Neural Substrates of Treatment Response to Cognitive-Behavioral Therapy in Panic Disorder With Agoraphobia

The study was part of the National Research Network PANIC-NET encompassing a randomized controlled clinical trial of CBT for panic disorder with agoraphobia (13). Eight German centers participated in the clinical trial (Aachen, Berlin-Adlershof, Berlin-Charité, Bremen, Dresden, Greifswald, Münster, and Würzburg) in which 369 patients who met DSM-IV-TR criteria for panic disorder with agoraphobia received treatment. Four of the centers (Aachen, Berlin-Charité, Dresden, and Münster) also participated in a functional MRI (fMRI) study. Baseline clinical characteristics were comparable between patients in the fMRI study and those in the non-fMRI sample. The manualized treatment protocol consisted of 12 twice-weekly sessions that focused on behavioral exposure in situ. Patients were randomly assigned to one of two CBT arms, which differed only with regard to therapist-guided or nonguided exposure sessions (six out of 12 sessions). Because participants in the treatment arms received comparable treatment and exhibited significant symptom reduction after CBT (13), the groups were collapsed in the present study. The frequency of treatment conditions was similar across responders and nonresponders (see Table S1 in the data supplement that accompanies the online edition of this article). In the clinical trial, treatment response was classified as a reduction >50% from baseline to posttreatment assessment on the primary outcome measure, the score on the Hamilton Anxiety Rating Scale (using the Structured Interview Guide for the Hamilton Anxiety Rating Scale [SIGH-A]) (14). We adopted this criterion to maintain comparability with previous studies (13, 15). The Clinical Global Impressions Scale (CGI) (16), the SIGH-A, the Panic and Agoraphobia Scale (17), the Anxiety Sensitivity Index (18), and the Beck Depression Inventory–II (BDI-II) (19) were used for sample characterization. Inclusion criteria were a primary diagnosis of panic disorder with agoraphobia as assessed by the Computer-Assisted Personal Interview version of the Munich-Composite International Diagnostic Interview (20) and validated by clinical experts, a SIGH-A score ≥18, a CGI score ≥4, and age 18–65 years. Patients were excluded if they were unable to comply with the study schedule; reported clinically significant suicidal intent; met diagnostic criteria for any psychotic or bipolar disorder, borderline personality disorder, or current alcohol dependence; had a medical condition that could explain their symptoms; or fulfilled MRI-related exclusion criteria. Current comorbid diagnoses, including unipolar depression and other anxiety disorders, were allowed unless they were of primary clinical concern. As such, our sample can be considered both to have relatively severe symptoms and to be representative of patients seen in clinical practice. Participants were required to discontinue all psychopharmacological medications, and those who were taking psychotropic medications underwent a 4-week washout period. Participants provided written informed consent after receiving a complete description of the study. The study was approved by all ethics committees of the participating fMRI centers. Of 369 patients enrolled in the clinical trial, 194 were recruited at fMRI centers, and 89 consented to participate in the present study. At baseline, 49 quality-controlled data sets were available; seven patients were excluded at the posttreatment assessment because of nonadherence or insufficient data quality, leaving 42 data sets for analysis of treatment-related changes. A detailed description of measures of quality control in this fMRI multicenter study and the CONSORT diagram for the study have been published elsewhere (3).

Parallel versions of a previously validated (3, 21) differential fear-conditioning task were applied before and after CBT. During differential conditioning, the reinforced conditioned stimulus (CS+) induces fear learning, while the nonreinforced conditioned stimulus, CS–, (which is never followed by the unconditioned stimulus [US]) acquires safety signal properties. The task consisted of three phases: familiarization, with 16 trials of each CS; acquisition, with 32 trials of each CS; and extinction, with 16 trials of each CS. Colored geometric stimuli represented the CSs (presentation time: 2,000 ms, with a variable intertrial interval of 4.785–7.250 seconds). An aversive auditory tone (white noise, 100 ms) was used as the US. It was pseudorandomly paired with one of the CSs during the acquisition phase (counterbalanced among participants). A partial reinforcement rate of 50% was employed. During acquisition, only those trials in which no US was delivered (CS+ unpaired) were analyzed. The task duration was approximately 17 minutes.

