Abnormal cortical motor excitability in dystonia

Motor cortical hyperexcitability in idiopathic scoliosis: could focal dystonia be a subclinical etiological factor?

The aetiology of idiopathic scoliosis (IS) remains unknown; however, there is a growing body of evidence suggesting that the spine deformity could be the expression of a subclinical nervous system disorder. A defective sensory input or an anomalous sensorimotor integration may lead to an abnormal postural tone and therefore the development of a spine deformity. Inhibition of the motor cortico-cortical excitability is abnormal in dystonia. Therefore, the study of cortico-cortical inhibition may shed some insight into the dystonia hypothesis regarding the pathophysiology of IS. Paired pulse transcranial magnetic stimulation was used to study cortico-cortical inhibition and facilitation in nine adolescents with IS, five teenagers with congenital scoliosis (CS) and eight healthy age-matched controls. The effect of a previous conditioning stimulus (80% intensity of resting motor threshold) on the amplitude of the motor-evoked potential induced by the test stimulus (120% of resting motor threshold) was examined at various interstimulus intervals (ISIs) in both abductor pollicis brevis muscles. The results of healthy adolescents and those with CS showed a marked inhibitory effect of the conditioning stimulus on the response to the test stimulus at interstimulus intervals shorter than 6 ms. These findings do not differ from those reported for normal adults. However, children with IS revealed an abnormally reduced cortico-cortical inhibition at the short ISIs. Cortico-cortical inhibition was practically normal on the side of the scoliotic convexity while it was significantly reduced on the side of the scoliotic concavity. In conclusion, these findings support the hypothesis that a dystonic dysfunction underlies in IS. Asymmetrical cortical hyperexcitability may play an important role in the pathogenesis of IS and represents an objective neurophysiological finding that could be used clinically.

Cortical disinhibition in diabetic patients with neuropathic pain

OBJECTIVES

Motor cortex disinhibition has a role in the mechanism of neuropathic pain. The duration of the cortical silent period (CSP) is used as a measure of excitability in cortical inhibitory circuits. We investigated cortical disinhibition in diabetic patients with and without neuropathic pain.

MATERIALS AND METHODS

We studied diabetic patients with (n = 20) and without (n = 50) neuropathic pain, and control subjects (n = 30). Transcranial magnetic stimulation (TMS) was performed on the right hemisphere at rest, and surface electromyography was recorded from the left first dorsal interosseous muscle for evaluation of motor evoked potential (MEP) latency and amplitude. CSP was recorded from the left FDI, and TMS was then delivered while the subject was performing a voluntary contraction.

RESULTS

We showed a low resting motor threshold, a short CSP duration, and a low CSP duration/MEP amplitude ratio in patients with neuropathic pain (P < 0.0001, P < 0.0001, P < 0.0001).

CONCLUSIONS

Our findings demonstrate that diabetic patients with neuropathic pain have a cortical disinhibition.

The subthalamic nucleus in primary dystonia: single-unit discharge characteristics

Most models of dystonia pathophysiology predict alterations of activity in the basal ganglia thalamocortical motor circuit. The globus pallidus interna (GPi) shows bursting and oscillatory neuronal discharge in both human dystonia and in animal models, but it is not clear which intrinsic basal ganglia pathways are implicated in this abnormal output. The subthalamic nucleus (STN) receives prominent excitatory input directly from cortical areas implicated in dystonia pathogenesis and inhibitory input from the external globus pallidus. The goal of this study was to elucidate the role of the STN in dystonia by analyzing STN neuronal discharge in patients with idiopathic dystonia. Data were collected in awake patients undergoing microelectrode recording for implantation of STN deep brain stimulation electrodes. We recorded 62 STN neurons in 9 patients with primary dystonia. As a comparison group, we recorded 143 STN neurons in 20 patients with Parkinson's disease (PD). Single-unit activity was discriminated off-line by principal component analysis and evaluated with respect to discharge rate, bursting, and oscillatory activity. The mean STN discharge rate in dystonia patients was 26.3 Hz (SD 13.6), which was lower than that in the PD patients (35.6 Hz, SD 15.2), but higher than published values for subjects without basal ganglia dysfunction. Oscillatory activity was found in both disorders, with a higher proportion of units oscillating in the beta range in PD. Bursting discharge was a prominent feature of both dystonia and PD, whereas sensory receptive fields were expanded in PD compared with dystonia. The STN firing characteristics, in conjunction with those previously published for GPi, suggest that bursting and oscillatory discharge in basal ganglia output may be transmitted via pathways involving the STN and provide a pathophysiologic rationale for STN as a surgical target in dystonia.

The role of inhibition from the left dorsal premotor cortex in right-sided focal hand dystonia

BACKGROUND

The left dorsal premotor cortex (PMd) plays an important role in movement selection and is abnormally activated in imaging studies in patients with right-sided focal hand dystonia (FHD).

OBJECTIVE

The aims of this study were to assess the role of left PMd in patients with FHD and in the genesis of surround inhibition, which is deficient in FHD.

METHODS

Single- and paired-pulse transcranial magnetic stimulation (TMS) was applied during different phases of an index finger movement using the abductor pollicis brevis muscle (APB), a surrounding, nonsynergistic muscle, as target muscle. To look at the effect of PMd on the primary motor cortex (M1), a subthreshold conditioning pulse was applied to PMd 6 milliseconds before stimulation over M1.

RESULTS

There was surround inhibition during movement initiation in controls, but not in FHD patients. In contrast, FHD patients, but not controls, showed premotor-motor inhibition (PMI) at rest. During movement, PMI was absent in both groups.

CONCLUSIONS

We conclude that PMI does not appear to play a key role in the formation of surround inhibition in normal subjects, because it was not enhanced during movement initiation. However, in FHD, inhibition from PMd on M1 was abnormally increased at rest and declined during movement initiation. The behavior of PMd can therefore partly explain the loss of surround inhibition in the FHD patients. The functional significance of increased PMI at rest is not clear, but might be an attempt of compensation for losses of inhibition from other brain areas.

Surround inhibition depends on the force exerted and is abnormal in focal hand dystonia

There is evidence that surround inhibition (SI), a neural mechanism to enhance contrast between signals, may play a role in primary motor cortex during movement initiation, while it is deficient in patients with focal hand dystonia (FHD). To further characterize SI with respect to different force levels, single- and paired-pulse transcranial magnetic stimulation was applied at rest and during index finger movement to evoke potentials in the nonsynergistic, abductor policis muscle. In Experiment 1, in 19 healthy volunteers, SI was tested using single-pulse transcranial magnetic stimulation. Motor-evoked potentials at rest were compared with those during contraction using four different force levels [5, 10, 20, and 40% of maximum force (F(max))]. In Experiments 2 and 3, SI and short intracortical inhibition (SICI) were tested, respectively, in 16 patients with FHD and 20 age-matched controls for the 10% and 20% F(max) levels. SI was most pronounced for 10% F(max) and abolished for the 40% F(max) level in controls, whereas FHD patients had no SI at all. In contrast, a loss of SICI was observed in FHD patients, which was more pronounced for 10% F(max) than for 20% F(max). Our results suggest that SI is involved in the generation of fine finger movements with low-force levels. The greater loss of SICI for the 10% F(max) level in patients with FHD than for the 20% F(max) level indicates that this inhibitory mechanism is more abnormal at lower levels of force.

Differences in physical characteristics and response to rehabilitation for patients with hand dystonia: musicians' cramp compared to writers' cramp

STUDY DESIGN

Pre-Post, Mixed Factorial Trial.

INTRODUCTION

Focal hand dystonia is a challenging movement disorder to rehabilitate in musicians and writers.

PURPOSE OF THE STUDY

To compare the neuromusculoskeletal characteristics of those with writers' cramp (WC) and musicians' cramp (MC), and evaluate responsiveness to learning-based sensorimotor training.

METHODS

Twenty-seven individuals (14 musicians, 13 writers) participated in 8 weeks of supervised therapy supplemented with a home program. Between-group differences on measures of musculoskeletal (physical), sensory, and motor performance were evaluated at baseline and post-intervention.

RESULTS

Subjects with MC had a higher level of functional independence and better range of motion, but less strength in the affected upper limb than those of subjects with WC. Subjects with MC demonstrated greater accuracy on graphesthesia, kinesthesia, and localization at baseline. No between-group differences in motor performance were noted at baseline or post-intervention. Following individually adapted learning-based sensorimotor training, both groups improved in musculoskeletal (physical) parameters, sensory processing, and motor control; however, improvements on certain subtests differed by group. At follow-up, differences in posture, ROM, strength, graphesthesia, and kinesthesia persisted between the groups.

CONCLUSIONS

Subjects with WC have different physical and performance risk factors compared with those of subjects with MC. Intervention paradigms are efficacious, but variable responses to rehabilitation occur.

Inter-hemispheric inhibition is impaired in mirror dystonia

Surround inhibition, a neural mechanism relevant for skilled motor behavior, has been shown to be deficient in the affected primary motor cortex (M1) in patients with focal hand dystonia (FHD). Even in unilateral FHD, however, electrophysiological and neuroimaging studies have provided evidence for bilateral M1 abnormalities. Clinically, the presence of mirror dystonia, dystonic posturing when the opposite hand is moved, also suggests abnormal interhemispheric interaction. To assess whether a loss of inter-hemispheric inhibition (IHI) may contribute to the reduced surround inhibition, IHI towards the affected or dominant M1 was examined in 13 patients with FHD (seven patients with and six patients without mirror dystonia, all affected on the right hand) and 12 right-handed, age-matched healthy controls (CON group). IHI was tested at rest and during three different phases of a right index finger movement in a synergistic, as well as in a neighboring, relaxed muscle. There was a trend for a selective loss of IHI between the homologous surrounding muscles in the phase 50 ms before electromyogram onset in patients with FHD. Post hoc analysis revealed that this effect was due to a loss of IHI in the patients with FHD with mirror dystonia, while patients without mirror dystonia did not show any difference in IHI modulation compared with healthy controls. We conclude that mirror dystonia may be due to impaired IHI towards neighboring muscles before movement onset. However, IHI does not seem to play a major role in the general pathophysiology of FHD.

Neuroimaging characteristics of patients with focal hand dystonia

NARRATIVE REVIEW: Advances in structural and functional imaging have provided both scientists and clinicians with information about the neural mechanisms underlying focal hand dystonia (FHd), a motor disorder associated with aberrant posturing and patterns of muscle contraction specific to movements of the hand. Consistent with the hypothesis that FHd is the result of reorganization in cortical fields, studies in neuroimaging have confirmed alterations in the topography and response properties of somatosensory and motor areas of the brain. Noninvasive stimulation of these regions also demonstrates that FHd may be due to reductions in inhibition between competing sensory and motor representations. Compromises in neuroanatomical structure, such as white matter density and gray matter volume, have also been identified through neuroimaging methods. These advances in neuroimaging have provided clinicians with an expanded understanding of the changes in the brain that contribute to FHd. These findings should provide a foundation for the development of retraining paradigms focused on reversing overlapping sensory representations and interactions between brain regions in patients with FHd. Continued collaborations between health professionals who treat FHd and research scientists who examine the brain using neuroimaging tools are imperative for answering difficult questions about patients with specific movement disorders.

