The effect of cutaneous input on intracortical inhibition in focal task‐specific dystonia

In normal subjects short interval intracortical inhibition (SICI) is topographically modulated by cutaneous input, which may be important for focusing muscle activation during tasks. In patients with writer's cramp, a task‐specific focal dystonia characterized by inappropriate and excessive muscle activation of the upper limb during certain motor tasks, intracortical inhibition is reduced at rest and lacks the normal topographically‐specific modulation during motor tasks. In the present study we investigated whether cutaneous input modulated SICI in a group of patients with writer's cramp and a control group of subjects. Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles. Brief electrical stimuli were applied to either digit II or digit V with ring electrodes. SICI was investigated using a paired transcranial magnetic stimulation paradigm employing interstimulus intervals of 1–15 ms. Cutaneous input from both digit II and digit V modulated motor evoked potentials and SICI in a topographically‐specific manner in control subjects. In contrast, cutaneous input failed to modulate motor evoked potentials or SICI in the focal hand dystonia patients. These results provide further evidence of abnormal sensorimotor integration in focal hand dystonia. © 2007 Movement Disorder Society     

An fMRI study of musicians with focal dystonia during tapping tasks

Musician’s dystonia is a type of task specific dystonia for which the pathophysiology is not clear. In this study, we performed functional magnetic resonance imaging to investigate the motor-related brain activity associated with musician’s dystonia. We compared brain activities measured from subjects with focal hand dystonia and normal (control) musicians during right-hand, left-hand, and both-hands tapping tasks. We found activations in the thalamus and the basal ganglia during the tapping tasks in the control group but not in the dystonia group. For both groups, we detected significant activations in the contralateral sensorimotor areas, including the premotor area and cerebellum, during each tapping task. Moreover, direct comparison between the dystonia and control groups showed that the dystonia group had greater activity in the ipsilateral premotor area during the right-hand tapping task and less activity in the left cerebellum during the both-hands tapping task. Thus, the dystonic musicians showed irregular activation patterns in the motor-association system. We suggest that irregular neural activity patterns in dystonic subjects reflect dystonic neural malfunction and consequent compensatory activity to maintain appropriate voluntary movements.     

Corticospinal excitability in patients with secondary dystonia due to focal lesions of the basal ganglia and thalamus

ObjectiveTo investigate the possible correlations between clinico-radiological features and pathophysiological mechanisms in patients with dystonia secondary to focal brain lesions.MethodsSingle and paired-pulse transcranial magnetic stimulation was used to assess corticospinal excitability in 10 patients (4 females; mean age 61) and a group of normal controls. Active threshold, latency and amplitude of motor evoked potentials (MEPs), silent period (SP) duration and short-interval intracortical inhibition (SICI) were evaluated.ResultsPatients with lesions involving the putamen and caudate presented with dystonic postures at rest. TMS assessment in these subjects showed increased MEP amplitude on the affected side and a bilateral decrease of SP duration and SICI. When the lesion spared the putamen and caudate, mainly involving the thalamus, the clinical picture was dominated by slow repetitive involuntary movements and tremor. In the affected side of these subjects the MEP amplitude was reduced and the MEP threshold was increased.ConclusionsWhen putamen and caudate were lesioned, the patients presented with dystonic postures at rest; furthermore the patients showed changes of corticospinal excitability in comparison to both healthy subjects and other dystonic patients.SignificanceThere are correlations between type of dystonia, site of the lesion and neurophysiological findings.     

Bilateral impairment of intracortical inhibition in delayed-onset posthemiplegic dystonia: pathophysiological implications.

