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BMJ No 7104 Volume 315 Letters Saturday 9 August 1997 Obstructive sleep apnoeaSome criticisms of studies are unfoundedEditor,At the Scottish National Sleep Laboratory we strongly agree with John Wright and colleagues that further research is required to clarify whether sleep apnoea causes vascular events,(1) and we look forward to contributing to these studies. However, we are disappointed that they are not prepared to support the treatment of sleep apnoea, which, at least in Edinburgh, is aimed at improving symptoms, quality of life, objective sleepiness, and daytime performance. We showed that all of these improved with continuous positive airways pressure(2) in the 'one out of 45 studies of continuous positive airways pressure [that was] a truly randomised controlled trial.' (1) We do not believe that the criticism that our study 'had important weaknesses' is justified. We are criticised for using an oral placebo instead of a sham continuous positive airways pressure machine. We did this for ethical and scientific reasons as continuous positive airways pressure at subtherapeutic pressure may worsen hypoxaemia during sleep; patients could differentiate between lower sham and higher effective pressure so unblinding the study; and wearing a placebo mask would impair sleep thus biasing in favour of active treatment. Furthermore, machine mystique - the idea that a physical device might have more of an effect than an oral placebo - would be unlikely to influence the objective variables measured more than our actively advocated placebo. We are criticised for the lack of a washout period, but we did not take any measurements until 28 days after crossover and the benefits of continuous positive airways pressure wear off within one day.(3) We tested for potential carryover by examining order effects using analysis of variance; we accept that between subject effects have lower power than within subject effects, but only the one variable indicated showed a significant (P0.05) order effect.(2) Critically, we found significant benefits from continuous positive airways pressure while any carryover effect would bias against such findings. Also, Wright and colleagues omit to indicate that we found significant improvements compared with the placebo in four measures of cognitive function, including intelligence quotient (IQ). The Australian National Health and Medical Research Council conducted a similarly detailed review and concluded that there was evidence from a randomised controlled trial (level II evidence) and from meta-analysis of other studies (level III evidence) that continuous positive airways pressure was effective and recommended treatment with it for sleepiness and impaired daytime function.(4) This was also the conclusion of the Scottish needs assessment programme.(5) All three reviews(1)(4,5) conclude that more evidence is needed about vascular events. Wright and colleagues' review seems excessively negative about the merits of continuous positive airways pressure for sleepiness and cognitive impairment and this negative conclusion largely centres on their criticism of our study. In contrast, the Australian investigation, while also concerned about the use of an oral placebo, concluded that there were 'no major methodological threats to the validity' of our study.(4) H M Engleman
University of Edinburgh, References 1 Wright J, Johns R, Watt I, Melville A, Sheldon T. Health effects of obstructive sleep apnoea and the effectiveness of continuous positive airways pressure: a systematic review of the research evidence. BMJ 1997;314:851-60. (22 March.) 2 Engleman H M, Martin S E, Deary I J, Douglas N J. Effect of continuous positive airway pressure treatment on daytime function in sleep apnoea/hypopnoea syndrome. Lancet 1994;343:572-5. 3 Kribbs N B, Pack A I, Kline L R, Getsy J E, Schuett J S, Henry J N, et al. Effects of one night without nasal CPAP treatment on sleep and sleepiness in patients with obstructive sleep apnoea. Am Rev Respir Dis 1993;147:1162-8. 4 Australian Health Technology Advisory Committee. The effectiveness and cost effectiveness of nasal continuous positive airway pressure (nCPAP) in the treatment of obstructive sleep apnoea in adults. Canberra: Government Public Service, 1996. 5 Scottish Needs Assessment Programme. Obstructive sleep apnoea and allied disorders. Glasgow: Scottish Forum for Public Health Medicine, 1997.
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