic hormone) normally acts on V2 receptors located in the renal collecting tubule, thick ascending limb of the loop of Henle, and periglomerular tubules.
• Activation of the Gs-coupled V2 receptor increases intracellular cyclic adenosine monophosphate (cAMP), leading to the insertion of aquaporin-2 (AQP2) water channels into the apical (luminal) membrane, which allows water movement into the hypertonic inner medullary interstitium.
• In SIADH, the inappropriate presence or action of vasopressin prevents normal free water clearance, leading to an inappropriately concentrated urine (often >800 mOsm/kg H2O).
• The retention of free water leads to a normal or slightly elevated plasma volume (euvolemic or mildly hypervolemic state).
• The relative hypervolemia suppresses aldosterone secretion and elevates atrial natriuretic peptide (ANP) concentrations, which subsequently promotes natriuresis.
• Consequently, patients exhibit a normal-to-high urine sodium concentration.
• Because of the hemodilution and increased renal clearance of uric acid driven by the effective circulating volume, serum uric acid levels are characteristically low in SIADH, distinguishing it from hyponatremia caused by systemic dehydration.
• Recent diagnostic advancements include the measurement of copeptin (the stable carboxy-terminus of the vasopressin precursor), which, when coupled with hypertonic saline infusion, can classify SIADH into different pathophysiological subtypes.

Etiology

• Central Nervous System Disorders: CNS insults are classic triggers for inappropriate vasopressin release.
• These include encephalitis, brain tumors (such as gliomas, optic chiasm gliomas, and ependymomas), head trauma, and psychiatric diseases.
• Brain malformations, such as alobar holoprosencephaly, have also been associated with refractory SIADH.
• SIADH frequently occurs in the postictal period following generalized seizures.
• Meningitis, particularly tuberculous and bacterial forms, is a well-documented cause; the presence of SIADH in tuberculous meningitis predicts more severe disease and a poorer outcome.
• Post-Neurosurgical "Triple Phase" Response: Transsphenoidal pituitary surgery or craniopharyngioma resection often triggers a characteristic triple-phase response.
• The first phase is transient diabetes insipidus (lasting 1.5 to 2 days) due to local edema.
• The second phase is SIADH, which can last up to 10 days, resulting from the unregulated release of vasopressin from degenerating and dying neurons. Up to 35% of patients develop this hyponatremic phase about one week after surgery.
• The final phase is permanent diabetes insipidus if more than 90% of the vasopressin neurons are destroyed.
• Pulmonary Disorders: Pulmonary infections and pathologies can trigger ectopic vasopressin release or stimulate central release.
• Documented causes include viral pneumonia, bacterial pneumonia, respiratory syncytial virus (RSV) infection, tuberculosis, and aspergillosis.
• Malignancies: Paraneoplastic production of vasopressin is seen in several cancers.
• Associated pediatric and adult malignancies include thymoma, lymphoma, Ewing sarcoma, small cell carcinoma of the lung, and bronchial carcinoid.
• Pharmacological Agents (Drug-Induced SIADH): Numerous drugs impair free water clearance either by augmenting vasopressin release, enhancing renal response to vasopressin, or acting directly on the nephron.
• Carbamazepine and chlorpropamide both increase vasopressin secretion and enhance the cellular response to vasopressin.
• High-dose cyclophosphamide (often used with forced water diuresis to prevent hemorrhagic cystitis), vinblastine, and cisplatin cause hyponatremia through a direct effect on the distal collecting tubule's water permeability, independent of plasma vasopressin concentrations.
• Other commonly implicated drugs include vincristine, oxcarbazepine, tricyclic antidepressants (e.g., imipramine, amitriptyline), selective serotonin reuptake inhibitors (e.g., fluoxetine, sertraline), haloperidol, and opiate derivatives.
• Iatrogenic Causes: The most common cause of SIADH-like hyponatremia in pediatric patients is the excessive therapeutic administration of vasopressin or desmopressin (dDAVP).
• This occurs during the treatment of central diabetes insipidus, bleeding disorders (e.g., von Willebrand disease, hemophilia A), or historically for primary nocturnal enuresis.
• Other Associations: Prolonged nausea is a potent nonosmotic stimulus for vasopressin release and can cause profound hyponatremia. AIDS, Guillain-Barre syndrome, and acute intermittent porphyria are also established causes.

Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD)

• NSIAD represents a rare genetic form of hyponatremia that perfectly mimics SIADH clinically but is characterized by undetectable or appropriately suppressed vasopressin levels.
• It is caused by gain-of-function missense mutations in the V2 vasopressin receptor gene (AVPR2) located on chromosome Xq28.
• Mutations typically involve a change in codon 137 from arginine to cysteine (R137C) or leucine (R137L), leading to constitutive activation of the V2 receptor in the absence of ligand binding.
• Because it is an X-linked disorder, it primarily affects male infants, presenting with severe hyponatremia in the first months of life, though some female carriers may also be affected.
• Treatment is challenging as vasopressin receptor antagonists (vaptans) are ineffective against the constitutively active mutant receptor; management relies primarily on fluid restriction and oral urea.

