Reference — Electrolytes
Hyponatremia vs Hypernatremia
Sodium disorders primarily cause neurological symptoms through osmotic shifts in brain tissue. Both hyponatremia and hypernatremia require gradual correction to prevent serious neurological complications from rapid osmotic changes.
Educational use only. Sodium disorder management requires careful calculation, provider orders, and close neurological monitoring. Rapid correction of either disorder can cause permanent neurological injury. Always follow institutional protocols. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Side-by-Side Comparison
| Hyponatremia | Hypernatremia | |
|---|---|---|
| Definition | Na⁺ < 136 mEq/L | Na⁺ > 145 mEq/L |
| Osmolality effect | Serum osmolality low → water moves INTO cells → brain swells | Serum osmolality high → water moves OUT of cells → brain shrinks |
| Fluid status (common) | Euvolemic (SIADH), hypovolemic (diuretics), hypervolemic (HF, cirrhosis) | Hypovolemic (dehydration, DI), rarely hypervolemic (hypertonic saline) |
| Correction rate | Max 8–10 mEq/L per 24 hours (risk: osmotic demyelination) | Max 10 mEq/L per 24 hours (risk: cerebral edema from rapid rehydration) |
Hyponatremia (Na⁺ < 136 mEq/L)
Causes
- SIADH (syndrome of inappropriate ADH) — most common cause in hospitalized patients; ADH causes excess free water retention. Triggers: pain, nausea, pulmonary disease, CNS disease, certain medications (opioids, SSRIs, carbamazepine), malignancy
- Diuretics (especially thiazides) — urinary sodium loss exceeds free water loss
- Heart failure, cirrhosis, nephrotic syndrome — dilutional hyponatremia from fluid overload
- Vomiting, diarrhea — GI sodium loss
- Excessive hypotonic IV fluids or water intake
- Hypothyroidism, adrenal insufficiency — hormonal causes
- Psychogenic polydipsia — excessive free water intake overwhelming renal capacity
Symptoms (by severity)
| Na⁺ Level | Symptoms |
|---|---|
| 130 – 135 | Often asymptomatic; mild nausea, headache, fatigue |
| 125 – 129 | Nausea, vomiting, headache, muscle cramps, disorientation |
| < 125 | Confusion, lethargy, seizures, coma, respiratory arrest — neurological emergency |
Key Nursing Interventions
- Fluid restriction is the primary treatment for euvolemic hyponatremia (SIADH) — restrict free water, not sodium
- For severe/symptomatic hyponatremia: hypertonic saline (3% NaCl) — requires ICU-level monitoring; administer via controlled infusion, not bolus
- Correct slowly — maximum 8–10 mEq/L per 24 hours to prevent osmotic demyelination syndrome (ODS)
- Monitor sodium levels every 4–6 hours during active correction
- Implement seizure precautions for severe hyponatremia
- Assess level of consciousness, orientation, and neurological status serially
- Identify and treat underlying cause — remove SIADH triggers when possible
Hypernatremia (Na⁺ > 145 mEq/L)
Causes
- Dehydration / insufficient water intake — most common cause; elderly patients, infants, and patients unable to access water are highest risk
- Diabetes insipidus (DI) — central (ADH deficiency) or nephrogenic (kidney unresponsive to ADH) — massive dilute urine output with sodium concentration
- Diarrhea (hypotonic fluid loss) — common in infants and elderly
- Excessive sodium administration — hypertonic IV fluids, IV sodium bicarbonate, hypertonic tube feeds
- Fever, sweating, mechanical ventilation — insensible free water losses
- Hyperaldosteronism — sodium retention with water excretion (rare cause of hypernatremia)
Symptoms
- Intense thirst (except in patients with altered thirst mechanism or impaired access to water)
- Dry mucous membranes, decreased skin turgor (if hypovolemic)
- Agitation, restlessness, irritability
- Muscle weakness, hyperreflexia, spasticity
- Confusion, disorientation, lethargy
- Seizures, coma (severe — Na⁺ > 160 mEq/L)
- Intracranial hemorrhage risk from brain cell shrinkage (brain pulls away from meninges)
Key Nursing Interventions
- Replace free water — oral water (if conscious and swallowing), D5W IV, or hypotonic saline (0.45% NaCl) per provider order
- Correct slowly — maximum 10 mEq/L per 24 hours to prevent cerebral edema from rapid rehydration
- Monitor sodium every 4–6 hours during active correction
- Assess neurological status serially — report changes in consciousness or new neurological symptoms
- For DI: desmopressin (DDAVP) for central DI; treat nephrogenic DI by addressing underlying cause and ensuring adequate free water intake
- Implement fall and seizure precautions for altered mental status
- Monitor urine output, urine specific gravity, and urine sodium — helps identify DI vs other causes
Correction Rate Warning
Both disorders: Correct no faster than 10 mEq/L per 24 hours
Rapid correction of hyponatremia → Osmotic Demyelination Syndrome (ODS): demyelination of brainstem neurons, permanent neurological injury, often irreversible
Rapid correction of hypernatremia → Cerebral edema: water rapidly entering previously dehydrated brain cells can cause fatal brain herniation
The brain adapts to chronic sodium disorders by adjusting intracellular osmoles. Rapid correction overwhelms these adaptations and causes osmotic injury in the opposite direction. Serial sodium monitoring and controlled infusion rates are essential safety measures.
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with Infusion Nurses Society (INS) Standards of Practice · Institute for Safe Medication Practices (ISMP) · Standard laboratory reference ranges. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →