In hypervolemic hyponatremia, there is an excess of total body water and sodium, although the increase in water isgreater than the increase in sodium. In renal failure, there isan inability to excrete sodium or water; the urine sodium maybe low or high, depending on the cause of the renal insufficiency. In other causes of hypervolemic hyponatremia, there is a decrease in the effective blood volume because of either third space fluid loss or poor cardiac output (see Chapter145). In response to the low effective blood volume, ADHcauses renal water retention, and the kidneys also retainsodium, leading to a low urine sodium concentration. Thepatient’s serum sodium concentration decreases when waterintake exceeds sodium intake, and ADH prevents the normalloss of excess water.
Hyponatremia causes a fall in the osmolality of the extracellular space. Because the intracellular space then has a higher osmolality, water moves from the extracellular space to the intracellular space to maintain osmotic equilibrium. The increase in intracellular water may cause cells to swell. Brain cell swelling is responsible for most of the symptoms of hyponatremia. Neurologic symptoms of hyponatremia include anorexia, nausea, emesis, malaise, lethargy, confusion, agitation, headache, seizures, coma, and decreased reflexes. Patients may develop hypothermia and Cheyne-Stokes respirations. Hyponatremia can cause muscle cramps and weakness. Symptoms are more severe when hyponatremia develops rapidly; chronic hyponatremia can be asymptomatic because of a compensatory decrease in brain cell osmolality, which limits cerebral swelling.
Rapid correction of hyponatremia can produce central pontine myelinolysis. Avoiding more than a 12-mEq/L increase in the serum sodium every 24 hours is prudent, especially in chronic hyponatremia. Treatment of hypovolemic hyponatremia requires administration of IV fluids with sodium to provide maintenance requirements and deficit correction, as well as to replace ongoing losses (see Chapter 33). For children with SIADH, water restriction is the cornerstone of therapy. Children with hyponatremia secondary to hypothyroidism or cortisol deficiency need specific hormone replacement. Acute water intoxication rapidly self-corrects with transient restriction of water intake, which is followed by introduction of a normal diet. Treatment of hypervolemic hyponatremia centers on restriction of water and sodium intake, but disease-specific measures, such as dialysis in renal failure, also may be necessary.
Emergency treatment of symptomatic hyponatremia, such as seizures, uses IV hypertonic saline to increase the serum sodium concentration rapidly, which leads to a decrease in brain edema. One milliliter per kilogram of 3% sodium chloride increases the serum sodium by approximately 1 mEq/L.
A child often improves after receiving 4 to 6 mL/kg of 3% sodium chloride.
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There are three basic mechanisms of hypernatremia (Fig. 35-2). Sodium intoxication is frequently iatrogenic in a hospital setting resulting from correction of metabolic acidosis with sodium bicarbonate. In hyperaldosteronism, there is renal retention of sodium and resultant hypertension; the hypernatremia is mild.
Hypernatremia resulting from water losses develops only if the patient does not have access to water or cannot drink adequately because of neurologic impairment, emesis, or anorexia. Infants are at high risk because of their inability to control their own water intake. Ineffective breastfeeding, often in a primiparous mother, can cause severe hypernatremic dehydration. High insensible losses of water are especially common in premature infants; the losses increase further as a result of radiant warmers or phototherapy for hyperbilirubinemia. Children with extrarenal causes of water loss have high levels of ADH and very concentrated urine.