Partial N-acetyl glutamate synthase deficiency presenting as postpartum hyperammonemia: Diagnosis and subsequent pregnancy management.

Abstract

N-acetyl glutamate synthase (NAGS) deficiency (OMIM #: 237310) is a rare urea cycle disorder that usually presents early in life with hyperammonemia. NAGS catalyzes the synthesis of N-acetyl glutamate (NAG) which functions as an activator of the carbamoyl phosphate synthetase-1 mediated conversion of ammonia to carbamoyl phosphate. The absence of NAG results in a proximal urea cycle disorder which can result in severe neurologic sequelae secondary to hyperammonemia and even death. Unlike the other urea cycle disorders, a specific pharmacological treatment for NAGS deficiency exists in the form of carglumic acid, an analog of NAG. Here we present a 29-year-old previously healthy female who presented with hyperammonemia and obtundation just after the birth of her first child. Exome sequencing revealed two novel variants in the NAGS gene, and plasma metabolomics revealed extremely low levels of NAG. Carglumic acid treatment led to prompt resolution of her biochemical abnormalities and symptoms. She tolerated two subsequent pregnancies, 2 years and 6 years after her initial presentation, while taking carglumic acid, and breastfed her third child, all without complications in the mother or children. This case report emphasizes the importance of considering urea cycle disorders in previously-healthy adults presenting with neurological symptoms during periods of metabolic stress, including the postpartum period. It also highlights the efficacious and safe use of carglumic acid during pregnancy and while breastfeeding.