Abnormal cortical motor excitability in dystonia

Modulation of motor surround inhibition during motor tasks

Surround inhibition (SI) in the motor system is important in individuation of actions, but is sometimes difficult to demonstrate. It has also not been evaluated in real life tasks. In this study, we use real life tasks and a new method where excitability of the surround muscle is assessed with respect to its current activity level rather than when it is at rest. Motor evoked potential (MEP) amplitudes were measured in the abductor digiti minimi (ADM) muscle while participants performed several motor tasks: "writing" on paper, "holding a pen" precisely and, "holding a water bottle" against gravity. These MEPs were compared to ADM MEPs amplitudes measured during a fifth finger abduction (ADM being the center muscle). SI was also measured in the traditional way, by comparing ADM MEPs during an index finger flexion and at rest. For the "writing" and "holding a pen" tasks, but not the "holding bottle" task, the MEP amplitudes were significantly smaller when compared to MEP amplitudes when the ADM was the center muscle with the same level of activation. The ADM MEP amplitudes were not different between rest and during index finger flexion. The new method employed here shows, that motor SI can be measured during tonic movements. The findings also show motor SI during two real-life motor tasks: "writing" and "holding a pen". The lack of modulation of MEP amplitude during "holding bottle" task seems to indicate that SI is action specific rather than muscle specific.

Applications of Transcranial Magnetic Stimulation for Understanding and Treating Dystonia

Transcranial magnetic stimulation (TMS)-based studies have led to an advanced understanding of the pathophysiology of dystonia. This narrative review summarizes the TMS data contributed to the literature so far. Many studies have shown that increased motor cortex excitability, excessive sensorimotor plasticity, and abnormal sensorimotor integration are the core pathophysiological substrates for dystonia. However, an increasing body of evidence supports a more widespread network dysfunction involving many other brain regions. Repetitive TMS pulses (rTMS) in dystonia have therapeutic potential as they can induce local and network-wide effects through modulation of excitability and plasticity. The bulk of rTMS studies has targeted the premotor cortex with some promising results in focal hand dystonia. Some studies have targeted the cerebellum for cervical dystonia and the anterior cingulate cortex for blepharospasm. We believe that therapeutic potential could be leveraged better when rTMS is implemented in conjunction with standard-of-care pharmacological treatments. However, due to several limitations in the studies conducted to date, including small samples, heterogeneous populations, variability in the target sites, and inconsistencies in the study design and control arm, it is hard to draw a definite conclusion. Further studies are warranted to determine optimal targets and protocols yielding the most beneficial outcomes that will translate into meaningful clinical changes.

Strengthening the GABAergic system through neurofeedback training suppresses implicit motor learning

Gamma-aminobutyric acid (GABA) activity within the primary motor cortex (M1) is essential for motor learning in cortical plasticity, and a recent study has suggested that real-time neurofeedback training (NFT) can self-regulate GABA activity. Therefore, this study aimed to investigate the effect of GABA activity strengthening via NFT on subsequent motor learning. Thirty-six healthy participants were randomly assigned to either an NFT group or control group, which received sham feedback. GABA activity was assessed for short intracortical inhibition (SICI) within the right M1 using paired-pulse transcranial magnetic stimulation. During the NFT intervention period, the participants tried to modulate the size of a circle, which was altered according to the degree of SICI in the NFT group. However, the size was altered independently of the degree of SICI in the control group. We measured the reaction time before, after (online learning), and 24 h after (offline learning) the finger-tapping task. Results showed the strengthening of GABA activity induced by the NFT intervention, and the suppression of the online but not the offline learning. These findings suggest that prior GABA activity modulation may affect online motor learning.

Transcranial Direct Current Stimulation as a Treatment Tool for Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) has been defined as a transient (<24 h) condition of confusion and/or loss of consciousness for less than 30 min after brain injury and can result in short- and long-term motor and cognitive impairments. Recent studies have documented the therapeutic potential of non-invasive neuromodulation techniques for the enhancement of cognitive and motor function in mTBI. Alongside repetitive transcranial magnetic stimulation (rTMS), the main technique used for this purpose is transcranial direct current stimulation (tDCS). The focus of this review was to provide a detailed, comprehensive (i.e., both cognitive and motor impairment) overview of the literature regarding therapeutic tDCS paradigms after mTBI. A publication search of the PubMed, Scopus, CINAHL, and PsycINFO databases was performed to identify records that applied tDCS in mTBI. The publication search yielded 14,422 records from all of the databases, however, only three met the inclusion criteria and were included in the final review. Based on the review, there is limited evidence of tDCS improving cognitive and motor performance. Surprisingly, there were only three studies that used tDCS in mTBI, which highlights an urgent need for more research to provide additional insights into ideal therapeutic brain targets and optimized stimulation parameters.

Transcranial direct current stimulation for balance and gait in repetitive mild traumatic brain injury in rats

BACKGROUND

Balance impairment and lack of postural orientation are serious problems in patients with repetitive mild traumatic brain injury (mTBI).

OBJECTIVE

To investigate whether anodal transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) can improve balance control and gait in repetitive mTBI rat models.

METHODS

In this prospective animal study, 65 repetitive mTBI rats were randomly assigned to two groups: the tDCS group and the control group. To create repetitive mTBI model rats, we induced mTBI in the rats for 3 consecutive days. The tDCS group received one session of anodal tDCS over the M1 area 24 h after the third induced mTBI, while the control group did not receive tDCS treatment. Motor-evoked potential (MEP), foot-fault test, and rotarod test were evaluated before mTBI, before tDCS and after tDCS. The Mann-Whitney U test and Wilcoxon signed rank test were used to assess the effects of variables between the two groups.

