Challenges in Systematic Reviews and Meta-Analyses of Mediation Analyses

Mediators of the Association Between Socioeconomic Status and Survival After Out-of-Hospital Cardiac Arrest: A Systematic Review

Low socioeconomic status (SES) is associated with poor outcomes after out-of-hospital cardiac arrest (OHCA). Patient characteristics, care processes, and other contextual factors may mediate the association between SES and survival after OHCA. Interventions that target these mediating factors may reduce disparities in OHCA outcomes across the socioeconomic spectrum. This systematic review identified and quantified mediators of the SES-survival after OHCA association. Electronic databases (MEDLINE, Embase, PubMed, Web of Science) and grey literature sources were searched from inception to July or August 2023. Observational studies of OHCA patients that conducted mediation analyses to evaluate potential mediators of the association between SES (defined by income, education, occupation, or a composite index) and survival outcomes were included. A total of 10 studies were included in this review. Income (n = 9), education (n = 4), occupation (n = 1), and composite indices (n = 1) were used to define SES. The proportion of OHCA cases that had bystander involvement, presented with an initial shockable rhythm, and survived to hospital discharge or 30 days increased with higher SES. Common mediators of the SES-survival association that were evaluated included initial rhythm (n = 6), emergency medical services response time (n = 5), and bystander cardiopulmonary resuscitation (n = 4). Initial rhythm was the most important mediator of this association, with a median percent excess risk explained of 37.4% (range 28.6%-40.0%; n = 5; 1 study reported no mediation) and mediation proportion of 41.8% (n = 1). To mitigate socioeconomic disparities in outcomes after OHCA, interventions should target potentially modifiable mediators, such as initial rhythm, which may involve improving bystander awareness of OHCA and the need for prompt resuscitation.

Potential Mediators for Treatment Effects of Novel Diabetes Medications on Cardiovascular and Renal Outcomes: A Meta-Regression Analysis

BACKGROUND

Prior research suggests clinical effects of glucagon-like peptide-1 receptor agonists (GLP-1RAs) and sodium-glucose cotransporter-2 inhibitors (SGLT2is) are mediated by changes in glycated hemoglobin, body weight, systolic blood pressure, hematocrit, and urine albumin-creatinine ratio. We aimed to confirm these findings using a meta-analytic approach.

METHODS AND RESULTS

We updated a systematic review of 9 GLP-1RA and 13 SGLT2i trials and summarized longitudinal mediator data. We obtained hazard ratios (HRs) for cardiovascular, renal, and mortality outcomes. We performed linear mixed-effects modeling of LogHRs versus changes in potential mediators and investigated differences in meta-regression associations among drug classes using interaction terms. HRs generally became more protective with greater glycated hemoglobin reduction among GLP-1RA trials, with average HR improvements of 20% to 30%, reaching statistical significance for major adverse cardiovascular events (ΔHR, 23%; P=0.02). Among SGLT2i trials, associations with HRs were not significant and differed from GLP1-RA trials for major adverse cardiovascular events (Pinteraction=0.04). HRs for major adverse cardiovascular events, myocardial infarction, and stroke became less efficacious (ΔHR, -15% to -34%), with more weight loss for SGLT2i but not for GLP-1RA trials (ΔHR, 4%-7%; Pinteraction<0.05). Among 5 SGLT2i trials with available data, HRs for stroke became less efficacious with larger increases in hematocrit (ΔHR, 123%; P=0.09). No changes in HRs by systolic blood pressure (ΔHR, -11% to 9%) and urine albumin-creatinine ratio (ΔHR, -1% to 4%) were found for any outcome.

CONCLUSIONS

We confirmed increased efficacy findings for major adverse cardiovascular events with reduction in glycated hemoglobin for GLP1-RAs. Further research is needed on the potential loss of cardiovascular benefits with increased weight loss and hematocrit for SGLT2i.

Inflammatory mediation of the relationship between early adversity and major depressive disorder: A systematic review

Early adverse experience is related to psychiatric illness that occurs decades later. The mechanisms underlying this phenomenon have not been fully identified. There is a translational and clinical literature linking early adversity with Major Depressive Disorder (MDD) and inflammation. We reviewed articles that examine whether inflammation mediates this relationship.

