Participants
Twenty-six patients experiencing a depressive episode (mean age = 41.1\(\pm\)14.1 years, 50% male) diagnosed by the Structured Clinical Interview for DSM-IV were assessed between February, 2015 and November, 2020. Patient clinical and demographic characteristics are outlined in Table 1. All patients provided written informed consent and the study procedure was approved by the local ethics committee of the Massachusetts General Hospital.
Table 1
Demographic and clinical information
N | 26 |
Age, mean (SD)* | 41.1 (14.1) |
Sex, male/female | 13/13 |
Clinical Rating Scales | |
HDRS-17 baseline, mean (SD) | 18.3 (5.5) |
HDRS-17 percent change, mean (SD) | -41.6% (33%) |
HDRS-6 baseline, mean (SD) | 10.0 (2.4) |
HDRS-6 percent change, mean (SD) | -41.2% (36%) |
CMA baseline, mean (SD) | 5.5 (1.9) |
CMA percent change, mean (SD) | -37.6% (51%) |
SoD baseline, mean (SD) | 8.8 (3.3) |
SoD percent change, mean (SD) | -39.8% (44%) |
INS baseline, mean (SD) | 2.5 (1.7) |
INS percent change, mean (SD) | -41.1% (45%) |
SHAPS baseline, mean (SD) | 6.2 (3.4) |
SHAPS percent change, mean (SD) | -28.8% (122%) |
HDRS-17 Responder/Non-Responder, N | 11/15 |
HDRS-17 Remitter/Non-Remitter, N | 9/17 |
Treatment Parameters | |
Left DLPFC Target, N* | 17 |
Right DLPFC Target, N | 5 |
Bilateral DLPFC Target, N | 3 |
Number of sessions, #Sessions (N) | 30 (n = 3) 36 (n = 18) 71 (n = 1) 72 (n = 4) |
10 Hz − 3000 pulses, N* | 19 |
1 Hz − 1800 pulses, N | 6 |
* Value missing for one participant Abbreviations: HDRS: Hamilton Depression Rating Scale; CMA: Core Mood/Anhedonia; SoD: Somatic Distrubances; INS: Insomnia; SHAPS: Snaith-Hamilton Pleasure Scale; DLPFC: Dorsolateral Prefrontal Cortex |
Table 1. Outline of patient demographic and clinical features.
Study inclusion criteria included that patients 1) were between the ages of 18–80 years old, 2) had a current DSM-IV diagnosis of a depressive episode, and 3) had previously failed four antidepressant treatments of two different classes. Exclusionary criteria included comorbid diagnoses of schizoaffective disorder, schizophrenia or dementia, substance use disorder, other severe medical illness, or contraindications to MRI or TMS.
Treatment
Patients received rTMS to the left DLPFC (n = 17), right DLPFC (n = 5), or bilateral DLPFC (n = 4). Stimuli were delivered at 120% resting motor threshold at 10Hz to the left DLPFPC (3000 stimuli per session) or 1Hz to the right DLPFC (1800 stimuli per session). Stimulation was delivered at 110% resting motor threshold for one patient receiving 1Hz stimulation to the right DLPFC due to poor tolerability. The number of rTMS sessions varied as outlined in Table 1. Depressive symptoms were assessed using the HDRS-17 the day of the first and final treatment. Similarly, patients underwent resting-state fMRI within one week prior to their first treatment. Patients were allowed to continue medication as usual during TMS.
Image Acquisition and Processing
Images were collected on a 3-Tesla Siemens Skyra MRI scanner (Siemens Healthcare, Malvern, PA, USA). Structural data was acquired with an anatomical T1-weighted multi-echo magnetization prepared rapid gradient-echo sequence with parameters: Repetition time (TR) = 2530ms, echo time (TE) = 1.69ms, inversion time (TI) = 1100ms, flip angle = 7.0°, number of excitations = 1, slice thickness = 1mm, field of view (FoV) = 256mm, in-plane resolution = 1.0 x 1.0mm, and a matrix of 256x256. Resting-state BOLD data was collected with a whole-brain echoplanar imaging sequence with the following parameters: TR = 3000ms, TE = 30ms, flip angle = 85°, slice thickness = 3.0mm, in-plane resolution = 3.0x3.0mm, FoV = 216mm. Subjects were instructed to keep eyes open and try to stay focused passively on a fixation cross displayed behind them during the resting-state scan (white cross on black background).
