Background: Diabetes insipidus (DI) is characterised clinically by the inappropriate production of large volumes of dilute urine even in the presence of clinical dehydration or deprivation of water. DI occurs either due to a deficiency or insufficiency of arginine vasopressin (AVP) hormone production. Hereditary DI accounts for <10% of the DI cases. As hyponatraemic seizures secondary to inappropriate use of desmopressin can occur, caution is required before a diagnosis of DI is made. On the other hand, primary polydipsia does not exclude the patient from possibly later developing DI and genetic testing within families may be indicated. The gold standard diagnostic test remains the water deprivation test.
Method: Case report.
Results: This is the case of a family with a known heterozygous pathogenic missense mutation, c.232>A (GLU78LYS) in the AVP gene. The mother and her eldest daughter have biochemically confirmed central DI and are symptomatically well controlled on desmopressin. The youngest daughter, aged 8, who is symptomatic has passed a water deprivation test on three occasions, mutation. A son, now 16, who had primary polydipsia as a toddler, is negative for the mutation. MRI in the mother and youngest daughter show normal posterior pituitary bright spot, but this is absent in the eldest daughter.
Conclusion: These cases illustrate that primary polydipsia can co-exist in families where a diagnosis of central DI has already been established in other family members. This can be challenging especially where there is a different perception of what normal fluid intake is. A child who exhibits water-seeking behaviour and who has a sibling or parent with known central DI can mistakenly be assumed to have the same condition. Complex diagnostic and management issues may be faced when a genetic mutation is found in the context of symptoms but negative biochemical confirmation.
20 - 22 Sep 2014
European Society for Paediatric Endocrinology