All studies which used montelukast intermittently reported USMA [2, 4, 15, 18]. The overall effect was not statistically significant (MD = −0.13, 95% CI −0.33 to 0.07, mean for montelukast 1.62 vs placebo 1.78 (p = 0.21)). There was a low level of heterogeneity (Fig. 3b, p = 0.30, I 2 statistic 18%).