OBJECTIVE: To study short interval intracortical inhibition (SICI) in a rare patient with segmental dystonia of the left upper limb due to a vascular lesion in the contralateral putamen without corticospinal tract involvement. METHODS: Paired-pulse transcranial magnetic stimulation (TMS) was applied to both hemispheres in a conditioning-test paradigm. Six interstimulus intervals (ISIs) and 4 conditioning stimulation intensities were investigated in two separate sessions. RESULTS: Motor evoked potentials upon single-pulse TMS were within the normal range, whereas paired-pulse TMS revealed major changes in cortical excitability, proving that SICI was bilaterally absent. CONCLUSIONS: The bilateral impairment of SICI cannot be considered the cause of dystonic contractions, but just a predisposing factor. SIGNIFICANCE: The absence of SICI might be regarded as a condition able to promote maladaptive plastic changes triggered by focal lesions in the putamen.     

Electrophysiological features of myoclonus‐dystonia

Inherited myoclonus‐dystonia (M‐D) is an autosomal dominant disorder characterized by myoclonus and dystonia that often improves with alcohol. To examine the electrophysiologic characteristics of M‐D, we studied 6 patients from 4 different families and 9 age‐matched healthy subjects. Neurophysiological studies performed include electromyography (EMG)‐electroencephalography (EEG) polygraphy, jerk‐locked back‐averaged EEG, somatosensory evoked potentials (SEP), long‐latency reflex (LLR) to median and digital nerve stimulation, and transcranial magnetic stimulation studies with short‐interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long‐interval intracortical inhibition (LICI). All 6 patients showed myoclonus and dystonia on clinical examination and EMG testing. The EMG burst durations ranged from 30.4 to 750.6 milliseconds (mean, 101.5 milliseconds). Jerk‐locked back‐averaged EEG failed to reveal any preceding cortical correlates. Median nerve SEP revealed no giant potential. No patients had exaggerated LLR to median or digital nerve stimulation. There was no significant difference in SICI, ICF, and LICI between M‐D patients and normal subjects. Myoclonus in inherited M‐D is likely of subcortical origin. Normal intracortical inhibition and facilitation suggest that the GABAergic circuits in the motor cortex are largely intact and that the mechanisms of myoclonus and dystonia are different from those for cortical myoclonus and other dystonic disorders. © 2008 Movement Disorder Society     

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. © 2010 Movement Disorder Society     

Separate ipsilateral and contralateral corticospinal projections in congenital mirror movements: Neurophysiological evidence and significance for motor rehabilitation.

The neurophysiological hallmark of congenital mirror movements (MM) are fast-conducting corticospinal projections from the hand area of one primary motor cortex to both sides of the spinal cord. It is still unclear whether the abnormal ipsilateral projection originates through branching fibres from the normal contralateral projection or constitutes a separate ipsilateral projection. To clarify this question, we used focal paired-pulse transcranial magnetic stimulation to test task-related modulation of short-interval intracortical inhibition (SICI) in the abductor pollicis brevis (APB) muscles of a 15-year-old girl (Patient 1) and a 40-year-old woman (Patient 2) with congenital MM. In both patients, during intended unilateral APB contraction, SICI decreased markedly in the "task" APB but remained unchanged in the "mirror" APB when compared to muscle rest. In contrast, spinal excitability as tested with H reflexes increased similarly in the task and mirror flexor carpi radialis muscles. This dissociation of task-related SICI modulation strongly supports the existence of a separate ipsilateral fast-conducting corticospinal projection. In Patient 1, we tested the functional significance of this separate ipsilateral projection during 7 months of motor rehabilitation training, which was designed to facilitate unilateral finger movements. A marked reduction of MM was observed after training, suggesting that unwanted mirror activity in the ipsilateral pathway can be suppressed by learning.     

