Hypernatremia Clinical Case Vignettes — Intermediate

A very dilute urine in the presence of marked hypernatremia indicates failure of renal water conservation, most consistent with diabetes insipidus. Inappropriately low ADH secretion suggests central disease, whereas nephrogenic disease reflects renal resistance. SIADH would produce concentrated urine and hyponatremia, not this pattern.

Hypernatremia that has been present for more than about 48 hours should generally be corrected gradually because rapid reduction can promote cerebral edema as brain osmolytes readjust. The fluid choice depends on hemodynamic status, but correction requires free water with close serial sodium monitoring.

A substantial increase in urine osmolality after desmopressin indicates that the kidneys can respond to ADH and that endogenous ADH availability is deficient. This supports central diabetes insipidus. Nephrogenic diabetes insipidus shows little response because the renal concentrating pathway is resistant.

In shock or significant hypovolemia, circulatory support takes priority. An isotonic crystalloid restores effective arterial volume; once perfusion is stabilized, the remaining free-water deficit can be corrected with an appropriate hypotonic fluid. Hypertonic saline would worsen hypernatremia.

Lithium can enter collecting-duct principal cells and impair ADH signaling, producing nephrogenic diabetes insipidus. The resulting inability to insert aquaporin-2 channels prevents urinary concentration and causes free-water loss. Desmopressin response is therefore limited.

A concentrated urine demonstrates preserved ADH-mediated water conservation. In severe hyperglycemia, glucosuria creates osmotic diuresis and can cause proportionally greater water loss than sodium loss, resulting in hypernatremia when intake cannot keep pace.

The brain gradually accumulates intracellular osmolytes during chronic hypernatremia, partially restoring cell volume and reducing symptoms despite a high sodium concentration. This adaptation is also why overly rapid correction can cause cerebral edema.

Desmopressin is a selective vasopressin analogue that restores V2-receptor stimulation and collecting-duct water reabsorption in central diabetes insipidus. It does not correct nephrogenic resistance, and the fluid deficit must still be replaced carefully.

The free-water deficit estimates the amount of electrolyte-free water needed to return total-body water toward a target sodium concentration. It is a starting estimate, not a substitute for serial sodium measurements, ongoing-loss assessment, and adjustment for comorbidities.

An unexpectedly rapid fall suggests that free-water delivery or ongoing losses have been misestimated. Hypotonic replacement should be reassessed and adjusted, with frequent sodium checks. Management must account for neurologic risk, hemodynamics, urine output, and the cause of the water loss.