RESULTS

Anodal tDCS over the M1 area significantly improved the amplitude of MEP in the tDCS group (p = 0.041). In addition, rotarod duration was significantly increased in the tDCS group (p = 0.001). The foot-fault ratio was slightly lower in the tDCS group, however, this was not statistically significant.

CONCLUSION

Anodal tDCS at the M1 area could significantly improve the amplitude of MEP and balance function in a repetitive mTBI rat model. We expect that anodal tDCS would have the potential to improve balance in patients with repetitive mTBI.

Muscle Tone Physiology and Abnormalities

The simple definition of tone as the resistance to passive stretch is physiologically a complex interlaced network encompassing neural circuits in the brain, spinal cord, and muscle spindle. Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia. The loss of supraspinal control mechanisms gives rise to hypertonia, resulting in spasticity or rigidity. On the other hand, dystonia and paratonia also manifest as abnormalities of muscle tone, but arise more due to the network dysfunction between the basal ganglia and the thalamo-cerebello-cortical connections. In this review, we have discussed the normal homeostatic mechanisms maintaining tone and the pathophysiology of spasticity and rigidity with its anatomical correlates. Thereafter, we have also highlighted the phenomenon of network dysfunction, cortical disinhibition, and neuroplastic alterations giving rise to dystonia and paratonia.

Opposite modulations of corticospinal excitability by intermittent and continuous peripheral electrical stimulation in healthy subjects

Peripheral electrical stimulation (PES) modulates the excitability of the corticospinal tract (CST). This modulation of CST excitability depends on the PES intensity, defined by the amplitude and the width of each pulse, the total pulse number, the stimulation frequency, and the intervention duration. Another key PES parameter is the stimulation pattern; little is known about how PES pattern affects CST excitability, as previous studies did not control other PES parameters. Here, we investigated the effect of the net difference in PES pattern on CST excitability. We use three controlled PESs, intermittent PES (30 Hz) (stimulation trains at 30 Hz with pauses), continuous PES (12 Hz) (constant stimulation at 12 Hz without pauses), and continuous PES (30 Hz) with the same stimulation frequency as the intermittent PES (30 Hz), to compare the effect of the stimulation frequency. The motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs) of healthy subjects were recorded before and after these three types of PESs in separate sessions. We found that intermittent PES (30 Hz) increased MEP amplitudes, whereas continuous PES (12 and 30 Hz) decreased amplitudes. A significant change in subcortical SEP component occurred during continuous PES (12 and 30 Hz), but not intermittent PES (30 Hz), whereas cortical SEP components showed similar behavior in three types of PESs. We conclude that (1) opposing modulations of CST excitability were induced by the differences in the PES pattern, and (2) these modulations appear to be mediated through different processes in the sensorimotor system. Our findings suggest the possibility that it may be preferable to select the PES pattern in therapeutic interventions based on the putative desired effect and the neural structure being targeted.

Alertness fluctuations when performing a task modulate cortical evoked responses to transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) has been widely used in human cognitive neuroscience to examine the causal role of distinct cortical areas in perceptual, cognitive and motor functions. However, it is widely acknowledged that the effects of focal cortical stimulation can vary substantially between participants and even from trial to trial within individuals. Recent work from resting state functional magnetic resonance imaging (fMRI) studies has suggested that spontaneous fluctuations in alertness over a testing session can modulate the neural dynamics of cortical processing, even when participants remain awake and responsive to the task at hand. Here we investigated the extent to which spontaneous fluctuations in alertness during wake-to-sleep transition can account for the variability in neurophysiological responses to TMS. We combined single-pulse TMS with neural recording via electroencephalography (EEG) to quantify changes in motor and cortical reactivity with fluctuating levels of alertness defined objectively on the basis of ongoing brain activity. We observed rapid, non-linear changes in TMS-evoked responses with decreasing levels of alertness, even while participants remained responsive in the behavioural task. Specifically, we found that the amplitude of motor evoked potentials peaked during periods of EEG flattening, whereas TMS-evoked potentials increased and remained stable during EEG flattening and the subsequent occurrence of theta ripples that indicate the onset of NREM stage 1 sleep. Our findings suggest a rapid and complex reorganization of active neural networks in response to spontaneous fluctuations of alertness over relatively short periods of behavioural testing during wake-to-sleep transition.

Mirtazapine-associated movement disorders: A literature review

Mirtazapine (MTZ) is an atypical antidepressant approved by the FDA, which mechanism of action involves the antagonism of alpha-2, H1, 5-HT2A, 5-HT2C, and 5-HT3 receptors. In this context, the aim of this literature review is to evaluate the clinical epidemiological profile, pathological mechanisms, and management of MTZ-associated movement disorders (MDs). Relevant reports of six databases were identified and assessed by two reviewers without language restriction. Fifty-two reports containing 179 cases from 20 countries were assessed. The mean age was 57 year (range, 17–85). The majority of the individuals were female (60%) and of European origin. The mean time from MTZ start to symptom onset was 7.54 days; the time from management to MD improvement was within one week in 82.60% of the individuals. The MDs associated with MTZ were 69 restless legs syndrome (RLS), 35 tremors, 10 akathisia (AKT), 9 periodic limb MD, 6 dystonia, 4 rapid eye movement sleep behavior disorders, 3 dyskinesia, 2 parkinsonism, and 1 tic, and in the group not clearly identified, 18 restlessness, 15 hyperkinesis, and 1 extrapyramidal symptom. In the literature, the majority of the reports lack important information about the neurological examination. The management should be the MTZ withdrawal, except in RLS that other options are possible. In AKT, the MTZ should not be rechallenge, and if available, the prescription of a benzodiazepine may reduce recovery time.

