💧 Hypernatremia

Definition:

• Serum sodium >145 mEq/L

• Reflects a relative water deficit compared with sodium, not always excess sodium.

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1️⃣ Pathophysiology

Hypernatremia occurs due to either:

1. Water loss (most common) → urine, GI, skin, or insensible losses

2. Sodium gain → hypertonic saline, sodium bicarbonate, salt tablets

3. Impaired thirst or inability to access water → elderly, infants, neurologically impaired

Effect on cells:

• Extracellular hypertonicity → water shifts out of cells → cellular dehydration, especially in the brain → neurologic symptoms.

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2️⃣ Clinical Manifestations

Mild Hypernatremia (145–155 mEq/L)

• Often asymptomatic

• Thirst, dry mucous membranes

Moderate to Severe (>155 mEq/L)

• Lethargy, irritability

• Muscle weakness, twitching

• Confusion, stupor

• Seizures, coma (especially if rapid onset)

Other signs

• Decreased skin turgor

• Hypotension or tachycardia (if hypovolemic)

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3️⃣ Causes

Type	Mechanism	Examples
Hypovolemic hypernatremia	Water loss > sodium loss	Diuretics, diarrhea, vomiting, sweating, burns
Hypervolemic hypernatremia	Sodium gain > water gain	Hypertonic saline, sodium bicarbonate, salt tablets
Euvolemic hypernatremia	Pure water loss	Diabetes insipidus (central or nephrogenic), osmotic diuresis

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4️⃣ Laboratory Evaluation

• Serum sodium >145 mEq/L

• Serum osmolality >295 mOsm/kg

• Urine osmolality:

  • High → appropriate renal response to conserve water

  • Low → diabetes insipidus (kidneys cannot concentrate urine)

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5️⃣ Management Principles

Step 1: Assess volume status

• Hypovolemic → restore intravascular volume first with isotonic saline.

• Hypervolemic → treat underlying cause, consider diuretics.

• Euvolemic → often due to diabetes insipidus, treat underlying cause.

Step 2: Correct water deficit

$$\text{Water deficit (L)} = 0.6 \times \text{body weight (kg)} \times \frac{\text{Na}^+-140}{140}$$

• Correct slowly: <10–12 mEq/L per 24h to avoid cerebral edema.

Step 3: Treat underlying cause

• Central diabetes insipidus → desmopressin

• Nephrogenic DI → thiazides, low-salt diet, treat hypercalcemia or hypokalemia if present

• Sodium overload → stop offending agent, diuretics

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6️⃣ Key Clinical Pearls

• Rapid onset hypernatremia → higher risk of neurologic symptoms.

• Chronic hypernatremia → brain adapts (osmotic adaptation), slower correction needed.

• Always calculate water deficit before fluid replacement.

• Monitor serum sodium closely during correction.