Undiagnosed partial ornithine transcarbamylase deficiency presenting as recurrent hyperammonaemic encephalopathy after capecitabine administration

Capecitabine is a widely used drug for cancer treatment. Capecitabine is a derivative of 5-fluorouracil (5-FU). A known complication of 5-FU is hyperammonaemia which can cause encephalopathy. To our knowledge, there are very few documented cases demonstrating hyperammonaemia associated with capecitabine. There are also cases suggesting capecitabine improves hyperammonaemia from 5-FU. We present the case of an older man taking capecitabine, who presented to our hospital with severe encephalopathy with elevated ammonia. Workup led to the diagnosis of a urea cycle disorder. This case highlights the importance of determining the aetiology of a patient's presentation, especially if they have presented repeatedly with the same issue. It is important to think of common causes first; however, we must broaden the differential diagnoses when the simple answer does not explain the clinical picture.