Hypernatremia Etiology
Hypernatremia in this patient is most likely caused by either excessive water loss (gastrointestinal, renal, or insensible losses) or iatrogenic sodium excess, with the specific etiology determined by assessing volume status, urine osmolality, and recent fluid/medication history. 1, 2
Primary Pathophysiological Mechanisms
Hypernatremia fundamentally reflects an imbalance where water content is deficient relative to sodium content in body fluids. 3, 4 The causes can be systematically categorized:
Water Loss (Most Common)
Gastrointestinal losses including diarrhea, vomiting, fistulas, or drainage tubes represent a major cause of hypernatremia through hypotonic fluid depletion. 1
Renal water losses occur through:
- Inadequate water intake due to impaired thirst mechanism or lack of access to water 2
- Diabetes insipidus (central or nephrogenic) 2, 3
- Osmotic diuresis 5
Insensible losses particularly from excessive sweating in endurance athletes or high fever states can lead to hypernatremia when water replacement is inadequate. 1
Transepidermal water loss is especially critical in very low birth weight infants and represents a common iatrogenic cause when replacement is incorrect. 6, 1
Sodium Excess (Less Common)
Iatrogenic sodium administration through:
- Excessive sodium in parenteral nutrition 6, 1
- Hypertonic saline infusions 6
- Inadvertent sodium loading in critically ill patients 1
Salt ingestion can cause severe hypernatremia, though fatal cases from table salt ingestion are rare but documented with surprisingly small amounts (70-90g can be lethal). 7
Clinical Context-Specific Causes
In hospitalized patients: Inadequate fluid prescription is the most common iatrogenic cause. 1
In pediatric patients: Incorrect replacement of transepidermal water loss in premature infants is a leading cause. 6, 1
In patients with liver disease: Excessive diuretic use can precipitate hypovolemic hypernatremia. 1
In athletes: Excessive sweating without adequate water replacement during endurance activities. 1
Diagnostic Algorithm to Identify the Cause
Step 1: Assess volume status clinically
- Hypovolemic (dry mucous membranes, decreased skin turgor, orthostatic hypotension): suggests water loss exceeding sodium loss 2, 5
- Euvolemic: suggests pure water loss (diabetes insipidus, insensible losses) 2
- Hypervolemic (edema, ascites): suggests sodium excess 5
Step 2: Measure urine osmolality
- Urine osmolality >800 mOsm/kg: appropriate renal response, suggests extrarenal water loss 1, 2
- Urine osmolality <300 mOsm/kg: inappropriate dilute urine, suggests diabetes insipidus 2, 3
Step 3: Review medication and fluid history
- Recent diuretic use 1
- Parenteral nutrition composition 6, 1
- Hypertonic saline administration 6
- Access to free water 2
Step 4: Measure urine sodium
- Helps differentiate renal versus extrarenal losses and guides fluid replacement strategy 1
Critical Pitfalls to Avoid
Do not assume dehydration is the only cause - while hypernatremia is usually associated with dehydration and water deficit, pure sodium excess can occur iatrogenically and requires different management. 4, 7
Do not overlook medication-induced causes - diuretics, particularly in cirrhotic patients, and inadvertent sodium administration in parenteral nutrition are frequently missed. 6, 1
Do not forget diabetes insipidus in euvolemic patients with inappropriately dilute urine, as this requires specific treatment with desmopressin rather than just fluid replacement. 2, 3
In neonates, anticipate hypernatremia in very low birth weight infants during the transition phase, as it mostly results from incorrect replacement of transepidermal water loss or inadequate water intake. 6