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- Serum sodium (Na) concentration >145 mEq/L (1)
- Usually represents a state of hypertonicity (1,2)
- Na concentration reflects balance between total body water (TBW) and total body Na. Hypernatremia occurs from deficit of water relative to Na.
- Hypernatremia results from net water loss or, more rarely, from primary Na gain (1).
- May exist with hypo-, hyper-, or euvolemia, although hypovolemia is by far most common type
- Hypovolemic: occurs with a decrease in TBW and a proportionately smaller decrease in total body Na
- Euvolemic: no change in TBW with a proportionate increase in total body Na
- Hypervolemic: increase in TBW and a proportionately greater increase in total body Na
- It has been shown to be an indicator for higher mortality in critically ill patients and patients with chronic kidney disease (CKD) (3)[B].
- Hypernatremia will not develop if thirst mechanism is intact and water is available.
Etiology and Pathophysiology
- Water loss (total body Na normal). Hypernatremia due to water loss occurs only in patients who can’t access water such as infants, elderly, patients with altered mental status and hypodipsia (5).This is called dehydration and differs from hypovolemia where both salt and water are lost. The following conditions lead to water loss:
- Insensible loss
- Excessive sweating, such as with fever, infants under radiant heaters, and exercise
- Renal loss
- Nephrogenic diabetes insipidus (DI) (congenital or due to renal dysfunction, hypercalcemia, hypokalemia, medication-related, e.g., lithium)
- Central DI (due to head trauma, stroke, meningitis) (4)
- Osmotic diuresis: glucose, urea, and mannitol
- Post-ATN diuresis
- Gastrointestinal loss
- Osmotic diarrhea: lactulose, malabsorption, and some types of infectious diarrhea
- Enterocutaneous fistula
- Vomiting, NG suction
- Hypothalamic disorders leading to impaired thirst or osmoreceptor function
- Primary hypodipsia
- Reset osmostat due to volume expansion in mineralocorticoid excess.
- Essential hypernatremia with loss of osmoreceptor function
- Insensible loss
- Excess Na (increase in total body Na) resulting from the following:
- IV NaCl or NaHCO3 during cardiopulmonary resuscitation, metabolic acidosis, or hyperkalemia (4)
- Sea water ingestion
- Excessive use of NaHCO3 antacid
- Incorrect infant formula preparation
- Intrauterine NaCl for abortion
- Excessive Na in dialysate solutions
- Disorders of the adrenal axis (Cushing syndrome, Conn syndrome, congenital adrenal hyperplasia)
- Tube feeding
- With acute hypernatremia, the rapid decrease in brain volume can cause rupture of the cerebral veins, leading to focal intracerebral and subarachnoid hemorrhages and possibly irreversible neurologic damage (2).
Some forms of DI may be hereditary.
- Patients at increased risk include those with an impaired thirst mechanism or restricted access to water as well as those with increased water loss
- Elderly patients (may also have a diminished thirst response to osmotic stimulation via an unknown mechanism)
- Patients who are intubated/have altered mental status
- Diabetes mellitus
- Prior brain injury
- Diuretic therapy, especially loop diuretics
- Lithium treatment
- Treatment/prevention of underlying cause
- Properly prepare infant formula and never add salt to any commercial infant formula.
- Keep patients well hydrated.
Commonly Associated Conditions
- Altered mental status
- Hypermetabolic conditions
- Head injury
- Renal dysfunction