Gradient washout and secondary nephrogenic diabetes insipidu
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Disorders of water and sodium balance can occur after brain injury. Prolonged polyuria resulting from central diabetes insipidus and cerebral salt wasting complicated by gradient washout and a type of secondary nephrogenic diabetes insipidus, however, has not been described previously, to the best of our knowledge. We report an unusual case of an infant with glioblastoma who, after tumor resection, was treated for concurrent central diabetes insipidus and cerebral salt wasting complicated by secondary nephrogenic diabetes insipidus.

A 5-month-old Hispanic girl was found to have a large, hemorrhagic, suprasellar glioblastoma causing obstructive hydrocephalus. Prior to mass resection, she developed central diabetes insipidus. Postoperatively, she continued to have central diabetes insipidus and concurrent cerebral salt wasting soon after. She was managed with a vasopressin infusion, sodium supplementation, fludrocortisone, and urine output replacements. Despite resolution of her other major medical issues, she remained in the pediatric intensive care unit for continual and aggressive management of water and sodium derangements.

Starting on postoperative day 18, her polyuria began increasing dramatically and did not abate with increasing vasopressin. Nephrology was consulted. Her blood urea nitrogen was undetectable during this time, and it was thought that she may have developed a depletion of inner medullary urea and osmotic gradient: a “gradient washout.” Supplemental dietary protein was added to her enteral nutrition, and her fluid intake was decreased. Within 4 days, her blood urea nitrogen increased, and her vasopressin and fluid replacement requirements significantly decreased. She was transitioned soon thereafter to subcutaneous desmopressin and transferred out of the pediatric intensive care unit.

Conclusively, Gradient washout has not been widely reported in humans, although it has been observed in the mammalian kidneys after prolonged polyuria. Although not a problem with aquaporin protein expression or production, gradient washout causes a different type of secondary nephrogenic diabetes insipidus because the absence of a medullary gradient impairs water reabsorption. In this case, infant developed complex water and sodium imbalances after brain injury.

Prolonged polyuria resulting from both water and solute diuresis with low enteral protein intake was thought to cause a urea gradient washout and secondary nephrogenic diabetes insipidus. The restriction of fluid replacements and supplementation of enteral protein appeared adequate to restore the renal osmotic gradient and efficacy of vasopressin.

Source: https://jmedicalcasereports.biomedcentral.com/articles/10.1186/s13256-020-02536-0
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