Bridging structural and functional biomarkers in functional movement disorder using network mapping

Abstract Background There are gaps in our neurobiological understanding of functional movement disorder (FMD). Objectives We investigated gray matter volumetric profiles in FMD, and related findings to resting‐state functional connectivity (rsFC) profiles using Human Connectome Project data. Methods Volumetric differences between 53 FMD patients and 50 controls were examined, as well as relationships between individual differences in FMD symptom severity and volumetric profiles. Atrophy network mapping was also used to probe whether FMD‐related structural alterations preferentially impacted brain areas with dense rsFC. Results Compared to controls without neurological comorbidities (albeit with mild depression and anxiety as a group), the FMD cohort did not show any volumetric differences. Across patients with FMD, individual differences in symptom severity negatively correlated with right supramarginal and bilateral superior temporal gyri volumes. These findings remained significant adjusting for FMD subtype or antidepressant use, but did not remain statistically significant adjusting for depression and anxiety scores. Symptom severity‐related structural alterations mapped onto regions with dense rsFC—identifying several disease epicenters in default mode, ventral attention, and salience networks. Conclusions This study supports that FMD is a multinetwork disorder with an important role for the temporoparietal junction and its related connectivity in the pathophysiology of this condition. More research is needed to explore the intersection of functional neurological symptoms and mood.


INTRODUCTION
Advances have been made in the diagnosis, treatment, and pathophysiology of functional movement disorder (FMD). (LaFaver et al., 2020, Demartini et al., 2021 Nonetheless, biomarkers of FMD symptom severity remain poorly understood-a factor that negatively impacts the development of biologically informed treatments. Neuroimaging studies support that FMD is associated with default mode, salience, limbic, attentional and sensorimotor network alterations-findings underscoring the importance of densely connected multimodal integration brain areas (e.g., temporoparietal junction [TPJ], cingulo-insular areas) in the neurobiology of this condition. (Sepulcre et al., 2012) Recently, use of a dimensional, symptom severity-informed perspective to elucidate the neural mechanisms underlying FMD has been encouraged.  Here, we hypothesized that FMD symptom severity and illness duration would relate to gray matter volumes in brain areas that would impact the resting-state functional connectivity (rsFC) of densely-connected multimodal integration brain areas. (Sepulcre et al., 2012, Fox, 2018 To test this hypothesis using a transdiagnostic approach (given that mixed symptoms are the norm rather than the exception , we first performed between-group analyses to examine gray matter volumetric differences in 53 patients with a range of different FMD phenotypes versus 50 controls without neurological comorbidities. Within-group analyses subsequently investigated relationships between individual differences in FMD symptom severity or illness duration and volumetric profiles in 50 FMD patients. Thereafter, we used Human Connectome Project (HCP) data and atrophy network mapping to identify the rsFC consequences of FMD symptom severityrelated atrophy maps.

Participants
The study was approved by General University Hospital ethics committee in Prague, and all participants provided written informed consent.
We enrolled 53 outpatients with nonparoxysmal FMD (42 females; age = 43.7 ± 10.1; illness duration = 5.3 ± 5.2 years) meeting clinically definite diagnostic criteria. (Gupta & Lang, 2009)  To provide a naturalistic control group that could account for common psychiatric comorbidities and medication use patterns, individuals with clinically salient depression, anxiety, and/or antidepressant use were included. Twenty-four patients and 16 controls were on antide-pressants. Exclusion criteria for all participants included age <18 years old, known magnetic resonance imaging (MRI) abnormality, intellectual disability, other major neurological/medical conditions, and psychotic/bipolar/substance use disorders.

Questionnaires and scales
Participants completed the Beck Depression Inventory-II (BDI) and Spielberger State-Trait Anxiety Inventory (STAI-trait). In patients with  Table S1 for additional clinical score details.

MRI acquisition and volumetric analyses
Brain scans were acquired on a Siemens 3T Trio scanner using magnetization-prepared rapid gradient-echo (see Supporting Information Methods for acquisition parameters). Gaussian kernel of 10 mm full-width at half-maximum was also applied to the subjects' cortical volumetric maps prior to statistical analyses.
Subcortical volumes were calculated using the FreeSurfer segmentation pipeline. All between-and within-group analyses were adjusted for age, sex, and estimated total intracranial volume (eTIV).
To investigate between-group differences, a two-class general antidepressant use (yes/no); (c) FMD subtype (i.e., functional weakness yes/no).

Atrophy network mapping
Published methods for rsFC preprocessing steps and atrophy network mapping procedures are given in Supporting Information Methods. (Larivière et al., 2020) In brief, publicly available rsFC MRI data from an HCP healthy adult cohort (n = 207; 83 males; mean age ± SD = 28.7 ± 3.7 years) were used as a template to secondarily investigate whether FMD symptom severity-related atrophy maps followed connectome organization principles (e.g., whether structural biomarkers of FMD severity preferentially mapped onto brain areas with dense rsFC profiles).
Specifically, weighted-degree centrality was used to identify highly connected brain areas by computing the sum of all weighted connections for every region in the HCP dataset (higher weighted-degree centrality denotes a region with greater network architecture influence).
Spatial similarity between FMD symptom severity atrophy maps and centrality distributions were then compared through Pearson correlations, and statistically assessed via spatial permutation test using 10,000 repetitions.
Additionally, we identified potential "disease epicenters"-regions whose rsFC profiles spatially resembled FMD symptom severity atrophy maps. (Larivière et al., 2020) Here, FMD-related disease epicenters were identified by spatially correlating every region's healthy rsFC profile derived from the HCP dataset to FMD symptom severity atrophy maps. This approach was repeated across the whole brain, assessing statistical significance using spatial permutation tests with 10,000 repetitions. A given brain region could be an epicenter if it is strongly connected to other high-atrophy regions and weakly connected to lowatrophy regions. Epicenters also do not necessarily represent the most highly connected regions (e.g., hubs) but could alternatively be closely connected to them. (Larivière et al., 2020)

