Shortened cortical silent period in adductor spasmodic dysphonia: evidence for widespread cortical excitability

Functional and structural abnormalities of the speech disorders: a multimodal activation likelihood estimation meta-analysis

Speech disorders are associated with different degrees of functional and structural abnormalities. However, the abnormalities associated with specific disorders, and the common abnormalities shown by all disorders, remain unclear. Herein, a meta-analysis was conducted to integrate the results of 70 studies that compared 1843 speech disorder patients (dysarthria, dysphonia, stuttering, and aphasia) to 1950 healthy controls in terms of brain activity, functional connectivity, gray matter, and white matter fractional anisotropy. The analysis revealed that compared to controls, the dysarthria group showed higher activity in the left superior temporal gyrus and lower activity in the left postcentral gyrus. The dysphonia group had higher activity in the right precentral and postcentral gyrus. The stuttering group had higher activity in the right inferior frontal gyrus and lower activity in the left inferior frontal gyrus. The aphasia group showed lower activity in the bilateral anterior cingulate gyrus and left superior frontal gyrus. Across the four disorders, there were concurrent lower activity, gray matter, and fractional anisotropy in motor and auditory cortices, and stronger connectivity between the default mode network and frontoparietal network. These findings enhance our understanding of the neural basis of speech disorders, potentially aiding clinical diagnosis and intervention.

Exploring Neurophysiological Mechanisms and Treatment Efficacies in Laryngeal Dystonia: A Transcranial Magnetic Stimulation Approach

Laryngeal dystonia (LD), known or termed as spasmodic dysphonia, is a rare movement disorder with an unknown cause affecting the intrinsic laryngeal muscles. Neurophysiological studies point to perturbed inhibitory processes, while conventional genetic studies reveal fragments of genetic architecture in LD. The study's aims are to (1) describe transcranial magnetic stimulation (TMS) methodology for studying the functional integrity of the corticospinal tract by stimulating the primary motor cortex (M1) for laryngeal muscle representation and recording motor evoked potentials (MEPs) from laryngeal muscles; (2) evaluate the results of TMS studies investigating the cortical silent period (cSP) in LD; and (3) present the standard treatments of LD, as well as the results of new theoretical views and treatment approaches like repetitive TMS and laryngeal vibration over the laryngeal muscles as the recent research attempts in treatment of LD. Neurophysiological findings point to a shortened duration of cSP in adductor LD and altered cSP duration in abductor LD individuals. Future TMS studies could further investigate the role of cSP in relation to standard laryngological measures and treatment options. A better understanding of the neurophysiological mechanisms might give new perspectives for the treatment of LD.

Probing the inhibitory motor circuits in adductor laryngeal dystonia during a dystonia-unrelated task

BACKGROUND

The pathophysiology of adductor laryngeal dystonia (AdLD) remains unknown; however, there is growing evidence that dystonia is associated with disruptions in the inhibitory regulation of sensorimotor cortical areas. Using functional MRI (fMRI) and transcranial magnetic stimulation (TMS) complementarily, we previously demonstrated an overly activated laryngeal motor cortex and revealed correlations between blood-oxygen-level dependent (BOLD) activation and intracortical inhibition in a phonation (dystonia-related) task in adductor laryngeal dystonia (AdLD).

OBJECTIVE

Here, we aimed to characterize the brain-based findings in the primary motor cortex (M1) during a dystonia-unrelated (finger tapping) task in AdLD and controls (CTL).

METHODS

We examined the between-group differences in task-dependent BOLD activation and intracortical inhibition, measured by the TMS-evoked cortical silent period (cSP), in the M1. The correlations between fMRI and TMS responses were assessed.

RESULTS

There is more broadly dispersed BOLD activation, not confined to the hand motor cortex, and reduced intracortical inhibition in AdLD compared to CTL. Further, there are more positive correlations between cSP and BOLD activation in a task unrelated to dystonic symptoms in AdLD compared with CTL. This is in contrast to our previous work that demonstrated fewer positive correlations in AdLD during a dystonic phonation task.

CONCLUSIONS

In unaffected musculature activation, there is dispersed BOLD activation that is correlated with intracortical inhibition suggesting a possible compensatory strategy in the non-dystonic muscles.

Laryngeal Sensory Symptoms in Spasmodic Dysphonia

OBJECTIVE

The purpose of this research was to determine whether an association between laryngeal hypersensitivity (LH) and spasmodic dysphonia (SD) exists using the LH Questionnaire (LHQ). This study also explored the prevalence of self-reported upper respiratory infection (URI) at the time of SD onset across SD phenotypes.

METHODS

Individuals with and without SD were recruited to complete an online survey measure. All respondents provided demographic information and completed the LHQ. Participants with SD were also asked to provide information about their diagnosed SD subtype and pattern of onset, including whether onset was associated with a URI. The percentage of respondents with and without SD who were classified with LH was determined based on the LHQ. Scores on the LHQ were also compared between the non-SD and the SD groups, as well as between SD phenotypes (adductor SD, abductor SD, and mixed)).