Short intracortical and surround inhibition are selectively reduced during movement initiation in focal hand dystonia

In patients with focal hand dystonia (FHD), pathological overflow activation occurs in muscles not involved in the movement. Surround inhibition is a neural mechanism that can sharpen desired movement by inhibiting unwanted movement in adjacent muscles. To further establish the phenomenon of surround inhibition and to determine whether short intracortical inhibition (SICI) reflecting inhibition from the local interneurons in primary motor cortex (M1), might play a role in its genesis, single- and paired-pulse transcranial magnetic stimulation (TMS), and Hoffmann reflex testing were applied to evaluate the excitability of the relaxed abductor pollicis brevis muscle (APB) at various intervals during a movement of the index finger in 16 patients with FHD and 20 controls. Whereas controls showed inhibition of APB motor-evoked potential (MEP) size during movement initiation and facilitation of APB MEP size during the maintenance phase, FHD patients did not modulate APB MEP size. In contrast, SICI remained constant in controls, but FHD patients showed reduced SICI during movement initiation. The H(max)/M(max) ratio in control subjects increased during movement initiation. The results provide additional evidence for the presence of surround inhibition in M1, where it occurs only during movement initiation, indicating that different mechanisms underlie movement initiation and maintenance. Thus, surround inhibition is sculpted both in time and space and may be an important neural mechanism during movement initiation to counteract increased spinal excitability. SICI may contribute to its generation, because in patients with FHD, the lack of depression of APB MEP size is accompanied by a reduction in SICI.

The pathophysiological basis of dystonias

Dystonias comprise a group of movement disorders that are characterized by involuntary movements and postures. Insight into the nature of neuronal dysfunction has been provided by the identification of genes responsible for primary dystonias, the characterization of animal models and functional evaluations and in vivo brain imaging of patients with dystonia. The data suggest that alterations in neuronal development and communication within the brain create a susceptible substratum for dystonia. Although there is no overt neurodegeneration in most forms of dystonia, there are functional and microstructural brain alterations. Dystonia offers a window into the mechanisms whereby subtle changes in neuronal function, particularly in sensorimotor circuits that are associated with motor learning and memory, can corrupt normal coordination and lead to a disabling motor disorder.

Abnormal cortical excitability with preserved brainstem and spinal reflexes in sialidosis type I

OBJECTIVE

To examine neurophysiological evidence of functional involvement of the brainstem and spinal cord and motor cortical excitability in sialidosis type I, a rare inherited neurodegenerative disorder caused by mutations in the NEU1 gene.

METHODS

We investigated particular pathways in the brainstem, spinal cord and motor cortex in 12 genetically proven cases of sialidosis type I by assessing blink reflex recovery cycle (BR), spinal reciprocal inhibition (RI), input-output curves (I/O), short interval intracortical inhibition (SICI), intracortical facilitation (ICF) and silent period (SP).

RESULTS

The BR and RI were normal. The slope of I/O was significantly increased, and SICI and the duration of SP were reduced in sialidosis patients.

CONCLUSIONS

Despite reports of pathology involving brainstem and anterior horn neurones, there were no obvious abnormalities in spinal and brainstem reflexes in the present patients, suggesting that the major clinical effects may be caused by changes at a level above the brainstem.

SIGNIFICANCE

For the first time, the integrity of certain brainstem and spinal cord reflexes in addition to motor cortical facilitatory and inhibitory circuits has been assessed in genetically proven type I sialidosis. This provides new data to aid in understanding of the pathophysiology of motor system dysfunction in this condition.

Cervical Dystonia

Cervical dystonia, which is the most common form of focal dystonia, presents with sustained neck spasms, abnormal head posture, head tremor, and pain. One of the interesting and unique features of cervical dystonia is the geste antagoniste. There is not a well-described pathophysiology for cervical dystonia, but several hypotheses report involvement at the central and peripheral level. Treatment options include: oral medical therapy, botulinum toxin injection, and surgery. Oral medical therapy has limited efficacy in control of the symptoms of cervical dystonia. Two types of botulinum toxin, types A and B, are being used for treatment of cervical dystonia, with equivalent benefit. Surgery is an option when other treatments fail or become ineffective. The surgical procedures are brain lesioning, brain stimulation, and peripheral surgical intervention. Several trials are currently ongoing in the United States and Europe to evaluate the efficacy of deep brain surgery in cervical dystonia.

Investigation of paroxysmal dystonia in a patient with multiple sclerosis: a transcranial magnetic stimulation study

OBJECTIVE

To study the pathogenesis of paroxysmal dystonia affecting the right body side in a patient with a demyelinating lesion in the descending motor pathways, also involving the basal ganglia.

METHODS

Single-pulse transcranial magnetic stimulation (TMS) was applied to study motor evoked potentials (MEPs) and the following silent periods (SPs) in the first dorsal interosseous muscle (FDI) of both sides and in the right extensor carpi radialis muscle (ECR) during voluntary contractions performed outside the dystonic attacks. During the dystonic paroxysms, single-pulse TMS was used to investigate the time course of MEPs and SPs in both FDI and ECR of the right side. Furthermore, paired-pulse TMS was applied at rest to investigate short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in both FDI muscles.

RESULTS

At rest SICI and ICF were normal in both motor cortices. During voluntary contraction the MEP was smaller and the SP was longer in the affected FDI than in the contralateral. During the paroxysms, the MEPs and SPs were suppressed in comparison with the responses elicited during voluntary contraction.

CONCLUSIONS

These results fit well with the theory of ephaptic excitement of corticospinal axons for the pathogenesis of paroxysmal dystonia due to a demyelinating lesion.

SIGNIFICANCE

Identification of the mechanisms underlying paroxysmal dystonia in demyelinating disorders extends our knowledge on the pathophysiology of dystonia.

