Eleven studies, comprising 290 participants were included.
Four studies were rated as being of high quality (score of 8) and the remaining 7 were of sufficient quality.
Baseline BHR levels were in the mild asthmatic range in five of the studies and in the moderate asthmatic range in the remaining studies. Effect sizes were all in favour of the inhaled corticosteroids, ranging from 0.44 to 2.40 DD of the bronchoconstricting agent. However, in five of the 11 studies, the inhaled corticosteroid did not have a significant effect on BHR compared with placebo.
The total effect size of inhaled corticosteroids versus placebo was 1.16 DD (95% CI: 0.76, 1.57, test of heterogeneity), which was statistically significant. To determine whether heterogeneity could be explained by the variation in age the total effect size was assessed without the two studies involving children. The total effect size remained statistically significant (0.88 DD of the bronchoconstricting agent; 95% CI: 0.64, 1.14).
A univariate regression analysis showed no statistically significant dose-response relationship between the dose of inhaled corticosteroid and the level of BHR (regression coefficient -0.007 DD/100 micrograms, p=0.87). Correcting for study duration did not improve the relationship between the dose of the inhaled corticosteroids and decrease in BHR, nor was there a statistically significant effect found when the patients were divided into two groups according to the dose of the inhaled corticosteroid (<1000 micrograms daily, four studies, total effect 1.25 DD; greater than or equal to 1000 micrograms, seven studies, total effect 1.13 DD; p =0.92, Wilcoxon rank test).
Inhaled corticosteroids were able to decrease BHR during short term treatment (2-8 weeks) in four of the eight studies, but a negative result was found in the other four. The separate study effects in these short term studies were combined to assess the overall effect size of inhaled corticosteroids compared with control on BHR. The effect size under the condition of homogeneity was 0.91 DD (95% CI: 0.65, 1.16) of the bronchoconstrictor in favour of the inhaled corticosteroid (p=0.14).
In addition, the effect sizes of individual studies were related to the dose of inhaled corticosteroids used in the short term studies by univariate regression analysis which gave a regression coefficient of 0.02 DD/100 micrograms (p =0.38). Correcting for study duration did not improve the relationship between the dose of inhaled corticosteroids and decrease in BHR. A comparison of low dose (<1000 micrograms, 3 of 8 studies) versus high dose inhaled corticosteroids also showed a lack of correlation between the dose used and the level of BHR (0.88 DD versus 1.21 DD, respectively; p = 0.55, Wilcoxon rank test.