Hypernatremia

Definition, Etiology, PathogenesisTop

Hypernatremia is usually defined as a serum [Na+] >145 mmol/L. It is classified as chronic when persisting >48 hours.

Hypernatremia is most frequently caused by loss of water or hypotonic fluids, insufficient water intake (in such cases total body sodium content is unchanged or decreased), or less frequently by excessive sodium intake (in such cases total body sodium content is increased).

Causes:

1) Loss of water: Fever, hypercatabolic conditions (thyrotoxicosis, sepsis).

2) Loss of hypotonic fluids via the skin (excessive sweating), gastrointestinal tract (vomiting, diarrhea), kidneys (central or nephrogenic diabetes insipidus; osmotic diuresis caused by hyperglycemia, mannitol, or urea).

3) Insufficient water intake in patients unable to drink fluids unassisted (unconscious patients, young children, residents of long-term care facilities) or due to impaired thirst.

4) Excessive sodium intake: Administration of excessive doses of NaHCO3 in patients with lactic acidosis or undergoing cardiopulmonary resuscitation, feeding infants high-salt foods (salt poisoning), drinking of seawater by marine accident survivors, use of dialysis solutions containing excessively high sodium levels in patients undergoing hemodialysis or peritoneal dialysis.

Extracellular fluid (ECF) volume may be reduced (hypovolemia), normal (euvolemia), or increased (hypervolemia).

In the early phase of hypernatremia, water is shifted from the intracellular fluid (ICF) to the ECF (cellular dehydration). With time, osmolytes are generated in cells and an influx of Na+­, K+, and Cl− ions into the cells occurs, which causes a decrease in the osmotic gradient between the ICF and the ECF. For this reason, in patients with chronic hypernatremia manifestations of central nervous system (CNS) dehydration may be absent. In physiologic conditions, kidneys respond to hypernatremia by achieving the maximum urine concentration (because of increased effective plasma osmolality).

Clinical FeaturesTop

Manifestations of hypernatremia depend on the rate of serum [Na+] increase, severity of hypernatremia, and coexisting blood volume changes. Signs and symptoms of the underlying condition causing hypernatremia are frequently seen.

Early manifestations of developing hypernatremia include loss of appetite as well as nausea and vomiting. In later stages patients develop impaired mental status and agitation or somnolence, which may progress to coma. Increased muscle tone and hyperreflexia may be present.

In patients with hypernatremia caused by hypotonic fluid loss or insufficient water intake, manifestations of hypovolemia may be present, urine volume is usually low, and urine is highly concentrated. Urine output is high in patients with diabetes insipidus (with low urine specific gravity) or osmotic diuresis.

Patients with chronic hypernatremia are often asymptomatic. A too rapid correction of chronic hypernatremia may result in cerebral edema, which is manifested by the onset of neurologic signs and symptoms in a previously asymptomatic patient.

DiagnosisTop

Hypernatremia is usually defined as a serum [Na+] >145 mmol/L.

In every patient consider total body water status to establish the cause of hypernatremia. Hypernatremia with hypovolemia suggests extrarenal or renal fluid loss or insufficient water intake. Hypernatremia with hypervolemia suggests excessive sodium intake (as dietary sodium or as solutions used for treatment of hyponatremia or acidosis). Hypernatremia with euvolemia occurs in the case of moderate extrarenal or renal fluid loss. In patients with renal water loss in whom osmotic diuresis has been excluded, establish the type and cause of diabetes insipidus.

TreatmentTop

General Measures

1. Try to control the cause of hypernatremia and correct serum [Na+] by administering fluids without effective osmolytes.

2. Correction of hypernatremia should correspond to the rate at which hypernatremia developed. The rate of the serum [Na+] decrease during the first 24 hours should not be >1 mmol/L/h in acute hypernatremia and >0.5 mmol/L/h in chronic hypernatremia.Evidence 1Strong recommendation (benefits clearly outweigh downsides; right action for all or almost all patients). Low Quality of Evidence (low confidence that we know true effects of the intervention). Quality of Evidence lowered due to indirectness (majority of evidence coming from pediatric studies). Fang C, Mao J, Dai Y, et al. Fluid management of hypernatraemic dehydration to prevent cerebral oedema: a retrospective case control study of 97 children in China. J Paediatr Child Health. 2010 Jun;46(6):301-3. doi: 10.1111/j.1440-1754.2010.01712.x. Epub 2010 Apr 16. PMID: 20412412.

Pharmacotherapy

1. Select infusion fluid based on volume status:

1) In patients with hypovolemia, administer a balanced crystalloid until blood pressure is normalized, then use a 1:1 mixture of 0.45% NaCl and 5% glucose (dextrose) solutions.

2) In patients with euvolemia or hypervolemia, administer a 5% glucose solution. In patients with hypervolemia, add oral or IV furosemide; repeat the dose every 6 to 8 hours if necessary.

2. Estimate the serum [Na+] change after the administration of 1 L of the solution, using the same formula as for hyponatremia. The resulting value will be negative (meaning that serum [Na+] is decreasing). Using the same method, calculate the volume of the solution to be infused over 1 hour to achieve the target reduction of serum [Na+]. Measure serum [Na+] frequently (initially every 1-2 hours) and adjust the management accordingly.

3. Another approach involves initial calculation of the water deficit using the following formula:

Where: [Na+]ser, current serum [Na+]; [Na+]target, target serum [Na+]; BM, body mass expressed in kilograms; BM × 0.6, total body water in liters.

Add the volume of the current water loss to the calculated water deficit and administer the resulting volume over 72 hours (half of the volume over the first 24 hours). Frequently measure serum [Na+].

4. In conscious patients with mild hypernatremia, water deficit may be corrected by the oral route.

5. In patients with extreme hypernatremia, you can consider removing sodium and water excess using dialysis.

PrognosisTop

Mortality rates in patients with severe hypernatremia are >50%, but death is most frequently caused by the underlying condition.