Abnormal reorganization in focal hand dystonia--sensory and motor training programs to retrain cortical function

Dystonia is a disabling movement disorder, which is characterized by an abnormal pattern of muscle activity with co-contraction of agonist and antagonist muscles. In the case of focal hand dystonia (FHD), these abnormal movements affect muscles of the forearm and hand while performing a specific task. Patients may initially present with dystonic symptoms occurring with a selective task (simple writer's cramp or musician's cramp), and may progress to develop symptoms with multiple tasks (dystonic writer's cramp). The underlying cause of this disabling condition remains unclear. This review examines recent studies designed to further elucidate the underlying pathophysiological processes in focal dystonia. Animal research work, and neurophysiological and neuroimaging studies in humans, have identified several possible mechanisms that may contribute to the underlying pathophysiology, including impaired sensorimotor integration, motor cortex activation and surround inhibition. Pharmacological treatment for dystonia is currently suboptimal. Based on these recent pathophysiological findings, several promising and novel non-pharmacological treatment modalities have recently been developed. Attempts at modulating impaired sensorimotor integration and cortical inhibition using sensorimotor retraining, and the range of sensory training techniques recently described, are further discussed in this review.     

Modulation of proprioceptive integration in the motor cortex shapes human motor learning

Sensory and motor systems interact closely during movement performance. Furthermore, proprioceptive feedback from ongoing movements provides an important input for successful learning of a new motor skill. Here, we show in humans that attention to proprioceptive input during a purely sensory task can influence subsequent learning of a novel motor task. We applied low-amplitude vibration to the abductor pollicis brevis (APB) muscle of eight healthy volunteers for 15 min while they discriminated either a small change in vibration frequency or the presence of a simultaneous weak cutaneous stimulus. Before and after the sensory attention tasks, we evaluated the following in separate experiments: (1) sensorimotor interaction in the motor cortex by testing the efficacy of proprioceptive input to reduce GABA(A)ergic intracortical inhibition using paired-pulse transcranial magnetic stimulation, and (2) how well the same subjects learned a ballistic thumb abduction task using the APB muscle. Performance of the vibration discrimination task increased the interaction of proprioceptive input with motor cortex excitability in the APB muscle, whereas performance in the cutaneous discrimination task had the opposite effect. There was a significant correlation between the integration of proprioceptive input in the motor cortex and the motor learning gain: increasing the integration of proprioceptive input from the APB increased the rate of motor learning and reduced performance variability, while decreasing proprioceptive integration had opposite effects. These findings suggest that the sensory attention tasks transiently change how proprioceptive input is integrated into the motor cortex and that these sensory changes drive subsequent learning behavior in the human motor cortex.     

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.     

Loss of topographic specificity of LTD-like plasticity is a trait marker in focal dystonia

In focal hand dystonia, long-term potentiation (LTP) and depression (LTD)-like neuronal plasticity, as assessed by paired associative stimulation (PAS) targeting the hand-associated motor cortex, is enhanced and the topographic organization of plasticity is lost. However, if any of these abnormalities alone is sufficient to cause focal dystonia (FD) remains unknown. Ten patients with cervical dystonia (CD), 9 with blepharospasm (BS) and 16 age- and sex-matched controls were examined. PAS was performed by combining repetitively electric stimulation of the median nerve with subsequent transcranial magnetic stimulation of the contralateral motor cortex at 21.5ms (PAS21.5) and 10ms (PAS10). Corticospinal excitability was indexed by the magnitude of motor evoked potentials (MEPs) recorded from abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. In controls, MEP size of the homotopically conditioned APB increased after PAS21.5 whereas the MEP size of the heterotopically conditioned ADM remained stable. PAS10 led to a decrease of MEP size of the APB and to an increase of the heterotopic ADM. In contrast, after PAS21.5 and PAS10 in CD and BS MEP size increased and decreased, respectively, in both muscles. The magnitude of excitability changes, however, did not differ between dystonic patients and healthy controls. In FD the topographic organization of PAS21.5 and PAS10-induced plasticity is deranged in cortical areas not involved in the control of the dystonic body part. Somatotopical disorganization of plasticity may represent an endophenotypic trait in FD but may not be sufficient to generate a dystonic phenotype. Development of a dystonic phenotype may require that the gain of plasticity is additionally enhanced. This article is part of a Special Issue entitled "Advances in dystonia".     

Difference in intracortical inhibition of the motor cortex between cortical myoclonus and focal hand dystonia.

