Endocrine Abstracts

Diabetes insipidus is characterised clinically by the inappropriate production of large volumes of dilute urine, even in the presence of clinical dehydration or depravation of water. DI occurs either due to deficiency or insufficiency of arginine vasopressin (AVP) hormone production. The gold standard test remains the water deprivation test. Hereditary DI accounts for <10% of all cases.

We present a family with a known heterozygous missense mutation, c232>A(GLU78LYS) in the AVP gene. The mother has biochemically confirmed central DI and the mutation. She has four children. Three of whom have been tested. The eldest daughter, who was symptomatic, tested positive for the mutation and also failed the water deprivation test, on MRI imaging she had absence of the bright spot in the posterior pituitary. The next daughter, also tested positive for the mutation, however although she was symptomatic she passed the water deprivation test. The third child is asymptomatic, does not have the mutation and passed the water deprivation test. The fourth child has yet to be tested.

This case illustrates that psychogenic polydipsia can co-exist in families, in whom a diagnosis of familial diabetes insipidus has already been established in other family members. A child exhibiting water seeking behaviour, can mistakenly be assumed to suffer from the same condition. Caution is required before prompt diagnosis, as there is a risk from hyponatraemic seizures with inappropriate desmopressin use.

However, primary polydipsia, does not preclude this child from developing diabetes insipidus in the future and genetic analysis still has an important role to play in identifying those at risk of developing DI in the future.

This case demonstrates the complex nature of diagnostic and management issues when faced with symptoms and a positive genetic test in the face of negative biochemistry.