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Signs of hypernatremia include warm, doughy, velvety skin, dry mucous membranes, muscular signs such as twitching and hyperreflexia and central nervous system symptoms such as lethargy, confusion, irritability, rigidity, generalized convulsions, and finally, coma. Patients will complain of extreme thirst. In the presence of markedly elevated serum sodium, electrocardiograms may be abnormal, and the CSF protein concentration may be elevated. The degree of dehydration associated with hypernatremia may be more than clinically apparent -- i.e., these patients may look better than they actually are. This is because intravascular volume is relatively preserved at the expense of intracellular water.
In hypernatremia, there is a greater loss of water than sodium. In the majority of cases, the total body sodium is actually below normal. Like hyponatremic dehydration, the causes can be divided into renal and extrarenal. The renal conditions result in inappropriate loss of both water and sodium; even in conditions of volume contraction, the urine is relatively dilute (sg < 1.015) and the urine sodium is high (> 20 mEq/L)
As in hyponatremia, the first step is to determine volume status, and then urine sodium. In the case of hypovolemia, both total body sodium and water are decreased, with the loss of water exceeding that of sodium. In hypervolemia, there is increased total body sodium; total body water may be increased, decreased or normal. In euvolemia, there is a decrease in total body water. The appearance of euvolemia belies intracellular dehydration. Total body sodium may be increased, decreased or normal. The urine sodium will be high if the kidneys are either not reclaiming sodium well or if they are appropriately dumping excessive sodium. Be aware that if other osmotically active substances are present (mannitol, glucose), hypotonic losses of urine will occur with resultant hypernatremia.
The following table is adapted from Pediatric Annals (1995) 24:23-30:
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The key to the treatment of hypernatremia is slow correction of the serum sodium concentration. When hypernatremic dehydration occurs, the brain shrinks. Tearing of the communicating vessels and hemorrhage may occur. To compensate, the brain produces "idiogenic osmoles" (aspartate, glutamate, taurine) which raise the tonicity of the brain cells and limit shrinkage. If the serum sodium falls too quickly, free water will move from the serum into the relatively hypertonic brain tissue, causing cerebral edema. If repletion is conducted slowly, the intracellular osmoles will have time to diminish. Unlike isotonic and hypotonic dehydration, hypernatremic dehydration involves fluid therapy calculated over at least 48 hours. The goal is to avoid dropping the sodium any faster than 2.5 mEq per four hours.
1. Calculate the water deficit
2. Replete water and electrolytes over 2 to 3 days.
3. Correct any associated abnormalities
1. The water deficit is calculated as:
| water deficit = 0.6 * body weight (kg) * (1-140/serum Na) |
In moderate hypernatremia (serum Na between 155 and 175), one half of the free water deficit is given in the first 24 hours. In severe hypernatremia (serum Na greater than 175), one third is given in the first 24 hours. Electrolyte-free solutions are never given, at a minimum, use 0.2% NS. During repletion, serial chemistries can be obtained to follow the rate of decrease of serum sodium concentration. For concentrations greater than 165 mEq/L, values should be obtained every 4 hours until they approach normal.
3. Correct associated abnormalities:
Hypernatremia is associated with hyperglycemia and hypocalcemia.
Insulin is rarely required to manage the hyperglycemia, which should improve
dilutionally. It may be necessary to use a solution with less than
5% dextrose, such as D2.5%1/3NS. The calcium level may fall under
9 mg/dL, but tetany is rare. Central nervous system complications
of hemorrhage or thrombosis may also occur.
| Example: Severe dehydration, moderate hypernatremia
A 3 month old is brought to the ER with a history of diarrhea, a small amount of vomiting, and decreased po intake. A rotavirus antigen assay is positive. Serum sodium is 165. The patient is rousable, but not very active. By physical exam, the patient looks about 5 to 10% dehydrated, but has poor skin turgor. The patient's weight is 4.7 kg. The assumption is that this patient is more dehydrated than she appears, perhaps as much as 15%. The patient's free water deficit is: 0.7 * (4.7 * 1.15) * (1-140/165) = 573 mL 0.7 is used because at this age, total body water comprises about 70%
of the patient's body weight. The patient's weight is corrected for
15% dehydration by multiplying by 1.15. The sodium is corrected in
the formula to an optimal value of 140.
The goal is to give 287 mL of free water on day #1. If the above were repleted with NS, there would be no free water (i.e., water in excess of isotonic), thus something with less Na must be chosen. ½NS is half free water; thus if 995 mL were infused in one day, it would be the equivalent of given 497 mL of free water. |
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