Clinical Manifestations

• The clinical presentation is strictly dependent on the rate of decline and the absolute nadir of the serum sodium concentration.
• Acute Hyponatremia: Defined as a rapid drop in serum sodium occurring in less than 12 to 24 hours, or an absolute level dropping below 120 mEq/L.
• The rapid hypotonicity of the extracellular fluid causes a massive influx of water into the relatively hyperosmolar intracellular space, leading to severe cell swelling (cerebral edema).
• This presents acutely as lethargy, altered mental status, psychosis, coma, generalized seizures, neuronal dysfunction, and potentially fatal cerebral herniation.
• Chronic Hyponatremia: If the hyponatremia develops gradually over more than 24 to 48 hours, the brain initiates compensatory mechanisms.
• Cells actively extrude intracellular organic osmolytes (such as glutamate, taurine, and myo-inositol) to reduce the intracellular osmotic gradient, thereby shedding water and partially restoring normal cell volume.
• Because of this adaptation, children with chronic SIADH are frequently asymptomatic or exhibit only mild, vague symptoms despite profound hyponatremia.

Diagnostic Evaluation

• Laboratory Criteria for SIADH:
• True hyponatremia (Serum Sodium < 135 mEq/L), frequently much lower at presentation.
• Decreased effective serum osmolality (< 275 mOsm/kg H2O).
• Inappropriately concentrated urine (urine osmolality typically >100 mOsm/kg H2O, and frequently >800 mOsm/kg H2O) despite serum hypotonicity.
• Elevated urine sodium (typically >30 mEq/L) reflecting a euvolemic or hypervolemic state with intact renal sodium excretion.
• Low serum uric acid and low blood urea nitrogen (BUN).
• Normal thyroid and adrenal function must be confirmed, as deficiencies in either can independently impair free water clearance and cause euvolemic hyponatremia.
• Exclusion of Pseudohyponatremia and Hyperglycemia:
• Hyperglycemia shifts water from the intracellular to the extracellular space, diluting serum sodium by approximately 1.6 mEq/L for every 100 mg/dL increment in blood glucose above 100 mg/dL.
• Hypertriglyceridemia and hyperproteinemia can cause factitious hyponatremia (pseudohyponatremia) when older laboratory methods are used, as the aqueous fraction of the serum is reduced relative to the total volume.
• Differentiation from Cerebral Salt Wasting (CSW):
• CSW is a critical differential diagnosis in neurosurgical or head trauma patients presenting with hyponatremia.
• CSW is caused by the primary hypersecretion of natriuretic peptides (e.g., ANP), leading to massive renal salt loss.
• Unlike SIADH, CSW is characterized by true hypovolemia, high urine output, orthostatic hypotension, normal or high serum uric acid, and extremely high urinary sodium excretion (often >150 mEq/L).
• Differentiating the two is crucial because CSW requires vigorous fluid and sodium replacement, whereas SIADH requires fluid restriction.
• Differentiation from Systemic Dehydration:
• Dehydration presents with physical signs of hypovolemia (tachycardia, poor skin turgor), high BUN, high uric acid, and appropriately low urine sodium (<20 mEq/L) due to maximal aldosterone-driven renal sodium conservation.

Management

• Management of Chronic or Asymptomatic SIADH:
• The cornerstone of therapy is strict oral fluid restriction.
• To prevent worsening hyponatremia in a state of full antidiuresis, fluid intake must be limited to match the volume required to excrete the daily obligate renal solute load plus insensible losses.
• In practice, this generally requires limiting total daily fluid intake to approximately 1,000 mL/m2/24 hr.
• Salt administration (e.g., normal saline infusion) is generally ineffective and can be dangerous; because the urine is highly concentrated and aldosterone is suppressed, the administered sodium is rapidly excreted in a small volume of urine, while the water is retained, potentially worsening the hyponatremia (desalination phenomenon).
• Emergency Management of Acute/Symptomatic Hyponatremia:
• Symptomatic hyponatremia (seizures, coma, altered sensorium) is a medical emergency that necessitates immediate intervention with hypertonic (3%) sodium chloride.
• The primary goal is to raise the serum sodium concentration only enough to reverse critical cerebral edema and improve mental status.
• Generally, administration of 12 mL/kg of 3% sodium chloride will acutely raise the serum sodium concentration by approximately 10 mEq/L.
• Prevention of Osmotic Demyelination Syndrome:
• In chronic hyponatremia, the brain has adapted by losing intracellular osmolytes.
• Rapid correction of serum sodium in this adapted state causes sudden extracellular hypertonicity, leading to severe cell shrinkage.
• This shrinkage causes Central Pontine Myelinolysis (osmotic demyelination syndrome), an irreversible and devastating neurological condition characterized by axonal demyelination in the basal pons and other regions.
• To prevent this, the rate of serum sodium correction must absolutely not exceed 0.5 mEq/L/hr, or a total of 12 mEq/L over a 24-hour period.
• Pharmacological Interventions:
• Urea: Oral urea has been safely and successfully used to induce an osmotic diuresis in infants and children with chronic SIADH and NSIAD. It allows for a more liberal fluid intake, which is essential for ensuring adequate caloric intake and growth in young children.
• Vasopressin Receptor Antagonists (Vaptans): Drugs like tolvaptan and conivaptan are nonpeptide V2 receptor antagonists (aquaretics) that block vasopressin action, leading to a rapid water diuresis without sodium loss.
• While effective in adults, vaptans are not FDA-approved for children and carry significant risks, including liver toxicity, extreme thirst, and the potential for dangerous overcorrection of serum sodium exceeding the safe limits for preventing myelinolysis. They are also completely ineffective in patients with NSIAD, as the mutant receptor is constitutively active independent of vasopressin binding.
• Demeclocycline and Lithium: Historically used to intentionally induce nephrogenic diabetes insipidus by inhibiting transepithelial water transport and cAMP generation. However, their use in pediatric patients is currently limited by significant nephrotoxic and systemic toxicities.