This child’s hypernatremia could be due to diabetes insipidus or due to hypertonic fluids. The way to prove would be to do serum and urine osmolality. If urine osmolality is equivalent to serum osmolality, it suggests intact
ADH mechanism and most likely cause of hypernatremia to be use of hypertonic fluids. If serum osmolality is more than urine osmolality, it suggests inability of kidneys to concentrate urine (diabetes insipidus) either due to deficiency of
ADH (central DI) or due to inability of
ADH to act on kidneys (Nephrogenic DI). To differentiate the two, one can either do serum
ADH levels (ADH is low in central DI and elevated in nephrogenic DI) or by giving
Vasopressin and measuring urine output and serum sodium (In central DI, with vasopressin, urine output should decrease and serum sodium should normalize. In Nephrogenic DI, there is no response).
In this child, serum osmolality was 350 mosm/L and simultaneous urine osmolality was 175 mosm/L confirming a diagnosis of DI.
Vasopressin administration led to decrease in urine output from 7 cc/kg/hour to 2.2 cc/kg/hour and normalizing of serum sodium. Thus, the cause of hypernatremia in this child is central DI most likely as sequelae of meningitis.