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The etiology of focal hand dystonia is still considered idiopathic, but there is agreement that the condition is multifactorial.
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Focal hand dystonia includes involuntary end-range twisting postures of the fingers and wrist due to co-contractions of agonists and antagonists. The onset of the problem may initially manifest as an abnormality in the quality of sound produced by a musical instrument, increasing errors in task performance, unusual fatigue or sense of weakness, or involuntary and/or involuntary excessive movement of one or more digits when performing a certain task.
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The objective of retraining patients with focal dystonia must be to restore normal differentiation of cortical topography of the hand, enhance the quality and accuracy of sensorimotor feedback, and enable efficient and effective performance of motor pathways for the hand.
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Treatment of focal hand dystonia should be interdisciplinary and comprehensive to include education, stress management, fitness, good biomechanics, appropriate ergonomics, medications (e.g., botulinum toxin), counseling, brain retraining, and sometimes brain stimulation.
Repetitive strain injuries, or cumulative trauma disorders, are commonly reported in individuals who perform jobs demanding repetitive fine-motor movements such as typing, playing a musical instrument, and writing. Some of these individuals resolve the problem by taking time off work, decreasing the time and intensity at the task, and modifying the ergonomics of the worksite and the task. When ergonomic issues cannot be resolved, chronic pain or unusual, disabling, painless involuntary movements of the hand may develop, referred to as focal hand dystonia (occupational hand cramps, musician’s cramp, keyboarder’s cramp, writer’s cramp, task-specific dystonia). Usually these symptoms develop as a result of an accumulation of intrinsic (e.g., genetics, neurophysiology, anatomic structure) and extrinsic (e.g., environment, situation, personality) factors. This chapter reviews the history, etiology, diagnosis, and treatment strategies of patients with focal hand dystonia. The aims of this chapter are to (l) increase the awareness of hand dystonia in the health care community; (2) encourage clinicians to participate in educational programs to prevent hand dystonia and other repetitive strain injuries; (3) enable clinicians to make an early diagnosis of hand dystonia; (4) prepare multidisciplinary clinicians to provide patient support and encouragement for recovery; (5) encourage physicians and therapists to develop creative, innovative, insightful, and effective learning-based retraining programs to maximize normal neural adaptation, quality of life, and independent function; and (6) challenge clinicians to participate in multidisciplinary clinical and translational research studies to better understand the etiology and improve the effectiveness of treatment for hand dystonia.
History
An ad hoc committee of the scientific advisory board of the Dystonia Medical Research Foundation agreed on the following definition of dystonia: Dystonia is a syndrome including sustained and repetitive muscle contractions that cause twisting, end-range abnormal postures. A dystonia that involves a particular body part (e.g., hand, neck, foot) is called a focal limb dystonia . When the limb dystonia occurs only during the performance of a target task, it is called task-specific or action dystonia . Fahn and colleagues added that action dystonia involves an involuntary posturing (dystonia) superimposed on a voluntary movement, with task-specific limb dystonias being a subset of the action dystonias. The majority of task-specific dystonias involve the upper limb. Invariably, the tasks affected usually require (1) highly repetitive movements; (2) extreme motor precision; (3) interplay between conscious or at least “feedback-related” modulation; and/or (4) a repetitively executed motor plan. Although not a painful disorder, task-specific focal hand dystonia can bring an abrupt halt to a promising occupational, professional, or performance career. Figure 135-1 summarizes typical patterns of dystonia in musicians.
Limb dystonia can be divided into three categories: simple cramp (movements abnormal in relation to a single task), dystonic cramp (movements abnormal in relation to more than one task), and progressive cramp (abnormal movement that begins in relation to a single task and later becomes abnormal in other tasks). The limb and action dystonias can be further classified by occupation (e.g., musician’s dystonia, golfer’s “yip”) or by the task performed (e.g., writer’s cramp, keyboarder’s cramp).
In 1833, Bell was the first to refer to hand dystonia as scrivener’s palsy . In 1861, Duchenne described a similar disability in French workers. As we moved into the 20th century, hand dystonia became a topic of growing interest among employers, employees, neurologists, educators, physical therapists, hand therapists, and behavioral scientists. It is estimated that some type of repetitive motion disorder of the upper limb may develop in 30% of musicians and aggressive computer users/data entry/programmers, but it is not clear how many of these individuals will develop a focal hand dystonia. Focal dystonia appears to be most common in males. Initially, writer’s cramp was considered the most common type of dystonia; however, quality epidemiologic studies are needed to clarify the incidence and prevalence as well as the etiologic intrinsic and extrinsic risk factors for the development of focal hand dystonia.
