Hyponatremia

Key Features

Essentials of Diagnosis

• Must know volume status as well as serum and urine osmolality to determine etiology

• Hyponatremia usually reflects excess water relative to sodium; total body sodium may be appropriate, low, or high

• Hyponatremia in hospitalized patients is commonly caused by administration of hypotonic fluids

General Considerations

• Most common electrolyte abnormality in hospitalized patients

• Defined as a serum sodium concentration < 135 mEq/L (135 mmol/L)

• Whether hyponatremia is symptomatic depends on both severity and acuity

• Severity can be subclassified as

  • Mild (130–134 mEq/L)

  • Moderate (125–129 mEq/L)

  • Severe (< 125 mEq/L)

• Acuity

  • Acute hyponatremia defined as present for< 48 hours

  • Chronic hyponatremia defined as present for > 48 hours (often diagnosed on routine electrolyte panels)

• A diagnostic algorithm separates the causes of hyponatremia using serum osmolality, urine osmolality and volume status (Figure 21–1)

• SIADH is a clinical diagnosis characterized by

  • Hyponatremia

  • Hypoosmolality (< 280 mOsm/kg)

  • Normal thyroid and adrenal function

  • Urine Na usually > 20 mEq/L

  • BUN may be < 10 mg/dL [3.6 mmol/L] and serum uric acid < 4 mg/dL [238 mcmol/L]

  • See Syndrome of Inappropriate Antidiuretic Hormone

Etiology

• Isotonic and Hypertonic Hypernatremia

  • Hyponatremia is typically associated with hypoosmolality with two exceptions: pseudohyponatremia and hypertonic hyponatremia.

• Hypotonic Hyponatremia

  • ADH-independent causes

    • Psychogenic polydipsia

    • Beer potomania and "tea and toast" diet

    • Renal impairment

  • ADH-dependent causes

    • Failure to suppress ADH action may be appropriate (eg. hypovolemia), or inappropriate (eg. in the absence of hypovelemia), which is the syndrome of inappropriate ADH secretion (SIADH)

    • Hypovolemic hyponatremia

      • Occurs with renal or extrarenal volume loss (sodium and water) and subsequent hypotonic fluid replacement

    • Hypervolemic hyponatremia (eg. edematous states of cirrhosis or heart failure)

      • Decreased effective arterial blood volume occurs despite an overall increase in extracellular volume (edema)

      • ADH secretion results

    • SIADH (see Syndrome of Inappropriate Antidiuretic Hormone)

      • ADH is secreted in the absence of an appropriate physiologic stimuli such as a decreased effective circulating volume or hyperosmolality

    • Reset osmostat

      • Vasopressin release is regulated around a lower, or hypotonic, set point

    • Adrenal insufficiency and hypothyroidism

      • Cortisol normally provides negative feedback on ADH release

      • Cortisol deficiency can lead to uninhibited ADH and hyponatremia

      • Hyponatremia in myxedema coma may be due to appropriate ADH release from reduced cardiac output and concurrent adrenal insufficiency

    • Nausea, pain, and surgery

      • Nausea and pain are potent stimulators of ADH release

      • Severe hyponatremia can develop after elective surgery in healthy patients due to excessive use of hypotonic fluids

    • Exercise-associated hyponatremia

      • Hyponatremia during or after exercise, especially endurance events (eg. triathlons and marathons)

      • May be caused by excessive hypotonic fluid intake and ADH secretion ...