RESULTS AND CONCLUSIONS

Significant associations were found between ADSD and LH, mixed SD and LH, and URI at time of SD onset and increased severity of LH symptoms. These findings suggest that laryngeal sensory symptoms may potentially contribute to or result from motor spasms in SD and/or have implications for its pathophysiology.

Temporal specificity of abnormal neural oscillations during phonatory events in laryngeal dystonia

Laryngeal dystonia is a debilitating disorder of voicing in which the laryngeal muscles are intermittently in spasm resulting in involuntary interruptions during speech. The central pathophysiology of laryngeal dystonia, underlying computational impairments in vocal motor control, remains poorly understood. Although prior imaging studies have found aberrant activity in the CNS during phonation in patients with laryngeal dystonia, it is not known at what timepoints during phonation these abnormalities emerge and what function may be impaired. To investigate this question, we recruited 22 adductor laryngeal dystonia patients (15 female, age range = 28.83-72.46 years) and 18 controls (eight female, age range = 27.40-71.34 years). We leveraged the fine temporal resolution of magnetoencephalography to monitor neural activity around glottal movement onset, subsequent voice onset and after the onset of pitch feedback perturbations. We examined event-related beta-band (12-30 Hz) and high-gamma-band (65-150 Hz) neural oscillations. Prior to glottal movement onset, we observed abnormal frontoparietal motor preparatory activity. After glottal movement onset, we observed abnormal activity in the somatosensory cortex persisting through voice onset. Prior to voice onset and continuing after, we also observed abnormal activity in the auditory cortex and the cerebellum. After pitch feedback perturbation onset, we observed no differences between controls and patients in their behavioural responses to the perturbation. But in patients, we did find abnormal activity in brain regions thought to be involved in the auditory feedback control of vocal pitch (premotor, motor, somatosensory and auditory cortices). Our study results confirm the abnormal processing of somatosensory feedback that has been seen in other studies. However, there were several remarkable findings in our study. First, patients have impaired vocal motor activity even before glottal movement onset, suggesting abnormal movement preparation. These results are significant because (i) they occur before movement onset, abnormalities in patients cannot be ascribed to deficits in vocal performance and (ii) they show that neural abnormalities in laryngeal dystonia are more than just abnormal responses to sensory feedback during phonation as has been hypothesized in some previous studies. Second, abnormal auditory cortical activity in patients begins even before voice onset, suggesting abnormalities in setting up auditory predictions before the arrival of auditory feedback at voice onset. Generally, activation abnormalities identified in key brain regions within the speech motor network around various phonation events not only provide temporal specificity to neuroimaging phenotypes in laryngeal dystonia but also may serve as potential therapeutic targets for neuromodulation.

Effects of low-frequency repetitive transcranial magnetic stimulation in adductor laryngeal dystonia: a safety, feasibility, and pilot study

PURPOSE

The effects of neuromodulation are virtually unexplored in adductor laryngeal dystonia (AdLD), a disorder characterized by involuntary contraction of intrinsic laryngeal muscles. Recent findings indicated that intracortical inhibition is reduced in people with AdLD. Low-frequency repetitive transcranial magnetic stimulation (rTMS) induces prolonged intracortical inhibition, but the effects in AdLD are unexplored. This pilot and feasibility study aimed to examine the safety, feasibility, and effects of a single session 1 Hz rTMS over the laryngeal motor cortex (LMC) in people with AdLD and healthy individuals.

METHODS

The stimulation location was individualized and determined through TMS-evoked responses in the thyroarytenoid muscles using fine-wire electrodes. 1200 pulses of 1 Hz rTMS were delivered to the left LMC in two groups: Control (n = 6) and AdLD (n = 7). Tolerance, adverse effects, intracortical inhibition, and voice recordings were collected immediately before and after rTMS. Voice quality was assessed with acoustic-based and auditory-perceptual measures.

RESULTS

All participants tolerated the procedures, with no unexpected adverse events or worsening of symptoms. No significant effects on intracortical inhibition were observed. In the AdLD group, there was a large-effect size after rTMS in vocal perturbation measures and a small-effect size in decreased phonatory breaks.

CONCLUSIONS

One rTMS session over the LMC is safe and feasible, and demonstrated trends of beneficial effects on voice quality and phonatory function in AdLD. These preliminary findings support further investigation to assess clinical benefits in a future randomized sham-controlled trial. CLINICALTRIALS.GOV: NCT02957942, registered on November 8, 2016.