Etiology of Focal Hand Dystonia
In the past 10 years, basic and clinical research as well as the integration of sophisticated genetic, neuroimaging, electrophysiologic, and psychophysical analyses have provided increased insight into the etiology of focal dystonia. Although the etiology of hand dystonia is still considered idiopathic, there is some consensus that the problem is multifactorial. It appears that intrinsic factors interact with extrinsic factors to produce the clinical phenotype of focal hand dystonia. For some individuals, intrinsic factors may be the strongest contributor (e.g., genetics, neurophysiologic dysfunction, anatomic restrictions, sensory deficits, aberrant homeostatic plasticity), whereas for others, extrinsic factors such as excessive repetition, trauma, or behavioral characteristics may drive the development of the phenotype.
Endophenotypic Factors
Genetics
One common intrinsic factor in dystonia is genetics. Some are known familial genes commonly characterizing primary general dystonia, but new genes for focal dystonia are being discovered. Patients with a genetic etiology of dystonia reportedly have a receptor abnormality for dopamine binding in the putamen. This genetic anomaly could decrease the dopamine normally secreted in rewarded, repetitive, associative, and nonassociative learning behaviors. Interestingly, despite a unilateral presentation of hand dystonia, abnormal neurophysiologic measures are reported in both cerebral hemispheres, reinforcing the possibility of a genetic etiology. In addition, approximately 30% of individuals with focal hand dystonia have a distant relative with dystonia. To date, no gene has been identified specifically for focal hand dystonia.
In Germany, a gene was identified in a family in whom many members had a cervical dystonia. All those with the cervical dystonia had the gene, but cervical dystonia did not develop in all family members with the gene. None of the family members reported a focal hand dystonia. This type of gene is classified as one with low penetrance. Another gene for general dystonia, the DYTI gene in Ashkenazi Jewish families, has also been studied. Although those with generalized dystonia have the gene, in some family members with the gene, dystonia does not develop, and only a hand dystonia may develop in others.
Basal Ganglia–Thalamocortical Dysfunction
Some researchers report specific dysfunction in the basal ganglia – thalamocortical motor circuit in patients with dystonia. Within the basal ganglia, the striatum receives major input from the motor cortex with the output modems including the globus pallidus (internal segment), substantia nigra, and pars reticulata. Direct and indirect pathways connect to the input and output nuclei. Except for one excitatory projection, all pathways interconnecting the basal ganglia nuclei are inhibitory. Dystonia could result from either excessive activity in the direct striatum–globus pallidus (internal segment) pathway or from reduced activity in the indirect striatum–globus pallidus (external segment) pathway. Focal dystonia is sometimes considered a prototype of a hyperkinetic disorder resulting from an imbalance of excitation and inhibition in the globus pallidus.
Hypoactivity of inhibitory pathways could also result from reduced basal ganglia output and/or reduced activity in the motor thalamus. Interestingly, the putamen and the pallidum show evidence of hypermetabolism in patients with general dystonia as well as some patients with hand dystonia. This may explain why a pallidotomy can reduce the severity of symptoms in patients with generalized dystonia.
Inadequate Inhibition
Lack of inhibition at multiple levels of the nervous system seems to be a fundamental problem in the genesis of dystonia. The nervous system requires a balance between excitation and inhibition of neural circuits to facilitate smooth, coordinated motor control. A variety of forms of inhibition are used to control the precision and smoothness of movement, particularly in the hand, where individual finger movements require selective and specific activation of muscles.
Reciprocal inhibition allows for control of muscles around a single joint. Lack of reciprocal inhibition at the spinal and peripheral levels leads to co-contractions of antagonistic muscles, which characterize patients with hand dystonia. Moving the intended fingers is associated with the firing of unnecessary adjacent fingers. Using electromyographic (EMG) technology, muscle overactivity has been documented with inappropriate co-contractions, prolongation of EMG bursts into muscle groups outside the intended movement, abnormal H reflexes, and abnormal long latency reflexes. These signs could also be viewed as a reflection of a loss of reflex inhibition, which is consistent with the deterioration of poor, fine, graduated movements observed in patients with hand dystonia.
Using transcranial magnetic stimulation (TMS), abnormal cortical inhibition has also been demonstrated bilaterally even in patients with unilateral focal hand dystonia. Both inhibitory interneuronal activity and surround inhibition have been reported abnormal in patients with focal hand dystonia. Surround inhibition allows selective control of individual muscles by simultaneous inhibition of surrounding muscles. The indirect pathway of the globus pallidus also plays a major role in surround inhibition . Dysfunction of inhibitory interneurons that use γ-aminobutyric acid as a neurotransmitter could further mediate the abnormal surround inhibition at the cortical level.
Anatomic Musculoskeletal Limitations
Some people have tight joints, muscles, and fascia and restrictions in the retinaculum in the hand. Restrictions in end-range finger spread, forearm rotation, and shoulder rotation have been documented in patients with focal hand dystonia. It is possible that anatomic restrictions put a patient at risk for the development of a hand dystonia, particularly under conditions of stressful, highly repetitious practice.
