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. 2025 Aug 12;16(1):2536973. doi: 10.1080/20008066.2025.2536973

The relationship between changes in emotional intensity and treatment outcome in PTSD patients in EMDR therapy

De relatie tussen veranderingen in emotionele intensiteit en behandelingsuitkomst bij PTSS-patiënten in EMDR-therapie

Valentijn V P Alting van Geusau a,b,CONTACT, Mae D Nuijs a, Ad de Jongh b,c,d,e,f, Mirjam Moerbeek g, Suzy J M A Matthijssen a,c
PMCID: PMC12344708  PMID: 40792392

ABSTRACT

Background: Eye movement desensitisation and reprocessing (EMDR) therapy primarily aims to reduce the emotional intensity or subjective disturbance associated with traumatic memories. However, to date, only one study has investigated whether a reduction in emotional intensity is related to a reduction in post-traumatic stress disorder (PTSD) symptoms.

Objective: Therefore, the purpose of the present study was to determine the relationship between changes in emotional intensity of traumatic memories during EMDR therapy and treatment outcomes.

Method: One hundred twenty-five patients (88.8% female, M age = 36.4 years, SD = 11.40) with PTSD due to multiple traumatisation participated in a six-day intensive treatment programme consisting of a combination of six 90 min EMDR therapy sessions, six 90 min prolonged exposure sessions, six 60 min sessions of physical activity, and six 60 min psychoeducation sessions delivered at an academic outpatient mental healthcare clinic.

Results: The results showed that a greater reduction in the emotional intensity of traumatic memory during EMDR therapy was associated with a larger decrease in PTSD symptoms at four weeks post-treatment.

Conclusions: Clinicians should monitor changes in emotional intensity during treatment to assess treatment progress. Furthermore, the findings justify the use of memory disturbance as an outcome measure in experimental studies on EMDR therapy. Future research should focus on EMDR therapy processes and their relationship to treatment outcome, whereas replication of the present findings in other trauma populations is warranted.

KEYWORDS: EMDR, PTSD, SUD, treatment, disturbance, distress, emotional intensity

HIGHLIGHTS

  • A reduction in the emotional intensity of traumatic memory during EMDR therapy was associated with a decrease in PTSD symptoms at post-treatment.

  • This is the first study to find this association in which a standardised measure of treatment outcome was used.

  • Therapists should consistently monitor subjective units of disturbance (SUD) decrease during EMDR therapy sessions, as SUD changes can serve as a valuable real-time indicator of the therapeutic process.

1. Introduction

Eye movement desensitisation and reprocessing (EMDR) therapy (Shapiro, 2018) is a protocolized treatment procedure in which patients are asked to recall a specific traumatic memory while simultaneously executing a task such as following the therapist’s fingers, which move from left to right in front of the patient’s eyes. Extensive research conducted on EMDR therapy supports its effectiveness in treating post-traumatic stress disorder (PTSD) and other mental health conditions (De Jongh et al., 2024; Valiente-Gómez et al., 2017). A recent meta-analysis found that EMDR therapy is one of the most effective interventions to improve PTSD symptoms (Yunitri et al., 2023). Accordingly, the use of EMDR therapy is recommended by most international guidelines for the treatment of PTSD (International Society of Traumatic Stress Studies, 2018; U.S. Department of Veterans Affairs, 2023; World Health Organization, 2013).

According to the adaptive information processing (AIP) model which underpins EMDR therapy, the brain has an inherent capacity to process and integrate distressing experiences into adaptive memory networks (Solomon & Shapiro, 2008). Traumatic experiences may overwhelm this system, leading to the dysfunctional storage of memories, along with associated thoughts, emotions, and somatic sensations. These unprocessed memories can continue to cause psychological distress when triggered. EMDR therapy aims to reprocess these memories so they can be adaptively integrated, thereby reducing their emotional intensity (i.e. subjective disturbance; Shapiro, 2018). A mechanism by which EMDR therapy facilitates this reprocessing is the working memory theory. This theory suggests that holding a disturbing memory in mind while simultaneously performing a task that taxes working memory, such as making horizontal eye movements, leads to competition for limited cognitive resources (Gunter & Bodner, 2008; van den Hout & Engelhard, 2012). Given that both holding the memory in mind and performing the dual task rely on limited working memory capacity, this competition leads to not being able to hold on to the memory with the same emotional disturbance. The disturbance of the memory reduces during recall and the memory becomes more amenable to adaptive integration, consistent with the AIP model.

