Methoxyflurane toxicity: historical determination and lessons for modern patient and occupational exposure

Historically methoxyflurane was used for anaesthesia. Evidence of nephrotoxicity led to abandonment of this application. Subsequently, methoxyflurane, in lower doses, has re-emerged as an analgesic agent, typically used via the Penthrox inhaler in the ambulance setting. We review the literature to consider patient and occupational risks for methoxyflurane.

Articles were located via PubMed, ScienceDirect, Google Scholar, Anesthesiology journal and the Cochrane Library.

Early studies investigated pharmacokinetics and considered the resulting effects to pose minimal risk. Pre-clinical rodent studies utilised a species not vulnerable to the nephrotoxic fluoride metabolite of methoxyflurane, so nephrotoxicity was not identified until almost a decade after its introduction, and was initially met with scepticism. Further evidence of nephrotoxicity led to abandonment of methoxyflurane use for anaesthesia. Subsequent research suggested there are additional risks potentially relevant to recurrent patient or occupational exposure. Specifically, greater than expected fluoride production after repeated low-dose exposure, increased fluoride production due to medication-caused hepatic enzyme induction, fluoride deposition in bone potentially acting as a slow-release fluoride compartment, which suggests a risk of skeletal fluorosis, and hepatotoxicity. Gestational risk is unclear.

Methoxyflurane poses a potentially substantial health risk in high (anaesthetic) doses, and there are a number of pathways whereby repeated exposure to methoxyflurane in lower doses may pose a risk. Single analgesic doses in modern use generally appear safe for patients. However, the safety of recurrent patient or occupational healthcare-worker exposure has not been confirmed, and merits further investigation.