*Purpose: To examine long-term outcomes after liver transplantation for organic acidemias and urea cycle defects at our center which utilizes standardized pre-, peri- and post-operative protocols to minimize morbidity and mortality in this population.

*Methods: We retrospectively studied children ages 0-17 years old who underwent liver transplantation for methylmalonic acidemia (MMA, n=29), propionic acidemia (PA, n=4) and urea cycle defects (UCD, n=39) between 2000-2019.

*Results: At the time of transplantation, median age was 1 year (range 0-17 years). Of the 72 patients with MMA, PA and UCD: 52 patients (72%) received whole liver grafts, 19 received partial grafts and 1 received a split graft. Living donation was used in one patient with citrullinemia. Thirteen patients underwent combined liver-kidney transplantation (LKT) all of whom were transplanted for methylmalonic acidemia (median age at combined LKT was 5 years). Of the 72 patients who received either a liver transplant or LKT, four patients developed hepatic artery thrombosis (HAT, 5.5%). All patients with HAT had urea cycle defects and received whole grafts and all required re-transplantation. Over a mean follow-up period of 6.9 years (range 1-19 years), 3 patients (4.2%) developed biliary strictures: 1 related to bile leak and 2 related to HAT. Of the 6 patients that developed graft failure: 4 were re-transplanted for HAT and 2 were re-transplanted for chronic rejection. 3 patients died: 1 from intra-operative cardiac arrest at the time of transplant, 1 from infectious complications and 1 from chronic rejection. Following this intra-operative death secondary to a peri-transplant metabolic crisis, our center instituted standardized protocols for pre-, peri- and post-operative management, specifically for consideration of hemodialysis 4 hours prior to liver transplantation in children with MMA to prevent life-threatening metabolic crises. Since the institution of this protocol, there have been no subsequent intra-operative deaths in this population.

*Conclusions: Transplant evaluation at our center is inclusive of the metabolic genetics team and early referral for transplant evaluation may explain our low median age at the time of transplant to confer neuroprotection for children at risk of recurrent metabolic crises. Incidence of HAT in this population is low, despite use of primarily whole grafts in the smallest children. Children with organic acidemias and urea cycle defects represent a metabolically fragile population at particular risk of decompensation in the peri-operative period. We report the experience of our center to highlight the importance of multidisciplinary protocols to minimize morbidity and mortality in this population and to continue to ensure excellent post-transplant outcomes.

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