TorsinB overexpression prevents abnormal twisting in DYT1 dystonia mouse models

Genetic redundancy can be exploited to identify therapeutic targets for inherited disorders. We explored this possibility in DYT1 dystonia, a neurodevelopmental movement disorder caused by a loss-of-function (LOF) mutation in the TOR1A gene encoding torsinA. Prior work demonstrates that torsinA and its paralog torsinB have conserved functions at the nuclear envelope. This work established that low neuronal levels of torsinB dictate the neuronal selective phenotype of nuclear membrane budding. Here, we examined whether torsinB expression levels impact the onset or severity of abnormal movements or neuropathological features in DYT1 mouse models. We demonstrate that torsinB levels bidirectionally regulate these phenotypes. Reducing torsinB levels causes a dose-dependent worsening whereas torsinB overexpression rescues torsinA LOF-mediated abnormal movements and neurodegeneration. These findings identify torsinB as a potent modifier of torsinA LOF phenotypes and suggest that augmentation of torsinB expression may retard or prevent symptom development in DYT1 dystonia.

Increased Finger-Tapping Related Cerebellar Activation in Cervical Dystonia, Enhanced by Transcranial Stimulation: An Indicator of Compensation?

Background: Cervical dystonia is a movement disorder causing abnormal postures and movements of the head. While the exact pathophysiology of cervical dystonia has not yet been fully elucidated, a growing body of evidence points to the cerebellum as an important node.

Methods: Here, we examined the impact of cerebellar interference by transcranial magnetic stimulation on finger-tapping related brain activation and neurophysiological measures of cortical excitability and inhibition in cervical dystonia and controls. Bilateral continuous theta-burst stimulation was used to modulate cerebellar cortical excitability in 16 patients and matched healthy controls. In a functional magnetic resonance imaging arm, data were acquired during simple finger tapping before and after cerebellar stimulation. In a neurophysiological arm, assessment comprised motor-evoked potentials amplitude and cortical silent period duration. Theta-burst stimulation over the dorsal premotor cortex and sham stimulation (neurophysiological arm only) served as control conditions.

Results: At baseline, finger tapping was associated with increased activation in the ipsilateral cerebellum in patients compared to controls. Following cerebellar theta-burst stimulation, this pattern was even more pronounced, along with an additional movement-related activation in the contralateral somatosensory region and angular gyrus. Baseline motor-evoked potential amplitudes were higher and cortical silent period duration shorter in patients compared to controls. After cerebellar theta-burst stimulation, cortical silent period duration increased significantly in dystonia patients.

Conclusion: We conclude that in cervical dystonia, finger movements—though clinically non-dystonic—are associated with increased activation of the lateral cerebellum, possibly pointing to general motor disorganization, which remains subclinical in most body regions. Enhancement of this activation together with an increase of silent period duration by cerebellar continuous theta-burst stimulation may indicate predominant disinhibitory effects on Purkinje cells, eventually resulting in an inhibition of cerebello-thalamocortical circuits.

Impaired white matter integrity between premotor cortex and basal ganglia in writer's cramp

INTRODUCTION

Writer's cramp (WC) as a focal hand dystonia is characterized by abnormal postures of the hand during writing. Impaired inhibition and maladaptive plasticity in circuits linking the basal ganglia and sensorimotor cortices have been described. In particular, a dysfunction of lateral premotor cortices has been associated with impaired motor control in WC. We applied diffusion tensor imaging to identify changes in white matter connectivity between premotor regions and important cortical and subcortical structures.

METHODS

Whole brain white matter tracts were reconstructed in 18 right-handed WC patients and 18 matched controls, using probabilistic fiber tracking. We restricted our analyses to left-hemispheric fibers between the middle frontal gyrus (MFG) and basal ganglia, thalamus, primary motor, and sensory cortex. Diffusion parameters (fractional anisotropy and linear anisotropy) were compared between both groups.

RESULTS

A significant reduction in fractional anisotropy values was shown for patients (mean ± SD: 0.37 ± 0.02) vs. controls (0.39 ± 0.03) regarding fibers between the left-sided MFG and the putamen (p < 0.05). The same applied for linear anisotropy values in this connection (p < 0.05).

CONCLUSIONS

Our results suggest an impaired structural connectivity between the left-hemispheric MFG and putamen with a loss of equally aligned fibers in WC patients. This could reflect a structural basis for functional findings interpreted as altered inhibition and plasticity, both within the premotor cortex and the basal ganglia, that at last lead to the clinical symptoms of WC.

Basal ganglia mechanisms in action selection, plasticity, and dystonia

Basal ganglia circuits are organized to selected desired actions and to inhibit potentially competing unwanted actions. This is accomplished through a complex circuitry that is modified through development and learning. Mechanisms of neural plasticity underlying these modifications are increasingly understood, but new mechanisms continue to be discovered. Dystonia, a movement disorder characterized by involuntary muscle contractions that cause abnormal postures and movements. Emerging evidence points to important links between mechanisms of plasticity and the manifestations of dystonia. Investigation of these mechanisms has improved understanding of the action of currently used medication and is informing the development of new treatments.

Dynamic head-neck stabilization in cervical dystonia

BACKGROUND

Effective sensorimotor integration is essential to modulate (adapt) neck stabilization strategies in response to varying tasks and disturbances. This study evaluates the hypothesis that relative to healthy controls cervical dystonia patients have an impaired ability to modulate afferent feedback for neck stabilization with changes in the frequency content of mechanical perturbations.

