Guide — Electrolytes
Sodium Disorders: Hyponatremia & Hypernatremia
Fluid balance concepts, neurological manifestations, causes, risk factors, correction rate safety, and nursing priorities for sodium imbalances in clinical nursing practice.
10 min read · Electrolytes
Educational use only. Sodium correction protocols are provider-driven. Never correct sodium faster than ordered — overly rapid correction causes irreversible neurological damage. 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.
Sodium and Fluid Balance
Normal serum sodium: 135–145 mEq/L. Sodium is the primary extracellular cation and the main determinant of serum osmolality and water distribution.
Key principle: Sodium disorders are primarily disorders of water balance, not sodium intake. The sodium level reflects the ratio of sodium to water — not just the absolute amount of sodium in the body.
| Concept | Hyponatremia | Hypernatremia |
|---|---|---|
| Serum osmolality | Low (< 280 mOsm/kg) | High (> 295 mOsm/kg) |
| Water movement | Water moves INTO cells (hypotonic environment) → cellular swelling | Water moves OUT OF cells (hypertonic environment) → cellular shrinkage |
| Brain impact | Cerebral edema → herniation if severe | Cerebral dehydration → vascular tearing → intracranial hemorrhage |
Hyponatremia (Na⁺ < 135 mEq/L)
Classification by Volume Status
| Type | Volume Status | Common Causes |
|---|---|---|
| Hypovolemic | Decreased total body water AND sodium (more Na loss) | Diuretics (thiazides most common), vomiting, diarrhea, burns, third-spacing |
| Euvolemic | Increased total body water, normal sodium | SIADH (most common cause), hypothyroidism, psychogenic polydipsia, glucocorticoid deficiency |
| Hypervolemic | Both increased (water retention > sodium retention) | Heart failure, cirrhosis, nephrotic syndrome, renal failure |
SIADH (Syndrome of Inappropriate ADH)
- ADH (vasopressin) is secreted inappropriately, causing water retention and dilutional hyponatremia
- Causes: CNS disorders (stroke, meningitis, brain tumors), pulmonary disease (pneumonia, TB), malignancy (small cell lung cancer produces ectopic ADH), pain, medications (SSRIs, carbamazepine, cyclophosphamide, vincristine, opioids), major surgery
- Key features: euvolemic, urine osmolality > serum osmolality, urine sodium > 40 mEq/L despite low serum sodium
- Treatment: fluid restriction (primary), vasopressin receptor antagonists (tolvaptan) for severe/symptomatic, 3% saline only for acute severe symptoms with seizure or coma
Neurological Manifestations (Correlate with Severity)
| Sodium Level | Symptoms |
|---|---|
| 130–134 mEq/L (mild) | Often asymptomatic; mild headache, nausea, fatigue |
| 125–129 mEq/L (moderate) | Confusion, disorientation, irritability, muscle cramps |
| < 125 mEq/L (severe) | Seizures, coma, herniation — medical emergency |
Correction Rate Safety — Osmotic Demyelination Syndrome (ODS)
Correcting sodium too rapidly in chronic hyponatremia causes osmotic demyelination syndrome (previously called central pontine myelinolysis). Brain cells adapted to low osmolality cannot tolerate rapid osmolality changes — myelin sheaths are destroyed.
Maximum safe correction: ≤ 10–12 mEq/L per 24 hours (≤ 6–8 mEq/L per 24 hours in very high-risk patients: alcoholism, malnutrition, liver disease)
Hypernatremia (Na⁺ > 145 mEq/L)
Causes
- Inadequate water intake: altered mental status, physical inability to drink, infant left without access to water, NPO without IV hydration
- Excessive water loss: diabetes insipidus (central or nephrogenic — hallmark cause of hypernatremia), fever (insensible loss), burns, diarrhea (especially in infants)
- Excess sodium intake: hypertonic saline administration, saltwater near-drowning, excessive sodium bicarbonate
- Diabetes insipidus (DI): ADH deficiency (central DI — post-neurosurgery, head trauma, pituitary tumor) or renal insensitivity to ADH (nephrogenic DI — lithium, demeclocycline, hypercalcemia). Produces large volumes of very dilute urine despite rising sodium.
Clinical Manifestations
- Thirst (hallmark early symptom — absent in impaired patients)
- Dry mucous membranes, decreased skin turgor, concentrated urine (except in DI)
- Neurological: agitation, restlessness, confusion, lethargy, seizures, coma
- Tachycardia, hypotension if volume-depleted
Treatment
- Identify and treat the underlying cause (stop free water losses first)
- Free water replacement: oral water (preferred if patient can drink), enteral water via tube, IV D5W or 0.45% NaCl
- Correct no faster than 10–12 mEq/L per 24 hours — rapid correction of chronic hypernatremia causes cerebral edema
- Central DI: desmopressin (DDAVP — synthetic ADH)
- Nephrogenic DI: address cause (stop lithium if possible); thiazide diuretics (paradoxically reduce urine output in DI); low-sodium, low-protein diet
DI vs SIADH — Key Differentiator for NCLEX
| Feature | Diabetes Insipidus | SIADH |
|---|---|---|
| Serum sodium | High (hypernatremia) | Low (hyponatremia) |
| Urine output | Massive (3–20 L/day) | Decreased |
| Urine specific gravity | Very low (< 1.005) | High (> 1.020) |
| Urine osmolality | Very low (< 200 mOsm/kg) | High (> serum osmolality) |
| ADH | Absent/insufficient | Excess |
| Treatment | DDAVP (central DI); free water | Fluid restriction; 3% saline if severe |
Nursing Priorities
- Monitor sodium levels frequently (q4–6h during active correction); accurate I&O
- Neurological assessments at least every 1–2 hours for moderate–severe dysnatremia
- Seizure precautions for severe hyponatremia or rapid sodium changes
- For hyponatremia: restrict free water per orders; patient education on water intake limits
- For hypernatremia: offer oral water or free water via tube q1–2h if not NPO; prevent further water loss; monitor urine specific gravity and output
- For DI post-neurosurgery: measure urine output hourly; report output > 200–250 mL/hr; monitor serum sodium and urine specific gravity; have DDAVP available
- For SIADH: daily weight; fluid intake/output balance; educate patient on fluid restriction rationale
NCLEX Pearls
Sodium = water balance: Think “too much water relative to sodium” for hyponatremia; “not enough water relative to sodium” for hypernatremia.
Neurological symptoms rule: Both hypo and hypernatremia cause neurological symptoms. Low Na = swelling (edema). High Na = shrinking (dehydration of brain cells). Both = altered consciousness → seizures → coma.
3% NaCl (hypertonic saline): Reserved for severe symptomatic hyponatremia (seizures, coma). Never given rapidly — maximum 1–2 mEq/L per hour, and only with close monitoring.
SIADH classic scenario: Patient post-surgery or with lung cancer presents with low Na, concentrated urine, and low urine output despite normal hydration → SIADH → fluid restriction is treatment.
DI classic scenario: Post-craniotomy or head trauma patient produces 5+ liters of dilute urine with rising serum sodium → central DI → DDAVP.
Correction rate: ≤ 10–12 mEq/L per 24 hours for BOTH hypo and hypernatremia. Faster correction of hyponatremia = ODS. Faster correction of hypernatremia = cerebral edema.
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 →