Relief of primary cervical dystonia symptoms by low frequency transcranial magnetic stimulation of the premotor cortex: case report

OBJECTIVE: To evaluate the effect of low-frequency repetitive transcranial magnetic stimulation (rTMS) on the symptoms of a patient with primary segmental dystonia (PSD). METHOD: 1200 TMS pulses at a frequency of 1Hz, over the premotor cortex, with an intensity of 90% of the motor threshold (MT), using an eight-shaped coil; a total of 5 sessions were carried out. RESULTS: A reduction of 50 percent in the neck subset of the Burke, Fahn and Marsden torsion dystonia scale (BFM) was observed in our patient. CONCLUSION: The reduction in the BFM scale supports the concept that rTMS of the premotor cortex may reduce specific motor symptoms in PSD.

The physiological effects of pallidal deep brain stimulation in dystonia

Dystonia is an involuntary movement disorder characterized by muscle contractions causing abnormal postures and spasms, affecting part or all of the body. Dystonia may be primary where an abnormal gene, most commonly DYT1, may be identified, or secondary to structural brain lesions or heredodegenerative disorders. The underlying defect is believed to be abnormal basal ganglia modulation of cortical motor pathways, and various motor and sensory physiological abnormalities have been demonstrated. The failure of medical treatment in many patients with the more severe and generalized forms of dystonia has led to renewed interest in neurosurgical treatment approaches. In recent years, deep brain stimulation (DBS) of globus pallidus internus (GPi) has emerged as an effective treatment for dystonia, particularly patients with primary generalized dystonia, where remarkable improvement may occur. In contrast to Parkinson's disease, the beneficial effects of DBS in dystonia are not immediate but progressive over weeks to months. Physiological and imaging studies in dystonia patients with GPi DBS have demonstrated both short and long-term effects of GPi DBS on motor cortex and subcortical circuits including progressive normalization of spinal and brainstem excitability after GPi DBS which correlate with clinical improvement. These effects, in light of existing physiological data in dystonia, suggest that GPi DBS acts primarily by major modification of basal ganglia output to brainstem, thalamus, and cortex resulting in neural reorganization, which may explain the characteristic progressive improvement in dystonia after GPi DBS.

Intermanual Differences in movement-related interhemispheric inhibition

Interhemispheric inhibition (IHI) between motor cortical areas is thought to play a critical role in motor control and could influence manual dexterity. The purpose of this study was to investigate IHI preceding movements of the dominant and nondominant hands of healthy volunteers. Movement-related IHI was studied by means of a double-pulse transcranial magnetic stimulation protocol in right-handed individuals in a simple reaction time paradigm. IHI targeting the motor cortex contralateral (IHI(c)) and ipsilateral (IHI(i)) to each moving finger was determined. IHI(c) was comparable after the go signal, a long time preceding movement onset, in both hands. Closer to movement onset, IHI(c) reversed into facilitation for the right dominant hand but remained inhibitory for left nondominant hand movements. IHI(i) displayed a nearly constant inhibition with a trough early in the premovement period in both hands. In conclusion, our results unveil a more important modulation of interhemispheric interactions during generation of dominant than nondominant hand movements. This modulation essentially consisted of a shift from a balanced IHI at rest to an IHI predominantly directed toward the ipsilateral primary motor cortex at movement onset. Such a mechanism might release muscles from inhibition in the contralateral primary motor cortex while preventing the occurrence of the mirror activity in ipsilateral primary motor cortex and could therefore contribute to intermanual differences in dexterity.

Chronic epidural motor cortical stimulation for movement disorders

Recent data suggest that epidural chronic motor cortical stimulation could improve movement disorders. Because the procedure is safe, it might be a valuable therapeutic option. Although the therapeutic effects of cortical stimulation still need to be assessed in controlled studies, we discuss its rationale and the possible physiological mechanisms involved. There are several factors that support the use of chronic cortical stimulation in patients with movement disorders, including the strategic position of the motor cortex, the improvement induced in some motor disorders by cortical lesions, the functional imaging findings documenting widespread cortical dysfunction in movement disorders, and the improvement induced in patients with Parkinson's disease and dystonia by repetitive transcranial magnetic stimulation. Among the possible mechanisms of action of chronic motor cortex stimulation, besides modifications in the motor cortex itself, the most probable is that of eliciting distant bilateral changes through efferents and afferents that bilaterally connect the motor cortex with other cortical and subcortical structures.

Effects of transcutaneous electrical nerve stimulation on motor cortex excitability in writer's cramp: neurophysiological and clinical correlations

We recently reported that transcutaneous electrical nerve stimulation (TENS) applied over forearm flexor muscles, a paradigm producing in physiological conditions transient changes in corticomotoneuronal excitability of forearm muscles, may improve motor symptoms in writer's cramp (WC). In the present study, we explored the possibility that one or repeated sessions of TENS might have beneficial effects on handwriting in WC by remodulating cortical excitability of forearm agonist and antagonist muscles. Motor evoked potentials (MEPs) after transcranial magnetic stimulation of the left motor cortex were recorded from the right flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles, before and after 1 and 15 sessions of TENS applied over flexor muscles in patients and in a control group. One session of TENS induced a significant smaller reduction of MEPs from the FCR and a smaller increase of the MEPs from the ECR in patients than in normal subjects. In WC, repeated sessions of TENS had the effect to decrease MEP amplitude in the FCR and to increase it in the ECR. This modulation was paralleled by a handwriting improvement. In conclusion, repeated TENS sessions may have the effect to re-modulate excitability of the motor cortex in WC and this modulation might partially play a role in temporarily improving the handwriting.