METHODS

We applied anterior-posterior displacement perturbations (110s) on the torso of seated subjects, while recording head-neck kinematics and muscular activity. We compared low bandwidth (0.2-1.2Hz) and high bandwidth (0.2-8Hz) perturbations where our previous research showed a profound modulation of stabilization strategies in healthy subjects. Cervical dystonia patients and age matched controls performed two tasks: (1) maintain head forward posture and (2) allow dystonia to dictate head posture.

FINDINGS

Patients and controls demonstrated similar kinematic and muscular responses. Patient modulation was similar to that of healthy controls (P>0.05); neck stiffness and afferent feedback decreased with high bandwidth perturbations. During the head forward task patients had an increased neck stiffness relative to controls (P<0.05), due to increased afferent feedback.

INTERPRETATION

The unaffected modulation of head-neck stabilization (both kinematic and muscular) in patients with cervical dystonia does not support the hypothesis of impaired afferent feedback modulation for neck stabilization.

Combination of Static Magnetic Fields and Peripheral Nerve Stimulation Can Alter Focal Cortical Excitability

For clinical application of transcranial static magnetic stimulation (tSMS), it is important to achieve a focal target cortical stimulation. Previous study suggested that the associative stimulation combining non-invasive stimulation of the motor cortex (M1) and the peripheral nerve stimulation (PNS) may be useful to produce cortical excitability change. To test this hypothesis, we measured the M1 excitability and intracortical circuits by using transcranial magnetic stimulation (TMS) before and after the tSMS of short duration (5 min) combined with PNS. Thirty-three normal volunteers were participated; tSMS+PNS (n = 11), sham+PNS (n = 11), and tSMS alone (n = 11). We found the transient suppression of the motor-evoked potential (MEP) of the right abductor pollicis brevis (APB) muscle, but not of the abductor digiti minimi (ADM) muscle, when combining tSMS with PNS over median nerve at the wrist. The lack of suppressive effect on APB in tSMS alone with short duration is in accord with the previous observation. In addition, the tendency of transient enhancement of the short-latency intracortical inhibition was observed immediately after intervention in the tSMS±PNS group. These findings show that the combination of tSMS and PNS can induce the cortical excitability change in target cortical motor area and potentiate the suppression effect.

Contribution of TMS and rTMS in the Understanding of the Pathophysiology and in the Treatment of Dystonia

Dystonias represent a heterogeneous group of movement disorders responsible for sustained muscle contraction, abnormal postures, and muscle twists. It can affect focal or segmental body parts or be generalized. Primary dystonia is the most common form of dystonia but it can also be secondary to metabolic or structural dysfunction, the consequence of a drug's side-effect or of genetic origin. The pathophysiology is still not elucidated. Based on lesion studies, dystonia has been regarded as a pure motor dysfunction of the basal ganglia loop. However, basal ganglia lesions do not consistently produce dystonia and lesions outside basal ganglia can lead to dystonia; mild sensory abnormalities have been reported in the dystonic limb and imaging studies have shown involvement of multiple other brain regions including the cerebellum and the cerebral motor, premotor and sensorimotor cortices. Transcranial magnetic stimulation (TMS) is a non-invasive technique of brain stimulation with a magnetic field applied over the cortex allowing investigation of cortical excitability. Hyperexcitability of contralateral motor cortex has been suggested to be the trigger of focal dystonia. High or low frequency repetitive TMS (rTMS) can induce excitatory or inhibitory lasting effects beyond the time of stimulation and protocols have been developed having either a positive or a negative effect on cortical excitability and associated with prevention of cell death, γ-aminobutyric acid (GABA) interneurons mediated inhibition and brain-derived neurotrophic factor modulation. rTMS studies as a therapeutic strategy of dystonia have been conducted to modulate the cerebral areas involved in the disease. Especially, when applied on the contralateral (pre)-motor cortex or supplementary motor area of brains of small cohorts of dystonic patients, rTMS has shown a beneficial transient clinical effect in association with restrained motor cortex excitability. TMS is currently a valuable tool to improve our understanding of the pathophysiology of dystonia but large controlled studies using sham stimulation are still necessary to delineate the place of rTMS in the therapeutic strategy of dystonia. In this review, we will focus successively on the use of TMS as a tool to better understand pathophysiology, and the use of rTMS as a therapeutic strategy.

Focal Hand Dystonia Affecting Musicians. Part I: An Overview Of Epidemiology, PathoPhysiology And Medical Treatments

In 1911, Oppenheim coined the term ‘dystonia’ to describe disordered motor control, characterised by an association of hypotonia and tonic muscle spasm. Focal hand dystonia is one form of this disorder, in which symptoms are often task-specific and occur during skilled movements such as writing (writer's cramp) or playing a musical instrument (musician's cramp). Much research has been conducted on the pathophysiology of dystonia, but the underlying mechanisms still remain unclear. Hypotheseses about functional central nervous system alterations continue to gain more support. Scientific treatment-based publications on focal dystonia are sparse, and progress in evidence-based treatment options are necessary in order to assist this patient group. This paper will review the literature, documenting dystonia classification criteria, manifestations, pathophysiology and medical treatment techniques for musicians affected by focal hand dystonia.

Effects of cerebellar theta-burst stimulation on arm and neck movement kinematics in patients with focal dystonia

OBJECTIVE

To investigate the cerebellar inhibitory influence on the primary motor cortex in patients with focal dystonia using a cerebellar continuous theta-burst stimulation protocol (cTBS) and to evaluate any relationship with movement abnormalities.