Exploring the connections between basal ganglia and cortex revealed by transcranial magnetic stimulation, evoked potential and deep brain stimulation in dystonia

We review the findings for motor cortical excitability, plasticity and evoked potentials in dystonia. Plasticity can be induced and assessed in cortical areas by non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and the invasive technique of deep brain stimulation (DBS), which allows access to deep brain structures. Single-pulse TMS measures have been widely studied in dystonia and consistently showed reduced silent period duration. Paired pulse TMS measures showed reduced short and long interval intracortical inhibition, interhemispheric inhibition, long-latency afferent inhibition and increased intracortical facilitation in dystonia. Repetitive transcranial magnetic stimulation (rTMS) of the premotor cortex improved handwriting with prolongation of the silent period in focal hand dystonia patients. Continuous theta-burst stimulation (cTBS) of the cerebellum or cTBS of the dorsal premotor cortex improved dystonia in some studies. Plasticity induction protocols in dystonia demonstrated excessive motor cortical plasticity with the reduction in cortico-motor topographic specificity. Bilateral DBS of the globus pallidus internus (GPi) improves dystonia, associated pain and functional disability. Local field potentials recordings in dystonia patients suggested that there is increased power in the low-frequency band (4-12 Hz) in the GPi. Cortical evoked potentials at peak latencies of 10 and 25 ms can be recorded with GPi stimulation in dystonia. Plasticity induction protocols based on the principles of spike timing dependent plasticity that involved repeated pairing of GPi-DBS and motor cortical TMS at latencies of cortical evoked potentials induced motor cortical plasticity. These studies expanded our knowledge of the pathophysiology of dystonia and how cortical excitability and plasticity are altered with different treatments such as DBS.

The effects of continuous oromotor activity on speech motor learning: speech biomechanics and neurophysiologic correlates

Sustained limb motor activity has been used as a therapeutic tool for improving rehabilitation outcomes and is thought to be mediated by neuroplastic changes associated with activity-induced cortical excitability. Although prior research has reported enhancing effects of continuous chewing and swallowing activity on learning, the potential beneficial effects of sustained oromotor activity on speech improvements is not well-documented. This exploratory study was designed to examine the effects of continuous oromotor activity on subsequent speech learning. Twenty neurologically healthy young adults engaged in periods of continuous chewing and speech after which they completed a novel speech motor learning task. The motor learning task was designed to elicit improvements in accuracy and efficiency of speech performance across repetitions of eight-syllable nonwords. In addition, transcranial magnetic stimulation was used to measure the cortical silent period (cSP) of the lip motor cortex before and after the periods of continuous oromotor behaviors. All repetitions of the nonword task were recorded acoustically and kinematically using a three-dimensional motion capture system. Productions were analyzed for accuracy and duration, as well as lip movement distance and speed. A control condition estimated baseline improvement rates in speech performance. Results revealed improved speech performance following 10 min of chewing. In contrast, speech performance following 10 min of continuous speech was degraded. There was no change in the cSP as a result of either oromotor activity. The clinical implications of these findings are discussed in the context of speech rehabilitation and neuromodulation.

Thalamic Deep Brain Stimulation for Spasmodic Dysphonia: A Phase I Prospective Randomized Double-Blind Crossover Trial

BACKGROUND

Adductor spasmodic dysphonia (SD) is a dystonia of the vocal folds causing difficulty with speech. The current standard of care is repeated botulinum toxin injections to weaken the adductor muscles. We sought to ameliorate the underlying neurological cause of SD with a novel therapy—deep brain stimulation (DBS).

OBJECTIVE

To assess the safety of DBS in SD through phase I trial, and to quantify the magnitude of any benefit.

METHODS

Six patients had left ventral intermediate nucleus (Vim) thalamic DBS and were randomized to 3 mo blinded-DBS “on” or “off” followed by a crossover. Primary outcomes were quality of life and quality of voice during the blinded phase. Patients continued with open-DBS “on.” Secondary outcomes were comparisons of pre- and 1-yr cognitive, mood, and quality of life. This trial was registered with ClinicalTrials.gov (NCT02558634).

RESULTS

There were no complications. Every patient reported an improvement in quality of life (P = .07) and had an improvement in quality of their voice (P = .06) when their blinded DBS was “on” versus “off.” The trend did not reach statistical significance with the small sample size. Secondary outcomes showed no difference in cognition, an improvement in mood, and quality of life at 1 yr.

CONCLUSION

This phase I randomized controlled trial confirmed that DBS can be performed safely in patients with SD. Blinded DBS produced a strong trend toward improved quality of life and objective quality of voice despite the small sample size. The cerebellar circuit, not the pallidal circuit, appears to be crucial for motor control of the vocal folds.

Laryngeal Dystonia: Multidisciplinary Update on Terminology, Pathophysiology, and Research Priorities

OBJECTIVE

To delineate research priorities for improving clinical management of laryngeal dystonia, the NIH convened a multidisciplinary panel of experts for a 1-day workshop to examine the current progress in understanding its etiopathophysiology and clinical care.

METHODS

The participants reviewed the current terminology of disorder and discussed advances in understanding its pathophysiology since a similar workshop was held in 2005. Clinical and research gaps were identified, and recommendations for future directions were delineated.

RESULTS

The panel unanimously agreed to adopt the term "laryngeal dystonia" instead of "spasmodic dysphonia" to reflect the current progress in characterizations of this disorder. Laryngeal dystonia was recognized as a multifactorial, phenotypically heterogeneous form of isolated dystonia. Its etiology remains unknown, whereas the pathophysiology likely involves large-scale functional and structural brain network disorganization. Current challenges include the lack of clinically validated diagnostic markers and outcome measures and the paucity of therapies that address the disorder pathophysiology.