OBJECTIVE: The short interval intracortical inhibition (SICI) of the motor cortex (M1) is reduced in both cortical myoclonus and focal hand dystonia. This reduction has been attributed to the dysfunction of GABAergic system within the motor cortex. However, the precise mechanisms underlying the reduction may not be entirely identical in these two disorders, being due to primary pathological involvement in M1 or secondary to functional changes outside M1. The aim of this study was to elucidate possible differences in intracortical inhibition between these two disorders. METHODS: Subjects were 11 patients with benign myoclonus epilepsy, 7 with focal hand dystonia, and 11 normal volunteers. We studied SICI using anterior-posterior (AP) directed and posterior-anterior (PA) directed induced currents in the brain. RESULTS: In both disorders, SICI with PA-directed currents was reduced as reported previously. In contrast, SICI studied with AP currents was normal in patients with focal hand dystonia, but reduced in patients with cortical myoclonus. CONCLUSIONS: The difference between the two disorders might reflect the underlying pathological difference. In cortical myoclonus, the inhibitory interneurons of the motor cortex are affected, whereas the same interneurons are intact in dystonia. The difference in SICI induced by AP and PA directed currents in dystonia may be explained by the following possibilities: the difference in composition of I-waves contributing to EMG generation and the difference in modulation of the interneuronal activity by voluntary contraction. These changes may be secondary to dysregulation of the motor cortex by the basal ganglia or related cortices in dystonia. SIGNIFICANCE: The SICI using AP directed currents together with the conventional SICI using PA directed currents was able to demonstrate some difference in the intrinsic circuits of M1 between myoclonus and focal hand dystonia. SICI using AP directed currents can provide additional information about the motor cortical excitability changes over those obtained by the previously reported methods.     

Does ulnar neuropathy predispose to focal dystonia?

We have observed a high incidence of ulnar neuropathy in musicians with dystonic flexion of the ipsilateral little and ring fingers. To investigate the relationship between ulnar neuropathy and focal dystonia, we compared the patterns of surface EMG activity in extensor digitorum communis (EDC4) and flexor digitorum superficialis (FDS4) during tapping of the ring finger in normal controls and patients with ulnar neuropathy or local dystonia. Ten of 10 normal subjects exhibited wellformed alternating EMG bursts in EDC4 and FDS4 separated by clear silent periods. Seven of 7 patients with dystonic flexion of the little and ring fingers showed loss of silent periods between poorly formed bursts in FDS or EDC. Surprisingly, 9 of 10 patients with ulnar neuropathy showed burst pattern abnormalities qualitatively similar to those observed in the dystonic patients. These data suggest that ulnar neuropathy alters the execution of a motor task involving multiple peripheral nerves. © 1995 John Wiley & Sons, Inc.     

Impaired modulation of motor cortex excitability by homonymous and heteronymous muscle afferents in focal hand dystonia.

A decrease of heteronymous median nerve-evoked inhibition of corticospinal projections to forearm extensor muscles was reported in a group of 10 dystonic patients by Bertolasi and colleagues in 2003. Here we tested the excitability of corticomotoneuronal connections to both wrist extensor (ECR) and flexor (FCR) muscles after conditioning stimulation of median and also radial nerve at rest in a group of 25 patients with focal hand dystonia compared to 20 healthy subjects. We also investigated the effect of the wrist dystonic posture, either in flexion or in extension, on the afferent modulation of ECR and FCR motor evolved potentials (MEPs). The heteronymous (median-induced) but also homonymous (radial-induced) inhibitions (interstimuli intervals 13-21 ms) of ECR MEP size observed in healthy subjects were decreased in patients. In addition, homonymous (median-induced) facilitation of FCR MEP size was also decreased in patients while heteronymous inhibition (radial-induced) was not. Neither the involvement of the target muscle in the dystonic posture nor the origin of the afferent volley (from a dystonic muscle) influenced the degree of impairment of afferent modulation of the MEP. These findings support the view that a global abnormal somatosensory coupling in focal hand dystonia may contribute to an inadequate motor command to wrist muscles.     