METHODS

Thirteen patients with focal hand dystonia, 13 patients with cervical dystonia and 13 healthy subjects underwent two sessions: (i) cTBS over the cerebellar hemisphere (real cTBS) and (ii) cTBS over the neck muscles (sham cTBS). The effects of cerebellar cTBS were quantified as excitability changes in the contralateral primary motor cortex, as well as possible changes in arm and neck movements in patients.

RESULTS

Real cerebellar cTBS reduced the excitability in the contralateral primary motor cortex in healthy subjects and in patients with cervical dystonia, though not in patients with focal hand dystonia. There was no correlation between changes in primary motor cortex excitability and arm and neck movement kinematics in patients. There were no changes in clinical scores or in kinematic measures, after either real or sham cerebellar cTBS in patients.

CONCLUSIONS

The reduced cerebellar inhibitory modulation of primary motor cortex excitability in focal dystonia may be related to the body areas affected by dystonia as opposed to being a widespread pathophysiological abnormality.

SIGNIFICANCE

The present study yields information on the differential role played by the cerebellum in the pathophysiology of different focal dystonias.

Abnormal interhemispheric inhibition in musician's dystonia - Trait or state?

INTRODUCTION

A clustering of relatives with dystonia has been reported in families with musician's dystonia suggesting a genetic contribution to this disease. The aim of the present study was to determine whether interhemispheric inhibition (IHI) measured with transcranial magnetic stimulation is impaired in healthy family members rendering it a suitable endophenotypic marker for musician's dystonia.

METHODS

Patients with musician's hand dystonia (n = 21), patients with sporadic writer's cramp (n = 15), their healthy family members (n = 27), healthy musicians (n = 12) and healthy non-musicians (n = 12) were included. An extended interview about the family history and musical activity was performed. IHI in both hemispheres was measured using transcranial magnetic stimulation.

RESULTS

A stepwise regression analysis revealed musical activity (p = 0.001) and a family history of dystonia (p = 0.008) but not dystonia per se, age, handedness or gender as relevant factors modulating IHI.

CONCLUSION

These data support the notion of a genetic background of musician's hand dystonia and suggests that reduced IHI is a possible endophenotypic marker of this disorder.

Dystonia and Paroxysmal Dyskinesias: Under-Recognized Movement Disorders in Domestic Animals? A Comparison with Human Dystonia/Paroxysmal Dyskinesias

Dystonia is defined as a neurological syndrome characterized by involuntary sustained or intermittent muscle contractions causing twisting, often repetitive movements, and postures. Paroxysmal dyskinesias are episodic movement disorders encompassing dystonia, chorea, athetosis, and ballism in conscious individuals. Several decades of research have enhanced the understanding of the etiology of human dystonia and dyskinesias that are associated with dystonia, but the pathophysiology remains largely unknown. The spontaneous occurrence of hereditary dystonia and paroxysmal dyskinesia is well documented in rodents used as animal models in basic dystonia research. Several hyperkinetic movement disorders, described in dogs, horses and cattle, show similarities to these human movement disorders. Although dystonia is regarded as the third most common movement disorder in humans, it is often misdiagnosed because of the heterogeneity of etiology and clinical presentation. Since these conditions are poorly known in veterinary practice, their prevalence may be underestimated in veterinary medicine. In order to attract attention to these movement disorders, i.e., dystonia and paroxysmal dyskinesias associated with dystonia, and to enhance interest in translational research, this review gives a brief overview of the current literature regarding dystonia/paroxysmal dyskinesia in humans and summarizes similar hereditary movement disorders reported in domestic animals.

Multiday Transcranial Direct Current Stimulation Causes Clinically Insignificant Changes in Childhood Dystonia: A Pilot Study

Abnormal motor cortex activity is common in dystonia. Cathodal transcranial direct current stimulation may alter cortical activity by decreasing excitability while anodal stimulation may increase motor learning. Previous results showed that a single session of cathodal transcranial direct current stimulation can improve symptoms in childhood dystonia. Here we performed a 5-day, sham-controlled, double-blind, crossover study, where we measured tracking and muscle overflow in a myocontrol-based task. We applied cathodal and anodal transcranial direct current stimulation (2 mA, 9 minutes per day). For cathodal transcranial direct current stimulation (7 participants), 3 subjects showed improvements whereas 2 showed worsening in overflow or tracking error. The effect size was small (about 1% of maximum voluntary contraction) and not clinically meaningful. For anodal transcranial direct current stimulation (6 participants), none showed improvement, whereas 5 showed worsening. Thus, multiday cathodal transcranial direct current stimulation reduced symptoms in some children but not to a clinically meaningful extent, whereas anodal transcranial direct current stimulation worsened symptoms. Our results do not support transcranial direct current stimulation as clinically viable for treating childhood dystonia.

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.

Static magnetic field can transiently alter the human intracortical inhibitory system

OBJECTIVE

Although recent studies have shown the suppressive effects of static magnetic fields (SMFs) on the human primary motor cortex (M1) possibly due to the deformed neural membrane channels, the effect of the clinical MRI scanner bore has not been studied in the same way.

METHODS

We tested whether the MRI scanner itself and compact magnet can alter the M1 function using single- and paired-pulse transcranial magnetic stimulation (TMS).

RESULTS

We found the transient suppression of the corticospinal pathway in both interventions. In addition, the transient enhancement of the short-latency intracortical inhibition (SICI) was observed immediately after compact magnet stimulation.

