Exploratory analyses: Within- and between session extinction

We did not find significant within-session extinction

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5 In a previous study (de Kleine et al., 2014) we identified two personality traits (NEO Extraversion and Conscientiousness) as moderators of DCS enhancement effects in our clinical trial. To examine the effects of extinction learning while controlling for these effects, we built models including these variables and their interaction with time and group. The results were no different from those presented in the main text, and for the sake of clarity, we present the simplest models.

Discussion

In contrast to previous work and our hypotheses we failed to find moderating effects of the degree of extinction learning on DCS administration for symptom improvement in PE for PTSD.

Our findings are incongruent with the putative mechanism of action of DCS augmentation (i.e., enhancement of extinction learning consolidation), and evidence from preclinical and prior clinical research that demonstrated that the degree of extinction learning during extinction training/exposure therapy moderated the efficacy of DCS enhancement for several anxiety disorders (Bolkan & Lattal, 2014; Bouton et al., 2008; Smits, Rosenfield, Otto, Marques, et al., 2013; Smits, Rosenfield, Otto, Powers, et al., 2013; Weber et al., 2007), including PTSD (Rothbaum et al., 2014). Interestingly, two additional studies have since documented that the efficacy of other cognitive enhancers of exposure therapy (e.g., yohimbine, methylene blue) also depends on the level of extinction learning as indexed by low end fear (Smits et al., 2014;

Telch et al., 2014). In our main trial we failed to establish overall DCS enhancement effects of exposure therapy for PTSD (de Kleine et al., 2012). In the current reanalysis, we found evidence that (change in) SUDs ratings were related to better outcome regardless of group assignment (Bluett, Zoellner, & Feeny, 2014; Rauch, Foa, Furr, & Filip, 2004; van Minnen & Hagenaars, 2002), indicating that the range in SUDs ratings in our study was sufficient to moderate treatment outcome. However, we found no confirmation for the DCS mechanism of extinction learning consolidation enhancement.

In order to replicate and extend extant research on extinction learning and DCS interactions in exposure therapy we opted to utilize the same extinction learning indices as in the studies that guided this paper. However, one explanation of our negative finding may be that in-session SUDS ratings are simply not a good index of the learning process that is enhanced by DCS in PE for PTSD. There is extensive evidence that DCS does not facilitate extinction learning as such, but rather enhances the consolidation of extinction learning (see for instance, Davis et al., 2006; Ledgerwood et al., 2003; Ressler et al., 2004; Walker et al., 2002). Although the SUDS ratings conceptually map on nicely to the traditional indices of extinction learning in animal research on fear extinction, and therefore make a reasonable index of exposure success, it is possible that SUDS levels (or their change) not always fully reflect the acquisition of a new extinction memory. Principally, SUDS levels gauge fear expression, which is not necessarily equivalent to fear extinction learning. Furthermore, SUDs ratings are the result of a subjective conscious rating process and therefore subject to cognitive appraisal, while in animal fear learning, extinction learning is indexed by automatic physiological or behavioral responding (i.e., decrements in freezing or fear potentiated startle (Myers & Davis, 2007). This translational

decreased physiological arousal) or cognitive change (e.g., decreased harm expectancy, see also Hofmann, 2008).

Even though animal studies provide a solid basis for the extinction consolidation enhancement hypothesis of DCS and the central role of the amygdala in this (Ledgerwood et al., 2003; Walker et al., 2002), the precise underlying mechanism of DCS remains to be determined.

In an animal model of PTSD, Yamamoto and colleagues (2008) demonstrated that, irrespective of its extinction consolidation enhancement effects, DCS might help to reverse hippocampal plasticity. The hippocampus is critically involved in new learning, and PTSD has been related to reduced hippocampal volume and functioning (see for review Bremner, Elzinga, Schmahl,

& Vermetten, 2008). In line with the idea that there are several mechanisms of action in PE (i.e. extinction learning and cognitive reappraisal), DCS mechanisms may go beyond extinction consolidation enhancement, implying that DCS might facilitate auxiliary therapeutic processes (e.g. cognitive restructuring) in PE. More studies are needed in this direction to further clarify this issue.

There are some limitations of the current work that deserve mentioning. First, while we assumed a linear relationship between DCS moderation of extinction learning and treatment outcome, this may not be necessarily a valid assumption. Following the idea that DCS might make “good” exposure sessions better and “bad” exposure sessions worse (Hofmann, 2014;

Litz et al., 2012), there might be a critical point for successful DCS enhancement, which should be established in future work. Second, for the purpose of treatment outcome prediction our sample was relatively small. Third, ideally, we would have assessed weekly symptom change by both self-report and clinician administered instruments. Fourth, exposure content may have differed largely between participants, dependent on their trauma history and index traumas, limiting the homogeneity of the results. Last, our predominantly women sample limits the generalizibility of our results.

Conclusion

DCS augmentation of exposure therapy shows promise in improving treatment efficacy for anxiety disorders, including PTSD. Nonetheless not all studies found positive effects and more work needs to be done to elucidate the conditions for successful augmentation. Some have suggested that DCS enhancement effects may depend on the “success of an exposure session”

(Smits, Rosenfield, Otto, Marques, et al., 2013, p. 1455), which was confirmed in several studies in anxiety disorders. However, we could not replicate these findings in our mixed-trauma PTSD sample. In PTSD, not one extinction learning index has been consistently linked to DCS enhancement effects, and more work needs to be done to unravel the complex interplay between extinction learning and DCS enhancement in this disorder.