The underlying clinical assumption is that a reduction in memory disturbance translates into a decrease in PTSD symptoms. This assumption is reflected in the use of the subjective units of disturbance (SUD) scale (Wolpe, 1990), an 11-point Likert scale ranging from 0 (no disturbance) to 10 (extreme disturbance), which is routinely used to assess within-session progress (Shapiro, 1989, 2018). Although within-session reductions in SUD scores are commonly interpreted as indicators of treatment effectiveness, the extent to which these reductions are predictive of actual treatment outcome remains unclear. Accordingly, the effectiveness of EMDR therapy will ultimately have to be assessed using appropriate, validated outcome measures, such as the Clinician-Administered PTSD Scale for DSM-5 (CAPS-5; Weathers et al., 2013a). The assumption that a reduction in the disturbance of traumatic memories, as assessed by a SUD-score, will alleviate PTSD symptoms is also evident in experimental research on EMDR therapy, its working mechanisms, and EMDR-related protocols with non-clinical samples (Houben et al., 2020; Lee & Cuijpers, 2013). These studies often rely on the SUD scale or Likert-type emotionality and vividness scales to assess the efficacy of interventions. Despite its central role in EMDR therapy, little empirical research has investigated whether decreases in SUD scores are meaningfully associated with symptom reduction across the course of treatment.

To date, only one study has been conducted that investigated the psychometric properties of the SUD scale for EMDR therapy (Kim et al., 2008). This study used a sample of 61 patients, 55% of whom were diagnosed with PTSD, and found that SUD scores at the end of the first treatment session were significantly correlated with treatment outcome. Specifically, patients who reported lower SUD scores after the first treatment session had better therapist-rated outcomes after treatment completion (Kim et al., 2008). However, the researchers did not use standardised measures of treatment outcomes to evaluate the predictive validity of the SUD scale. Instead, the therapist rated the change in symptoms on a 7-point scale from 1 (very much improved) to 7 (very much worse) at the termination of treatment. Unfortunately, the study also included patients with a primary psychiatric diagnosis other than PTSD (e.g. depressive and anxiety disorders), which impedes conclusions regarding the predictive validity of the SUD scale for PTSD outcomes. Another study found that psychophysiological changes (e.g. changes in heart rate) during EMDR therapy sessions were positively correlated with changes in SUD (Sack et al., 2008). Hence, these results suggest that SUD scores might be a valid measure for assessing physiological disturbances during EMDR therapy. Changes in psychophysiological activity were not significantly correlated with changes in PTSD symptoms. However, the authors stated that the study might have been underpowered to detect significant effects due to the small sample size. Sack et al. (2008) did not investigate the relationship between SUD change and treatment outcomes. Therefore, there is a clear need for further research that employs standardised PTSD outcome measures and larger PTSD-specific samples to assess the relationship between SUD changes and treatment outcomes in EMDR therapy.

The purpose of the current study was to investigate the relationship between memory disturbance during EMDR therapy and PTSD treatment outcome. Specifically, we investigated whether changes in SUD scores during EMDR therapy sessions could predict PTSD treatment outcome four weeks after therapy. Based on the findings of Kim et al. (2008), we hypothesised that patients who, on average, exhibited a larger reduction in SUD scores during EMDR treatment would show a greater decrease in PTSD symptoms. Furthermore, we hypothesised that patients who achieved a higher number of traumatic images desensitised to zero disturbance would demonstrate a greater decrease in PTSD symptoms.

