Hypernatremia Comprehensive Review - Urine Nephrology Now

Andrew Bland, MD, MBA, MS

Pathophysiology

Fundamental Concept

Hypernatremia is ALWAYS a water deficit state. Serum [Na+] reflects the ratio of total body sodium to total body water. Elevated [Na+] indicates loss of water in excess of sodium, not sodium excess. Treatment fundamentally involves restoring water balance.

Free Water Deficit Calculation

Free Water Deficit = TBW × ([current Na] - 140) / 140
TBW ≈ 0.6 × weight (kg) in males; 0.5 × weight in females

Volume Status Classification

Pure water loss: Hypernatremic hypovolemia (most common)
Hypotonic fluid loss: Loss of both water and electrolytes with water > sodium
Hypernatremic hypervolemia: Rare; excessive hypertonic saline or sodium gain

Etiology Classification

Renal Water Loss (Polyuric Hypernatremia)

Central Diabetes Insipidus (CDI)

Pathophysiology: ADH deficiency from hypothalamic-pituitary insufficiency
Causes: Idiopathic (30%), traumatic brain injury, neurosurgery, pituitary tumors, infiltrative diseases (sarcoidosis, histiocytosis), granulomatous meningitis
Features: Abrupt onset if traumatic; polyuria (3–20 L/day), polydipsia; partial CDI exists

Nephrogenic Diabetes Insipidus (NDI)

Pathophysiology: Kidney resistance to ADH; collecting duct cannot concentrate urine
Genetic: X-linked (V2 receptor/AVPR2 mutations, >50%); autosomal recessive (AQP2 mutations, rare)
Acquired (common):
- Medications: Lithium (most common; chronic use), amphotericin B, cisplatin, ifosfamide
- Electrolytes: Hypercalcemia, hypokalemia
- Renal disease: Pyelonephritis, reflux nephropathy, post-obstructive polyuria
Features: Large volumes of dilute urine; poor response to desmopressin

Osmotic Diuresis

Causes: Hyperglycemia/DKA (glucose >180 mg/dL), high protein/urea (tube feeding), mannitol/sorbitol, contrast agents
Feature: Large urine volume; urine osmolality may be >300 if solute present

Extrarenal Water Loss

GI losses: Diarrhea (most common cause in elderly), vomiting, NG suctioning
Insensible losses: Mechanical ventilation, tachypnea, hyperthermia, burns, excessive sweating

Iatrogenic (Hospital/ICU Acquired)

Hypertonic saline administration
Tube feeding formulas with inadequate free water supplementation
Normal saline in setting of ongoing insensible losses

Clinical Presentation

Neurologic manifestations result from osmotic stress on brain cells. Cells shrink as extracellular osmolarity rises; traction on meningeal vessels creates intracranial bleeding risk (especially elderly).

Onset	Symptoms
Acute (<48h)	Thirst, irritability, restlessness, lethargy, muscle weakness, tremors, seizures (>160 mEq/L), altered mental status
Chronic (slow onset)	Often asymptomatic (brain adapts via osmolyte accumulation); mild confusion, lethargy; may present with complication (subdural hematoma)
Pediatric	Irritability, high-pitched cry, poor feeding; higher risk of CNS injury

Diagnostic Approach

Step 1: Confirm and Assess Acuity

Serum [Na+] >145 mEq/L (mild 145–150; moderate 150–160; severe >160). Assess acute (<48h) vs chronic onset.

Step 2: Assess Volume Status

Volume Status	Findings	Differential
Hypovolemic	Low JVP, orthostatic changes	GI losses, insensible losses, osmotic diuresis
Euvolemic	Normal vitals and exam	DI (central or nephrogenic), insensible only
Hypervolemic	Edema, elevated JVP, HTN	Hypertonic saline infusion, Na+ gain

Step 3: Urine Osmolality (Key Diagnostic Test)

Urine Osm	Interpretation	Likely Diagnosis
>800 mOsm/kg	Kidney concentrating appropriately	GI losses, insensible losses (extrarenal)
400–800	Partial renal response	Partial CDI, early osmotic diuresis, partial NDI
<300 mOsm/kg	Dilute urine; cannot concentrate	Complete CDI, NDI, osmotic diuresis

Step 4: Desmopressin Challenge (Polyuric Cases)

Give desmopressin 10 mcg IV/SC or 20 mcg intranasal. Measure urine osmolality at 30–60 minutes.

