*Purpose: There are limited options for superior mesenteric vein (SMV) outflow in isolated intestine transplant (ITx), the two most common being anastomosis to the recipient superior mesenteric vein or to the vena cava. Previous research comparing these two options demonstrated no difference in early clinical outcomes. This paper reports four cases of symptomatic late hyperammonemia occurring in ITx patients more than 1-year from transplant. Each affected patient experienced marked encephalopathy. In each case, a very low protein diet was effective in lowering ammonia levels and ameliorating symptoms. Each affected ITx patient had direct SMV-vena cava drainage of their graft. This study reports an analysis of all living ITx patient at this single center to determine if hyperammonia is associated with vena cava drainage alone or if it occurs generally in this population.

*Methods: ITx recipients at a single center were identified. Random serum ammonia levels were obtained with routine blood draws. Ammonia levels were analyzed in relation to both (1) SMV drainage technique and to (2) time from transplant. Liver function was also assessed using standard lab values.

*Results: There were 71 ITx patients between 2003 and 2018, 26 patients with SMV-SMV anastomosis (37%) and 45 with SMV-vena cava anastomosis (63%). Of these patients, 37 are currently living (52%). For SMV-vena cava patients, the available ammonia levels were consistently greater than 50mcg/dL. Among affected patients, the peak levels would reach as high as 150-200mcg/dL. For SMV-SMV patients, the levels were always less than 50mcg/dL. There was one SMV-SMV patient with intermittent encephalopathy, but she had peak ammonia levels of 60mcg/dL. She was ultimately diagnosed with a rare neurocognitive disorder.

*Conclusions: This study demonstrates that hyperammonemia is not uncommon among late survivors of ITx, but only in those patients with SMV to vena cava anastomosis. This hyperammonia may result in marked encephalopathy as seen in four patients in this cohort. A very low protein diet is effective in minimizing this process. In ITx patients with SMV-vena cava anastomosis, routine monitoring of serum ammonia levels is indicated.

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