The clinical manifestations of hyperammonemia are usually easily identifiable to the clinician when associated with liver disease and lead to prompt diagnosis and treatment. However, hyperammonemia-induced encephalopathy is rare in adults in the absence of overt liver disease, thus diagnosis is often delayed or missed leading to potentially life threatening complications. Without proper treatment, such patients can decompensate rapidly with poor outcomes including seizures, coma, and death. Early assessment of plasma ammonia levels in patients with normal hepatic function and characteristic symptoms of encephalopathy can lead to early intervention while investigating the underlying etiology. We describe a patient who presented with a 2-year progression of waxing and waning acute mental status changes after a Roux-en-Y gastric bypass surgery. He was found to have elevated ammonia level as well as orotic aciduria; results consistent with a urea cycle disorder. After consulting neurology as well as toxicology, he ultimately improved after dietary protein restriction, sodium benzoate and lactulose therapy. While rare, clinicians should have a high index of suspicion for late onset urea cycle disorders in symptomatic patients presenting with encephalopathy secondary to hyperammonemia.

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