Leijinse and Hallett proposed a musculoskeletal etiologic model of focal hand dystonia in which defects in the musculoskeletal system combined with environmental factors such as overuse may accumulate in a focal hand dystonia. In this model, the assumption is that the acquisition of instrumental techniques requires high levels of physiologic movements under high performance demands with ergonomic limitations of the instrument and the individual. This model predicts that the development of focal dystonia is preceded by gradual changes in playing technique over a long period of time.
Based on this musculoskeletal hypothesis, ineffective or physiologically infeasible playing movements must be modified by voluntary (teaching) or involuntary (systemic) feedback. If the movement modification process does not converge to using muscle synergies that satisfy all constraints, movement modifications will continue until overcompensated muscle synergies are produced. Antagonists of the intended movements are recruited, and dystonic symptoms develop. Based on this model, musculoskeletal limitations should be addressed in the individual hand to resolve peripheral conflicts between constraints and tasks. These musculoskeletal limitations should be addressed before neurologic retraining.
In a post-training anatomic study of nonhuman primates with behaviorally induced focal hand dystonia, one monkey had an anatomic defect of the profundus tendon, with adhesions on the middle and distal segments of the fourth digit on the trained side and of the third digit on the untrained side. On the side trained at a highly repetitive, attended, stereotypical task, movement dysfunction developed in 5 weeks, significantly earlier than in the other monkeys. The somatosensory representation of the hand was degraded, particularly the receptive fields for D4. However, on the untrained side, there were no signs of involuntary motor control or somatosensory degradation of the third digit. If biomechanical demands on the hands are not stressful, movement dysfunction might not develop, even when there are anatomic restrictions. These findings are consistent with a multifactorial theory of origin for focal hand dystonia.
Abnormal Motor Preparation
A striking characteristic of focal hand dystonia is its task specificity. Initially, symptoms are manifested only when patients are performing a specific task (e.g., writing, playing an instrument, using the keyboard). This specificity can extend to certain passages and not all aspects of playing an instrument. This interesting task specificity was initially thought to be psychiatric in origin.
Some researchers report a deficiency in the preparation and/or organization of established motor programs in patients with hand dystonia, not simply a deficiency during movement initiation. Using neuroimaging techniques, underactivity of motor areas have been reported during writing in patients with writer’s cramp. In further support for this hypothesis of a defective motor program, Stinear and Byblow observed defective intracortical inhibition only during voluntary movements, never at rest. In patients with dystonia, there are also some differences in set shifting compared with controls. Further, some researchers have reported abnormal sequential learning in patients with DYTI dystonia. We know even less about neurophysiologic processing differences between healthy controls and patients with hand dystonia during the acquisition of movement control.
Sensory Abnormalities
Although focal hand dystonia is characterized as a movement disorder, there is significant evidence of dysfunction in sensory processing in patients with focal hand dystonia. Cortical somatosensory receptive fields are abnormally enlarged and disorganized in patients with hand dystonia. In patients with unilateral focal hand dystonia, bilateral difficulty with discriminating sensory stimuli has been measured in both the spatial and temporal domains. These perceptual abnormalities have also been reported in the hands of patients with blepharospasm and those with cervical dystonia.
Sensory processing also may play a modulatory role in dystonia. For example, a sensory trick such as touching, holding, or taping can quiet the dystonic symptoms in an involved digit. In addition, tonic vibration can lead to a worsening of dystonia, whereas anesthetic blocks can relieve symptoms. In terms of intervention, sensory retraining in the form of tactile discrimination practice including reading Braille, stereotactic matching tasks, and interpreting information drawn on the skin can help ameliorate motor symptoms.
There is also evidence of abnormal sensorimotor integration in patients with focal hand dystonia. The modulation of sensory processing in response to movement (referred to as sensory gating) is reported as abnormal in patients with focal hand dystonia. This could contribute to problems of motor control as seen in dystonia. However, Nowak and colleagues evaluated the potential generalized impairment of sensorimotor integration in patients with writer’s cramp or musician’s cramp. These researchers measured grip force behavior. In this study, the researchers looked at adapting grip force when lifting a new object or adjusting grip force in anticipating or reacting to a change in load force when catching a weight. Interestingly, patients with focal dystonia and normal controls showed similar predictive grip force adjustments to expected changes in object load.
Patients with dystonia produced a grip force overshoot during the initial lifts. Patients with dystonia also had a shorter latency of grip force response than controls after an unexpected load increase. These researchers suggested those with dystonia had a greater level of preparatory motor activity or a disinhibited spinal reflex response. The researchers suggested increased grip force was likely to be a prelearned phenomenon rather than a primary disorder of sensorimotor integration.
