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ESSENTIALS OF DIAGNOSIS
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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. The serum sodium concentration is not a measure of total body sodium.
Hyponatremia in hospitalized patients is commonly caused by administration of hypotonic fluids.
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GENERAL CONSIDERATIONS
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Hyponatremia is defined as a serum sodium concentration less than 135 mEq/L (135 mmol/L) and is the most common electrolyte abnormality encountered in clinical practice. Hyponatremia represents an excess of water relative to sodium in the plasma leading to a reduction in plasma osmolality and subsequent movement of water from the extracellular fluid into the intracellular fluid compartment. If this movement of water happens acutely, cerebral edema can occur, increasing the risk of seizures and even brain herniation.
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Chronic hyponatremia is often asymptomatic or presents with mild confusion, nausea, or falls; cerebral adaptation occurs as the brain cells excrete intracellular osmoles to limit cellular swelling. In this chronic setting, hyponatremia is traditionally corrected slowly due to concern for instigating profound neurologic abnormalities (osmotic demyelination syndrome). However, this paradigm has been challenged in recent studies; in practice, osmotic demyelination is rare, suggesting factors other than correction rates are likely implicated. Regardless, caution is still recommended when correcting chronic hyponatremia in terms of the rate of hypertonic fluid administration.
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A common misconception is that hyponatremia is secondary to a deficiency in total body sodium, when this state actually usually reflects an excess of total body water. The basic pathophysiologic principle is that more water (oral or intravenous) is ingested than the kidney can excrete (commonly due to the action of ADH). A diagnostic algorithm (Figure 23–1) separates the causes of hyponatremia using serum osmolality, urine sodium, and volume.
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A. Isotonic and Hypertonic Hyponatremia
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Hyponatremia is typically associated with hypoosmolality with two exceptions: pseudohyponatremia and hypertonic hyponatremia.
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1. Pseudohyponatremia
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This represents a rare laboratory artifact in patients with marked hypertriglyceridemia or hypergammaglobulinemia. In these settings, there is an increase in the solid components of plasma, relative to plasma water, resulting in a lower sodium level per given volume. This issue has become less prevalent since most laboratories now use direct ion selective electrodes without blood dilution. Consult the clinical laboratory if this condition is suspected.
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2. Hypertonic hyponatremia
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The best clinical examples of this situation occur in the setting of hyperglycemia. Glucose is an active osmole that increases the osmolality of the extracellular fluid, pulling water from inside cells into the extracellular ...