CONCLUSION

Research priorities should be guided by challenges in clinical management of laryngeal dystonia. Identification of disorder-specific biomarkers would allow the development of novel diagnostic tools and unified measures of treatment outcome. Elucidation of the critical nodes within neural networks that cause or modulate symptoms would allow the development of targeted therapies that address the underlying pathophysiology. Given the rarity of laryngeal dystonia, future rapid research progress may be facilitated by multicenter, national and international collaborations.

Contemporary clinical neurophysiology applications in dystonia

The complex phenomenological understanding of dystonia has transcended from the clinics to genetics, imaging and neurophysiology. One way in which electrophysiology will impact into the clinics are cases wherein a dystonic clinical presentation may not be typical or a "forme fruste" of the disorder. Indeed, the physiological imprints of dystonia are present regardless of its clinical manifestation. Underpinnings in the understanding of dystonia span from the peripheral, segmental and suprasegmental levels to the cortex, and various electrophysiological tests have been applied in the course of time to elucidate the origin of dystonia pathophysiology. While loss of inhibition remains to be the key finding in this regard, intricacies and variabilities exist, thus leading to a notion that perhaps dystonia should best be gleaned as network disorder. Interestingly, the complex process has now spanned towards the understanding in terms of networks related to the cerebellar circuitry and the neuroplasticity. What is evolving towards a better and cohesive view will be neurophysiology attributes combined with structural dynamic imaging. Such a sound approach will significantly lead to better therapeutic modalities in the future.

Reduced Interhemispheric Coherence in Cerebellar Kainic Acid-Induced Lateralized Dystonia

The execution of voluntary muscular activity is controlled by the primary motor cortex, together with the cerebellum and basal ganglia. The synchronization of neural activity in the intracortical network is crucial for the regulation of movements. In certain motor diseases, such as dystonia, this synchrony can be altered in any node of the cerebello-cortical network. Questions remain about how the cerebellum influences the motor cortex and interhemispheric communication. This research aims to study the interhemispheric cortical communication between the motor cortices during dystonia, a neurological movement syndrome consisting of sustained or repetitive involuntary muscle contractions. We pharmacologically induced lateralized dystonia to adult male albino mice by administering low doses of kainic acid on the left cerebellar hemisphere. Using electrocorticography and electromyography, we investigated the power spectral densities, cortico-muscular, and interhemispheric coherence between the right and left motor cortices, before and during dystonia, for five consecutive days. Mice displayed lateralized abnormal motor signs, a reduced general locomotor activity, and a high score of dystonia. The results showed a progressive interhemispheric coherence decrease in low-frequency bands (delta, theta, beta) during the first 3 days. The cortico-muscular coherence of the affected side had a significant increase in gamma bands on days 3 and 4. In conclusion, lateralized cerebellar dysfunction during dystonia was associated with a loss of connectivity in the motor cortices, suggesting a possible cortical compensation to the initial disturbances induced by cerebellar left hemisphere kainate activation by blocking the propagation of abnormal oscillations to the healthy hemisphere. However, the cerebellum is part of several overly complex circuits, therefore other mechanisms can still be involved in this phenomenon.

Hyperactive sensorimotor cortex during voice perception in spasmodic dysphonia

Spasmodic dysphonia (SD) is characterized by an involuntary laryngeal muscle spasm during vocalization. Previous studies measured brain activation during voice production and suggested that SD arises from abnormal sensorimotor integration involving the sensorimotor cortex. However, it remains unclear whether this abnormal sensorimotor activation merely reflects neural activation produced by abnormal vocalization. To identify the specific neural correlates of SD, we used a sound discrimination task without overt vocalization to compare neural activation between 11 patients with SD and healthy participants. Participants underwent functional MRI during a two-alternative judgment task for auditory stimuli, which could be modal or falsetto voice. Since vocalization in falsetto is intact in SD, we predicted that neural activation during speech perception would differ between the two groups only for modal voice and not for falsetto voice. Group-by-stimulus interaction was observed in the left sensorimotor cortex and thalamus, suggesting that voice perception activates different neural systems between the two groups. Moreover, the sensorimotor signals positively correlated with disease severity of SD, and classified the two groups with 73% accuracy in linear discriminant analysis. Thus, the sensorimotor cortex and thalamus play a central role in SD pathophysiology and sensorimotor signals can be a new biomarker for SD diagnosis.

Cortical neurophysiology of primary isolated dystonia and non-dystonic adults: A meta-analysis

