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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.

  • The administration of hypotonic fluids commonly cause hyponatremia in hospitalized patients.

GENERAL CONSIDERATIONS

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. Acutely, this movement of water can result in cerebral edema, increasing the risk of seizures and even brain herniation.

Chronic hyponatremia is often asymptomatic or present with mild confusion, nausea, or falls. In these patients, cerebral adaptation has occurred as the brain cells have excreted intracellular osmoles to limit cell swelling. In this setting, over-rapid correction of chronic hyponatremia may produce profound neurologic abnormalities (osmotic demyelination syndrome).

A common misconception is that hyponatremia is secondary to a deficiency in total body sodium, when in reality it reflects an excess of total body water in the vast majority of cases. The basic pathophysiologic principle is the ingestion of water (oral or intravenous) in excess of the amount the kidney can excrete, commonly, but not always, due to the presence of ADH. A diagnostic algorithm using serum osmolality, urine sodium, and volume status separates the causes of hyponatremia into therapeutically useful categories (Figure 21–1).

Figure 21–1.

A diagnostic algorithm for the causes of hyponatremia using serum osmolality, urine osmolality, and urine sodium. ADH, antidiuretic hormone; GFR, glomerular filtration rate; SIADH, syndrome of inappropriate antidiuretic hormone.

ETIOLOGY

A. Isotonic and Hypertonic Hyponatremia

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

1. Pseudohyponatremia

This represents a laboratory artifact that rarely occurs 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 per given volume. This issue is becoming less prevalent as most laboratories are now using direct ion selective electrodes without blood dilution. Consultation with the clinical laboratory is necessary if this condition is suspected.

2. Hypertonic hyponatremia

The best clinical examples of this situation are hyperglycemia, and less commonly, mannitol infusion. Both glucose and mannitol are active osmoles, increasing the osmolality of the extracellular fluid, which pulls water from inside cells into the extracellular space. Note, this leads to a reduction in intracellular volume, and cerebral edema is not caused by hyponatremia ...

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