fMRI images were acquired using 3-T Philips Achieva (Aachen, Münster, Germany), 3-T Siemens Trio (Dresden, Germany), and 3-T General Electric Healthcare (Berlin) scanners. A total of 505 axial functional images (matrix=64×64; 30 slices interleaved; field of view=230; voxel size=3.6×3.6×3.8 mm; TE=30 ms; TR=2 seconds), covering the whole brain and positioned parallel to the intercommissural line (anterior commissure-posterior commissure), were recorded, and a three-dimensional structural data set was used (matrix=128×112; 88 slices; field of view=256; voxel size=2×2×2 mm; TE=3.93 ms; TR=1,100 ms; flip angle=9°). MR images were analyzed with statistical parametric mapping using SPM5 (www.fil.ion.ucl.ac.uk) implemented in MATLAB, version 7.1 (MathWorks, Natick, Mass.). The first five volumes were discarded to minimize T₁ saturation effects. A high-pass filter (cutoff period, 128 seconds) was applied to remove low-frequency fluctuations in the blood-oxygen-level-dependent (BOLD) signal. Following slice time correction, functional images were temporally and spatially aligned and normalized into a standard stereotactic space (Montreal Neurological Institute template: 2×2×2 mm). Accounting for differences in intrinsic smoothness between scanners, an iterative smoothness equalization (22) procedure was performed (12-mm full-width-at-half-maximum Gaussian isotropic kernel). Thus, data from all centers were iteratively smoothed until a smoothness of 12 mm full width at half maximum was reached. Assuming an intrinsic smoothness of 4–6 mm, smoothing is comparable to a predefined kernel of 8 mm full width at half maximum in a normal smoothing procedure.

At the single-subject level, realignment parameters were included as regressors to account for movement artifacts. The BOLD response for each event type (CS+ paired, CS+ unpaired, CS–, and US) and phase (familiarization, acquisition, and extinction) was modeled by the canonical hemodynamic response function used in SPM5 within the framework of the general linear model. Parameter estimates (β) and t-statistic images were calculated for each subject. Group analyses were performed by entering contrast images into a flexible factorial analysis, in which subjects were treated as random variables. The first group analysis focused on baseline characteristics and included data for 49 patients (baseline sample). A second analysis of changes from baseline to posttreatment assessment included data sets for 42 patients (baseline and posttreatment assessment sample). Since main effects during the baseline assessment were observed during the extinction phase, treatment-related changes were investigated during extinction. As used in previous analyses (3), the models included an fMRI center variable to account for scanner differences between sites and another covariate on educational level. Responders and nonresponders did not differ in baseline scores on the SIGH-A but did differ in baseline scores on the Panic and Agoraphobia Scale. We aimed to investigate correlates of treatment response independent of differences in panic symptoms and comorbid depressive symptoms, including baseline scores on the Panic and Agoraphobia Scale and BDI-II as covariates. However, results were comparable to those using a model without the Panic and Agoraphobia Scale and BDI-II, revealing that findings were unrelated to panic severity and depressive symptoms at baseline. F contrasts were computed for the interaction effects of group-by-CS (baseline model, separately for the experimental phases), group-by-time, and group-by-CS-by-time (baseline and posttreatment assessment model), followed by post hoc t tests (inclusive of masking by the respective F contrast at a p value <0.005) to explore the direction of effects. As in previous analyses (3), a Monte Carlo simulation of the brain volume was conducted to establish an appropriate voxel contiguity threshold (23). Assuming an individual voxel type I error at a p value <0.005, a cluster extent of 142 contiguous resampled voxels was indicated as sufficient to correct for multiple voxel comparisons at a p value <0.05. For all analyses, voxels with a significance level <0.005, uncorrected, belonging to clusters with at least 142 voxels are reported. Since this correction algorithm could bias findings toward larger brain regions, a region-of-interest analysis of the amygdala was conducted using the Wake Forest University PickAtlas (24) (p<0.05, family-wise-error corrected).