2. Method

2.1. Participants

Data were collected from 160 patients diagnosed with PTSD. All patients had been exposed to multiple traumatic events (e.g. sexual and physical abuse) and were treated at the Altrecht Academic Anxiety Centre in Utrecht, the Netherlands. They participated in an intensive PTSD treatment programme between April 2018 and September 2020. Data from 35 patients were excluded from analysis. Reasons for exclusion were dropout during treatment (n = 13), objection to the use of their data in research (n = 15), and unreliable or invalid data (n = 7). We included a sample of 125 patients (88.8% identified as female, mean age = 36.4 years, SD = 11.40) diagnosed with PTSD in the study. Nearly half of the patients (48.8%) met the criteria for the dissociative subtype of PTSD and 86.4% had at least one comorbid psychiatric disorder (Table 1). The inclusion criteria for treatment were (1) being diagnosed with PTSD according to the DSM-5 (APA, 2013), (2) a minimum age of 18 years, and (3) sufficient proficiency in the Dutch language. The exclusion criteria were (1) acute suicidality risk, (2) the presence of comorbid psychiatric disorders that would interfere with treatment, and (3) alcohol or drug use during treatment. The use of sedating medication during treatment was considered a contraindication and tapered as much as possible.

Table 1.

Frequencies of DSM-5 Diagnoses Comorbid to PTSD.

Disorder or subtype n %
Dissociative subtype of PTSD 61 48.8
Dissociative identity disorder 1 0.8
Unspecified or other specified dissociative disorder 4 3.2
Obsessive-compulsive disorder 2 1.6
Anxiety disorder 17 13.6
Eating disorder 11 8.8
Somatic symptom disorder 11 8.8
Conversion disorder 4 3.2
Attention-deficit disorder 18 14.4
Personality disorders 65 52.0
 Cluster A personality disorder 2 1.6
 Cluster B personality disorder 16 12.8
 Cluster C personality disorder 13 10.4
 Unspecified or other specified personality disorder 34 27.2
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2.2. Treatment

The patients took part in an intensive outpatient treatment programme for PTSD which consisted of two consecutive weeks of treatment, with three treatment days per week. The programme consisted of two evidence-based treatments for PTSD. Patients received 90 min of prolonged exposure (PE; Foa et al., 2019) in the morning and 90 min of EMDR therapy (De Jongh & Ten Broeke, 2019; Shapiro, 2018) in the afternoon. EMDR therapy sessions were performed according to the Dutch version of the EMDR standard eight-phase protocol for EMDR therapy (De Jongh & Ten Broeke, 2019; Shapiro, 2018). Therapists used techniques within the framework of EMDR 2.0 that generally use a higher dose in terms of working memory load than regular EMDR therapy (see Matthijssen et al., 2021). Compared to classical EMDR therapy (Shapiro, 2018), free association in the protocol used in this study is limited, focusing on the target memory. This structured approach aims to improve efficiency, although it may limit the spontaneous processing of related material. If new memories emerged during EMDR therapy sessions that had not been identified during the initial treatment planning, they were included and processed when clinically appropriate and feasible. Nonetheless, the primary focus of the treatment remained on the predetermined target memories.

Treatment-interfering anticipatory fear was addressed using the flashforward protocol (Logie & De Jongh, 2014). The treatment programme also included physical activity (60 min, e.g. running) and psychoeducation (60 min), both delivered in a group format (see Table 2 for the format of the treatment programme). To obtain more information regarding the treatment programme and its effectiveness, see Matthijssen et al. (2024).

Table 2.

Overview of the Treatment Programme.

Time Activity
09:45–11:15 Prolonged Exposure
11:15–11:30 Short break
11:30–12:30 Physical activity
12:30–13:15 Lunch break
13:15–14:45 EMDR therapy
14:45–15:00 Short break
15:00–16:00 Psychoeducation
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2.3. Measurements

2.3.1. Subjective Units of Disturbance (SUD) scale

The SUD scale (Wolpe, 1990) was used to measure the disturbance or distress experienced by patients during the EMDR sessions in relation to the recalled image, which is a standard practice in EMDR therapy (Shapiro, 2018). Patients were asked to rate how much disturbance they felt when bringing up a traumatic image on an 11-point Likert scale. The scores range from 0 (no disturbance) to 10 (extreme disturbance). The SUD scale has good psychometric properties when used in EMDR therapy (Kim et al., 2008).

