This study adds to a growing body of evidence about patients with BPD and co-occurring PTSD. According to our results, PTSD is significantly related to dissociation (DES) and a history of suicide attempts in BPD patients. A trend was found for the impact of PTSD on the symptom severity (BSL) and the history of self-mutilation in patients with BPD.

Our finding that patients with co-occurring current PTSD scored significantly higher on the DES than patients without a co-occurring diagnosis, is in line with the findings from Heffernan and Cloitre, who also reported higher scores on the DES in the group of BPD patients with co-occurring PTSD [9]. However, this association became non-significant in our study after controlling for CSA, suggesting that the effect of PTSD on dissociation in BPD patients may be associated with the history of sexual abuse in childhood [38, 39]. Our finding that significantly more BPD patients with PTSD reported having attempted suicide than patients without PTSD diagnosis is consistent with the results of several previous studies [4, 8]. When controlling for CSA, the connection between PTSD and the history of suicide attempts disappeared, suggesting that CSA and not PTSD, is influencing suicidality rates in BPD patients. Similar previous results have been reported stating that severity of CSA is associated with higher rates of suicide attempts [19, 20]. For example, Ferraz and colleagues revealed that in BPD patients, the presence, number and severity of previous suicide attempts are significantly predicted by CSA [40]. In recent studies [8, 13] it has been assumed that CSA could be associated with the effects of PTSD in BPD patients and even account for the presence of PTSD symptoms among patients with BPD [18]. Interestingly, BPD patients with and without PTSD did not differ in terms of the frequency of suicide attempts. However, our findings indicate that the frequency of suicide attempts was significantly related to CSA.

While self-mutilation tended to be higher in BPD patients with co-occurring PTSD, this trend was not statistically significant, despite the relatively large number of patients included in our study. This is not fully consistent with previous studies, which revealed that BPD patients with co-occurring PTSD had a higher frequency of intentional self-injury, typically being triggered by flashbacks, nightmares or thoughts about sexual abuse or rape [13, 14]. The slight differences between our finding and previous results might relate to the very high prevalence of self-mutilation in our study (>90% in both subsamples of BPD patients with and without PTSD), which might have caused a ceiling effect. Self-mutilation was unrelated to CSA in our sample.

With respect to the impact of PTSD on BPD severity, our study provided preliminary evidence that some specific aspects (rather than overall severity) might be impacted by co-occurring PTSD. With respect to overall severity, we only found a non-significant trend towards higher total scores in the BSL. We found no evidence for higher scores with respect to the number of BPD criteria and the dimensional IPDE score in the subgroup of patients with co-occurring PTSD. This is in line with previous reports that PTSD does not necessarily affect the extent of BPD psychopathology [11, 13, 18]. With respect to the facets of psychopathology (BSL), we did, however, detect higher scores in the subscales “intrusion” and “affect regulation” in BPD patients with co-occurring PTSD. This indicates that patients with both diagnoses report more intrusions and elevated problems in affect regulation than BPD patients without a co-occurring PTSD diagnosis. Overall, these results are in line with the findings by Harned and colleagues, who did not find an effect for PTSD on the number of BPD criteria and the dimensional score, but on emotion regulation [13]. However, these results do not support the outcomes of other studies, which indicated that co-occurring PTSD has a negative effect on general BPD symptomatology [8, 10, 14].

Our study has both strengths and limitations. A strength of our study is the relatively large sample of well diagnosed participants. All patients underwent standardized diagnostics including structured interviews for BPD (IPDE) and Axis-I disorders (SCID-I), which were conducted by experienced diagnosticians. With respect to the limitations, we would like to emphasize that our sample only included treatment-seeking female BPD patients recruited at specialized university settings. Hence, generalizations beyond the population investigated in this study should be made with caution. Furthermore, as most of our dependent variables were based on retrospective self-reports, there is a possibility of bias. One significant limitation concerns the assessment of the presence and frequency of previous suicide attempts and the history of self-mutilation in this study. As we did not use a well-established and validated measure, the validity of our results on suicide attempts and self-mutilation might be constrained. Future studies investigating this topic should make use of measures such as the Columbia-Suicide Severity Rating Scale (C-SSRS), which have been developed to consistently define and classify these behaviours and show good psychometric properties [41]. Furthermore, as we conducted an observational study, the possibility of making any causal conclusions regarding the impact of PTSD and CSA in BPD patients is precluded. Finally, multiple testing poses the question regarding the inflation of alpha-error. To address this issue, a Bonferroni-correction was applied. We found that dissociation and a history of at least one suicide attempt would still hold after correction for multiple testing.