Hyponatremia
A confused postoperative patient with a sodium of 128 mEq/L doesn't need more fluids — they likely need less. Correcting hyponatremia too fast can be more dangerous than the imbalance itself.
Core Concept
Hyponatremia is serum sodium below 135 mEq/L. Sodium is the primary extracellular electrolyte controlling water balance, so low sodium really means too much water relative to sodium — the blood is diluted. The most common cause in hospitalized clients is excessive hypotonic IV fluid administration or SIADH (syndrome of inappropriate antidiuretic hormone), where the body retains free water inappropriately. Other causes include diuretic use (especially thiazides), heart failure, and psychogenic polydipsia. Symptoms track with how fast sodium drops, not just how low it goes. Mild (130–134): often asymptomatic. Moderate (120–129): nausea, headache, confusion, lethargy. Severe (<120): seizures, coma, respiratory arrest. Assessment findings reflect cellular swelling from water shifting into cells via osmosis — the brain is most vulnerable because it's confined by the skull. Fluid restriction (typically 1,000–1,500 mL/day) is the first-line nursing intervention for dilutional hyponatremia. For severe symptomatic cases, hypertonic saline (3% NaCl) is given via IV pump with frequent sodium monitoring — correction must not exceed 8–12 mEq/L per 24 hours to prevent osmotic demyelination syndrome (central pontine myelinolysis), an irreversible neurological injury.
Watch Out For
Don't confuse hyponatremia (confusion, seizures, cellular swelling) with hypernatremia (thirst, dry mucous membranes, cellular shrinkage) — both cause neurological changes but the fluid shift direction is opposite. Students often think hyponatremia means 'give sodium,' but dilutional hyponatremia is treated by restricting water, not adding salt. Osmotic demyelination comes from correcting too fast, not from the low sodium itself — the treatment is the danger.
Clinical Pearl
Low sodium = swollen cells. Think of waterlogged brain tissue pushing against the skull — that's why seizures happen. Restrict water first, replace sodium slowly.
Test Your Knowledge
3 quick questions — see how well you understood Hyponatremia