Brief psychotherapies are efficacious for patients with multiple medically unexplained physical symptoms [e.g., Ref. (1)]. However, some patients with functional somatic syndromes undergoing psychotherapy also experience negative effects that can be attributed to factors within and outside the context of the psychotherapy (2). Patient variables have been shown to contribute to a more positive or more negative psychotherapy. For example, the short-term outcome of cognitive behavioral therapy (CBT) was more positive for patients suffering from somatoform disorders when they had a psychiatric history, higher psychological symptom severity, less characteristics related to personality-disorder, and a higher mental quality of life (3). Leibbrand et al. (4), however, failed to show associations between the treatment outcome of patients with somatoform disorder and their comorbid anxiety, depression, and personality disorders. In another study, long-term treatment outcomes were more negative for patients with somatization syndrome when the patients had a low acceptance of psychotherapy and less treatment expectations (5). Another patient variable associated with a less favorable outcome of psychodynamic psychotherapy is alexithymia (6). In CBT, alexithymia either did not influence the outcome (7) or had a beneficial effect on the outcome (8). Alexithymia can be described as having difficulties in identifying and describing emotions as well as by an externally oriented thinking style and affect regulation deficits (9–11). Karukivi and Saarijärvi (12) reviewed factors related to the development of alexithymia and identified genetic, environmental, and individual developmental factors. Some studies have shown a correlation between alexithymia and symptoms related to somatization [e.g., Ref. (11, 13)]. Although this association remained significant even when controlling for depression in the study by Mattila et al. (13), other studies reported that this correlation diminishes when controlling for negative affect such as depression [e.g., Ref. (14–16)].

In psychotherapy, therapists show predominately contempt when working with alexithymic patients (17). Such reactions of the therapists might contribute to the detrimental effect alexithymia exerts on the outcome of some psychotherapies (18). Moreover, alexithymia correlated negatively with the therapeutic alliance, which in turn correlated positively with the outcome in a current study on CBT and interpersonal therapy for patients with depressive disorder (8). A strong therapeutic alliance is associated with a more favorable psychotherapy outcome across all patients (19); yet, some patients benefit more than others from a strong therapeutic alliance. For example, Lorenzo-Luaces et al. (20) found that the therapeutic alliance affected the outcome only in depressed patients with 0–2 prior episodes but not in depressed patients with at least three prior episodes. Zilcha-Mano and Errázuriz (21) reported that symptom severity moderated the alliance-outcome link with more severely distressed patients benefiting more from a strong alliance.

To investigate alexithymia as a moderator of the alliance-outcome relationship, the current study re-analyzed data from a multicenter randomized controlled trial (RCT) on brief psychodynamic-interpersonal therapy (PIT) for patients with multisomatoform disorder (22). A multisomatoform disorder diagnosis requires at least three current, functionally disabling somatoform symptoms (on at least half of the days over at least 2 years) not sufficiently explained by an organic disease or another mental disorder, and intensive health-care use (23). Although this research question was not initially planned in the context of the RCT, we hypothesized that the therapeutic alliance exerts a more beneficial effect on the outcome of PIT in patients with higher alexithymia than in patients with lower alexithymia. This hypothesis bases on findings that therapists experience more difficulties in the relationship with alexithymic patients (8, 17, 18) suggesting that it is more important to reduce these difficulties/to establish a strong alliance in alexithymic than non-alexithymic patients. Moreover, we investigated whether alexithymia moderates the relationship between the alliance and the outcome of PIT even when controlling for depression. We had no specific hypothesis here because research on the question whether alexithymia and depression overlap has produced inconsistent results [e.g., Ref. (24, 25)].

The multicenter randomized controlled trial [“PISO trial” (22)] the data was drawn from was registered (ISRCTN23215121) and approved by the ethics committees of the six participating sites. Informed consent was obtained from all patients.

