Diagnostic Dilemma


A 2½ months old boy born of non-consanguineous marriage presented with fever and staring look. He was admitted in a hospital and CSF was suggestive of pyomeningitis. He was treated with IV antibiotics. However on 7th day of illness was found to have hypernatremia (serum sodium = 184 mEq/L). He was given hypernatremic dehydration correction following which his sodium decreased to 159 mEq/L in next 3 days.

How should one investigate for the cause of his hypernatremia?
Expert Opinion :
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.
Answer Discussion :
Karuppasamy TMC
diabetes Insipidus
5 months ago
Josef Zakkour
Diabetes insipidus
5 months ago

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