Abnormality of motor cortex excitability in peripherally induced dystonia

It is widely accepted that peripheral trauma such as soft tissue injuries can trigger dystonia, although little is known about the underlying mechanism. Because peripheral injury only rarely appears to elicit dystonia, a predisposing vulnerability in cortical motor areas might play a role. Using single and paired-pulse pulse transcranial magnetic stimulation, we evaluated motor cortex excitability of a hand muscle in a patient with peripherally induced foot dystonia, in her brother with craniocervical dystonia, and in her unaffected sister, and compared their results to those from a group of normal subjects. In the patient with peripherally induced dystonia, we found a paradoxical intracortical facilitation at short interstimulus intervals of 3 and 5 milliseconds, at which regular intracortical inhibition (ICI) occurred in healthy subjects. These findings suggest that the foot dystonia may have been precipitated as the result of a preexisting abnormality of motor cortex excitability. Furthermore, the abnormality of ICI in her brother and sister indicates that altered motor excitability may be a hereditary predisposition. The study demonstrates that the paired-pulse technique is a useful tool to assess individual vulnerability, which can be particularly relevant when the causal association between trauma and dystonia is less evident.

Dystonia

Dystonia is a disorder of involuntary sustained muscle contractions. It is commonly classified by age of onset, distribution of involved body regions, and etiology. The pathophysiolgy of this condition is complex and imperfectly understood. This article reviews the epidemiology, genetics, clinical features, and approach to diagnosis and treatment of dystonia.

A systematic review on the diagnosis and treatment of primary (idiopathic) dystonia and dystonia plus syndromes: report of an EFNS/MDS-ES Task Force

To review the literature on primary dystonia and dystonia plus and to provide evidence-based recommendations. Primary dystonia and dystonia plus are chronic and often disabling conditions with a widespread spectrum mainly in young people. Computerized MEDLINE and EMBASE literature reviews (1966-1967 February 2005) were conducted. The Cochrane Library was searched for relevant citations. Diagnosis and classification of dystonia are highly relevant for providing appropriate management and prognostic information, and genetic counselling. Expert observation is suggested. DYT-1 gene testing in conjunction with genetic counselling is recommended for patients with primary dystonia with onset before age 30 years and in those with an affected relative with early onset. Positive genetic testing for dystonia (e.g. DYT-1) is not sufficient to make diagnosis of dystonia. Individuals with myoclonus should be tested for the epsilon-sarcoglycan gene (DYT-11). A levodopa trial is warranted in every patient with early onset dystonia without an alternative diagnosis. Brain imaging is not routinely required when there is a confident diagnosis of primary dystonia in adult patients, whereas it is necessary in the paediatric population. Botulinum toxin (BoNT) type A (or type B if there is resistance to type A) can be regarded as first line treatment for primary cranial (excluding oromandibular) or cervical dystonia and can be effective in writing dystonia. Actual evidence is lacking on direct comparison of the clinical efficacy and safety of BoNT-A vs. BoNT-B. Pallidal deep brain stimulation (DBS) is considered a good option, particularly for generalized or cervical dystonia, after medication or BoNT have failed to provide adequate improvement. Selective peripheral denervation is a safe procedure that is indicated exclusively in cervical dystonia. Intrathecal baclofen can be indicated in patients where secondary dystonia is combined with spasticity. The absolute and comparative efficacy and tolerability of drugs in dystonia, including anticholinergic and antidopaminergic drugs, is poorly documented and no evidence-based recommendations can be made to guide prescribing.

Botulinum toxin therapy of writer's cramp

The pathophysiology and management of writer's cramp is one of the most challenging amongst the various forms of focal dystonias. Frequently, the dystonic postures are confounded by compensatory muscle activity. Correct identification of target muscles for botulinum toxin (BT) injections determines the treatment success. The dosages of different preparations vary, with 1 unit of Botox roughly equalling 3.5 units of Dysport. Electromyographic guided injections yield better results and may also decrease the amount of toxin required. Weakness of target muscles interfering with other non-writing activities is a frequently encountered adverse effect. Studies have shown that BT is a safe long-term therapy option.

Bilateral mirror writing movements (mirror dystonia) in a patient with writer's cramp: Functional correlates

A recent prospective analysis on writer's cramp showed that up to 44.6% of patients in a series of 65 presented mirror dystonia, defined as involuntary movements of the resting hand, abnormal posture, tremor, and jerks occurring while writing with the opposite hand. A clinical case is presented, with videotape evidence of right-handed writer's cramp, with mirror movements elicited while writing using either hand. Functional magnetic resonance imaging studies are compared both to those of a normal patient and to those from a patient with writer's cramp but lacking mirror dystonia. Widespread bilateral activation of cortical motor areas contralateral to the mirror movements in patients with writer's cramp and mirror movements suggests, that bilateral activation of the primary motor cortex may account for the appearance of these mirror movements. Further studies need to be conducted to determine whether mirror movements in dystonic patients appear as a result of loss of intra- and/or interhemispheric cortical inhibition or are simply a consequence of the sustained effort these patients need to exert while writing using a dystonic hand.

The effect of repetitive transcranial magnetic stimulation on dystonia: a clinical and pathophysiological approach

Dystonia is characterized by sustained muscle contraction, which frequently causes repetitive, twisting movements or abnormal posture. The precise pathophysiological mechanisms of dystonia are still unknown. Several studies did demonstrate that, although motor cortex hyperexcitability appears to be responsible for abnormal co-contraction and overflow to adjacent muscles, plasticity mechanisms and integrative sensorimotor processing are also likely to be involved in this condition. Current dystonia treatments are based on oral medication, injection of botulinum toxin and, in a low proportion of cases, bi-pallidal deep brain stimulation. However, treatment outcome is generally disappointing. A few researchers have reported the application of repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex or the premotor cortex, with the goal of decreasing motor cortex hyperexcitability. This article reviews all studies using this technique in dystonia and discusses rTMS therapeutic impact and its possible mechanisms of action in this indication. Currently, the premotor cortex seems to be the best target for rTMS in dystonia. Rather than merely reducing the hyperexcitability of the primary motor cortex, this technique's clinical benefit seems to result from modifications in plasticity and restoration of sensorimotor integration. The corollary technique for chronic rTMS is electrical cortical stimulation. Even though this new therapeutic tool may have therapeutic promise, more studies are required to confirm it. In particular, we need to broaden our knowledge of rTMS impact on the various forms of dystonia and to optimize target localization.

