Challenging case of hypernatraemia in infancy.

Area of Science:

• Pediatric Endocrinology
• Nephrology
• Genetics

Background:

• Infants presenting with fever and shock can exhibit symptoms mimicking dehydration, complicating initial diagnosis.
• High urine output with discrepant plasma and urine osmolalities suggests a disorder of arginine vasopressin (AVP), formerly diabetes insipidus (DI).
• Differentiating between AVP deficiency (cranial DI) and AVP resistance (nephrogenic DI) is crucial for appropriate management.

Purpose of the Study:

• To present a case of X-linked vasopressin resistance in an infant.
• To illustrate the diagnostic difficulties in distinguishing AVP deficiency from resistance in early infancy.
• To emphasize the utility of copeptin and genetic testing in diagnosing AVP disorders.

Main Methods:

• Clinical presentation of a 1-month-old male infant with fever, shock, and electrolyte abnormalities.
• Endocrine testing including plasma and urine osmolality, thyroid function tests, and response to desmopressin.
• Measurement of copeptin levels and genetic screening for renal tubulopathy, specifically the AVP receptor 2 gene.

Main Results:

• The infant presented with hypernatraemia, hyperchloraemia, and signs of shock, initially treated for sepsis-induced dehydration.
• Poor response to desmopressin despite dose escalation indicated AVP resistance.
• Significantly elevated copeptin levels and a pathogenic variant in the AVP receptor 2 gene confirmed X-linked vasopressin resistance.

Conclusions:

• Distinguishing between AVP deficiency and resistance in infants can be challenging.
• Copeptin serves as a valuable surrogate marker for AVP levels.
• Genetic testing is essential for confirming the diagnosis of inherited AVP receptor disorders and guiding management.