Transcranial magnetic stimulation (TMS) is a non-invasive method to assess neurophysiology of the primary motor cortex in humans. Dystonia is a poorly understood neurological movement disorder, often presenting in an idiopathic, isolated form across different parts of the body. The neurophysiological profile of isolated dystonia compared to healthy adults remains unclear. We conducted a systematic review with meta-analysis of neurophysiologic TMS measures in people with isolated dystonia to provide a synthesized understanding of cortical neurophysiology associated with isolated dystonia. We performed a systematic database search and data were extracted independently by the two authors. Separate meta-analyses were performed for TMS measures of: motor threshold, corticomotor excitability, short interval intracortical inhibition, cortical silent period, intracortical facilitation and afferent-induced inhibition. Standardized mean differences were calculated using a random effects model to determine overall effect sizes and confidence intervals. Heterogeneity was explored using dystonia type subgroup analysis. The search resulted in 78 studies meeting inclusion criteria, of these 57 studies reported data in participants with focal hand dystonia, cervical dystonia, blepharospasm or spasmodic dysphonia, and were included in at least one meta-analysis. The cortical silent period, short-interval intracortical inhibition and afferent-induced inhibition was found to be reduced in isolated dystonia compared to controls. Reduced GABAergic-mediated inhibition in the primary motor cortex in idiopathic isolated dystonia's suggest interventions targeted to aberrant cortical disinhibition could provide a novel treatment. Future meta-analyses require neurophysiology studies to use homogeneous cohorts of isolated dystonia participants, publish raw data values, and record electromyographic responses from dystonic musculature where possible.

Transcranial magnetic stimulation and functional magnet resonance imaging evaluation of adductor spasmodic dysphonia during phonation

BACKGROUND

Reduced intracortical inhibition is a neurophysiologic finding in focal dystonia that suggests a broader problem of impaired cortical excitability within the brain. A robust understanding of the neurophysiology in dystonia is essential to elucidate the pathophysiology of the disorder and develop new treatments. The cortical silent period (cSP) is a reliable, non-invasive method to measure intracortical inhibition in the primary motor cortex associated with a muscle of interest. In adductor spasmodic dysphonia (AdSD), cSP of the laryngeal motor cortex (LMC) which directly corresponds to the affected musculature, the thyroarytenoid (TA), has not been examined.

OBJECTIVE

This work evaluated the cSP of the LMC and the relationship between cSP and functional magnetic resonance imaging (fMRI) blood-oxygen-level dependent (BOLD) activation in people with AdSD (n = 12) compared to healthy controls (CTL, n = 14).

RESULTS

Shortened LMC cSP were observed bilaterally in people with AdSD vs CTL (F(1, 99) = 19.5226, p < 0.0001), with a large effect size (η2 = 0.1834). Between-group fMRI analysis revealed greater activation in bilateral LMC in the AdSD > CTL contrast as compared to CTL > AdSD contrast. Correlation analysis showed that people with AdSD have positive correlation of left LMC BOLD activation and the cSP. Further, the right LMC cSP lacks either positive or negative associations with BOLD activation. CTL individuals displayed both positive and negative correlations between cSP and BOLD activation in the left LMC. In CTL, the LMC cSP and BOLD activation showed exclusively negative correlations in both hemispheres.

CONCLUSION

In AdSD, the cortical activation during phonation may not be efficiently or effectively associated with inhibitory processes, leading to muscular dysfunction. These findings may give insight into the maladaptive cortical control during phonation in people with AdSD.

Evidence for normal intracortical inhibitory recruitment properties in cervical dystonia

OBJECTIVE

Dystonia is associated with reduced intracortical inhibition as measured by the cortical silent period (cSP); however, this may be due to abnormal cSP threshold or input-output properties. This study evaluated cSP recruitment properties in people with cervical dystonia (CD).

METHODS

Bilateral electromyographic recordings were collected in the upper trapezius muscle in response to transcranial magnetic stimulation of the left and right primary motor cortex in a group with CD (n = 19) and controls (n = 21). cSP threshold, cSP input-output properties at stimulation intensities from 1 to 1.4x the cSP threshold, ipsilateral silent period duration (iSP) and timing and magnitude of the contralateral and ipsilateral motor evoked potential (MEP) were assessed.

RESULTS

The cSP threshold, input-output properties, and contralateral MEP magnitude were not significantly different between groups (all p > 0.07). Hemispheric symmetry was present in the control group while the CD group had reduced iSP (p < 0.01) and a trend for reduced ipsilateral MEP response (p = 0.053) in the left hemisphere.

CONCLUSIONS

Recruitment properties of intracortical inhibition are similar between control and CD groups. Transcallosal inhibition is asymmetric between hemispheres in people with CD.

SIGNIFICANCE

Evidence of normal intracortical inhibition recruitment properties challenge the commonly held view that cortical inhibition is reduced in dystonia.

Laryngeal vibration as a non-invasive neuromodulation therapy for spasmodic dysphonia

Spasmodic dysphonia (SD) is an incurable focal dystonia of the larynx that impairs speech and communication. Vibro-tactile stimulation (VTS) alters afferent proprioceptive input to sensorimotor cortex that controls speech. This proof-of-concept study examined the effect of laryngeal VTS on speech quality and cortical activity in 13 SD participants who vocalized the vowel /a/ while receiving VTS for 29 minutes. In response to VTS, 9 participants (69%) exhibited a reduction of voice breaks and/or a meaningful increase in smoothed cepstral peak prominence, an acoustic measure of voice/speech quality. Symptom improvements persisted for 20 minutes past VTS. Application of VTS induced a significant suppression of theta band power over the left somatosensory-motor cortex and a significant rise of gamma rhythm over right somatosensory-motor cortex. Such suppression of theta oscillations is observed in patients with cervical dystonia who apply effective sensory tricks, suggesting that VTS in SD may activate a similar neurophysiological mechanism. Results of this feasibility study indicate that laryngeal VTS modulates neuronal synchronization over sensorimotor cortex, which can induce short-term improvements in voice quality. The effects of long-term VTS and its optimal dosage for treating voice symptoms in SD are still unknown and require further systematic study.