Functional repetitive transcranial magnetic stimulation increases motor cortex excitability in survivors of stroke

ObjectiveTo determine if repetitive transcranial magnetic stimulation (rTMS) applied to the motor cortex with simultaneous voluntary muscle activation, termed functional-rTMS, will promote greater neuronal excitability changes and neural plasticity than passive-rTMS in survivors of stroke.MethodsEighteen stroke survivors were randomized into functional-rTMS (EMG-triggered rTMS) or passive-rTMS (rTMS only; control) conditions. Measures of short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), force steadiness (coefficient of variation, CV) at 10% of maximum voluntary contraction, and pinch task muscle activity were assessed before and after rTMS. Functional-rTMS required subjects to exceed a muscle activation threshold to trigger each rTMS train; the passive-rTMS group received rTMS while relaxed.ResultsSignificant interactions (time × condition) were observed in abductor pollicis brevis (APB) SICI, APB ICF, CV of force, and APB muscle activity. Functional-rTMS decreased APB SICI (p < 0.05) and increased ICF (p < 0.05) after stimulation, whereas passive-rTMS decreased APB muscle activity (p < 0.01) and decreased CV of force (p < 0.05). No changes were observed in FDI measures (EMG, ICF, SICI).Conclusion(s)Functional-rTMS increased motor cortex excitability, i.e., less SICI and more ICF for the APB muscle. Passive stimulation significantly reduced APB muscle activity and improved steadiness.SignificanceFunctional-rTMS promoted greater excitability changes and selectively modulated agonist muscle activity.     

Eyes always attract attention but gaze orienting is task-dependent: evidence from eye movement monitoring

Eyes and gaze are central to social cognition but whether they attract attention differently depending on the task is unknown. Here, the shift in attention towards the eye region and gaze direction of a perceived face was studied in two tasks by monitoring eye movements. The same face stimuli in front- or 3/4-view, with direct or averted gaze, were used in both tasks. In the Gaze task, subjects performed an explicit gaze direction judgment (gaze straight or averted) while in the Head task they performed a head orientation judgment (front- or 3/4-view). Gaze processing was evident in both tasks as shown by longer RTs and lower accuracy when head and gaze directions did not match. In both tasks the eye region was the most attended area but the amount of viewing was task-dependent. Most importantly, approximately 90% of the initial saccades landed in the eye region in the Gaze task but only approximately 50% of them did so in the Head task. These saccades were made in the direction signaled by gaze in the Gaze task but in the direction signaled by head orientation in the Head task. Altogether, these task-modulated behaviors argue against a purely exogenous and automatic orienting-to-gaze mechanism. Based on patient work and neuroimaging studies of gaze processing, we suggest that this task-dependent orienting behavior is rather endogenous and subtended by cortical areas amongst which frontal regions play a central role. We discuss the implications of this finding for clinical populations.     

Effects of Two Weeks of Cerebellar Theta Burst Stimulation in Cervical Dystonia Patients

Dystonia is generally regarded as a disorder of the basal ganglia and their efferent connections to the thalamus and brainstem, but an important role of cerebellar-thalamo-cortical (CTC) circuits in the pathophysiology of dystonia has been invoked. Here in a sham controlled trial, we tested the effects of two-weeks of cerebellar continuous theta burst stimulation (cTBS) in a sample of cervical dystonia (CD) patients. Clinical evaluations were performed by administering the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) and the Burke-Fahn-Marsden Dystonia Rating Scale (BFMDRS). We used TMS to measure the inhibitory connectivity between the cerebellum and the contralateral motor cortex (cerebellar brain inhibition [CBI]), and the excitability of the contralateral primary motor cortex assessing intracortical inhibition (SICI), intracortical facilitation (ICF) and cortical silent period (CSP). Paired associative stimulation (PAS) was tested to evaluate the level and the topographical specificity of cortical plasticity, which is abnormally enhanced and non-focal in CD patients. Two weeks of cerebellar stimulation resulted in a small but significant clinical improvement as measured by the TWSTRS of approximately 15%. Cerebellar stimulation modified the CBI circuits and reduced the heterotopic PAS potentiation, leading to a normal pattern of topographic specific induced plasticity. These data provide novel evidence CTC circuits could be a potential target to partially control some dystonic symptoms in patients with cervical dystonia.     