CONCLUSIONS

The present results suggest that not only the inhomogeneous SMFs induced by a compact magnet but also the homogeneous SMF produced by the MRI scanner bore itself can produce the transient cortical functional change.

SIGNIFICANCE

Static magnetic stimulation can modulate the intracortical inhibitory circuit of M1, which might be useful for clinical purposes.

Cortical activation and inter-hemispheric sensorimotor coherence in individuals with arm dystonia due to childhood stroke

OBJECTIVE

Dystonia is a disabling motor disorder often without effective therapies. To better understand the genesis of dystonia after childhood stroke, we analyzed electroencephalographic (EEG) recordings in this population.

METHODS

Resting spectral power of EEG signals over bilateral sensorimotor cortices (Powrest), resting inter-hemispheric sensorimotor coherence (Cohrest), and task-related changes in power (TRPow) and coherence (TRCoh) during wrist extension were analyzed in individuals with dystonia (age 20±3years) and healthy volunteers (age 17±5years).

RESULTS

Ipsilesional TRPow decrease was significantly lower in patients than controls during the more affected wrist task. Force deficits of the affected wrist correlated with reduced alpha TRPow decrease on the ipsilesional and not the contralesional hemisphere. Cohrest was significantly lower in patients than controls, and correlated with more severe dystonia and poorer hand function. Powrest and TRCoh were similar between groups.

CONCLUSIONS

The association between weakness and cortical activation during wrist extension highlights the importance of ipsilesional sensorimotor activation on function. Reduction of Cohrest in patients reflects a loss of inter-hemispheric connectivity that may result from structural changes and neuroplasticity, potentially contributing to the development of dystonia.

SIGNIFICANCE

Cortical and motor dysfunction are correlated in patients with childhood stroke and may in part explain the genesis of dystonia.

Associative plasticity in surround inhibition circuits in human motor cortex

Surround inhibition is a physiological mechanism that is hypothesised to improve contrast between signals in the central nervous system. In the human motor system, motor surround inhibition (mSI) can be assessed using transcranial magnetic stimulation (TMS). We evaluated whether it is possible to modulate mSI, using a paradigm able to induce plastic effects in primary motor cortex (M1). Fifteen healthy volunteers participated in the experiments. To assess mSI, we delivered single pulses at rest and at the onset of a right thumb abduction. TMS pulses over abductor digiti minimi (ADM; surround muscle) hotspot were delivered when EMG activity in right abductor pollicis brevis (APB; active muscle) > 100 μV was detected. Paired associative stimulation (PAS) was delivered using peripheral median nerve electric stimulation and TMS over APB M1 area at an interstimulus interval of 21.5 ms for the real PAS (PAS21.5) and 100 ms for the sham PAS (PAS100). To verify the effect of PAS21.5 on mSI we collected 20 MEPs from ADM at rest and during APB movements before (T0) and 5 (T1), 15 (T2) and 30 (T3) minutes after PAS21.5. mSI from APB to ADM was present at baseline. PAS21.5 increased the amount of mSI compared with baseline whereas there was no effect after PAS100. Our results suggest that mSI is an adaptable phenomenon depending on prior experience.

Does abnormal interhemispheric inhibition play a role in mirror dystonia?

The presence of mirror dystonia (dystonic movement induced by a specific task performed by the unaffected hand) in the dominant hand of writer's cramp patients when the nondominant hand is moved suggests an abnormal interaction between the 2 hemispheres. In this study we compare the level of interhemispheric inhibition (IHI) in 2 groups of patients with writer's cramp, one with the presence of a mirror dystonia and the other without as well as a control group. The level of bidirectional IHI was measured in wrist muscles with dual-site transcranial magnetic stimulation with a 10-millisecond (short IHI) and a 40-millisecond (long IHI) interstimulus interval during rest and while holding a pen in 9 patients with mirror dystonia 7 without mirror dystonia, and 13 controls. The group of patients without mirror dystonia did not differ from the controls in their IHI level. In contrast, IHI was significantly decreased in the group of patients with mirror dystonia in comparison with the group without mirror dystonia and the controls in both wrist muscles of both the dystonic and unaffected hand whatever the resting or active condition (P = 0.001). The decrease of IHI level in the group of patients with mirror dystonia was negatively correlated with the severity and the duration of the disease: the weaker the level of IHI, the more severe was the disease and the longer its duration. Interhemispheric inhibition disturbances are most likely involved in the occurrence of mirror dystonia. This bilateral deficient inhibition further suggests the involvement of the unaffected hemisphere in the pathophysiology of unilateral dystonia.

Somatosensory cortectomy induces motor cortical hyperexcitability and scoliosis: an experimental study in developing rats

BACKGROUND CONTEXT

Dysfunctions in sensorimotor integration, reminiscent to those described in idiopathic dystonia, have been found in idiopathic scoliosis (IS) and might be involved in its pathogenesis. Studying the effects of experimental disruption of sensory cortex may shed further insight into the etiopathology of IS.

PURPOSE

To evaluate whether disruption of central sensorimotor integration through partial ablation of the somatosensory cortex leads to scoliosis in developing rats and to describe the effects of such an intervention on motor cortico-cortical inhibition and facilitation.

METHODS

Fifty Wistar rats aged 3 weeks were used in the study. Twenty-four rats underwent craniotomy and electrocoagulation of the sensory cortex (PAR1) in the right hemisphere. A second group of 16 rats underwent a sham operation with craniotomy but no electrocoagulation. A third group of 10 rats was used as intact controls. Four weeks after surgery, motor cortical excitability was assessed with paired-pulse electrical cortical stimulation. Neurologic and behavioral examinations were completed serially, and 10 weeks after surgery, X-ray examinations were performed in anesthetized rats to assess spinal curvature. Electromyographic recordings of paravertebral muscle activity were performed in waking rats. At the end of the study, rats were sacrificed, and histologic examinations of brain tissue were performed to confirm the extent of the lesion. A grant from a Government Health Research Fund without salaries assignment financed the study.

