The review question was clear in terms of the study design, intervention, participants and outcomes. Several relevant sources were searched and the keywords were listed. By limiting the included studies to those published in English, some relevant studies may have been omitted. No attempt was made to locate unpublished studies, thus raising the possibility of publication bias. More than one reviewer independently selected the studies, which reduces the potential for bias and errors. However, the methods used to extract the data were not described; hence, any efforts made to reduce errors and bias in the data extraction cannot be judged. Only RCTs were included but validity was not formally assessed. Limited information on the individual studies was presented. Some indication of the severity of asthma among participants, the age range, drug doses and concomitant treatments would have been useful in judging which populations and which treatment regimens the results of the review might apply to. It was not stated whether the data were extracted on an intention-to-treat basis and the issue of drop-outs was not addressed. Most of the included RCTs were crossover studies, but no comment was made on the adequacy of the washout period in preventing carry-over effects of the first drug. Data for the main outcome were appropriately combined in a meta-analysis and statistical heterogeneity was tested. Subgroup analyses were conducted to explore the influence on the results of drug type, stimulus type and timing of the outcome.
The evidence presented appears to support the authors' conclusions but, given the lack of detail of the patients studied, it is not possible to judge the generalisability of the results. In addition, the outcome is a proxy outcome and the clinical importance of the review's findings is unknown.