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For further information, see CMDT Part 21-02: Hyponatremia
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Essentials of Diagnosis
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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
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General Considerations
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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
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Isotonic and hypertonic hyponatremia
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Hypotonic hyponatremia
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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
ADH-independent causes of hypotonic hyponatemia
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