Atypical somatosensory-motor cortical response during vowel vocalization in spasmodic dysphonia

OBJECTIVE

Spasmodic dysphonia (SD) is a debilitating voice/speech disorder without an effective cure. To obtain a better understanding of the underlying cortical neural mechanism of the disease we analyzed electroencephalographic (EEG) signals of people with SD during voice production.

METHOD

Ten SD individuals and 10 healthy volunteers produced 50 vowel vocalization epochs of 2500 ms duration. Two EEG features were derived: (1) event-related change in spectral power during vocalization relative to rest, (2) inter-regional spectral coherence.

RESULTS

During early vocalization (500-1000 ms) the SD group showed significantly larger alpha band spectral power over the left motor cortex. During late vocalization (1000-2500 ms) SD patients showed a significantly larger gamma band coherence between left somatosensory and premotor cortical areas.

CONCLUSIONS

Two atypical patterns of cortical activity characterize the pathophysiology of spasmodic dysphonia during voice production: (1) a reduced movement-related desynchronization of motor cortical networks, (2) an excessively large synchronization between left somatosensory and premotor cortical areas.

SIGNIFICANCE

The pathophysiology of SD is characterized by an abnormally high synchronous activity within and across cortical neural networks involved in voice production that is mainly lateralized in the left hemisphere.

Motor cortical circuits in Parkinson disease and dystonia

We review the motor cortical and basal ganglia involvement in two important movement disorders: Parkinson's disease (PD) and dystonia. Single and paired pulse transcranial magnetic stimulation studies showed altered excitability and cortical circuits in PD with decreased silent period, short interval intracortical inhibition, intracortical facilitation, long afferent inhibition, interhemispheric inhibition, and cerebellar inhibition, and increased long interval intracortical inhibition and short interval intracortical facilitation. In dystonia, there is decreased silent period, short interval intracortical inhibition, long afferent inhibition, interhemispheric inhibition, and increased intracortical facilitation. Plasticity induction protocols revealed deficient plasticity in PD and normal and exaggerated plasticity in dystonia. In the basal ganglia, there is increased β (14-30Hz) rhythm in PD and characteristic 5-18Hz band synchronization in dystonia. These motor cortical circuits, cortical plasticity, and oscillation profiles of the basal ganglia are altered with medications and deep brain stimulation treatment. There is considerable variability in these measures related to interindividual variations, different disease characteristics, and methodological considerations. Nevertheless, these pathophysiologic studies have expanded our knowledge of cortical excitability, plasticity, and oscillations in PD and dystonia, improved our understanding of disease pathophysiology, and helped to develop new treatments for these conditions.

Short Interval Intracortical Inhibition Responses to Low-Frequency Repetitive Transcranial Magnetic Stimulation Under Multiple Interstimulus Intervals and Conditioning Intensities

BACKGROUND

The extent to which short interval intracortical inhibition (SICI) responds to low-frequency repetitive transcranial magnetic stimulation (rTMS) remains inconclusive with reports of increased, decreased and unchanged response following modulation. The aim of this study was to systematically investigate if the variability of SICI following rTMS is explained by the interstimulus interval (ISI) and/or the conditioning stimulus intensity (CSI).

METHODS

Two experiments with pretesting/posttesting and an rTMS session (1 Hz, 90% RMT, 900 pulses) were done. Experiment I (N = 15): SICI with multiple ISIs (1.0-4.0 msec, 0.2 msec increment). Experiment II (N = 15): SICI with CSIs (50-95% of RMT, 5% increment). In both experiments, the cortical silent period (cSP) was also collected.

RESULTS

After low-frequency rTMS, no significant change (p > 0.10) in SICI at any specific ISI or CSI was observed, nor did the optimal ISI or CSI change. However, a significant decrease was observed in SICI responses when assessed under the range of ISIs (p = 0.0001), but not CSIs. cSP inhibition increased significantly (p < 0.0015) for both experiments.

CONCLUSIONS

The optimal ISI or CSI did not shift or reveal SICI changes after inhibitory rTMS. However, when the whole curve of SICI responses were evaluated from a wide range of ISIs, a decrease in inhibition was found. The contrast between the results of individual ISI tests and the wide range of ISI assessment may be due to higher intersubject variability of SICI and/or sample size, rendering traditional SICI testing methods ineffective for measuring changes in inhibition. Further, it is possible that rTMS modulates GABA_A and GABA_B mediated inhibitory processes differently, which would explain the conflicting results for SICI and cSP.

Review of differential diagnosis and management of spasmodic dysphonia

PURPOSE OF REVIEW

The recent literature on spasmodic dysphonia is reviewed with regard to pathogenesis, differential diagnosis, treatment options, audits, and current methods of management.