2.3.2. Clinician-Administered PTSD Scale for DSM-5 (CAPS-5)

The Dutch version of the CAPS-5 (Boeschoten, Bakker, Jongedijk, and Olff, 2014; Weathers et al., 2013a) is a clinician-rated structured interview that was used to assess PTSD diagnosis and PTSD symptom severity. Given that 20 symptoms are scored on a 0–4 scale, the total PTSD severity score ranges from 0 to 80. The CAPS-5 can be used to measure symptoms over the past month as well as over the past week. The Dutch CAPS-5 has adequate validity and reliability (α = .90; Boeschoten et al., 2018).

2.3.3. Life Events Checklist for the DSM-5 (LEC-5)

The Dutch version of the LEC-5 (Boeschoten, Bakker, Jongedijk, van Minnen, et al., 2014; Weathers et al., 2013b) was used to index experienced potential traumatic events. The LEC-5 is a self-report questionnaire that consists of 17 items. Sixteen items check for different types of traumatic events (e.g. natural disasters), and one is a residual category. Participants were asked if they had experienced, witnessed, and/or learned about these traumatic events, and/or if they were a part of their job. Although the psychometric properties of the LEC-5 have not yet been established, the LEC-5 differs very little from the LEC for DSM-IV which shows adequate psychometrics (Gray et al., 2004).

2.4. Procedure

Prior to starting treatment, participants entered a diagnostic phase in which they were assessed and screened based on the inclusion and exclusion criteria. The Dutch version of the Clinician-Administered PTSD Scale for DSM-5 (CAPS-5; Boeschoten, Bakker, Jongedijk, & Olff, 2014; Weathers et al., 2013a) and the Life Events Checklist for DSM-5 (LEC-5; Boeschoten, Bakker, Jongedijk, van Minnen, et al., 2014; Weathers et al., 2013b) were administered to determine whether the patient fulfilled the diagnostic criteria for PTSD. After the diagnostic phase, an individualised treatment plan was established. This involved the assessment of the participants’ traumatic experiences. Participants were asked to select at least six of their most disturbing traumatic memories for treatment (i.e. one traumatic memory per treatment day). In addition, the inclusion criteria were discussed and agreements were made considering suicidality, self-harming behaviour, and the use of sedating medication.

At least one traumatic memory was treated in every EMDR therapy session. Based on the fact that traumatic memories likely consist of several disturbing images, the most disturbing mental image of the memory was selected with the purpose to be processed and desensitised. This is in accordance with the EMDR protocol by De Jongh and Ten Broeke (2019), but differs from the protocol by Shapiro (2018) in which the traumatic memory is targeted as a whole. Pre- and post-SUD scores were assessed to monitor the progress of the treatment process. Once a mental image reached a SUD score of zero, another image of the same memory was desensitised. If all images of the memory reached a SUD score of zero before the end of the session, the mental images of the next most disturbing memory were desensitised. To determine treatment outcome, the CAPS-5 was readministered after one week, and at four weeks post-treatment.

2.5. Data Analyses

Hierarchical multiple linear regression analysis was used to determine whether changes in emotional intensity or subjective disturbance of the traumatic memory (i.e. SUD scores) of the patients’ most disturbing images during the EMDR therapy sessions were related to the overall treatment outcome. In the first step of the model, the following predictor/baseline variables were added: ‘’age’’, ‘’gender’’, ‘’presence of the dissociative subtype of PTSD’’, and ‘’baseline CAPS-5 score’’. In the second step of the model, the main predictors of interest were added: ‘’Change in SUD score’’ and the ‘’number of disturbing images desensitised to a SUD score of 0’’. The change in the SUD score was computed by calculating the average change in the SUD scores of all disturbing images during the six EMDR sessions. The dependent variable was ‘treatment outcome’ and was defined as the difference in CAPS-5 severity score at the 4-week follow-up, compared to the CAPS-5 severity score at the pre-treatment level. In addition, a separate multiple linear regression analysis was conducted to explore whether the baseline variables were associated with the average SUD change, to provide descriptive insight into potential influences on the within-treatment SUD reduction. During the intensive treatment programme, at least six images of the memories were treated. However, some participants who showed a rapid decrease in SUD scores received EMDR therapy on more than six disturbing images. We analysed only six primary images that were targeted during the EMDR sessions. The analyses were performed using IBM SPSS (Version 29).

