5-fluorouracil (5-FU) is one of the most common adjuvant antineoplastic agents used in the treatment of localized and metastatic colon cancer. Frequent side effects of 5-FU include myelosuppression, mucositis, nausea, vomiting, and diarrhea. However, hyperammonemic encephalopathy is a rare neurologic toxicity that can occur after 5-FU chemotherapy administration. Patients with 5-FU-induced hyperammonemic encephalopathy often exhibit symptoms of altered mental status with no radiologic abnormalities or laboratory abnormalities except for significantly elevated ammonia levels with occasional lactic acidosis and respiratory alkalosis. We report a case of a patient with stage IV colon adenocarcinoma who experienced altered state of consciousness due to hyperammonemia during the administration of palliative chemotherapy with 5-FU, bevacizumab, and leucovorin. On cycle 1 day 2 of chemotherapy, the patient became drowsy and confused at home, prompting a visit to the emergency department and ultimately hospital admission. Laboratory tests revealed an elevated blood ammonia level (838 μg/dL). After an extensive negative workup, his altered state of consciousness was thought to be secondary to 5-FU-induced hyperammonemia. Upon admission, 5-FU was immediately discontinued and the patient was treated with lactulose enemas, intravenous fluids, rifaximin, and continuous renal replacement therapy with gradual recovery to baseline mental status. It is crucial for advanced practitioners to be aware of this rare side effect to ensure prompt diagnosis and maximize treatment effectiveness.
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