Magnesium sulfate may unmask subclinical excess vasopressinase-induced arginine vasopressin disorder

Abstract Context Gestational arginine vasopressin (AVP) disorder is classified into 3 categories: AVP deficiency (AVP-D), AVP resistance, and excess vasopressinase-induced AVP disorder. This is the first case report of gestational AVP disorder linked with magnesium sulfate administration. Case description A 27-year-old woman pregnant with twins was admitted to our hospital with threatened preterm labor (TPL). The day after continuous intravenous magnesium sulfate administration for TPL was initiated, the patient suddenly developed thirst, polydipsia, and polyuria. Because the plasma AVP level was extremely low and the urine was hypotonic, AVP-D was initially suspected. Her symptoms were effectively controlled using desmopressin. However, its administration became unnecessary the day after delivery, inconsistent with the typical clinical course of AVP-D. The removed placenta was nearly twice as heavy as that in typical singleton cases, and serum vasopressinase levels immediately before delivery were markedly elevated. The results of the hypertonic saline infusion test performed soon after delivery suggested an underestimation of plasma AVP levels due to AVP degradation by excess vasopressinase. We have encountered 8 similar cases since 2021, suggesting that coexisting subclinical excess vasopressinase-induced AVP disorder may contribute to the development of magnesium sulfate-associated gestational AVP disorder. The inhibition of AVP secretion from the posterior pituitary gland by markedly increasing circulating magnesium ions may be associated with the manifestation of subclinical excess vasopressinase-induced AVP disorder. Conclusion We encountered 9 cases of previously unreported magnesium sulfate-associated gestational AVP disorder in the past 4 years alone, suggesting that many similar cases may be overlooked in clinical practice.