Asthma prevention ineffective

“Dust mite products fail to help asthmatics” is the headline in The Daily Telegraph . People with asthma should “stop wasting money on special mattress protectors and vacuum cleaners to tackle dust mites”, because they do not reduce the allergens that can trigger asthma attacks, the newspaper says.

As some asthmatics are allergic to the tiny mites that live in dusty carpets, bedding and soft furnishings around the house, it seems sensible to try to reduce the levels of the mites by enclosing mattresses and pillows in protectors, washing bedding at high temperatures, using special cleaning products, high-powered vacuum cleaners and air filters. However, this comprehensive Cochrane review of 54 studies has concluded that such measures can no longer be recommended as they are not effective in reducing asthma symptoms or other measures of asthma. The review goes further by saying that it is doubtful if future research studies in this area would be worthwhile, unless they are particularly rigorous and use new methods for reducing allergen levels.

Where did the story come from?

Drs Peter Gøtzsche and Helle Johansen from the Nordic Cochrane Centre in Denmark carried out this research with support from The Swedish Heart Lung Foundation, the Nordic Council of Ministers and the Sygekassernes Helsefond in Denmark. The study was published in the Cochrane Database of Systematic Reviews , a publication of the Cochrane Collaboration.

What kind of scientific study was this?

This was a systematic review in which the authors searched for and assessed all the randomised trials they could find that looked at dust mite control measures compared with placebo (dummy measures) or no treatment, in people with asthma who are known to be sensitive to house dust mites.

The researchers used the reference lists in the Cochrane library and all trials that were available in a publicly available database of trials (PubMed) from 1966 until November 2007. They accepted trials in any language and included any additional trials found by searching through the reference lists of the studies retrieved.

In all, the researchers found 54 trials involving 3,002 participants. Among these, 26 trials assessed the effect of covering mattresses, 10 looked at chemical methods of controlling mites and eight looked at a combination of chemical and physical methods. In general, the researchers judged the trials were of poor quality: they rarely described how the patients involved in the trials had been randomised and only eight trials described how systems, such as sealed envelopes or computer programmes, were used to conceal the allocation of participants from the researchers. Both of these factors are important ways of preventing bias in randomised controlled trials.

Some of the studies were suitable for further statistical analysis and the differences between the average outcomes were summarised into a single figure using a technique known as meta-analysis.

What were the results of the study?

Poor quality trials, such as these, might be expected to exaggerate the reported effect; however, this was not the case. The researchers report that they did not find any effect of the interventions.

There was no difference in peak flow in the morning amongst the 1,565 patients examined. The peak flow test measures the maximum amount of air that a person with asthma can breathe out through a simple hand held device and it is commonly used to measure the severity of the asthma. This was the most frequently reported outcome in the studies reviewed. There were also no statistically significant differences either in the number of patients who improved, in asthma symptom scores or in medication usage.

What interpretations did the researchers draw from these results?

The review concludes that none of the methods thought to eradicate the dust mite work. Specifically, the researchers mention that washing bedding at temperatures higher than 60C and using specialist cleaning products are not recommended. Although the best products can get rid of 50% of dust mites, more than 90% eradication would be needed to prevent asthma attacks in people who are sensitive to dust mites.

What does the NHS Knowledge Service make of this study?

This is a comprehensive, systematic review of many randomised trials. It provides the best level of proof yet regarding the effectiveness, or lack thereof, for commonly used methods thought to reduce the problems caused by house dust mites in people with asthma.

The quality of the studies retrieved by the authors is a concern, as reporting of randomisation, allocation and how the patients in trials are “blinded”, ie. kept in the dark about which group they were allocated is important. It appears to be relatively simple to build this into the design of most trials and to report it. Some of the more recent trials included as part of this review, including one by this review’s lead author, have reported these features more completely.

The researchers are clear in suggesting that further studies of these eradication methods will be of dubious value and instead researchers should concentrate on new methods to eradicate dust mites. This seems reasonable, since this review provides good evidence of no effect.

Sir Muir Gray adds...

The systematic review is the highest quality evidence, especially if performed by people in the Cochrane Collaboration; it is unlikely that their conclusion is wrong.