Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Key Features

Essentials of Diagnosis

• Euvolemic hypotonic hyponatremia, particularly among hospitalized patients

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

• Urine sodium > 20 mEq/L, urine osmolality usually > 300 mOsm/kg

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

General Considerations

• Nonphysiologic ADH secretion: water is inappropriately retained leading to relatively concentrated urine (Uosm is usually > 300 mOsm/kg)

• Urinary solute handling remains preserved, thus, urine sodium is generally not low (U_Na > 20 mEq/L), though it does depend on dietary ingestion of sodium

• Hyponatremia after exercise, especially endurance events such as triathlons and marathons, is generally thought to be a combination of excessive hypotonic fluid intake and inappropriate ADH secretion

• Specific universal recommendations for fluid replacement rates are not possible given the variability of sweat production, renal water excretion, and environmental conditions

Clinical Findings

Symptoms and Signs

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

• Symptoms mainly relate to brain edema: nausea, malaise, headache, lethargy, disorientation

• Most serious symptoms: seizures, coma, brainstem herniation, death

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

Diagnosis

• Clinical in a euvolemic patient

• Characterized by

  • Hypotonic hyponatremia

  • Absence of heart, kidney, or liver disease

  • Normal thyroid and adrenal function

  • Urine sodium usually > 20 mEq/L, urine osm usually > 300 mOsm/kg

• In clinical practice, ADH levels are not measured

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

Treatment

• Acute (< 24–48 hours, eg, postoperatively) hyponatremia

  • Generally, the 24-hour correction goal rate of 4–6 mEq/L (for chronic hyponatremia) is still recommended for acute hyponatremia, so after a brief period of rapid correction, no further correction may be needed until the next day

  • Monitor serum sodium every 2–8 hours, depending on severity, and hourly urinary output

  • High urinary output suggests ...