For connectivity analyses, we extracted eigenvectors (adjusted for the effect of movement parameters) from the anterior cingulate cortex cluster on the first level across the entire task, serving as regressors in a separate first-level model. Individual activation maps reflected the correlation of each voxel time course with the time course of the anterior cingulate cortex. These contrast images were used in the group analysis focusing on group differences (49 patients) and group-by-time interactions (42 patients) in connectivity between the anterior cingulate cortex as a seed region and the amygdala (exploratory whole-brain analysis: p<0.005, uncorrected; region-of-interest analysis: p<0.05, family-wise-error corrected).

Classification of treatment response by the magnitude of brain activation at baseline in three clusters of interest (the anterior cingulate cortex, the hippocampus, and the amygdala) and anterior cingluate cortex-amygdala connectivity was tested using a receiver operating characteristic curve analysis. A Bonferroni-corrected alpha set at 0.0125 indicated statistical significance. Analyses were carried out using SPSS, version 19.0 (IBM, Armonk, N.Y.).

Demographic and clinical characteristics of the baseline sample are summarized in Table 1. We observed a significant group-by-CS interaction during extinction (Table 2). Post hoc t tests (responders > nonresponders [CS+ unpaired > CS–]) found elevated neural activation encompassing the right pregenual anterior cingulate cortex, the hippocampus, the pre- and postcentral gyri and amygdala (region-of-interest analysis), the left middle temporal gyrus, the fusiform gyrus, and the inferior occipital gyrus. Beta values indicated that this interaction effect was driven by a pronounced increase in activation during CS– processing, compared with CS+ unpaired processing, in nonresponders that was not evident in responders (Figure 1A). Results on baseline differences between responders and nonresponders could be replicated in the smaller subsample of 42 patients (see the online data supplement). No effects were present during familiarization or acquisition.

Group differences in functional connectivity with the anterior cingulate cortex cluster as a seed region were found in the left amygdala, the right superior medial frontal and precentral gyri, and the left superior frontal and parahippocampal gyri, with responders exhibiting a negative connectivity but nonresponders exhibiting a positive connectivity. Region-of-interest analyses confirmed findings in the amygdala using a conservative statistical threshold (Table 3, Figure 1B).

The anterior cingulate cortex and hippocampus cluster yielded good classification accuracies for treatment response, with an area under the curve exceeding the value that would be expected by chance (anterior cingulate cortex: area under the curve=0.758, p=0.002; hippocampus: area under the curve=0.726, p=0.007; amygdala: area under the curve=0.563, p=0.447; anterior cingulate cortex-amygdala connectivity: area under the curve=0.642, p=0.089) (Figure 1C).

Demographic and clinical characteristics of the baseline and posttreatment assessment sample are presented in the online data supplement. Significant clinical improvement was present in both responders and nonresponders, albeit more pronounced in responders (Figure 2A). We observed a significant two-way interaction of group-by-time and three-way interaction of group-by-time-by-CS. Post hoc tests on the three-way interaction revealed increased activation in the right hippocampus in the nonresponder group in response to the safety stimulus at baseline, but this activation was reduced at the posttreatment assessment (contrast: nonresponder group: posttreatment assessment > baseline [CS+ > CS–]; Figure 2C). A similar response pattern was evident for the two-way interaction in the right anterior cingulate cortex cluster, which revealed reduced activity in nonresponders at the posttreatment assessment (contrast: nonresponder group: baseline > posttreatment assessment). Particular successful treatment response was characterized by enhanced activation in the right hippocampus during the processing of both the CS+ and CS– (contrast: responder group: posttreatment assessment > baseline; Figure 2B). No changes over time were observed in functional anterior cingulate cortex-amygdala connectivity (Table 3).