RECENT FINDINGS

Advances in technology have enabled clinicians to better understand the connection between brain and laryngeal function and dysfunction. Refinements in imaging and genetic investigation techniques have led to advances in the understanding of the underlying mechanism of this neurolaryngeal disorder. Development of diagnostic assessment tools and measures of quality of life hold the potential to improve treatment and care.

SUMMARY

Fifty articles published between 2014 and 2015 were selected for this review. The sources were drawn from several clinical specialties: 54% come under the scope of laryngology, 32% from neurology, and 14% from other areas. It remains poorly understood, misdiagnosed, and underdiagnosed. Its identification, diagnosis, treatment selection, and coordination of care require an expert specialist multidisciplinary team. More training is required to help people who have this chronic and psychosocially disabling voice disorder, which impinges on all aspects of their lives. Spasmodic dysphonia is now classified as a 'rare' disease in the United States. This designation will assist in international standards of diagnosis, assessment, treatment, and management.

Evaluation of the Cortical Silent Period of the Laryngeal Motor Cortex in Healthy Individuals

Objective: This work aimed to evaluate the cortical silent period (cSP) of the laryngeal motor cortex (LMC) using the bilateral thyroarytenoid (TA) muscles with transcranial magnetic stimulation (TMS).

Methods: In 11 healthy participants, fine-wire electromyography (EMG) was used to record bilateral TA muscle responses to single pulse TMS delivered to the LMC in both hemispheres. Peripheral responses to stimulation over the mastoid, where the vagus nerve exits the skull, were collected to verify the central origin of the cortical stimulation responses by comparing the latencies.

Results: The cSP duration ranged from 41.7 to 66.4 ms. The peripherally evoked motor-evoked potential (MEP) peak occurred 5–9 ms earlier than the cortical responses (for both sides of TAs: p < 0.0001) with no silent period. The right TA MEP latencies were earlier than the left TA responses for both peripheral and cortical measures (p ≤ 0.0001).

Conclusion: These findings demonstrate the feasibility of measuring cSP of LMC based on intrinsic laryngeal muscles responses during vocalization in healthy volunteers.

Significance: The technique could be used to study the pathophysiology of neurological disorders that affect TA muscles, such as spasmodic dysphonia. Further, the methodology has application to other muscles of the head and neck not accessible using surface electrodes.

Cortical sensorimotor alterations classify clinical phenotype and putative genotype of spasmodic dysphonia

BACKGROUND AND PURPOSE

Spasmodic dysphonia (SD), or laryngeal dystonia, is a task-specific isolated focal dystonia of unknown causes and pathophysiology. Although functional and structural abnormalities have been described in this disorder, the influence of its different clinical phenotypes and genotypes remains scant, making it difficult to explain SD pathophysiology and to identify potential biomarkers.

METHODS

We used a combination of independent component analysis and linear discriminant analysis of resting-state functional magnetic resonance imaging data to investigate brain organization in different SD phenotypes (abductor versus adductor type) and putative genotypes (familial versus sporadic cases) and to characterize neural markers for genotype/phenotype categorization.

RESULTS

We found abnormal functional connectivity within sensorimotor and frontoparietal networks in patients with SD compared with healthy individuals as well as phenotype- and genotype-distinct alterations of these networks, involving primary somatosensory, premotor and parietal cortices. The linear discriminant analysis achieved 71% accuracy classifying SD and healthy individuals using connectivity measures in the left inferior parietal and sensorimotor cortices. When categorizing between different forms of SD, the combination of measures from the left inferior parietal, premotor and right sensorimotor cortices achieved 81% discriminatory power between familial and sporadic SD cases, whereas the combination of measures from the right superior parietal, primary somatosensory and premotor cortices led to 71% accuracy in the classification of adductor and abductor SD forms.

CONCLUSIONS

Our findings present the first effort to identify and categorize isolated focal dystonia based on its brain functional connectivity profile, which may have a potential impact on the future development of biomarkers for this rare disorder.

Abnormal motor cortex excitability during linguistic tasks in adductor-type spasmodic dysphonia

In healthy subjects (HS), transcranial magnetic stimulation (TMS) applied during 'linguistic' tasks discloses excitability changes in the dominant hemisphere primary motor cortex (M1). We investigated 'linguistic' task-related cortical excitability modulation in patients with adductor-type spasmodic dysphonia (ASD), a speech-related focal dystonia. We studied 10 ASD patients and 10 HS. Speech examination included voice cepstral analysis. We investigated the dominant/non-dominant M1 excitability at baseline, during 'linguistic' (reading aloud/silent reading/producing simple phonation) and 'non-linguistic' tasks (looking at non-letter strings/producing oral movements). Motor evoked potentials (MEPs) were recorded from the contralateral hand muscles. We measured the cortical silent period (CSP) length and tested MEPs in HS and patients performing the 'linguistic' tasks with different voice intensities. We also examined MEPs in HS and ASD during hand-related 'action-verb' observation. Patients were studied under and not-under botulinum neurotoxin-type A (BoNT-A). In HS, TMS over the dominant M1 elicited larger MEPs during 'reading aloud' than during the other 'linguistic'/'non-linguistic' tasks. Conversely, in ASD, TMS over the dominant M1 elicited increased-amplitude MEPs during 'reading aloud' and 'syllabic phonation' tasks. CSP length was shorter in ASD than in HS and remained unchanged in both groups performing 'linguistic'/'non-linguistic' tasks. In HS and ASD, 'linguistic' task-related excitability changes were present regardless of the different voice intensities. During hand-related 'action-verb' observation, MEPs decreased in HS, whereas in ASD they increased. In ASD, BoNT-A improved speech, as demonstrated by cepstral analysis and restored the TMS abnormalities. ASD reflects dominant hemisphere excitability changes related to 'linguistic' tasks; BoNT-A returns these excitability changes to normal.