RESULTS

Almost half of the animals with somatosensory cortectomy (46%) developed scoliosis, with an average Cobb angle of 23 ± 8°. None of the animals in the sham or control groups developed scoliosis. Despite cortical lesions, no motor or behavioral deficits were apparent in the experimental group, and cortectomized rats were neurologically indistinguishable from sham or control animals, except for the presence of scoliosis. Cortico-cortical inhibition was significantly reduced in the hemisphere of scoliotic concavity in the cortectomized group but was normal in the other groups.

CONCLUSIONS

These findings indicate that altered sensorimotor integration may cause scoliosis without noticeable motor impairment. Reduced cortico-cortical inhibition was observed in cortectomized rats. This finding is consistent with results in adolescents with IS and suggests that alteration of cortical hemispheric balance of sensorimotor integration may play an important role in the pathogenesis of IS.

Anatomy of handedness and the laterality of seizure onset: surgical implications of new understandings in motor control

OBJECTIVES

This article pursues another corollary of the anatomy of handedness, a code for the laterality of motor control. The latter indicates the absence of any motor communication from the minor (right, in the vast majority of population) to the major hemisphere (left, in the vast majority of right handers). It also indicates that all communications between the two hemispheres are excitatory in nature. This arrangement prohibits initiation of seizure within the minor and its propagation to the major hemisphere, via the callosum.

METHODS

A comprehensive review of the literature is undertaken regarding theoretical and technical reasons for the failure of seizure surgery in subjects undergoing the same for intractable epilepsy.

RESULTS

Whereas the laterality of motor control is heavily biased towards the left hemisphere (approximately 80%), the operation is performed equally on both hemispheres. Failures of surgery in some series were substantially higher among those who had undergone operations on the right hemisphere. Technical reasons for this are traced to the unreliability of tests commonly employed in securing laterality of seizure onset, which is the same as that of motor control. Accordingly, the failure rate of seizure surgery may equal the rate of false lateralization of the major hemisphere in these circumstances.

CONCLUSION

Given the dichotomous anatomy of handedness, the most robust test for lateralizing the hemisphere of onset of seizure is that of determining the reaction times of two symmetrically located effectors, one on each side of the body. The side with the shorter reaction time will always be opposite to the major hemisphere. The difference between the two values is commensurate to the inter-hemispheric transfer time.

Interhemispheric control of unilateral movement

To perform strictly unilateral movements, the brain relies on a large cortical and subcortical network. This network enables healthy adults to perform complex unimanual motor tasks without the activation of contralateral muscles. However, mirror movements (involuntary movements in ipsilateral muscles that can accompany intended movement) can be seen in healthy individuals if a task is complex or fatiguing, in childhood, and with increasing age. Lateralization of movement depends on complex interhemispheric communication between cortical (i.e., dorsal premotor cortex, supplementary motor area) and subcortical (i.e., basal ganglia) areas, probably coursing through the corpus callosum (CC). Here, we will focus on transcallosal interhemispheric inhibition (IHI), which facilitates complex unilateral movements and appears to play an important role in handedness, pathological conditions such as Parkinson's disease, and stroke recovery.

Cortical silent period duration and its implications for surround inhibition of a hand muscle

Surround inhibition is a neural mechanism that assists in the focusing of excitatory drive to muscles responsible for a given movement (agonist muscles) by suppressing unwanted activity in muscles not relevant to the movement (surround muscles). The purpose of the study was to determine the contribution of γ-aminobutyric acid(B) receptor-mediated intracortical inhibition, as assessed by the cortical silent period (CSP), to the generation of surround inhibition in the motor system. Eight healthy adults (five women and three men, 29.8 ± 9 years) performed isometric contractions with the abductor digiti minimi (ADM) muscle in separate conditions with and without an index finger flexion movement. The ADM motor evoked potential amplitude and CSP duration elicited by transcranial magnetic stimulation were compared between a control condition in which the ADM was activated independently and during conditions involving three phases (pre-motor, phasic, and tonic) of the index finger flexion movement. The motor evoked potential amplitude of the ADM was greater during the control condition compared with the phasic condition. Thus, the presence of surround inhibition was confirmed in the present study. Most critically, the CSP duration of the ADM decreased during the phasic stage of finger flexion compared with the control condition, which indicated a reduction of this type of intracortical inhibition during the phasic condition. These findings indicate that γ-aminobutyric acid(B) receptor-mediated intracortical inhibition, as measured by the duration of the CSP, does not contribute to the generation of surround inhibition in hand muscles.

The cerebellum in dystonia - help or hindrance?

Dystonia has historically been considered a disorder of the basal ganglia. This review aims to critically examine the evidence for a role of the cerebellum in the pathophysiology of dystonia. We compare and attempt to link the information available from both clinical and experimental studies; work detailing cerebellar connectivity in primates; data that suggests a role for the cerebellum in the genesis of dystonia in murine models; clinical observation in humans with structural lesions and heredodegenerative disorders of the cerebellum; and imaging studies of patients with dystonia. The typical electrophysiological findings in dystonia are the converse to those found in cerebellar lesions. However, certain subtypes of dystonia mirror cerebellar patterns of increased cortical inhibition. Furthermore, altered cerebellar function can be demonstrated in adult onset focal dystonia with impaired cerebellar inhibition of motor cortex and abnormal eyeblink classical conditioning. We propose that abnormal, likely compensatory activity of the cerebellum is an important factor within pathophysiological models of dystonia. Work in this exciting area has only just begun but it is likely that the cerebellum will have a key place within future models of dystonia.