Response	Diagnosis
Urine osm increases >50%	Central DI (responds to ADH)
Urine osm increases 10–50%	Partial CDI or nephrogenic DI
Urine osm unchanged	Nephrogenic DI (resistant to ADH)

Treatment

Correction Rate Rules

Acute hypernatremia (<48h) with symptoms: Target decrease 10–12 mEq/L in 24 hours.
Chronic hypernatremia: Slow correction MANDATORY. Target decrease 8–10 mEq/L per 24 hours.
NEVER lower [Na+] >12 mEq/L in 24 hours—rapid correction of chronic hypernatremia causes cerebral edema (brain has accumulated protective osmolytes; removing osmotic stress too fast causes water influx → herniation risk).

Hypovolemic Hypernatremia

Initial: 0.9% NaCl bolus 500–1000 mL if hypotensive (restore intravascular volume first)
Transition: Once stable, switch to hypotonic fluids (D5W or 0.45% NaCl)
Calculate: Free water deficit + ongoing losses (insensible ~500–1000 mL/day); infuse over 48–72 hours

Example Calculation

70 kg male; [Na+] 155 mEq/L
TBW = 0.6 × 70 = 42 L
Free water deficit = 42 × (155 - 140) / 140 = 4.5 L
Add 500–1000 mL insensible losses → infuse ~5.5 L D5W over 48–72 hours (~100–120 mL/hr)

Euvolemic Hypernatremia (Diabetes Insipidus)

Central DI

Desmopressin (DDAVP): Intranasal 5–10 mcg BID–TID; oral 0.1–0.2 mg TID; IV/SC 2–4 mcg daily
Adjust dose to achieve urine output 2–3 L/day
Patient drinks to thirst; desmopressin provides background DI suppression

Nephrogenic DI

Desmopressin ineffective—do NOT give
Thiazide diuretics (paradoxically helpful): HCTZ 25 mg BID → volume depletion → proximal reabsorption increases → less fluid reaches collecting duct
NSAIDs (indomethacin): Inhibit prostaglandins → decrease solute delivery to medulla
Amiloride (for lithium-induced NDI): Blocks lithium entry into collecting duct cells; 5–10 mg daily
Low sodium diet; adequate free water (often need 2–3 L additional daily)

Specific Clinical Scenarios

Post-Operative/ICU Hypernatremia

Common iatrogenic; from normal saline, tube feeding, insensible losses
Prevention: Regular free water (IV dextrose, enteral water flushes)
Correct slowly (typically chronic, develops over days)

Lithium-Induced NDI

Chronic use; 15–50% incidence
Partially reversible if lithium discontinued
Amiloride most effective pharmacologic option
Monitor [Na+] and renal function closely if continuing lithium

Neonatal Hypernatremia

Causes: Inadequate breastfeeding, insensible losses, diarrhea
Higher risk of CNS injury (brain more susceptible to osmotic changes)
Correct slowly; target decrease 10–15 mEq/L over 48 hours

Clinical Pearls

Hypernatremia = water deficit, not sodium excess—frame therapy as water replacement
Urine osmolality is the key diagnostic test—high urine osm = extrarenal cause; low = DI
Desmopressin challenge effectively distinguishes CDI (responds) from NDI (does not)
Correct slowly in chronic hypernatremia (8–10 mEq/L/24h); faster acceptable in acute + symptomatic
Partial CDI exists—intermediate response to desmopressin
Thiazides paradoxically help NDI by creating mild volume depletion
Amiloride is specific for lithium-NDI; blocks lithium entry into collecting duct cells
Always address the underlying cause—DI management is futile if active osmotic diuresis continues

References

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Sterns RH. Disorders of plasma sodium—causes, consequences, and correction. N Engl J Med. 2015;372(1):55-65. PubMed
Chauhan K, Chau T, Levy G, et al. Prevalence, etiology, and outcomes of hypernatremia in severe sepsis. Medicine (Baltimore). 2019;98(28):e16336. PubMed
Cheema-Dhadli S, Halperin ML, Kamel KS. Hypernatremia—a systematic approach. Kidney Int. 2014;85(2):268-273. PubMed
Zerbe RL, Stropes L, Robertson GL. Vasopressin function in SIADH. Annu Rev Med. 1980;31:315-327. PubMed