RESULTS
Compared to controls, the FMD cohort did not show any volumetric differences. There were no statistically significant differences in symptom severity scores between patients with isolated functional weakness and other FMD phenotypes. Across FMD patients, individual differences in symptom severity negatively correlated with gray matter volumes in the right supramarginal/posterior aspect of the superior temporal gyrus (r = −0.43, p corrected = .0002) and posterior aspect of the left superior temporal gyrus (r = −0.59, p corrected = .004; Figure 1).
Both clusters remained significant after adjusting for FMD subtypes or anti-depressant use; however, these clusters did not remain significant adjusting for depression and trait anxiety scores. In a post hoc analysis, there were no statistically significant associations between TPJ gray matter volume and depression or trait anxiety scores in patients with FMD. See Figure S1 for additional information. An additional post hoc analysis examining gray matter-symptom severity relationships in only the subset of patients with functional weakness showed similar right TPJ findings ( Figure S2). Across FMD patients, there were no statistically significant relationships between gray matter volumes and illness duration.
Spatial similarity testing revealed that brain areas displaying reduced volumes correlated with FMD symptom severity tended to be regions showing dense rsFC profiles based on HCP data (r = −0.45, p perm = .02; Figure 2 Panel A). In patients with FMD, the bilateral superior frontal and temporal gyri, right insular cortex and inferior frontal gyrus, and left middle cingulate cortex, paracentral lobule postcentral gyrus showed disease epicenter rsFC properties (p perm < .01; Figure 2 Panel B).

DISCUSSION
Here, the FMD cohort did not show any volumetric differences compared to controls. However, FMD symptom severity negatively correlated with volumetric profiles in the TPJ-specifically the right supramarginal and bilateral superior temporal gyri. Atrophy network mapping showed that these structural findings preferentially impacted higher order brain areas exhibiting increased rsFC influence (weighteddegree centrality) based on the healthy human functional connectome.
The finding relating the TPJ to individual differences in FMD severity fits well with the neuroimaging literature implicating abnormal activity and connectivity of the TPJ in FMD. (Demartini et al., 2021, Voon et al., 2010, Maurer et al., 2016, Baek et al., 2017 The TPJ, a core node of the default mode and ventral attention networks, is an important higher order region implicated in multisensory integration, self-agency, and stimulus-driven attention.  Our atrophy network mapping analyses help contextualize the symptom severity findings by noting that these structural alterations would be expected to impact several higher order (integrative) brain areas including the insula, middle cingulate cortex, dorsomedial prefrontal cortex, and inferior frontal gyrus. (Sepulcre et al., 2012) Given that a heterogeneous (and inconsistently identified) range of structural neuroimaging findings have been reported in FMD, we believe that it is unlikely that one definitive, microscopic structural "lesion" will be showed that a range F I G U R E 1 Correlations between symptom severity and gray matter volumes in 50 patients with functional movement disorder. Reduced cortical volumes in the right supramarginal gyrus and posterior aspect of the bilateral superior temporal gyri correlated with increased functional motor symptom severity. These findings were adjusted for age, sex, and estimated total intracranial volume, as well as corrected for multiple comparisons.
Abbreviations: SMG, supramarginal gyrus; STG, superior temporal gyrus F I G U R E 2 Atrophy network mapping contextualized the potential resting-state functional connectivity relevance of identified individual differences in gray matter volumes in patients with functional movement disorder. Panel A shows that symptom severity-related cortical atrophy spatially correlated with brain areas showing greater weighted-degree centrality as measured using human connectome healthy subject resting-state functional connectivity data. Panel B shows that the bilateral superior frontal and temporal gyri, right insular cortex and inferior frontal gyrus, and left middle cingulate cortex, paracentral lobule and postcentral gyrus emerged as potential disease epicenters. Note: The Desikan-Killiany atlas was used to parcellate cortical areas for these analyses of broadly distributed structural findings exhibited rsFC to the middle cingulate cortex. (Darby et al., 2018) Limitations include psychiatric comorbidities, medication use, phenotypic heterogeneity, and reliance on HCP rsFC data. We did not perform a structured psychiatric interview limiting description of categorical psychiatric comorbidities. Given that patients with FMD are known to have elevates rates of depression and anxiety, we allowed controls with these mental health symptoms to be enrolled to help limit false positive between-group findings; nonetheless, additional research is needed to further contextualize between-group findings in patients with FMD compared to neurological and psychiatric controls matched for the severity of depression and anxiety scores. Given that phenotypic overlap is common across FMD presentations, we used a transdiagnostic approach across hyperkinetic and hypokinetic phenotypes.  However, whether different outward presenting phenotypes are driven by the same biological mechanisms remains to be determined. Additionally, more research is needed to investigate relationships between illness duration and volumetric profiles in patients with FMD. Lastly-the within-group volumetric findings did not remain significant adjusting for BDI and STAI-trait scores-suggesting that the results are at the intersection of functional motor symptoms and negative emotions. This is supported by a recent study with 152 patients with FMD that found significant correlations between S-FMDRS and both depression and anxiety scores underscoring that both motor and nonmotor symptoms are possibly generated by the same underlying neural processes. (Forejtová et al., 2022) In conclusion, this study supports that the default mode, ventral attention, and salience networks are important in the pathophysiology of FMD-identifying correlations between TPJ volumes and functional motor symptom severity.