Hypernatremia : Causes, Signs and Symptoms, Treatment

Hypernatremia is defined as a serum sodium concentration >145 mmol/L. It represents a hyperosmolar state typically due to a net water deficit relative to sodium content, rather than excessive sodium accumulation.

🔑 Key Concept: Hypernatremia is fundamentally a “water problem”, not a sodium excess issue.

Classification

• Acute Hypernatremia: Develops in <24 hours.
• Chronic Hypernatremia: Persists for >48 hours.
  • Correction must be cautious in chronic cases to avoid cerebral edema.

Etiology (Causes of Hypernatremia)

1. Water Losses

   • Insensible Losses (Extrarenal):
     • Excessive sweating, burns, fever, tachypnea, respiratory infections.
   • Gastrointestinal Losses:
   • Vomiting, infectious diarrhea, nasogastric suctioning.
   • Transcellular Shifts:
     • Conditions like rhabdomyolysis or seizures can shift water into cells, concentrating extracellular sodium.

2. Renal Losses

   • Diabetes Insipidus (DI):
   • Central DI: Deficient ADH (vasopressin) secretion.
     • Causes: Trauma, neurosurgery, tumors, infections, hypoxic brain injury, granulomatous disease.
   • Nephrogenic DI: Renal unresponsiveness to ADH.
     • Causes: Chronic kidney disease, hypercalcemia, hypokalemia, sickle cell disease, lithium, demeclocycline.
3. Osmotic Diuresis:
   • Seen in DKA, hyperosmolar hyperglycemic state (HHS), mannitol use, glycosuria, urea.

Clinical Presentation

Neurologic Symptoms

• Result from cellular dehydration in the CNS.
• Common signs and symptoms:
  • Lethargy, weakness
  • Irritability, confusion
  • Seizures, coma (especially if Na+ >160 mmol/L)

⚠️ High-Risk Group: Elderly patients and those with impaired thirst or access to fluids.

Diabetes Insipidus Clue

• Patients with DI may report polyuria (3–20 L/day) and polydipsia.
• Urine is markedly dilute despite hypernatremia.

Diagnosis

Laboratory Evaluation

• Serum Sodium: >145 mmol/L
• Serum Osmolality: Elevated
• Urine Osmolality: Helps differentiate etiology:
  • Low (<300 mOsm/kg): Suggests DI or primary polydipsia
  • High (>600 mOsm/kg): Suggests extrarenal losses
• Urine Sodium and Potassium
• Glucose, Urea, Creatinine
• 24-hour urine volume
• Plasma Arginine Vasopressin (AVP) or copeptin levels (if available)
• Desmopressin (DDAVP) Test: Differentiates between Central and Nephrogenic DI

Management

General Principles

• Correct underlying cause
• Gradual correction of sodium to avoid cerebral edema
  • Acute hypernatremia: Can correct faster (up to 1–2 mEq/L per hour in symptomatic cases)
  • Chronic hypernatremia: Correct no more than 0.5 mEq/L per hour or 12 mEq/L per 24 hours

Fluid Replacement

• Initial resuscitation (if hypovolemic): Use isotonic fluids (0.9% saline)
• Free water replacement: Use 5% dextrose (D5W) or hypotonic saline (0.45%)
• Oral rehydration if mild and patient can drink

Specific Treatments

Central Diabetes Insipidus (CDI)

• Desmopressin (DDAVP): Intranasal, subcutaneous, IV, or IM
• Monitor sodium and fluid balance carefully

Nephrogenic Diabetes Insipidus (NDI)

• Address underlying cause (e.g., stop lithium)
• Low-sodium diet
• Thiazide diuretics (reduce polyuria via volume contraction)
• NSAIDs (e.g., indomethacin): Reduce prostaglandin-mediated inhibition of ADH

Complications

• Cerebral edema (from overly rapid correction)
• Seizures, permanent neurologic damage
• Coma if untreated severe hypernatremia

HIGH YIELD

• Hypernatremia = water deficit, not sodium excess.
• Always assess volume status first.
• Desmopressin test distinguishes CDI from NDI.
• Correct sodium slowly in chronic cases to avoid brain swelling.
• Elderly and neurologically impaired patients are at highest risk.

Mnemonic: "MODEL" for Hypernatremia Causes

Medications/Meals (high sodium)
Osmotic diuretics
Diabetes insipidus
Excessive water loss
Low water intake

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