3. Results

3.1. Sample characteristics and descriptive statistics

Data from 125 participants were analysed. Twenty participants showed missing data on the CAPS-5. Missing data were caused by technical errors when saving the CAPS-5 in the electronic patient file (n = 3), no follow-up measurements (n = 12), follow-up measurements that were conducted too late (n = 3), and no follow-up measurements because the participants were following additional trauma treatment at that time (n = 2). At the pre-treatment level, participants had an average CAPS-5 score of M = 43.32 (SD = 9.63), whereas at post-treatment, participants had an average CAPS-5 score of M = 24.87 (SD = 16.77). Post-treatment, 60 participants (48%) lost their PTSD diagnosis as assessed with the CAPS-5, 48 participants (38.4%) retained their PTSD diagnosis, and 17 participants (13.6%) had missing data. At the start of EMDR therapy sessions, traumatic mental images had an average SUD score of M = 8.03 (SD = 1.11), whereas at the end of EMDR therapy sessions, these images had an average SUD score of M = 1.99 (SD = 2.33). On average, the SUD decreased by M = 5.85 (SD = 2.01) points. By the end of treatment, at least one traumatic mental image had been desensitised to a SUD score of 0 in 85.6% of participants, while in 22.4% of participants all six primary images were desensitised to a SUD score of 0. Participants who no longer met the criteria for PTSD post-treatment showed an average SUD decrease of M = 6.71 (SD = 1.13), whereas those who retained their PTSD diagnosis showed a mean decrease of M = 5.15 (SD = 2.32).

3.2. Relationship between baseline characteristics and SUD change

Multiple linear regression was conducted to determine whether baseline characteristics ‘’age’’, ‘’gender’’, ‘’dissociative subtype of PTSD’’, and ‘’baseline CAPS-5 score’’ were significant predictors of SUD change. The assumptions of the regression analysis (i.e. linearity, homoscedasticity, normality and multicollinearity) were met.

We entered baseline predictor variables in the model to examine their influence on SUD change during EMDR therapy. The model did not explain a significant proportion of the variance in SUD change, R² = .038, F(4, 107) = 1.06, p = .378. See Table 3 for all the regression coefficients. None of the baseline predictors were significant. Age, gender, dissociative subtype, and baseline PTSD severity were not associated with the average reduction in SUD scores across EMDR sessions.

Table 3.

Results of Multiple Linear Regression Model: Predicting SUD Change during EMDR Therapy Sessions.

Predictor B SE β t p 95% CI
Age 0.01 0.02 .04 0.46 .645 [−0.03, 0.04]
Gender 0.30 0.63 .05 0.47 .637 [−0.95, 1.54]
Presence of dissociative subtype of PTSD −0.50 0.44 −.12 −1.14 .256 [−1.37, 0.37]
Baseline CAPS-5 score −0.02 0.02 −.11 −1.01 .313 [−0.07, 0.02]
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Note. PTSD = post-traumatic stress disorder; SUD = subjective units of disturbance; CAPS-5 = Clinician-Administered PTSD Scale for DSM-5.

3.3. Relationship between emotional intensity of traumatic memories during EMDR and treatment outcome

Hierarchical multiple linear regression was conducted to determine whether changes in SUD scores and the number of traumatic images desensitised to zero were significant predictors of CAPS-5 change. Then, the assumptions of the regression analysis (i.e. linearity, homoscedasticity, normality and multicollinearity) were met.

Using a hierarchical selection procedure, we entered baseline predictor variables ‘’age’’, ‘’gender’’, ‘’presence of the dissociative subtype of PTSD’’, and ‘’baseline CAPS-5 score’’ in the first step of the model. The first step did not explain a significant proportion of the variance in the CAPS-5 difference, R2 = .07, F(4,92) = 1.80, p = .136. In the second and final step, predictor variables ‘’number of traumatic images desensitised to a SUD score of 0’’ and ‘’change in SUD score’’ were added to the model, improving the fit. The model explained a significant additional proportion of the variance, ΔR2 = .33 F(2,90) = 9.98, p < .001. See Table 4 for all the regression coefficients. The final model showed that when the baseline CAPS-5 score increased, the CAPS-5 change also increased. When the number of images desensitised to a SUD score of 0 increased, the CAPS-5 change increased. Finally, when change in SUD score increased, the CAPS-5 change also increased. When comparing the standardised coefficients (Beta values), ‘’number of traumatic images desensitised to a SUD score of 0’’ was the strongest predictor of treatment outcome, followed by ‘’change in SUD score’’ and ‘’baseline CAPS-5 score’’ respectively.

