Background and aim Familial Neurohypophyseal Diabetes Insipidus (FNDI) is a rare autossomal dominant disorder caused by mutations in the vasopressin (AVP) gene and is characterised by childhood onset of polyuria and polydipsia. The authors present a case of FNDI caused by a new mutation in AVP gene in a Portuguese family.

Case Report Twelve year-old boy with unremarkable past medical history admitted at our outpatient clinic with complaints of polydipsia (20 Litres/day) and polyuria since early childhood. Polyphagia, weight loss, enuresis and urinary incontinence were denied. His mother, grandmother, great grandfather and second/third degree uncles/aunts presented the same symptoms. Basal laboratorial tests revealed: plasma osmolality 292 mOsm/KgH₂O, plasma Na + 137 mEq/L and a normal renal function. 24-hour urinalysis (volume 7450 mL) showed a low urine osmolality (150 mOsm/KgH₂O) and normal Na+ and creatinine concentrations. Water deprivation test was positive for Diabetes Insipidus. The positive response after 20 mcg of intranasal desmopressin confirmed the diagnosis of neurohypophyseal Diabetes Insipidus. The MRI showed a hyper-intense signal corresponding to an ectopic neurohypophysis in the supra-celar cistern. The genetic test confirmed the diagnosis of FNDI: mutation c.293G > C (p. Cys98Ser) in the AVP gene (heterozygous) in the proband, mother and grandmother. The patient started daily oral desmopressin with relieve of complaints. Genetic counselling was given to the family.

Conclusion The presented case revealed an AVP gene mutation not described in the literature. The diagnosis, not recognised until the fourth generation, allowed a prompt treatment and genetic counselling of the family and an improvement in quality of life of affected members.