Posttraumatic stress disorder (PTSD) involves acute distress associated with a traumatic event, and is primarily characterized by (1) involuntary, intrusive re-experiencing (e.g., memories, dreams, flashbacks) of the traumatic event, (2) persistent avoidance of associated stimuli, (3) negative alterations in associated cognitions and affect, and (4) marked alterations in associated arousal and reactivity; wherein a traumatic event constitutes direct or indirect exposure to actual or threatened death, serious injury, or sexual violence (APA, 2013). Considering that lifetime prevalence rates of traumatic event exposure range from 54 % to 73.8 % globally (World Mental Health Surveys; Atwoli et al., 2015), PTSD is arguably a common psychiatric disorder cross-nationally (Gillihan et al., 2014). Further, PTSD symptoms bear adverse implications for the development of physical health conditions (Keyes et al., 2013; Pacella et al., 2013), comorbid mood disorders (Spinhoven et al., 2014), as well as risk of suicidal behaviors (Nepon et al., 2010; Wilcox et al., 2009). In view of the severe repercussions associated with PTSD, further research is warranted to understand the efficacy of relevant intervention efforts. While the importance of evidence-informed interventions for PTSD (e.g., trauma-focused cognitive behavior therapy) has gained increasing recognition (Sareen et al., 2011; Sareen, 2014; Watts et al., 2013), recent systematic and literature reviews have identified several barriers that hinder their implementation. These include temporal and financial constraints, trauma-specific barriers such as avoidance related to trauma disclosure, and concerns related to stigmatization and adverse social consequences (Finch et al., 2020; Kantor et al., 2017; Kazlauskas, 2017; Trusz et al., 2011).

In light of these barriers, emerging research has identified the potentials of digital mental health interventions in circumventing constraints associated with traditional treatment (Bakker et al., 2016; Chandrashekar, 2018). These refer to psychological interventions— delivered via mobile applications or web-based platforms—that incorporate therapeutic techniques including, inter alia, psychoeducation, coping skills training, trauma-focused or non-trauma-focused cognitive restructuring, expressive writing, and cognitive training. In particular, digital mental health interventions may either be self-managed or therapist-assisted, with the latter involving synchronous (e.g., immediate feedback via virtual meetings) or asynchronous (e.g., delayed feedback via an electronic messaging platform or online discussion forum) therapeutic support to intervention users. Notably, these digital interventions can be administered independent of time and place (Struthers et al., 2015), are more cost-efficient than traditional treatment options (Binhadyan et al., 2016; Oyebode et al., 2020), and allow for discreet or even anonymous access to mental healthcare (Lal and Adair, 2014), hence critically addressing barriers of accessibility and stigmatization associated with traditional PTSD interventions (Koh et al., 2022). Moreover, digital interventions have the potential to complementarily augment therapeutic outcomes through functions such as psychoeducation (Sander et al., 2020), symptom monitoring (Hwang et al., 2021), and automatic reminder notifications which facilitate habit-tracking and goal attainment (Donker et al., 2013; Oyebode et al., 2020). Given burgeoning support for the advantages of digital mental healthcare, it is crucial that we gain deeper insight into the efficacy of digital mental health interventions for the treatment of PTSD symptoms.

While a handful of meta-analytic reviews demonstrate support for the efficacy of digital interventions for PTSD symptoms (e.g., Simblett et al., 2017; Wang et al., 2018), others present inconclusive results (e.g., Goreis et al., 2020; Kuester et al., 2016; Sander et al., 2020; Weisel et al., 2019). For example, Simblett et al.'s (2017) meta-analysis of randomized controlled trials (N = 3832, K = 38) found significant reductions in PTSD symptoms in active web-based intervention conditions (i.e., interventions with varying levels of therapeutic guidance) compared to waitlist or active controls, in a sample of adults who had experienced at least a single traumatic event. In contrast, in Goreis et al.'s (2020) meta-analytic review of the effects of self-managed smartphone-based apps on PTSD symptoms in adults with varying PTSD symptom severity, they demonstrated a moderate reduction in PTSD symptoms across pre-post comparison studies (209 unique participants across 4 studies), but inconclusive results based on randomized controlled studies (169 unique participants across 2 studies).

