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Key Features

Essentials of Diagnosis

  • Serum sodium concentration < 130 mEq/L (< 130 mmol/L)

  • Hypotonic, euvolemic hyponatremia

General Considerations

  • Most common cause of hyponatremia in hospitalized patients

  • Table 21–2 lists other causes of SIADH

  • Hyponatremia occurs from abnormal water balance rather than abnormal sodium balance

  • Inappropriate antidiuretic hormone (ADH) excess and consequent retention of water due to impaired excretion results in hyponatremia and low serum osmolality

  • Hospitalized patients treated with hypotonic fluid are at increased risk for hyponatremia

  • Patterns of abnormal ADH secretion

    • Random secretion (eg, carcinomas)

    • Reset osmostat (eg, elderly, pulmonary diseases)

    • Leak of ADH (eg, basilar skull fractures)

Table 21–2.Causes of syndrome of inappropriate ADH secretion (SIADH).

Clinical Findings

Symptoms and Signs

  • Frequently asymptomatic

  • Symptoms usually seen with serum sodium levels < 120 mEq/L

  • If symptomatic, primarily CNS symptoms of lethargy, weakness, confusion, delirium, and seizures

  • Symptoms are often mistaken for primary neurologic or metabolic disorders

Diagnosis

Laboratory Tests

  • Serum Na+ concentration < 135 mEq/L (< 135 mmol/L)

  • Decreased serum osmolality (< 280 mOsm/kg) with inappropriately increased urine osmolality (> 150 mOsm/kg)

  • Normal thyroid and adrenal function tests

  • Low blood urea nitrogen (BUN) (< 10 mg/dL [or < 3.6 mmol/L]) and hypouricemia (< 4 mg/dL [or < 238 mcmol/L]), are not only dilutional but result from increased urea and uric acid clearances in response to the volume-expanded state

  • High BUN suggests a volume-contracted state, which excludes the diagnosis of SIADH

Treatment

Medications

Symptomatic hyponatremia

  • Initial goal: Achieve serum sodium concentration of 125–130 mEq/L, guarding against overly rapid correction

  • Increase serum sodium concentration by ≤ 1–2 mEq/L/h and not > 25–30 mEq/L in first 2 days to prevent cerebral osmotic demyelination

  • Rate should be reduced to 0.5–1.0 mEq/L/h as neurologic symptoms improve

  • With CNS symptoms, hyponatremia should be immediately treated at any level of serum sodium concentration

    • Hypertonic (eg, 3%) saline plus furosemide (0.5–1.0 mg/kg intravenously) indicated for symptomatic hyponatremia

    • To determine how much 3% ...

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