Task-dependent modulation of silent period duration in focal hand dystonia

Focal hand dystonia (FHD) is a movement disorder that is also associated with impaired sensory function and sensorimotor integration. The aim of this study was to assess the modulation of inhibitory function in the motor cortex during the performance of four motor tasks (isometric pinch grip, writing, texture discrimination, and a phasic control task) in 8 FHD and 8 control subjects. The affected hands of the FHD subjects and the dominant hands of the control subjects were tested. Inhibitory function was assessed using transcranial magnetic stimulation to evoke a silent period in the ongoing electromyographic activity of the target muscle (first dorsal interosseous). There was no difference between FHD subjects and control subjects in silent period duration, which was significantly longer during the phasic texture discrimination and phasic control task than during the isometric pinch or writing. This finding suggests that the phasic nature of the task may increase cortical inhibitory function, rather than the sensory discrimination task itself. The accuracy of texture discrimination was significantly lower in FHD subjects than in control subjects. Sensory discrimination tasks do not appear to directly modulate the inhibitory processes responsible for the duration of the silent period.

Silent event-related fMRI reveals deficient motor and enhanced somatosensory activation in orofacial dystonia

Previous studies showed cortical dysfunction and impaired sensorimotor integration in primary generalized and focal hand dystonia. We used a whistling task and silent event-related fMRI to investigate functional changes in patients with blepharospasm and patients with a combination of blepharospasm and oromandibular dystonia (Meige's syndrome). Whistling served as a model for a skilful orofacial movement with a high demand on sensorimotor integration. It allowed us to study the oromandibular motor system that is clinically affected in Meige's syndrome but not in isolated blepharospasm. In Meige's syndrome, functional MRI revealed deficient activation of the primary motor and ventral premotor cortex within the mouth representation area during whistling. Compared with healthy controls, both forms of orofacial dystonia had increased activation of bilateral somatosensory areas and the caudal supplementary motor area (SMA) in common. While overactivity of somatosensory areas and caudal SMA in Meige patients was partly reversed by botulinum toxin treatment, impaired motor activation was not. We conclude that impaired motor activation appears to be specific for the clinically affected oromandibular motor system in Meige's syndrome while enhanced somatosensory activation is a common abnormality in both forms of orofacial dystonia independent of the affected motor system. Somatosensory overactivity indicates an altered somatosensory representation in orofacial dystonia while impaired motor activation may be a functional correlate of reduced cortical inhibition during oromandibular motor execution in Meige's syndrome.

Chronic administration of selective serotonin reuptake inhibitor (SSRI) paroxetine modulates human motor cortex excitability in healthy subjects

The aim of the study was to investigate the effect of chronic administration of paroxetine (selective serotonin reuptake inhibitor: SSRI) on motor cortex excitability in healthy subjects by means of transcranial magnetic stimulation (TMS), functional magnetic resonance imaging (fMRI) and behavioral motor tests. In a randomized, double-blind, crossover study, twenty-one right-handed subjects received 20 mg daily of either paroxetine or a placebo over a period of 30 days separated by a period of 3 months wash-out. The TMS study is presented here correlated with some results of the motor behavior study (finger tapping test) and the fMRI study (primary sensorimotor cortex (S1M1) volume of activation). TMS was used to test motor threshold (MT), motor evoked potential recruitment curve (RC), cortical silent period (CSP) and paired-pulse intracortical inhibition and facilitation (ICI, ICF). Chronic administration of paroxetine did not modulate ICI or CSP but induced a significant enhancement of mean ICF (ANOVA P=0.04), which significantly correlated with increase of speed in a finger tapping test (P=0.02). This suggests a modulation of cortical interneuronal excitatory pathways without changes in the excitability of cortical inhibitory GABAergic interneurons. A decrease of RC (ANOVA P=0.05) was also observed after 30 days intake of paroxetine in comparison with placebo and was associated with changes of fMRI activation intensity (left S1M1 hypoactivation, ), without changes of S1M1 activation volume. Finally, the different modulation of RC and ICF after chronic administration of paroxetine compared to single dose (opposite effects) emphasizes the different pharmacological action of the drug at cortical level depending on its acute or long-term administration.

Motor cortex excitability after thalamic infarction

OBJECTIVE

We examined 8 patients with hemihypesthesia due to an ischemic thalamic lesion to explore the effects of a central sensory dysfunction on motor cortex excitability.

METHODS

Motor excitability was assessed using transcranial magnetic stimulation techniques and electrical peripheral nerve stimulation. Motor function was evaluated by the Nine-Hole-Peg Test and measurement of hand grip strength. The affected side was compared with the non-lesioned side and with an age-matched control group.

RESULTS

Patients had a loss of inhibition and an increase of facilitation in the motor cortex of the affected side. The silent period was prolonged and motor function was impaired on the affected side.

CONCLUSIONS

A thalamic lesion can modulate motor cortical excitability.

SIGNIFICANCE

This study suggests that, under normal conditions, somatosensory afferents influence inhibitory and excitatory properties in the motor cortex.