Resting-state fMRI scans were acquired for 6 minutes. FMRI data were preprocessed using fMRIprep software package v. 20.2.1 [21]. Anatomical images were corrected for intensity bias with Advanced Normalization Tools N4BiasFieldCorrection (ANTs v. 2.3.3) (22). Images were segmented across tissue types in both ANTs and Freesurfer, and the previously estimated brain mask was refined by reconciling these two tissue segmentations using Mindboggle [23]. The T1 images were warped to MNI space using nonlinear registration with antsRegistration (ANTs 2.3.3).
Functional images were skull-stripped and aligned to the anatomical reference image using boundary-based registration with six degrees of freedom in Freesurfer. Functional images were then slice-time corrected (AFNI v. 201660207)[24] and motion-corrected (FSL v. 5.0.9) [25]. These images were then normalized to MNI space, using the warp previously computed for transforming the T1 image to MNI space. Fmriprep’s implementation of anatomical CompCor[26] was used to estimate principle components for signal originating in white matter and CSF, respectively. The data were then blurred using a 5mm smoothing kernel and submitted to confound regression using nilearn.glm.first_level. This regression included regressors for six motion parameters and their derivatives, and enough aCompCor principle components to capture 50% of the variance of signal from white matter and CSF, respectively (a maximum of five components). Regressors were also added to scrub the first 3 TRs and high motion TRs. High motion TRs were defined as those having a framewise displacement over .3 mm. Runs with greater than 50% of TRs meeting this threshold were excluded from analysis. Sessions with fewer the 5 minutes of data after censoring were excluded.
Predictive Features
Pretreatment rs-fMRI scans were parcellated using multiple atlases. We included 200 cortical regions using the Schaefer atlas [27], 24 regional parcellations from the Kelly insula atlas [28], 21 subcortical parcellations from the Harvard-Oxford Atlas [29], 16 nuclei from the Pauli subcortical atlas [30], 10 parcellations from the Pauli amygdala nuclei atlas [31], the bilateral bed nucleus of the stria terminalis, and the grey matter of the TMS target for 274 regional measures. Supplementary Table 2 lists each predictor. Global connectivity (GC) values for each ROI were computed subjectwise using the graph theoretic node degree [20]. This measure has the advantage of preserving the feature set size at p = 274 rather than p = 37401 [i.e., 274 * (274–1) / 2] given by region-to-region connectivity models. NiLearn [32] scripts were used to compute correlations of time series data between each ROI. Subjectwise correlation matrices were thresholded at Z\(\ge\)|0.4| to create a binary adjacency matrix. The node degree for each parcellation was computed as the number of other regions with which the given region was correlated above the threshold of Z\(\ge\)|0.4|, serving as a proxy of GC for each ROI.
Outcome Measures
Primary outcomes included changes along the depression severity HDRS-17 total score [33] and the HDRS-6 subscale [34]. We also included a previously identified three-factor solution [19, 35] for the HDRS-17 which captures dimensions of core mood and anhedonia (CMA), somatic disturbances (SoD), and insomnia. Items of the HDRS composing each subscale are outlined in Supplementary Table 1. As a secondary outcome, we investigated changes in hedonic tone using the Snaith-Hamilton Pleasure Scale (SHAPS) [36].
Predictive Modeling
Random forest regression (RFR) models were trained to predict change in each outcome. Models were trained and tested using 10-repeated 10-fold cross validation with a nested grid search for parameter optimization. The parameter grid tuned the number of features selected, k, using mutual information regression with k={10, 50, 100, p}, where p is the number of all predictors; number of regression trees n_tree={50, 100, 500, 1000}. Model performance was evaluated using the sums of squares formulation of \({R}^{2}\); i.e., 1 – \(\frac{{{\sum }_{i}\left({y}_{i}- {\widehat{y}}_{i}\right)}^{2}}{{{\sum }_{i}\left({y}_{i}- \stackrel{-}{y}\right)}^{2}}\), where \({\widehat{y}}_{i}\) is the predicted outcome of the i-th subject and \({\stackrel{-}{y}}_{i}\)is the average outcome across all subjects in the testing fold [37]. The significance of model performance was tested using permutation tests with B = 1000 permutations. Model significance was adjusted for multiple comparisons across primary outcomes using a Bonferroni correction, yielding a critical value of 0.05/5 = 0.01. Models were constructed using Scikit-learn v1.1.0 [38].
Post-hoc Analysis
Components of the DMN and dorsal attention network (DAN) were predictive of CMA, HDRS-17, and HDRS-6 outcomes and the activity of both networks have been associated with ruminative symptoms [39–41]. Therefore, as post-hoc analyses, we conducted Pearson correlation tests to determine whether the pretreatment Rumination Response Scale (RRS) [42] total score and brooding and reflection subscales were associated with (1) changes in the CMA, HDRS-17, and HDRS-6 scales, and (2) with the top six most predictive GC measures across those outcomes.