Abnormalities in grey matter structure in embouchure dystonia

Embouchure Dystonia as a Network Disease

While the pathophysiology of embouchure dystonia, a sub-entity of musician's dystonia, is still not fully understood, recent research has shown that it involves alterations of several brain functions and networks. Maladaptive plasticity in sensorimotor integration, sensory perception, and deficient inhibitory mechanisms at cortical, subcortical, and spinal level seem to contribute to its pathophysiology. Furthermore, functional systems of the basal ganglia and the cerebellum are involved, clearly pointing toward a network disorder. We therefore propose a novel network model, based on electrophysiological and recent neuroimaging studies highlighting embouchure dystonia.

Structural magnetic resonance imaging in dystonia: A systematic review of methodological approaches and findings

BACKGROUND AND PURPOSE

Structural magnetic resonance techniques have been widely applied in neurological disorders to better understand tissue changes, probing characteristics such as volume, iron deposition and diffusion. Dystonia is a hyperkinetic movement disorder, resulting in abnormal postures and pain. Its pathophysiology is poorly understood, with normal routine clinical imaging in idiopathic forms. More advanced tools provide an opportunity to identify smaller scale structural changes which may underpin pathophysiology. This review aims to provide an overview of methodological approaches undertaken in structural brain imaging of dystonia cohorts, and to identify commonly identified pathways, networks or regions that are implicated in pathogenesis.

METHODS

Structural magnetic resonance imaging studies of idiopathic and genetic forms of dystonia were systematically reviewed. Adhering to strict inclusion and exclusion criteria, PubMed and Embase databases were searched up to January 2022, with studies reviewed for methodological quality and key findings.

RESULTS

Seventy-seven studies were included, involving 1945 participants. The majority of studies employed diffusion tensor imaging (DTI) (n = 45) or volumetric analyses (n = 37), with frequently implicated areas of abnormality in the brainstem, cerebellum, basal ganglia and sensorimotor cortex and their interconnecting white matter pathways. Genotypic and motor phenotypic variation emerged, for example fewer cerebello-thalamic tractography streamlines in genetic forms than idiopathic and higher grey matter volumes in task-specific than non-task-specific dystonias.

DISCUSSION

Work to date suggests microstructural brain changes in those diagnosed with dystonia, although the underlying nature of these changes remains undetermined. Employment of techniques such as multiple diffusion weightings or multi-exponential relaxometry has the potential to enhance understanding of these differences.

Voxel-based meta-analysis of gray matter abnormalities in idiopathic dystonia

BACKGROUND

Neuroimaging studies have reported gray matter changes in patients with idiopathic dystonia but with considerable variations. Here, we aimed to investigate the convergence of dystonia-related gray matter changes across studies.

METHODS

The whole brain voxel-based morphometry studies comparing idiopathic dystonia and healthy controls were systematically searched in the PubMed, Web of Science and Embase. Meta-analysis of gray matter changes was performed using the anisotropic effect size-based signed differential mapping.

RESULTS

Twenty-eight studies comparing 701 idiopathic dystonia patients and 712 healthy controls were included in the meta-analysis. Compared to healthy controls, idiopathic dystonia patients showed increased gray matter in bilateral precentral and postcentral gyri, bilateral putamen and pallidum, right insula, and left supramarginal gyrus, while decreased gray matter in bilateral temporal poles, bilateral supplementary motor areas, right angular gyrus, inferior parietal gyrus and precuneus, left insula and inferior frontal gyrus. These findings remained robust in the jackknife sensitivity analysis, and no significant heterogeneity was detected. Subgroup analyses of different phenotypes of dystonia were performed to further confirm the above findings.

CONCLUSION

The meta-analysis showed that consistent widespread gray matter abnormalities were shared in different subtypes of idiopathic dystonia and were not restricted to the corticostriatal circuits.

Dystonia in Performing Artists: Beyond Focal Hand Dystonia

Overuse of specific muscles in perfecting movements in performing arts makes an artist prone to many medical conditions. Musicians' hand dystonia is focal task-specific dystonia (FTSD) of hand among musicians that has been extensively studied. However, embouchure, lower limbs, and laryngeal muscles can also be affected among musicians. Embouchure dystonia (ED) refers to dystonia of the perioral and facial muscles that occurs in musicians while playing embouchure instruments. It is essential to identify ED since the dystonia might become persistent and non-task-specific if the musician continues to play the instrument. Task-specific dystonia of lower limbs among musicians has been exclusively reported among drummers. The diagnosis rests on electromyogram (EMG) of the involved muscles during the task. Singer's dystonia (SD) refers to task-specific laryngeal dystonia that occurs only while singing. The diagnosis of SD is based on laryngeal EMG and spectrographic analysis. Cortical hyperexcitability, loss of inhibition, and aberrant plasticity are central to the pathogenesis in both ED and musicians' hand dystonia. The pathophysiological studies in SD are limited. This review aims to discuss the lesser known dystonias among performing artists - ED, FTSD of lower limb, and SD.

Structure-function abnormalities in cortical sensory projections in embouchure dystonia

BACKGROUND

Embouchure dystonia (ED) is a task-specific focal dystonia in professional brass players leading to abnormal orofacial muscle posturing/spasms during performance. Previous studies have outlined abnormal cortical sensorimotor function during sensory/motor tasks and in the resting state as well as abnormal cortical sensorimotor structure. Yet, potentially underlying white-matter tract abnormalities in this network disease are unknown.

OBJECTIVE

To delineate structure-function abnormalities within cerebral sensorimotor trajectories in ED.

METHOD

Probabilistic tractography and seed-based functional connectivity analysis were performed in 16/16 ED patients/healthy brass players within a simple literature-informed network model of cortical sensorimotor processing encompassing supplementary motor, superior parietal, primary somatosensory and motor cortex as well as the putamen. Post-hoc grey matter volumetry was performed within cortices of abnormal trajectories.

RESULTS

ED patients showed average axial diffusivity reduction within projections between the primary somatosensory cortex and putamen, with converse increases within projections between supplementary motor and superior parietal cortex in both hemispheres. Increase in the mode of anisotropy in patients was accompanying the latter left-hemispheric projection, as well as in the supplementary motor area's projection to the left primary motor cortex. Patient's left primary somatosensory functional connectivity with the putamen was abnormally reduced and significantly associated with the axial diffusivity reduction. Left primary somatosensory grey matter volume was increased in patients.

CONCLUSION

Correlates of abnormal tract integrity within primary somatosensory cortico-subcortical projections and higher-order sensorimotor projections support the key role of dysfunctional sensory information propagation in ED pathophysiology. Differential directionality of cortico-cortical and cortico-subcortical abnormalities hints at non-uniform sensory system changes.