These inconsistent findings may be attributed to heterogeneity in features of specific interventions examined (e.g., provision of therapeutic guidance), outcome measures used, and inclusion criteria used across meta-analyses. More specifically, meta-analytic reviews of digital PTSD interventions vary in two key aspects. First, characteristics of digital mental health interventions examined vary across meta-analyses in terms of therapeutic techniques applied (e.g., CBT-based versus non-CBT-based interventions; Lewis et al., 2019, Sijbrandij et al., 2016, Simon et al., 2021), intervention timeframes across included studies (i.e., ranging from 4 to 24 weeks; Lewis et al., 2019; Steubl et al., 2021), and the provision of therapeutic guidance in interventions (Kuester et al., 2016; Wang et al., 2018).

Given the heterogenous characteristics of examined interventions across meta-analyses, we sought to examine how each characteristic moderates the efficacy of digital interventions in alleviating severity of PTSD symptoms. Notably, the provision of therapeutic guidance in digital interventions, which refers to electronically-delivered human guidance related to therapeutic content, has been found to promote stronger adherence to interventions and enhance treatment outcomes (Baumeister et al., 2014; Moshe et al., 2021; Richards and Richardson, 2012). Further, some meta-analytic evidence suggests that internet-based interventions with a greater number of sessions are associated with lower attrition rates and stronger effects on symptoms of stress and pathological grief (Spijkerman et al., 2016; Wagner et al., 2020); though it remains unclear whether the timeframe of digital interventions for PTSD moderate their effectiveness (Sijbrandij et al., 2016; Lewis et al., 2017). Moreover, though digital interventions vary in their therapeutic approach—including cognitive behavior therapy, expressive writing, and cognitive training—existing meta-analyses are equivocal as to whether therapeutic techniques moderate their effects on PTSD symptoms (e.g., Barak et al., 2008; Kuester et al., 2016; Steubl et al., 2021). Given that therapeutic technique, intervention timeframe, and provision of guidance are key modifiable characteristics of digital mental health interventions, it is thus important that we examine their role in the efficacy of interventions aimed at reducing PTSD symptom severity.

Second, with regard to heterogenous methodological characteristics, meta-analyses employ different outcome measures to assess treatment efficacy (e.g., self-reported questionnaires vs. clinician-administered interviews; Lewis et al., 2019; Simblett et al., 2017; Simon et al., 2021; Zhou et al., 2021) and varying inclusion criteria related to sample characteristics (e.g., age, gender, SES). For instance, Steubl et al. (2021) focused on adults with PTSD or subthreshold PTSD assessed by a validated clinician-administered or self-rated measure of PTSD symptoms, whereas Simblett et al. (2017) examined adults who had experienced a possible single-event trauma and did not impose restrictions in terms of pre-intervention severity of PTSD symptoms.

Hence, it is vital that we examine how various user characteristics shape the effects of specific digital interventions for PTSD.

In this systematic review, we sought to provide a synthesis of meta-analyses published up till 27 December 2023 which investigate the effects of digital mental health interventions on PTSD symptoms. Further, we sought to explore the moderating effects of specific intervention characteristics (i.e., intervention technique, intervention timeframe, and therapeutic guidance) and methodological characteristics including intervention outcome measures assessed and sample inclusion criteria used (i.e., age, gender, SES, country, symptom severity, and trauma characteristics). In so doing, we seek to synthesize a comprehensive scope of published evidence to discern the clinical outcomes of digital mental health interventions for PTSD symptoms across various populations and therapeutic approaches. Furthermore, we aim to advance understanding into a) whether and how modifiable features of digital interventions (e.g., intervention timeframe or provision of therapeutic guidance) shape the efficacy of DMHIs, and b) how users from varying populations (e.g., those from clinical vs. nonclinical populations) can be matched with compatible interventions that optimize their treatment outcomes (Cuijpers et al., 2013a, Cuijpers et al., 2013b; Donker et al., 2013). These insights serve to inform existing or novel digital mental health interventions for PTSD symptoms to improve their clinical outcomes.