This study re-analyzed data from a multicenter randomized controlled trial on PIT for multisomatoform disorder to investigate whether alexithymia moderates associations between the therapeutic alliance and the outcome of PIT. Across all patients, neither alexithymia [in contrast to Ref. (6)] nor the therapeutic alliance [contrary to Ref. (19)] correlated with the outcome of PIT. However, alexithymia moderated the effect a stronger therapists’ alliance exerted on the outcome of PIT: significantly beneficial effects of a stronger therapists’ alliance on the outcome emerged only for patients scoring above the TAS-20 threshold of 61 points (9, 35) diagnostic for clinically relevant alexithymia. These results fit to other studies highlighting the role of the person of the therapist when working with alexithymic patients (6, 17, 18). However, alexithymia was no significant moderator of the relationship between the therapists’ alliance and the outcome anymore when controlling for depression. The non-significant effect of alexithymia when taking depression into account is in line with other studies reporting that symptoms related to somatization did not correlate anymore with alexithymia [e.g., Ref. (14, 16)] or associated affect regulation deficits (15) when controlling for negative affect such as depression. Yet, this was not the case in the study by Mattila et al. (13). It has been discussed that alexithymia and depression are overlapping constructs [e.g., Ref. (24, 25)] and the correlation between alexithymia and depression became also significant in the current study (r = 0.41) fitting to the moderate relationship (r = 0.43) found in the meta-analysis by Li et al. (36). Another study investigating alexithymia as a moderator between psychotherapeutic mechanisms of change and the outcome did not control for depression but for general symptom distress: Terock et al. (37) found that the patients’ psychotherapy motivation (more specifically “degree of suffering”) improved the short-term outcome only for patients with lower TAS-20 scores (37). As a stronger alliance in the therapists’ view was beneficial for patients with higher TAS-20 scores in the present study (before controlling for depression), it could be speculated that different psychotherapeutic mechanisms of change have a different impact on the outcome depending on the patients’ level of alexithymia. However, such conclusions have to be drawn with caution, since the sample used to analyze interactions between motivation, alexithymia, and the outcome was more heterogeneous and received a more multimodal/less manualized (psychiatric day hospital) treatment (37) than the outpatient sample of the present study on interactions between the alliance, alexithymia, and the outcome. More research is needed to inform therapists which psychotherapeutic mechanism/s of change [alliance, motivation, resource activation, problem actuation, mastery, clarification, insight, installation of hope, expectations, therapy techniques, …; see for example, Ref. (38–41)] is/are more or less beneficial for which kind of patients within a diagnostic category.

The result that alexithymia did not moderate—before and after controlling for depression—associations between the patients’ alliance ratings and the outcome might be attributable (at least to a certain extent) to the lacking ability of patients with higher alexithymia to adequately rate the relationship with the therapist due to their emotion-processing deficits.

One limitation of the present study is that the therapeutic alliance was assessed only once at the end of the intervention. The therapeutic alliance can fluctuate during psychotherapy and it would be more appropriate to measure the alliance on a session-to-session basis. Another shortcoming of the present study is the solely use of a self-rating to assess alexithymia. Although the TAS-20 is a frequently used alexithymia questionnaire, observer-based ratings might be less biased measures of alexithymia (11). A further limitation is the correlational design with its associated threats to the internal validity. One potential confounder in the context of the present study might be a difference in the therapists’ treatment adherence between patients with higher and lower alexithymia, especially regarding the PIT component “establishment of a therapeutic alliance.” However, the non-significant correlations between the therapists’ adherence ratings and patients’ alexithymia for all 12 PIT sessions indicate that treatment adherence was independent from alexithymia. The results that the therapists’ as well as the patients’ ratings of the therapeutic alliance were not significantly associated with alexithymia—contrary to Quilty et al. (8)—offer further evidence that therapists did not differ in their adherence to the PIT component “establishment of a therapeutic relationship” between patients with higher and lower alexithymia. Nevertheless, there are numerous other potential confounders and the internal validity of the results would have been stronger if patients with higher and also patients with lower alexithymia had been randomized to either a PIT condition including the component “establishment of the therapeutic relationship” or to a PIT condition excluding this component. Yet, it is also questionable whether it is ethical and feasible to conduct such component studies with regard to the therapeutic alliance.

The multicenter randomized controlled trial (“PISO trial”) the data was drawn from was registered (ISRCTN23215121) and approved by the ethics committees of the six participating sites. Informed consent was obtained from all patients.

TP drafted and revised the manuscript, contributed to the study design, and performed statistical analyses; HS revised the manuscript, contributed to the study design, and performed statistical analyses; HG revised the manuscript and contributed to the study design; PH was the grant recipient of the PISO trial, revised the manuscript, and contributed to the study design; JK revised the manuscript and contributed to the study design; GS revised the manuscript and contributed to the study design; CL drafted and revised the manuscript and contributed to the study design.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.