Effect of motion imagery to counter rest-induced suppression of F-wave as a measure of anterior horn cell excitability

OBJECTIVE: To test if motor imagery prevents the rest-induced suppression of anterior horn cell excitability. METHODS: Ten healthy subjects underwent two separate experiments, each consisting of stimulating the median nerve 100 times and recording F-waves from abductor pollicis brevis (APB) in three consecutive sessions: (1) after muscle exercise to standardize the baseline, (2) after immobilization of APB for 3h and (3) after muscle exercise to check recovery. We instructed the subject to volitionally relax APB in experiment 1 (relaxation task), and to periodically simulate thumb abduction without actual movement in experiment 2 (imagery task). RESULTS: F-wave persistence and amplitude declined after relaxation task and recovered quickly after exercise, but changed little with imagery task. F-wave latencies showed no change when analyzed individually. The frequency distribution of collective F-waves recorded from all subjects remained the same after relaxation task, but showed a shift toward longer latencies after imagery task. CONCLUSIONS: Mental imagery without overt motor output suffices to counter the effect of sustained volitional muscle relaxation, which would, otherwise, cause a reversible reduction in anterior horn cell excitability. SIGNIFICANCE: This finding documents the importance of central drive for spinal excitability, which affects F-wave studies of a paretic muscle.     

Human brain mapping in dystonia reveals both endophenotypic traits and adaptive reorganization

Dystonia has a wide clinical spectrum from early-onset generalized to late-onset sporadic, task-specific forms. The genetic origin of the former has been clearly established. A critical role of repetitive skilled motor tasks has been put forward for the latter, while underlying vulnerability traits are still being searched for. Using magnetoencephalography, we looked for structural abnormalities reflecting a preexisting dysfunction. We studied finger representations of both hands in the primary sensory cortex, as compared in 23 patients with unilateral task-specific dystonia and 20 control subjects. A dramatic disorganization of the nondystonic hand representation was found in all patients, and its amount paralleled the severity of the dystonic limb motor impairment. Abnormalities were also observed in the cortex coding the dystonic limb representation, but they were important only in the most severely affected patients. The abnormal cortical finger representations from the nondystonic limb appear to be endophenotypic traits of dystonia. That finger representations from the dystonic limb were almost normal for the less severely affected patients may be due to intrinsic beneficial remapping in reaction against the primary disorder.     

Normal Motor Adaptation in Cervical Dystonia: A Fundamental Cerebellar Computation is Intact

The potential role of the cerebellum in the pathophysiology of dystonia has become a focus of recent research. However, direct evidence for a cerebellar contribution in humans with dystonia is difficult to obtain. We examined motor adaptation, a test of cerebellar function, in 20 subjects with primary cervical dystonia and an equal number of aged matched controls. Adaptation to both visuomotor (distorting visual feedback by 30°) and forcefield (applying a velocity-dependent force) conditions were tested. Our hypothesis was that cerebellar abnormalities observed in dystonia research would translate into deficits of cerebellar adaptation. We also examined the relationship between adaptation and dystonic head tremor as many primary tremor models implicate the cerebellothalamocortical network which is specifically tested by this motor paradigm. Rates of adaptation (learning) in cervical dystonia were identical to healthy controls in both visuomotor and forcefield tasks. Furthermore, the ability to adapt was not clearly related to clinical features of dystonic head tremor. We have shown that a key motor control function of the cerebellum is intact in the most common form of primary dystonia. These results have important implications for current anatomical models of the pathophysiology of dystonia. It is important to attempt to progress from general statements that implicate the cerebellum to a more specific evidence-based model. The role of the cerebellum in this enigmatic disease perhaps remains to be proven.