Applying causal mediation methods to clinical trial data: What can we learn about why our interventions (don't) work?

Area of Science:

• Medical research
• Psychosocial interventions
• Causal inference

Background:

• Randomized controlled trials (RCTs) for low back pain (LBP) show limited effects from psychosocial interventions.
• There's a need to investigate the mechanisms driving intervention outcomes.

Purpose of the Study:

• To apply causal inference and mediation analysis to a cluster RCT for LBP.
• To explore potential mediators of treatment effects on disability.

Main Methods:

• A cluster RCT compared usual GP care with a minimal psychosocial intervention for (sub)acute LBP.
• Mediation analysis examined pain catastrophizing, fear-avoidance beliefs, distress, and advice-following as mediators at 6 weeks.
• The causal pathway between treatment and disability at 3 months was investigated.

Main Results:

• Unadjusted mediation analysis identified fear-avoidance beliefs as a mediator of treatment on disability.
• The indirect effect of fear-avoidance beliefs was -0.30 (95% CI: -0.86, -0.03) in unadjusted models.
• This mediation effect was not significant after adjusting for covariates.

Conclusions:

• Mediation analysis can reveal factors in the intervention-outcome pathway, aiding future intervention design.
• Fear-avoidance beliefs mediate the LBP treatment-disability relationship, but the intervention's impact on these beliefs was insufficient.