Identifying potential treatment effect modifiers of the effectiveness of chiropractic care to infants with colic through prespecified secondary analyses of a randomised controlled trial

BACKGROUND: A recent trial identified large variation in effect of chiropractic care for infantile colic. Thus, identification of possible effect modifiers could potentially enhance the clinical reasoning to select infants with excessive crying for chiropractic care. Therefore, the aim of this study is to identify potential treatment effect modifiers which might influence the effect of chiropractic care for excessive crying in infancy. METHODS: Design: Prespecified secondary analyses of data from a randomised controlled trial. The analyses are partly confirmative and partly exploratory. Setting: Four chiropractic clinics in Denmark. Participants: Infants aged 2–14 weeks with unexplained excessive crying. Of the 200 infants randomised (1:1), 103 were assigned to a chiropractic care group and 97 to a control group. Intervention: Infants in the intervention group received chiropractic care for 2 weeks, while the control group was not treated. Main analyses: The outcome was change in daily hours of crying. Fifteen baseline variables and 6 general variables were selected as potential effect modifiers, and indices based on these were constructed. Factor analyses, latent class analyses and prognosis were used to construct other potentially modifying variables. Finally, an attempt at defining a new index aiming at optimal prediction of the treatment effect was made. The predictive value for all resulting variables were examined by considering the difference in mean change in crying time between the two treatment groups, stratified by the values of the candidate variables, i.e. interaction analyses. RESULTS: None of the predefined items or indices were shown to be useful in identifying colicky infants with potentially larger gain from manual therapy. However, more baseline hours of crying (p = 0.029), short duration of symptoms (p = 0.061) and young age (p = 0.089) were all associated with an increased effect on the outcome of hours of crying. CONCLUSION: Musculoskeletal indicators were not shown to be predictive of an increased benefit for colicky infants from chiropractic treatment. However, increased benefit was associated with early treatment and a high level of baseline crying, suggesting that the most severely affected infants have the greatest potential of benefiting from manual therapy. This finding requires validation by future studies. TRIAL REGISTRATION: Clinical Trials NCT02595515, registered 2 November 2015. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12998-021-00373-6.