hypernatremia

Hypernatremia is an elevated serum sodium concentration, defined as [Na+] above 145 mEq/L. It results from a net water deficit relative to body sodium and reflects impaired free water balance rather than a fixed sodium load. It most often affects the elderly, children, or others with reduced access to water, but can occur at any age.

Causes include decreased water intake and increased water loss. Water loss can be insensible (fever, sweating), renal (diuretics, osmotic diuresis from hyperglycemia or mannitol), gastrointestinal losses (diarrhea, vomiting), or burns. Endocrine causes include central or nephrogenic diabetes insipidus. Iatrogenic causes include infusion of hypertonic saline or rapid administration of concentrated fluids without free water.

Pathophysiology and presentation: Increased extracellular osmolality draws water from cells, leading to cellular dehydration, especially of brain cells. Symptoms range from thirst and restlessness to confusion, lethargy, seizures, or coma. Hypovolemic hypernatremia shows signs of volume depletion; euvolemic or hypervolemic forms reflect the underlying condition.

Diagnosis: measure serum sodium and osmolality, assess volume status, and obtain urine osmolality and sodium to classify the type and cause. Tests for diabetes insipidus (e.g., water deprivation test) may be used when indicated.

Management: correct the free water deficit gradually to avoid cerebral edema. In hypovolemic hypernatremia, restore volume with isotonic saline, then switch to hypotonic fluids (0.45% saline or 5% dextrose in water). In euvolemic or hypervolemic cases, provide free water and treat the underlying cause. Target a correction rate of about 10–12 mEq/L per 24 hours (slower in chronic cases); overly rapid correction can cause complications. Monitor sodium, fluid balance, and neurologic status.