Motor training as treatment in focal hand dystonia

Focal hand dystonia may arise as a result of aberrant plasticity from excessive repetitive use. Improvement might be possible with appropriate motor training. Focusing on trying to decrease abnormal overflow of movement to fingers not involved in a task, we developed a motor training program for individualized finger movements. Ten patients with writer's cramp participated in the motor training program. Evaluation was done with the Fahn dystonia scale, kinematic analysis of handwriting, transcranial magnetic stimulation (TMS), and electroencephalography (EEG). Clinical improvement of dystonia was significant using the Fahn dystonia scale, and 6 patients reported an improvement in writing. The handwriting analysis showed a trend for improvement after training in simple exercises. There were no changes in cortical excitability measured by TMS and EEG. Whereas this method of motor training for 4 weeks led to mild subjective improvement and some improvement in handwriting, it is not sufficient to reverse motor cortex abnormalities measured by TMS and EEG.

Central motor conduction in cervical dystonia with cervical spondylotic myelopathy

OBJECTIVES

It has been known that cervical dystonia develops secondarily to spinal cord injuries as secondary dystonia. However, little is known about the pathophysiological mechanism.

PATIENTS AND METHODS

We examined motor and sensory conduction in six patients with symptomatic cervical dystonia by transcranial magnetic stimulation (TMS). All of the patients exhibited unilateral head rotation. They had symptoms corresponding to cervical myelopathy and felt discomfort in the neck, shoulders or arms before involuntary movement occurred.

RESULTS

Although the overall central motor conduction time (CMCT) was not different from that of normal controls, contralateral CMCT was significantly delayed compared to ipsilateral CMCT (p<0.05). The results of somatosensory evoked potential study demonstrated that contralateral central conduction time (CCT) was not significantly different from ipsilateral CCT.

CONCLUSION

These findings indicate that there is a selective interference with the contralateral corticospinal tract in patients with symptomatic cervical dystonia.

Task-specific impairment of motor cortical excitation and inhibition in patients with writer's cramp

Abnormalities in motor cortical excitation and inhibition have been reported in patients with writer's cramp, at rest and during muscle activation. We were interested in whether such abnormalities might be task-specific and depended on the type of movement task used to activate the dystonic hand. We therefore assessed motor-evoked potentials (facilitation/rest MEP amplitude ratio) and duration of the cortical silent period (CSP) from the right first dorsal interosseus (FDI) muscle to transcranial magnetic stimulation (TMS) in 10 patients with writer's cramp and in 10 healthy volunteers performing pincer and power gripping tasks. The mean facilitation/rest MEP amplitude ratio measured during the pincer grip task was significantly larger in dystonic subjects than in controls, but in the power grip condition was similar in the two groups. The CSP measured in the power grip condition was of similar length in normal controls and dystonic subjects, but in the pincer grip condition was significantly shorter in patients than in controls. These results indicate a task-specific impairment of motor cortical excitation and inhibition in writer's cramp.

Primary writing tremor: motor cortex reorganisation and disinhibition

BACKGROUND

Primary writing tremor (PWT) is a task-specific tremor of uncertain origin. There has been debate as to whether PWT represents a variant of essential tremor or a tremulous form of focal dystonia related to writer's cramp. In writer's cramp there is evidence of changes in intracortical inhibition (ICI), as well as cortical motor reorganisation.

OBJECTIVE

To study corticomotor organisation and short-latency ICI in a patient with typical task-specific PWT.

METHODS

Transcranial magnetic stimulation mapping of the corticomotor representation of the hand and studies of ICI using paired-pulse stimulation were performed in a 47-year-old right-handed woman with a pure task-specific writing tremor.

RESULTS

The motor maps for the hand were displaced posteriorly on both sides and reverted to a normal position after treatment with botulinum toxin. Short-latency ICI was reduced for the dominant hand.

CONCLUSION

The findings indicate reorganisation and disinhibition of the corticomotor projection to the hand and point to the participation of cortical centres in the origin of PWT.

Stimulation du cortex moteur, Parkinson et dystonie : que nous enseigne la stimulation magnétique transcrânienne? revue de la littérature

INTRODUCTION

Over the last few years, deep brain stimulation techniques, with targets such as the subthalamic nucleus or the pallidum, have bee found to be beneficial in the treatment of Parkinson's disease and dystonia. Conversely, therapeutic strategies of cortical stimulation have not yet been validated in these diseases, although they are known to be associated with various cortical dysfunctions. Transcranial magnetic stimulation (TMS) is a valuable tool for non-invasive study of the role played by the motor cortex in the pathophysiology of movement disorders, in particular by assessing various cortical excitability determinants using single or paired pulse paradigms. In addition, repetitive TMS (rTMS) trains can be used to study the effects of transient activity changes of a targeted cortical area.

BACKGROUND

Studies with TMS revealed significant motor cortex excitability changes, particularly regarding intracortical inhibitory pathways, both in Parkinson's disease and in dystonia, and these changes can be distinguished owing to the resting state or to the phases of movement preparation or execution. However, more specific correlation between electrophysiological features and clinical symptoms remains to be established. In addition, the stimulation of various cortical targets by rTMS protocols applied at low or high frequencies have induced some clear clinical effects.

PERSPECTIVES

The TMS effects are and will remain applied in movement disorders to better understand the role played by the motor cortex, to assess various types of treatment and appraise the therapeutic potential of cortical stimulation.

CONCLUSION

TMS provides evidence for motor cortex dysfunction in Parkinson's disease or dystonia. Moreover, rTMS results have opened new perspectives for therapeutic strategies of implanted cortical stimulation. By these both aspects, TMS techniques show their usefulness in the assessment of movement disorders.

