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For further information, see CMDT Part 21-02: Hyponatremia

Key Features

Essentials of Diagnosis

  • Euvolemic hypotonic hyponatremia, particularly among hospitalized patients

  • Serum sodium concentration < 135 mEq/L (< 135 mmol/L)

  • Plasma osmolality < 280 mOsm/kg and UNa > 20 mEq/L

  • Risk of osmotic demyelination syndrome from overly rapid correction of serum sodium

General Considerations

  • ADH is secreted in the absence of an appropriate physiologic stimuli (eg. hypovolemia or hyperosmolality)

  • There is failure to suppress ADH action; water is inappropriately retained leading to relatively concentrated urine

  • SIADH is a diagnosis of exclusion; must rule out other causes of hyponatremia (eg, hypovolemia, decreased solute intake, cortisol deficiency, and severe hypothyroidism)

  • Major causes of SIADH are disorders affecting the CNS, lungs (eg. cancer or infection) and medications (Table 21–2)

Table 21–2.Common causes of syndrome of inappropriate ADH secretion.

Clinical Findings

Symptoms and Signs

  • Frequently asymptomatic; symptoms depend on severity and acuity of the hyponatremia

  • Acute hyponatremia (present for < 48 hours)

    • Symptoms mainly relate to brain cell swelling and rise in intracranial pressure: headache, inattentiveness, lethargy, disorientation, and nausea

    • Most serious symptoms: marked confusion, decreased level of consciousness, vomiting, seizures, coma, brainstem herniation, death

  • Chronic hyponatremia (present for > 48 hours)

    • Often asymptomatic

    • Subtle abnormalities, (eg. mild concentration and cognitive deficits) and gait disturbances that can lead to falls, may be present

Differential Diagnosis

  • Severe hypothyroidism and glucocorticoid insufficiency can cause hyponatremia that cannot be differentiated from SIADH by urine or serum electrolytes alone

  • Hypovolemic or hypervolemic hyponatremia


  • SIADH is a clinical diagnosis in a euvolemic patient

  • Characterized by

    • Hypotonic hyponatremia (Figure 21–1)

    • Decreased plasma osmolality (< 280 mOsm/kg)

    • UNa > 20 mEq/L, urine osm > 100 mOsm/kg

    • Absence of heart, kidney, or liver disease

    • Normal thyroid and adrenal function

  • In clinical practice, ADH levels are not measured; urine osmolality is a surrogate for ADH activity

  • Low blood urea nitrogen (BUN) (< 5–10 mg/dL) and hypouricemia (< 4 mg/dL)

    • Not only dilutional

    • Increased urea and uric acid clearances in response to volume-expanded state

Figure 21–1.

A diagnostic algorithm for the causes of hyponatremia using serum osmolality, urine osmolality, and urine sodium. ...

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