Table 4.

Results of Hierarchical Multiple Linear Regression Model: Predicting CAPS-5 Change from Pre-treatment to 4-Week Follow-Up.

Predictor B SE β t p 95% CI R2
Step 1             .07
Age 0.28 0.14 .21 2.03 .045 [0.01, 0.56]  
Gender −1.29 5.16 −.03 −0.25 .803 [−11.54, 8.96]  
Presence of dissociative subtype of PTSD 0.36 3.54 .01 0.10 .920 [−6.67, 7.38]  
Baseline CAPS-5 score 0.26 0.18 .16 1.44 .154 [−1.00, 0.61]  
Step 2             .40
Age 0.17 0.12 .13 1.50 .137 [−0.06, 0.41]  
Gender −1.58 4.21 −.03 −0.37 .709 [−9.95, 6.79]  
Presence of dissociative subtype of PTSD 1.23 2.91 .04 0.42 .973 [−4.55, 7.02]  
Baseline CAPS-5 score 0.40 0.15 .25 2.75 .007 [0.11, 0.70]  
Number of images to SUD 0 1.40 0.43 .34 3.26 .002 [0.55, 2.26]  
Change in SUD score 2.42 0.80 .31 3.01 .003 [0.82, 4.02]  
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Note. PTSD = post-traumatic stress disorder; SUD = subjective units of disturbance; CAPS-5 = Clinician-Administered PTSD Scale for DSM-5.

4. Discussion

The present study aimed to investigate the relationship between memory disturbance during EMDR treatment, rated using the SUD scale, and EMDR therapy outcomes, as assessed by PTSD symptoms. As hypothesised, a larger decrease in memory disturbance during EMDR therapy sessions was significantly associated with a larger decrease in PTSD symptoms. Furthermore, a higher number of traumatic images that were desensitised to zero disturbance was significantly associated with a larger decrease in PTSD symptoms, and emerged as the strongest predictor of treatment success. Age, sex, and presence of the dissociative subtype of PTSD did not predict treatment outcomes. Higher pre-treatment PTSD symptomatology predicted stronger PTSD symptom reduction.

The current findings align with those of Kim et al. (2008), confirming that a reduction in emotional disturbance during EMDR therapy is associated with improved PTSD outcomes. The observed changes in disturbance suggest that the processing and integration of traumatic memories during EMDR therapy allow for adaptive resolution of the traumatic material and symptom change, as posited by the AIP model (Solomon & Shapiro, 2008). These results further support the notion that emotional processing is central to recovery from PTSD and the integration of previously unprocessed traumatic memories into adaptive memory networks aligns with theories suggesting that such resolution is key to the improvement of PTSD symptoms (Hase et al., 2017; Shapiro, 2018). Moreover, the current study’s clinical sample, characterised by a wide range of comorbid diagnoses and histories of multiple traumatisation, strengthens the generalisability of the findings to clinical practice in general. An important clinical implication of these results is that therapists should consistently monitor SUD decrease during EMDR therapy sessions, as SUD changes can serve as a valuable real-time indicator of the therapeutic process. Therapists should prioritise the complete desensitisation of all traumatic images which cause disturbance, as this was the strongest predictor of PTSD symptom improvement. If SUD scores do not decrease, therapists should adapt the treatment delivery accordingly. For instance, therapists can use cognitive interweaves (De Jongh & Hafkemeijer, 2024; Shapiro, 2018) or increase working memory taxation (Littel & van Schie, 2019; Matthijssen et al., 2021) to facilitate SUD decrease. The current findings also justify the use of SUD scores in experimental studies on the efficacy of EMDR therapy in nonclinical populations (Houben et al., 2020; Lee & Cuijpers, 2013).

