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 retention rather than sodium deficiency; total body sodium may be appropriate, low, or high

• Hypotonic fluids commonly contribute to hyponatremia in hospitalized patients

General Considerations

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

• Most common electrolyte abnormality encountered in clinical practice

• Represents an excess of water relative to sodium in the plasma leading to

  • Hypoosmolality (most common)

  • Subsequent movement of water from the extracellular fluid into the intracellular fluid (which can result in cerebral edema)

Classification

Isotonic and hypertonic hyponatremia

• Hyponatremia is typically hypoosmolar with two exceptions:

  • Pseudohyponatremia

    • A laboratory artifact that occurs rarely in patients with marked hypertriglyceridemia or hypergammaglobulinemia

    • Consultation with the clinical laboratory is necessary if this condition is suspected

  • Hypertonic hyponatremia

    • The best clinical examples of this situation are hyperglycemia, and less commonly, mannitol infusion

    • To determine if the hyponatremia can be entirely attributed to hyperglycemia, a sodium correction factor is often used

    • Many guidelines recommend using a decrease in the serum sodium concentration of 1.6 mEq/L (or 1.6 mmol/L) for every 100 mg/dL (5.5 mmol/L) rise in plasma glucose above normal

Hypotonic hyponatremia

• Most cases of hyponatremia

• Hypotonic hyponatremia can be subclassified as ADH dependent or ADH independent based on kidney's ability to excrete dilute urine

• ADH-dependent causes of hypotonic hyponatremia involve a failure to appropriately or inappropriately suppress ADH action

  • The most common cause of hypotonic hyponatremia

  • Appropriate suppression of ADH action: hypovolemia or hypervolemia (with a reduced effective arterial volume secondary to cirrhosis or heart failure)

    • Hypovolemic hyponatremia:

    • Occurs with renal or extrarenal volume loss (sodium and water) and subsequent hypotonic fluid replacement (Figure 21–1)

    • The reduced blood pressure results in an increase in ADH secretion by the pituitary gland, limiting free water excretion

    • Cerebral salt wasting is a rare subset of hypovolemic hyponatremia that occurs with intracranial disease (eg, infections, cerebrovascular accidents, tumors, and neurosurgery)

    • Hypervolemic hyponatremia:

    • Commonly occurs in the edematous states of cirrhosis and heart failure, and rarely, in nephrotic syndrome (Figure 21–1)

    • In these settings, a decreased effective arterial blood volume (typically low blood pressure) occurs despite an overall increase in extracellular volume (edema) resulting in ADH secretion

    • In cirrhosis and heart failure, effective circulating volume is decreased due to systemic vasodilation and reduced cardiac output, respectively

  • Inappropriate suppression of ADH action: In the syndrome of inappropriate ADH secretion (SIADH), ADH is secreted in the absence of appropriate physiologic stimuli, such as a decreased effective circulating volume from hypovolemia or edematous states

    • Table 21–2 lists major causes of SIADH

    • See Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

  • ADH-independent causes of hypotonic hyponatemia

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