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(See also Fluid and Electrolyte Metabolism: Hypernatremia.)
Hypernatremia
is a serum Na concentration > 150
mEq/L, usually from dehydration. Signs include lethargy and seizures.
Treatment is cautious hydration with 0.45% saline solution.
Etiology
and Pathophysiology
Hypernatremia develops when water is lost in excess of Na (hypernatremic dehydration), when Na intake exceeds Na losses (salt poisoning), or both.
Water loss in excess of Na intake is most commonly caused by diarrhea, vomiting, or high fever. It may also be caused by poor feeding in the early days of life (eg, when mother and infant are both learning to breastfeed) and may occur in very low-birth-weight (VLBW) infants born at 24 to 28 wk. In VLBW infants, insensible water losses through an immature, water-permeable stratum corneum combine with immature renal function and a reduced ability to produce concentrated urine to facilitate free water loss. Insensible water loss through the skin is also significantly increased by radiant warmers and phototherapy lights; exposed VLBW infants may require up to 250 mL/kg/day of water IV in the 1st few days, after which the stratum corneum develops and insensible water loss decreases.
Solute overload most commonly results from adding too much salt when preparing homemade infant formula or from administering hyperosmolar solutions. Fresh frozen plasma and human albumin contain Na and can contribute to hypernatremia when given repeatedly to very premature infants.
Symptoms,
Signs, and Diagnosis
Symptoms and signs include lethargy, restlessness, hyperreflexia, spasticity, and seizures. Intracranial hemorrhage, venous sinus thrombosis, and acute renal tubular necrosis are major complications.
Diagnosis is suspected by symptoms and signs and confirmed by measuring serum Na concentration. Additional laboratory findings may include an increase in BUN, a modest increase in serum glucose, and, if serum K is low, a depression in the level of serum Ca.
Treatment
Treatment is with 5% dextrose/0.3% to 0.45% saline solution IV in volumes equal to the calculated fluid deficit (see Dehydration and Fluid Therapy: Treatment), given over 2 to 3 days to avoid a rapid fall in serum osmolality, which would cause rapid movement of water into cells and potentially lead to cerebral edema. The goal of treatment is to decrease serum Na by about 10 mEq/day. Body weight, serum electrolytes, and urine volume and specific gravity must be monitored regularly so that fluid administration can be adjusted appropriately. Once adequate urine output is demonstrated, K is added to provide maintenance requirements or replace urinary losses. Maintenance fluids should be provided concurrently.
Extreme hypernatremia (Na > 200 mEq/L) caused by salt poisoning should be treated with peritoneal dialysis, especially if poisoning causes a rapid rise in serum Na.
Prevention
Prevention requires attention to the volume and composition of unusual fluid losses and of solutions used to maintain homeostasis. In neonates and young infants, who are unable to signal thirst effectively and to replace losses voluntarily, the risk of dehydration is greatest. The composition of feedings whenever mixing is involved (eg, some infant formulas and concentrated preparations for tube feeding) requires particular attention, especially when the potential for developing dehydration is high, such as during episodes of diarrhea, poor fluid intake, vomiting, or high fever.
Last full review/revision November 2005
Content last modified November 2005
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