Hyponatremia

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What this article covers

• Overview

  Definition: hyponatremia is a serum sodium concentration below 135 mEq/L (135 mmol/L), the most common electrolyte disorder in hospitalized patients.

• Epidemiology

  The most common electrolyte abnormality in hospitalized patients, and drugs are its most common cause overall, particularly thiazide diuretics and antidepressants causing SIADH (syndrome of inappropriate antidiuretic hormone secretion).

• Risk factors

  Medications, the leading cause overall: thiazide diuretics, SSRIs, phenothiazines, haloperidol, carbamazepine, cyclophosphamide, tricyclic antidepressants, valproic acid, and MDMA.

• Pathophysiology

  Hypertonic hyponatremia results from an osmotically active solute (typically glucose) pulling water out of cells and diluting serum sodium; for every 100 mg/dL rise in glucose, sodium falls by about 1.6-2.2 mEq/L.

• Clinical features

  Symptom severity depends on the rate and magnitude of the sodium fall, not solely the absolute value.

• Differential diagnosis

  Hypertonic hyponatremia from hyperglycemia: serum osmolality above 295 mOsm/kg with markedly elevated glucose, unlike the low osmolality of true hypotonic hyponatremia.

• Diagnosis

  Hyponatremia is confirmed by a serum sodium below 135 mEq/L, and the workup identifies the mechanism, since treatment differs sharply by category.

• Management

  First, determine whether hyponatremia is acute (under 48 hours) or symptomatic/severe, since these require emergent treatment; unless documented acute, treat as chronic to avoid overly rapid correction.

• Complications

  Osmotic demyelination syndrome from overly rapid correction of chronic hyponatremia: paresis, movement disorders, behavioral abnormalities, seizures, and obtundation, usually irreversible, from…

• Prognosis

  Outcome depends primarily on the rate and adequacy of correction, not the nadir sodium alone: appropriately paced correction resolves symptoms and avoids demyelination.

• Prevention

  Correct chronic hyponatremia no faster than 8.0 mEq/L in any 24-hour period, targeting 4.0-6.0 mEq/L per day, checking sodium every 2-4 hours during correction.