METHODS

Literature review of PUB MED, CINAHL and APA Psycinfo articles that explicitly examine inflammation as a mediator between early adversity and depression using ((((((((((adversity) OR (trauma)) OR (maltreatment)) OR (child abuse)) AND (inflammation)) OR (inflammatory cytokines)) OR (crp)) OR (il-6)) OR (tnf)) AND (mediates)) AND (depression))))))))) as key words.

RESULTS

2842 articles were initially identified. 1338 non-human studies were excluded and 512 more were filtered out as reviews. The remaining 992 titles and, when necessary, abstracts and manuscripts were reviewed and 956 were removed as being of other non-related phenomena. Four additional studies were added by hand searching the references of remaining studies. Out of these 40, 15 explicitly examined inflammation as a mediator of the relationship between early adversity and later depression. Approximately half (8/15) showed evidence that inflammation mediated the relationship between early adversity and depression. Sensitivity analyses showed that studies taking place in clinical populations, in youth and those that used the Adverse Childhood Events Scale to measure adversity, and IL-6 and TNF-α (as opposed to CRP) to measure inflammation were most likely to show mediation.

CONCLUSIONS

There is evidence to support the model of inflammation mediating the relationship between early adversity and depression. Certain measures in clinical populations appear more likely to support this model. Further study with more standardized, robust methods will help to answer this question more definitively and may elucidate a subtype of depression related to early adversity by alterations in immune function.

Concerns about a systematic review of mediators of PTSD treatments: A commentary on Alpert and colleagues (2023)

The systematic review by Alpert and colleagues (Clinical Psychology Reviews, https://doi.org/10.1016/j.cpr.2023.102300) suggests methodological standards for mediation analysis and evaluates the evidence for mediation of PTSD treatment effects in published research. Although the article articulates important methodological guidelines related to timing of treatment, mediator, and outcome, it fails to incorporate the insights of causal mediation analysis into the methodological standards. In particular, the systematic review does not recognize the limitations of using time, rather than PTSD treatment type, in mediation analyses and does not include an assessment of the potential impact of uncontrolled confounding between the mediator and the outcome in the included studies. The conclusions about mediators of PTSD treatments from this systematic review should be considered carefully.

Mechanisms of action of therapeutic exercise for knee and hip OA remain a black box phenomenon: an individual patient data mediation study with the OA Trial Bank

OBJECTIVES

To evaluate mediating factors for the effect of therapeutic exercise on pain and physical function in people with knee/hip osteoarthritis (OA).

METHODS

For Subgrouping and TargetEd Exercise pRogrammes for knee and hip OsteoArthritis (STEER OA), individual participant data (IPD) were sought from all published randomised controlled trials (RCTs) comparing therapeutic exercise to non-exercise controls in people with knee/hip OA. Using the Counterfactual framework, the effect of the exercise intervention and the percentage mediated through each potential mediator (muscle strength, proprioception and range of motion (ROM)) for knee OA and muscle strength for hip OA were determined.

RESULTS

Data from 12 of 31 RCTs of STEER OA (1407 participants) were available. Within the IPD data sets, there were generally statistically significant effects from therapeutic exercise for pain and physical function in comparison to non-exercise controls. Of all potential mediators, only the change in knee extension strength was statistically and significantly associated with the change in pain in knee OA (β -0.03 (95% CI -0.05 to -0.01), 2.3% mediated) and with physical function in knee OA (β -0.02 (95% CI -0.04 to -0.00), 2.0% mediated) and hip OA (β -0.03 (95% CI -0.07 to -0.00), no mediation).

CONCLUSIONS

This first IPD mediation analysis of this scale revealed that in people with knee OA, knee extension strength only mediated ±2% of the effect of therapeutic exercise on pain and physical function. ROM and proprioception did not mediate changes in outcomes, nor did knee extension strength in people with hip OA. As 98% of the effectiveness of therapeutic exercise compared with non-exercise controls remains unexplained, more needs to be done to understand the underlying mechanisms of actions.