2025 – PAGE 377 – FLUIDS & ELECTROLYTES

HYPERNATREMIA

Hypernatremia is much less common than hyponatremia. Correct at no more than 12 mEq/L/day to avoid an intracranial hemorrhage due to fluid shifts resulting in the tearing of bridging blood vessels. The patient can also get pulmonary edema from fluid shifts. The most common causes of hypernatremia include arginine vasopressin deficiency, excessive sweating, and increased intake. Hypernatremia can also occur due to diarrhea-related dehydration when more water is lost than sodium. If a patient is noted to have hypernatremic dehydration, assume s/he has at least 10% dehydration. Body fluids will be hypertonic with serum osmolarity often in excess of 300 mOsm/kg (300 mmol/kg).

ARGININE VASOPRESSIN DEFICIENCY AND RESISTANCE (AKA DIABETES INSIPIDUS)

If a baby has arginine vasopressin deficiency or resistance, he or she may have feeding problems and develop FTT. If the urine SG is > 1.008, this diagnosis is highly unlikely. GET IMAGING of the brain once the diagnosis is confirmed and obtain all of the pituitary hormone levels (FLATPiG).

• ARGININE VASOPRESSIN DEFICIENCY (AVP-D, previously central diabetes insipidus): Insufficient ADH leads to constant water loss. Look for hypernatremia when the patient does not have access to free water. Polyuria occurs, and the dilute urine has a very low urine osmolality. There is NO glycosuria in AVP-D. Serum osmolality will be HIGH once the patient is not drinking enough free water. Treat with DDAVP/ADH by mouth or intranasally.
  • PEARL: If the patient is an infant, s/he is dependent on an adult to provide enough free water to prevent HYPERNATREMIA. The child cannot walk to the kitchen when s/he feels that strong sense of thirst, and thus can get hypernatremic during some of the longer breaks between feeds. A 10-year-old child, though, has the ability to respond to thirst and obtain free water in the middle of the night. Polyuria (and nocturia) will exist in BOTH examples, but only the baby will get hypernatremic. The key is whether or not a patient has access to free water.
• ARGININE VASOPRESSIN RESISTANCE (AVP-R, previously nephrogenic diabetes insipidus): Usually X-linked recessive, so look for a male patient. He will have massive urine output but NO response to DDAVP/ADH supplementation. Water is lost, and the labs will show hypernatremia. Treat with hydrochlorothiazide and salt restriction.