Polyuria and polydipsia in a young child: diagnostic considerations and identification of novel mutation causing familial neurohypophyseal diabetes insipidus.

A 3-year 5-month-old boy was seen for second opinion regarding polydipsia and polyuria. Previously, a diagnosis of primary polydipsia was made after normal urine concentration after overnight water deprivation testing. The boy's father, paternal grandfather, and paternal aunt had diabetes insipidus treated with desmopressin acetate. Based on this young boy's symptoms, ability to concentrate urine after informal overnight water deprivation, and family history of diabetes insipidus, we performed AVP gene mutation testing. Analysis of the AVP gene revealed a novel mutation G54E that changes a normal glycine to glutamic acid, caused by a guanine to adenine change at nucleotide g.1537 (exon 2) of the AVP gene. Commonly, patients with familial neurohypophyseal diabetes insipidus (FNHDI) present within the first 6 years of life with progressively worsening polyuria and compensatory polydipsia. Since these patients have progressive loss of arginine vasopressin (AVP), they may initially respond normally to water deprivation testing and have normal pituitary findings on brain MRI. Genetic testing may be helpful in these patients, as well as preemptively diagnosing those with a mutation, thereby avoiding unnecessary surveillance of those unaffected.