Reducing the risk of prion transmission in anaesthesia

Wilkes, despite the title of his editorial, only looks at one way of reducing prion transmission in anaesthesia – filters in breathing circuits – and he is not sure all of them can be effective barriers [1]. He does not deal with other, perhaps more likely, ways of transmitting prions such as reusable laryngeal mask airways, for which there is a completely effective risk-avoiding strategy – use disposable ones. The risk from circuits and laryngeal mask airways is probably very small, otherwise we would be seeing consequences by now, but my contention is that when reusable airways were introduced, the risk of transmitting viruses, and later on prions, was overlooked and maybe we are just lucky that things have turned out as well as they seem to have done, so far. Prions are highly resistant to decontamination by routine cleaning and autoclaving [2]. Routine cleaning plus scrubbing, or ultrasonic cleaning followed by sterilisation, leaves proteinaceous material on some laryngeal mask airways [3]. Presoaking in potassium permanganate 2 mg.l for 20 min followed by a six-step decontamination process, but not the six-step process alone, removed protein from the dorsal surface of 24 of 30 laryngeal mask airways, but the rest were still mildly contaminated [4]. Standard sterilisation procedures for laryngeal mask airways in hospitals are unlikely to be as detailed as in that paper. The residual protein could host the variant Creutzfeldt-Jakob disease (vCJD) prion protein or other pathogens such as hepatitis C or HIV. About 12–18% of laryngeal mask airways show macroscopic, and 76–93% microscopic, blood on removal [5, 6]. Biofilm, an exopolysaccharide matrix containing bacteria, which also accumulates on airways [7] and other foreign surfaces in the body, could protect viruses. Duck hepatitis virus in biofilm on reused dental syringes was resistant to ultrasonic treatment, glutaraldehyde disinfection or short autoclaving cycles at 134 C [8]. When laryngeal mask airways are reused, they carry protein and biofilm on them past lymphoid tonsil tissue, and remain in contact with pharyngeal tissues for the duration of the anaesthetic. They could contaminate, be contaminated or both. It seems odd, therefore, that the UK government insists we use disposable laryngoscope blades in paediatric tonsil cases to reduce the risk of vCJD but we are free to use reusable laryngeal mask airways. What is not clear is why the risk of infection by viruses and prions from reusable laryngeal mask airways did not receive more attention until 2000 [5, 9] and why the general public was never asked whether they were happy for anaesthetists to use second-hand airways on them [6, 10]. A straw poll of people working in my theatres showed that almost all would prefer an unused laryngeal mask airway for their anaesthetic, because they see them contaminated with blood and secretions being removed from patients and know that they can be reused 40 times or more. Yet patients have never had that option. Even before laryngeal mask airways were introduced, the practice of reusing airway devices such as red rubber tracheal tubes was declining because of worries about cross infection from viruses such as hepatitis B, and later on hepatitis C. Laryngeal mask airways were introduced in 1988 apparently without published evidence (that I can find) that viruses such as hepatitis could be safely removed by the recommended method of resterilisation and I am not aware of any paper on this subject since then. Even though vCJD became a problem in 1996, we still do not know if prions can be removed from reusable airways; indeed, the first paper on the risk appeared 5 years later, in 2001 [9]. The Association of Anaesthetists was still calling for an effective decontamination and sterilisation process for reusables in 2003 [11]. Of course, the laryngeal mask airway was a brilliant invention and has many