After traumatic brain injury, polyuria and plasma sodium 159 mEq/L are noted. Which pathologic condition is associated with these findings?

After traumatic brain injury, polyuria and plasma sodium 159 mEq/L are noted. Which pathologic condition is associated with these findings?

• A. Syndrome of inappropriate antidiuretic hormone (SIADH)
• B. Diabetes mellitus
• C. Diabetes insipidus
• D. Cerebral salt wasting syndrome

Answer: (C)

Polyuria with a high serum sodium after brain injury points to a deficiency of antidiuretic hormone, i.e., central diabetes insipidus. Injury to the hypothalamic–posterior pituitary axis reduces vasopressin release, so the kidneys can’t concentrate urine and excrete large volumes of dilute water. The continuous water loss raises serum osmolality and sodium, explaining the hypernatremia. In contrast, SIADH involves excess ADH with hyponatremia and concentrated urine, diabetes mellitus causes osmotic diuresis from hyperglycemia, and cerebral salt wasting presents with hyponatremia and volume depletion due to renal salt loss. Thus, the scenario best fits central diabetes insipidus.

Polyuria with a high serum sodium after brain injury points to a deficiency of antidiuretic hormone, i.e., central diabetes insipidus. Injury to the hypothalamic–posterior pituitary axis reduces vasopressin release, so the kidneys can’t concentrate urine and excrete large volumes of dilute water. The continuous water loss raises serum osmolality and sodium, explaining the hypernatremia. In contrast, SIADH involves excess ADH with hyponatremia and concentrated urine, diabetes mellitus causes osmotic diuresis from hyperglycemia, and cerebral salt wasting presents with hyponatremia and volume depletion due to renal salt loss. Thus, the scenario best fits central diabetes insipidus.