Cortical Silent Period Reveals Differences Between Adductor Spasmodic Dysphonia and Muscle Tension Dysphonia

BACKGROUND

The pathophysiology of adductor spasmodic dysphonia (AdSD), like other focal dystonias, is largely unknown.

OBJECTIVE

The purposes of this study were to determine (a) cortical excitability differences between AdSD, muscle tension dysphonia (MTD), and healthy controls; (b) distribution of potential differences in cranial or skeletal muscle; and (c) if cortical excitability measures assist in the differential diagnosis of AdSD and MTD.

METHODS

Ten participants with adductor spasmodic dysphonia, 8 with muscle tension dysphonia, and 10 healthy controls received single and paired pulse transcranial magnetic stimulation (TMS) to the primary motor cortex contralateral to tested muscles, first dorsal interosseus (FDI), and masseter. We tested the hypothesis that cortical excitability measures in AdSD would be significantly different from those in MTD and healthy controls. In addition, we hypothesized that there would be a correlation between cortical excitability measures and clinical voice severity in AdSD.

RESULTS

Cortical silent period duration in masseter and FDI was significantly shorter in AdSD than MTD and healthy controls. Other measures failed to demonstrate differences.

CONCLUSION

There are differences in cortical excitability between AdSD, MTD, and healthy controls. These differences in the cortical measure of both the FDI and masseter muscles in AdSD suggest widespread dysfunction of the GABAB mechanism may be a pathophysiologic feature of AdSD, similar to other forms of focal dystonia. Further exploration of the use of TMS to assist in the differential diagnosis of AdSD and MTD is warranted.

Impaired Limb Proprioception in Adults With Spasmodic Dysphonia

OBJECTIVES

Focal dystonia of the head and neck are associated with a loss of kinesthetic acuity at muscles distant from the dystonic sites. That is, while the motor deficits in focal dystonia are confined, the associated somatosensory deficits are generalized. This is the first systematic study to examine, if patients diagnosed with spasmodic dystonia (SD) show somatosensory impairments similar in scope to other forms of focal dystonia.

METHODS

Proprioceptive acuity (ability to discriminate between two stimuli) for forearm position and motion sense was assessed in 14 spasmodic dystonia subjects and 28 age-matched controls using a passive motion apparatus. Psychophysical thresholds, uncertainty area (UA), and a proprioceptive acuity index (AI) were computed based on the subjects' verbal responses.

RESULTS

The main findings are as follows: first, the SD group showed significantly elevated thresholds and UAs for forearm position sense compared with the control group. Second, 9 of 14 dystonia subjects (64%) exhibited an AI for position sense above the control group maximum. Three SD subjects had a motion sense AI above the control group maximum.

CONCLUSIONS

The results indicate that impaired limb proprioception is a common feature of SD. Like other forms of focal dystonia, spasmodic dystonia does affect the somatosensation of nondystonic muscle systems. That is, SD is associated with a generalized somatosensory deficit.

Enhanced dorsal premotor-motor inhibition in cervical dystonia

OBJECTIVE

This study aims to understand whether the enhanced dPMI, seen in writer's cramp patients previously, extends to other populations of focal dystonia patients (e.g. cervical dystonia) as an endophenotypic marker.

METHODS

We studied 9 healthy subjects and 9 patients with CD. dPMI was tested by applying conditioning transcranial magnetic stimulation to the left dorsal premotor cortex and then a test pulse to the ipsilateral motor cortex at an interval of 6ms. We also looked at the duration of the cortical silent period (CSP)-a measure of cortical excitability.

RESULTS

CD patients had enhanced dPMI at rest (mean 57.0%, SD 16.2) in contrast to healthy volunteers (mean 124.1%, SD 35.7) (p<0.001). CSP latencies (in ms) in CD patients (mean 108.0, SD 33.1) were significantly shorter than in healthy volunteers (mean 159.1, SD 55.2) (p<0.05).

CONCLUSIONS

CD patients showed enhanced dPMI in a hand muscle-distant from their affected body part-similar to writer's cramp patients. This enhanced inhibition was independent of disease severity and neck posture. This suggests that enhanced dPMI may be an endophenotypic marker of dystonia.

SIGNIFICANCE

The abnormal dorsal premotor-motor connection in cervical dystonia is a potential novel and important avenue for therapeutic targeting.