S5824 Chlorzoxazone: The Culprit in a Case of Acute Liver Failure

Introduction: Chlorzoxazone is a centrally acting skeletal muscle relaxant that undergoes extensive hepatic metabolism. It is used in both adult and pediatric patients for muscle spasms and musculoskeletal conditions including lower back pain. Adverse effects include central nervous system depression as well as rare and unpredictable liver injury. We present a case of acute liver failure due to chlorzoxazone. Case Description/Methods: A 48-year-old man with a history of recently diagnosed cirrhosis and acute liver failure of unknown etiology presented with abdominal pain and encephalopathy. Lab testing showed aspartate transaminase (AST) and alanine transaminase (ALT) of approximately 1000 as well as elevated total bilirubin and international normalized ratio (INR). The patient had been admitted for a similar presentation 1 month prior and at that time liver function tests (LFTs) had slowly trended down throughout admission. Extensive workup for elevated LFTs was unremarkable. Trans jugular liver biopsy revealed extensive centrilobular necrosis, consistent with drug-induced liver disease due to chlorzoxazone. Chart review revealed long-term use of muscle relaxants. Chlorzoxazone was discontinued and subsequent follow-up showed normalization of LFTs and resolution of encephalopathy. Discussion: The link between chlorzoxazone and hepatotoxicity has not been clearly defined; however there have been case reports suggestive of a direct relationship between the 2. The result is most often a hepatocellular pattern of injury. In some cases, hepatic toxicity has been demonstrated following a single dose of chlorzoxazone. Discontinuation of the medication can result in full hepatic recovery; however even with early discontinuation some cases have resulted in acute liver failure and death. It is important to recognize the link between chlorzoxazone and liver injury. This allows proper identification of patients in whom this medication should be avoided as well as proper lab monitoring and discontinuation if necessary. Additionally, if liver injury is demonstrated chlorzoxazone should not be prescribed in the future even if LFTs have normalized.