Several limitations of this study should be considered when interpreting these results. First, the treatment programme included not only EMDR therapy, but also PE, psychoeducation, and physical activity. This combination introduces potential variability in the data, as these additional treatments, PE in particular, likely confounded direct attribution to CAPS-5 post-treatment scores. However, since memory disturbance was assessed only after the PE sessions concluded, we believe this timing allowed us to draw meaningful conclusions about the relationship found. Notably, even with the influence of PE on treatment outcomes, the relationship between the decrease in SUD and treatment outcomes remained evident. In addition, the average memory disturbance at the start of the EMDR therapy sessions was high, even though these memories were treated during PE beforehand. A limitation of the regression analysis is that, while it provides insight into the average relationship between changes in memory disturbance and treatment outcome, it does not account for individual variability. Specifically, it does not indicate the number of patients who experienced SUD changes without corresponding changes in PTSD symptoms, or vice versa. A strength of the current study is the utilisation of a clinician-rated, standardised measure to assess treatment effectiveness. Standardised measures are carefully designed and validated assessment tools that provide a more objective and systematic approach for evaluating treatment outcomes. Another strength pertains to the fact that each patient with PTSD received an equal high-quality therapeutic dose of trauma-focused therapy. Within the unique framework of the intensive treatment programme, all patients underwent the same treatment protocol with an identical number of session hours, ensuring consistent therapeutic dosing. Additionally, therapy sessions were delivered by different highly trained and supervised therapists. By implementing a therapist rotation model, the potential influence of the therapeutic relationship on outcomes was minimised and evenly distributed across sessions, reducing the likelihood of confounding bias.

When considering directions for future research, we would argue that the study should be replicated using a sample which includes specific populations such as refugees or military personnel, who have high prevalence rates of PTSD (Blackmore et al., 2020; Hines et al., 2014). Including such populations would further enhance the generalisability of our findings, as the current sample mainly consisted of survivors of sexual and physical abuse. Furthermore, we argue that a replication of the study using a treatment programme that consists solely of EMDR therapy instead of a combination of treatment methods, or using a control group, would strengthen causal inference further.

Moreover, future research should focus on investigating factors which may influence SUD score changes during EMDR therapy. Some studies have been conducted that investigated the processes during EMDR therapy which can influence treatment outcomes. For example, the responses that patients give after a set of eye movements have been found to be related to treatment outcomes, as measured by SUD scores and PTSD symptoms. Participants who responded with ‘’distancing’’ material (i.e. describing stimuli in a matter that it is not happening right now; Lee et al., 2006), or ‘’resolution’’ material (McCullough, 2002) showed greater symptom improvement during EMDR therapy sessions than those who responded with ‘’trauma’’ material. When combining these results with the findings of the current study that memory disturbance decrease predicts PTSD symptom decrease, we argue that we should focus future research on processes during EMDR therapy which could influence SUD decrease, and thus, overall treatment outcome.

In conclusion, the present study suggests that larger reductions in traumatic memory disturbance during EMDR predicted better treatment outcomes in a sample of patients who experienced multiple traumatisation, were diagnosed with PTSD using a structured interview, and were diagnosed with comorbid psychiatric disorders. Furthermore, a higher number of traumatic images desensitised to zero disturbance predicted better treatment outcomes. This has clinical implications, in that clinicians should monitor changes in SUD during treatment to assess treatment progress. Furthermore, the findings justify the use of memory disturbance as an outcome measure in experimental studies on EMDR therapy. Future research should focus on EMDR therapy processes and their relationship to treatment outcomes, whereas replication of the present findings in other trauma populations, such as refugees and military personnel, is warranted.

Acknowledgements

We would like to thank colleagues Sophie Menses and Mathilde Huisman for managing the data. We want to thank students Pepijn van der Post and Sanne Hofman for their aid in collecting the data.

Disclosure statement

No potential conflict of interest was reported by the authors.

Ethics

The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Scientific Board of Altrecht GGz. Informed consent was obtained from all subjects involved in the study.

Data availability statement

The anonymised datasets generated and analysed during the current study will be available upon request from v.alting-van-geusau@altrecht.nl

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The anonymised datasets generated and analysed during the current study will be available upon request from v.alting-van-geusau@altrecht.nl

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