Severe Hypomagnesemia in Pregnancy

Introduction: Hypomagnesemia in pregnancy has been associated with a higher risk for perinatal complications. We report a case of severe hypomagnesemia of unknown etiology in a woman presenting for prenatal care. Genetic testing revealed mutations in the transient receptor potential cation channel membrane 6 (TRPM6) gene leading to intestinal and renal magnesium(Mg) wasting with secondary hypocalcemia. Case Description: A 37-year-old woman with a history of infantile seizures was referred to nephrology clinic for prenatal evaluation of hypomagnesemia. She reported pediatric work-up revealing hypomagnesemia and one episode of breakthrough seizure in adolescence after stopping chronic Mg supplements. Family history was noted to be unremarkable. Laboratory values were notable for Creatinine 0.65mg/dL, fractional excretion(Fe) of Mg 2.5%, Fe calcium(Ca) 1.9%. Trends for Mg, Ca, and potassium are noted in the image. Kidney-centered genetic testing identified two mutations in TRPM6: one known to cause intestinal losses (c.1081C>T; p. Gln361*) and a variant of uncertain significance (c.1732-3C>G; p.?). At 36 weeks 3 days, the patient developed preeclampsia with severe features and was treated with intravenous (IV) Mg, maintaining levels > 4 mg/dL at time of delivery, and was discharged on oral Mg supplements. At 8 months postpartum, she presented to the hospital with seizures and was noted to have a Mg level of <0.1 mg/dL requiring IV Mg supplementation. Discussion: For most women in the United States, prenatal care is their sole contact with the health care system, offering a rare opportunity to diagnose and optimize chronic disease. Through nephrology referral and genetic testing, our patient was able to secure a diagnosis and understand implications for inheritance in her offspring.Though it is uncertain if the mutations found in our case are biallelic, her clinical history and lab findings suggest these mutations together are pathogenic in an autosomal recessive manner. We advocate for early genetic testing in patients presenting with severe hypomagnesemia to improve diagnosis and management. Aggressive management of hypomagnesemia during the prenatal period is also warranted, given its association with pre-eclampsia and preterm birth.