Episode 198: Hypernatremia

Show Notes

Episode Overview:

• Introduction to Hypernatremia
• Definition and basic concepts
• Clinical presentation and risk factors
• Diagnosis and management strategies
• Special considerations and potential complications

Definition and Pathophysiology:

• Hypernatremia is defined as a serum sodium level over 145 mEq/L.
• It can be acute or chronic, with chronic cases being more common.
• Symptoms range from nausea and vomiting to altered mental status and coma.

Causes of Hypernatremia based on urine studies:

• Urine Osmolality > 700 mosmol/kg
  • Causes:
    • Extrarenal Water Losses: Dehydration due to sweating, fever, or respiratory losses
    • Unreplaced GI Losses: Vomiting, diarrhea
    • Unreplaced Insensible Losses: Burns, extensive skin diseases
    • Renal Water Losses with Intact AVP Response:
    • Diuretic phase of acute kidney injury
    • Recovery phase of acute tubular necrosis
    • Postobstructive diuresis
• Urine Osmolality 300-600 mosmol/kg
  • Causes:
    • Osmotic Diuresis: High glucose (diabetes mellitus), mannitol, high urea
    • Partial AVP Deficiency: Incomplete central diabetes insipidus
    • Partial AVP Resistance: Nephrogenic diabetes insipidus
• Urine Osmolality < 300 mosmol/kg
  • Causes:
    • Complete AVP Deficiency: Central diabetes insipidus
    • Complete AVP Resistance: Nephrogenic diabetes insipidus
• Urine Sodium < 25 mEq/L
  • Causes:
    • Extrarenal Water Losses with Volume Depletion: Vomiting, diarrhea, burns
    • Unreplaced Insensible Losses: Sweating, fever, respiratory losses
• Urine Sodium > 100 mEq/L
  • Causes:
    • Sodium Overload: Ingestion of salt tablets, hypertonic saline administration
    • Salt Poisoning: Deliberate or accidental ingestion of large amounts of salt
• Mixed or Variable Urine Sodium
  • Causes:
    • Diuretic Use: Loop diuretics, thiazides
    • Adrenal Insufficiency: Mineralocorticoid deficiency
    • Osmotic Diuresis with Renal Water Losses: High glucose, mannitol

Risk Factors:

• Patients with impaired thirst response or those unable to access water (e.g., altered or ventilated patients) are at higher risk.
• Important to consider underlying conditions affecting thirst mechanisms.

Diagnosis:

• Initial assessment includes history, physical examination, and laboratory tests.
• Key tests: urine osmolality and urine sodium levels.
• Lab errors should be considered if the clinical picture does not match the lab results.

Management Strategies:

• Calculate the Free Water Deficit (FWD) to guide treatment. 

• Administration routes include oral, NGT, G-tube, or IV with D5W for larger deficits.
• Safe correction rate is 10-12 mEq/L per day or 0.5 mEq/L per hour to avoid cerebral edema.
• Address hypovolemia with isotonic fluids before correcting sodium.

Monitoring and Follow-Up:

• Monitor sodium levels every 4-6 hours.
• Assess urine output and adjust free water administration as needed.
• Admission to ICU for symptomatic patients or those with severe hypernatremia (sodium >160 mEq/L).
• Decision to discharge vs admit is a complicated one that factors in symptoms, etiology, degree of hypernatremia, patient preference, access to follow up, etc.

Take Home Points:

• Hypernatremia is a serum sodium level over 145 mEq/L, with symptoms ranging from nausea to coma.
• It is primarily caused by water loss exceeding intake due to various factors like sweating, vomiting, diarrhea, and renal issues.
• Correcting hypernatremia too quickly can lead to cerebral edema, so a safe correction rate is essential.
• Initial treatment involves calculating the Free Water Deficit and selecting the appropriate administration route.
• Monitor sodium levels frequently and decide on admission or discharge based on symptoms, sodium levels, and patient’s ability to follow up.