Deep brain stimulation effects in dystonia: time course of electrophysiological changes in early treatment

Deep brain stimulation to the internal globus pallidus is an effective treatment for primary dystonia. The optimal clinical effect often occurs only weeks to months after starting stimulation. To better understand the underlying electrophysiological changes in this period, we assessed longitudinally 2 pathophysiological markers of dystonia in patients prior to and in the early treatment period (1, 3, 6 months) after deep brain stimulation surgery. Transcranial magnetic stimulation was used to track changes in short-latency intracortical inhibition, a measure of excitability of GABA(A) -ergic corticocortical connections and long-term potentiation-like synaptic plasticity (as a response to paired associative stimulation). Deep brain stimulation remained on for the duration of the study. Prior to surgery, inhibition was reduced and plasticity increased in patients compared with healthy controls. Following surgery and commencement of deep brain stimulation, short-latency intracortical inhibition increased toward normal levels over the following months with the same monotonic time course as the patients' clinical benefit. In contrast, synaptic plasticity changed rapidly, following a nonmonotonic time course: it was absent early (1 month) after surgery, and then over the following months increased toward levels observed in healthy individuals. We postulate that before surgery preexisting high levels of plasticity form strong memories of dystonic movement patterns. When deep brain stimulation is turned on, it disrupts abnormal basal ganglia signals, resulting in the absent response to paired associative stimulation at 1 month. Clinical benefit is delayed because engrams of abnormal movement persist and take time to normalize. Our observations suggest that plasticity may be a driver of long-term therapeutic effects of deep brain stimulation in dystonia.

Impaired interhemispheric inhibition in writer's cramp

OBJECTIVES

Reduced cortical inhibition is a feature of focal hand dystonia and this likely contributes to excessive muscle contractions. Inhibition from the opposite hemisphere, known as interhemispheric inhibition (IHI), was studied bidirectionally in 7 right-handed patients with writer's cramp (WC) and age-matched healthy controls in a cross-sectional physiologic study.

METHODS

IHI was measured with paired transcranial magnetic stimulation with the conditioning stimulus applied to the motor cortex and the test stimulus applied to the contralateral motor cortex. Surface EMG was measured in right and left first dorsal interosseous muscles during rest, and while holding a pen between the thumb and index finger at 20% maximum voluntary contraction with the right dystonia-affected hand. The time course and magnitude of IHI was studied at interstimulus intervals of 6, 8, 10, 12, 30, 40, and 50 msec between the conditioning stimulus and test stimulus.

RESULTS

In WC at rest, IHI was significantly reduced in the dystonia-affected right hand (IHI from right to left motor cortex) at both short (SIHI, 10-12 msec) and long (LIHI, 30-40 msec) intervals compared to the unaffected hand. Compared to controls, SIHI and LIHI were reduced in the dystonia-affected hand only. There was no difference in IHI between controls and WC during the task of holding a pen.

CONCLUSIONS

In WC, both SIHI and LIHI are reduced in the dystonia-affected hand compared to the unaffected hand and to healthy controls. Impaired IHI may contribute to excessive muscle contraction in WC.

Restoration of motor inhibition through an abnormal premotor-motor connection in dystonia

To clarify the rationale for using rTMS of dorsal premotor cortex (PMd) to treat dystonia, we examined how the motor system reacts to an inhibitory form of rTMS applied to the PMd in healthy subjects and in a group of patients with focal hand dystonia and DYT1 gene carriers. Continuous theta burst transcranial magnetic stimulation (cTBS) with 300 and 600 pulses (cTBS300 and cTBS600) was applied to PMd, and its after-effects were quantified by measuring the amplitude of MEPs evoked by single pulse transcranial magnetic stimulation (TMS) over the primary motor cortex (M1), short interval intracortical inhibition/facilitation (SICI/ICF) within M1, the third phase of spinal reciprocal inhibition (RI), and writing tests. In addition, in DYT1 gene carriers, the effects of cTBS300 over M1 and PMd on MEPs were studied in separate experiments. In healthy subjects, cTBS300 and cTBS600 over PMd suppressed MEPs for 30 min or more and cTBS600 decreased SICI and RI. In contrast, neither form of cTBS over PMd had any significant effect on MEPs, while cTBS600 increased effectiveness of SICI and RI and improved writing in patients with writer's cramp. NMDYT1 had a normal response to cTBS300 over left PMd. We suggest that the reduced PMd to M1 interaction in dystonic patients is likely to be due to reduced excitability of PMd-M1 connections. The possible therapeutic effects of premotor rTMS may therefore involve indirect effects of PMd on SICI and RI, which this study has shown can be normalised by cTBS.

Therapeutic challenges in dystonia

Because dystonia can vary in clinical presentation and etiology, proper diagnosis and classification of these disorders are important in making therapeutic decisions. In primary dystonia, treatment is generally geared toward alleviating symptoms rather than curing the underlying condition, therefore severity of contractions, pain, and functional and social impact are also factors to consider in determining if and how to initiate therapy. On the other hand, if a secondary cause is identified, then it is often appropriate to direct treatment toward the underlying disorder. Treatment options include physical and occupational therapy, oral medications, botulinum toxin, and surgery. This article briefly reviews the clinical features, pathophysiology, and classification of dystonia before reviewing current therapeutic options.