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Hyperammonaemic encephalopathy and rebound effect: case report An 18-year-old man developed hyperammonaemic encephalopathy (HAE) during treatment with cisplatin, doxorubicin, fluorouracil and interferon-α for fibrolamellar hepatocellular carcinoma (FLHCC). Additionally, he developed rebound effect following sodium benzoate dose reduction [not all routes stated]. The man presented with a history of abdominal pain, distension, fatigue and vomiting. He was diagnosed with FLHCC. In May 2019, he was initiated on systemic chemotherapy with doxorubicin 30 mg/m2 on days 1 and 2, cisplatin 80 mg/m2 on day 1, interferon-α 5×108 U/m2 on days 1–4 and fluorouracil [5-fluorouracil] 400 mg/m2 on days 1–4. At midnight on the third day of chemotherapy, he became agitated; he was walking and running around the ward and was not responding to verbal stimuli. Laboratory testing showed AST and ALT levels of 201 and 126 U/L, respectively, total bilirubin level of 0.3 mg/dL and a serum ammonia level of 393 µg/dL. A diagnosis of HAE was considered. Chemotherapy (fourth dose of fluorouracil and interferon-α) was discontinued. The man was was initiated on rifaximin and lactulose treatment. However, the serum ammonia level rapidly increased, and his mental status deteriorated to the semicoma state. He was transferred to the ICU and was placed on mechanical ventilator support under sedation using vecuronium, midazolam and remifentanil. The treatment for hyperammonaemia was initiated with continuous venovenous hemofiltration, IV infusion of sodium benzoate at a loading dose of 8.8g (170 mg/kg), followed by continuous infusion (8.8 g/day) and sodium phenylbutyrate, glucose [dextrose] for caloric intake and administration of lactulose and rifaximin via Levin tube. His ammonia level decreased, but rebounded to 442 µg/dL on the third day of ICU care. IV sodium benzoate 8.8g was loaded again, leading to reduction in ammonia level. On the sixth day of the above treatment, continuous venovenous haemofiltration was stopped. On day 8, he was extubated, and sodium phenylbutyrate and sodium benzoate administration was changed to an intermittent schedule, with IV sodium benzoate 1.8g every 8 hours, sodium phenylbutyrate and arginine. He was initiated on a low-protein diet through a Levin tube. Two days afterwards (day 10 of ICU care), he became disoriented and was not able to respond properly to verbal stimuli, with an ammonia level of 173 µg/dL (rebound). The low-protein diet was stopped, and a continuous sodium benzoate infusion 5.5 g/day, following a loading dose of 5.5g was started again, along with citrulline. No focal neurologic signs were observed, and brain MRI did not show any abnormality. As the serum ammonia level started to reduce, his conscious level ameliorated. On day 12, he started Levin tube feeding and oral medications (levocarnitine [L-carnitine] and sodium phenylbutyrate); citrulline was stopped. On day 14 of ICU care, infusion of sodium benzoate was switched to an intermittent schedule (1.8g every 8 hours). He restarted the low-pr