Patterns of EMG-EMG coherence in limb dystonia

Dystonia of the limbs may be due to a wide range of aetiologies and may cause major functional limitation. We investigated whether the previously described pathological 4 to 7 Hz drive to muscles in cervical dystonia is present in patients with aetiologically different types of dystonia of the upper and lower limbs. To this end, we studied 12 symptomatic and 4 asymptomatic carriers of the DYT1 gene, 6 patients with symptomatic dystonia due to focal basal ganglia lesions, and 11 patients with fixed dystonia, a condition assumed to be mostly psychogenic in aetiology. We evaluated EMG-EMG coherence in the tibialis anterior (TA) of these and 15 healthy control subjects. Ten of 12 (83%) of symptomatic DYT1 patients had an excessive 4 to 7 Hz common drive to TA, evident as an inflated coherence in this band. This drive also involved the gastrocnemius, leading to co-contracting electromyographic bursts. In contrast, asymptomatic DYT1 carriers, patients with symptomatic dystonia, patients with fixed dystonia, and healthy subjects showed no evidence of such a drive or any other distinguishing electrophysiological feature. Moreover, the pathological 4 to 7 Hz drive in symptomatic DYT1 patients was much less common in the upper limb, where it was only present in 2 of 6 (33%) patients with clinical involvement of the arms. We conclude that the nature of the abnormal drive to dystonic muscles may vary according to the muscles under consideration and, particularly, with aetiology.

Short latency afferent inhibition and facilitation in patients with writer's cramp

Patients with writer's cramp (WC) show abnormalities of sensorimotor integration possibly contributing to their motor deficit. We studied sensorimotor integration by determining short-latency afferent inhibition (SAI) in 12 WC patients and 10 age-matched healthy controls. A conditioning electrical median nerve stimulus was followed 14 to 36 msec later by transcranial magnetic stimulation of the contralateral primary motor cortex, and motor evoked potentials (MEP) were recorded from the relaxed or contracting abductor pollicis brevis muscle (APB). SAI was normal in WC but during APB relaxation SAI was followed by abnormal MEP facilitation, which was absent during APB contraction and in the controls. These findings suggest that somatosensory short-latency inhibitory input into the primary motor cortex is normal in WC, whereas a later excitatory input, which very likely reflects the long-latency reflex II, is exaggerated.

Disinhibition of the somatosensory cortex in cervical dystonia—decreased amplitudes of high-frequency oscillations

OBJECTIVE

To determine whether patients with cervical dystonia have electrophysiological signs of disinhibition in the somatosensory cortex by recording high-frequency oscillations (HFOs) in somatosensory evoked potentials (SEPs).

METHODS

HFOs were recorded in 13 patients and 10 age-matched control subjects, and the data were analyzed statistically by paired comparison and by Pearson's correlation.

RESULTS

In patients with cervical dystonia, the early part of HFOs showed a significant decrease in amplitude, and the amplitude ratios of both early and late parts of HFOs/N20 potential were also significantly decreased. The amplitudes of HFOs and N20 potential were linearly correlated in the control subjects but not in dystonia patients.

CONCLUSIONS

Patients with cervical dystonia may suffer from a disturbance of inhibition in the sensory cortex. This disturbance is reflected by decreased HFO amplitude, representing decreased activities of inhibitory interneurons in area 3b.

Modulation of human cervical premotoneurons during bilateral voluntary contraction of upper-limb muscles

A greater proportion of the voluntary command to the affected upper limb of well-recovered stroke patients descends via the putative C3/4 propriospinal system compared with the unaffected limb or the limbs of control subjects. We have examined whether there are differences in the pattern and extent of the modulation in intact subjects when the upper-limb muscles are bilaterally activated compared with unilateral activation. Motor evoked potentials (MEPs) recorded from flexor carpi radialis (FCR) in response to transcranial magnetic stimulation (TMS) were assessed for an effect of cocontraction with biceps brachii (BB) and subthreshold electrical conditioning of the musculocutaneous (MC) nerve. No between-hand differences were revealed, but there was less facilitation during bilateral than unilateral contraction. The findings suggest that the C3/4 propriospinal system is more active during unilateral muscle contraction and less active during bilateral contraction of upper-limb muscles. Therefore, bimanual rehabilitation strategies for patients with upper-limb hemiparesis should be developed with caution.

The effects of repetitive proprioceptive stimulation on corticomotor representation in intact and hemiplegic individuals

OBJECTIVE

To determine the effects of a passive wrist movement intervention on cortical representation of forearm musculature.

METHODS

Transcranial magnetic stimulation was used to map cortical representation of a forearm flexor muscle in healthy individuals and in individuals following stroke before and immediately after a 30 min session of passive wrist movement.

RESULTS

In the healthy individuals, no changes in map area or map centre of gravity were noted after the intervention; however, map volume increased significantly across all subjects. In the stroke patient group there were no significant changes in any parameters following the intervention.

CONCLUSIONS

It is speculated that the enlargement in map volume following the passive movement intervention arose through a heightened synaptic efficacy of the corticospinal pathway in response to the increase in afferent information. Short-term proprioceptive stimulation can induce alterations in corticomotor excitability in the target musculature.

SIGNIFICANCE

These findings provide a potential neural substrate to account for alterations in motor and sensory function in stroke patients in response to long-term passive movement interventions.

Basal ganglia activity remains elevated after movement in focal hand dystonia

Although previous studies of focal hand dystonia have detected cortical sensorimotor abnormalities, little is known about the role of the basal ganglia in this disorder. We report here that when focal hand dystonic patients performed finger-tapping tasks, functional magnetic resonance imaging showed persisting elevations of basal ganglia activity after the tasks ended. We posit that inhibitory control of the basal ganglia may be faulty in focal hand dystonia, and that the increases we observe in "resting" activity may mask basal ganglia abnormalities in standard imaging contrast analyses.