Unfortunately, there are no studies that document neurosensory processing competencies or abnormalities in patients before the onset of hand dystonia. Thus, it is unclear whether somatosensory and sensorimotor dysfunction predisposes a patient to the development of dystonia under conditions of highly stressful repetitive hand use or whether repetitive, nearly simultaneous overuse of the hands can degrade the somatosensory hand representation and lead to involuntary dystonic movements. In primate and rat animal studies, repetitive overuse can be associated with peripheral and central inflammatory responses as well as changes in somatosensory, sensorimotor, and motor cortices bilaterally, even when the training involves only one side and the dystonia is documented only on one side.
Sensory abnormalities may drive dystonic movements. However, directed sensory training can relieve the dystonia. It is not clear how the sensory abnormalities alone can directly lead to the motor manifestations of dystonia. Although this discussion is still under debate at this time, more and more evidence suggests that there are both sensory and motor abnormalities in patients with hand dystonia.
Maladaptive Homeostatic Plasticity
The central nervous system is plastic. New motor skills are acquired throughout one’s lifetime. With sensory, motor, and mental learning, the plasticity of the nervous system changes circuitry to accommodate new skill development. These changes in synaptic connections and circuitry occur with maturation, but can also be purposely driven by learning-based training activities in a dynamic environment. However, plasticity is not infinite. Further, although plasticity is usually controlled by homeostatic mechanisms, it seems that plasticity can potentially be excessive, leading to loss of control and destabilization. In patients with focal dystonia, there is some evidence that these homeostatic mechanisms may be abnormal.
It was the hypothesis of Quartarone and colleagues that patients with dystonia had an impairment in the ability to keep cortical excitability within a normal physiologic range. Usually, anodal stimulation enhances the inhibitory effect of transmagnetic stimulation (TMS) in terms of corticospinal excitability and cathodal stimulation reverses the aftereffects of TMS, producing an increase in corticospinal excitability. In patients with writer’s cramp, after preconditioning with transcranial direct current stimulation (tDCS), there were no consistent changes in corticospinal excitability after TMS stimulation. Quartarone and colleagues interpreted these findings to mean that the homeostatic mechanisms stabilizing excitability levels are impaired in patients with writer’s cramp. Thus, there is excessive corticospinal excitability.
Another interpretation of the aberrant plasticity findings is that patients with focal dystonia have an exceptionally adaptive nervous system. Neural changes can exceed the neural operating limits. This abnormally enhanced plasticity could explain the abnormal organization of the sensory, sensorimotor, and motor maps and loss of motor control of the hand in patients with focal hand dystonia after repetitive hand use.
Aberrant plasticity may also be generated by an environmental trigger. For example, practice and repetition usually lead to improved performance. Positive feedback and behavioral rewards increase acetylcholine, dopamine, and other modulatory neurotransmitters to generate continual positive neural adaptation. However, because of competition between neuronal pathways, neural adaptation is not infinite. When something changes in the practice cycle (e.g., modification of the equipment or technique, task complexity, time on task), there may be a marked turn in behavior. Now, increased investment of time leads to deterioration rather than to improvement in performance. As the operative movements become faster, the temporal inputs become nearly simultaneous, losing their individual differentiation. At some point, stereotypic repetition can lead to a degradation in the response. The more the practice, the more the fatigue, and ultimately the patient begins to sense some emergent incoordination. Finally, the fingers seem to develop a life of their own, uncontrollably curling when performing the target task.
Cortical networks engage both excitatory and inhibitory neurons by strong input perturbation. Within a given cortical area, cortical pyramidal cells cannot be effectively re-excited by another perturbation for tens to hundreds of milliseconds. These integration times are dictated primarily by the time for recovery from inhibition, which dominates poststimulus excitability. The cortex continues to define its representation of the temporal aspects of behaviorally important inputs by generating more synchronous representations of sequenced input perturbations or events. These time constants both govern and limit the ability to “chunk” (i.e., to separately represent by distributed coordinated discharge) successive events within its processing channels.
Researchers such as Bara-Jimenez and colleagues propose that the abnormal reorganization of the cortex in patients with focal hand dystonia may not be based on rigidly learned highly repetitive movements. Rather, a congenital or remote acquired abnormal cortical deficit may explain the abnormal reorganization. Experience-based reshaping of cortical representation can be mediated by dynamic plastic operations of the brain, with subject–environment interactions affecting the organization of the somatosensory cortex.
Exophenotypic Factors
Repetitive Use
The common association between highly skilled manual performance and the development of focal hand dystonia is suggestive of an environmental contribution to focal dystonia, such as repetitive use. The theory was that while practice and repetition should normally have a positive effect on learning, it is possible that the repetition could be excessive or even associated with some peripheral damage and have a negative effect on the nervous system. A primate and a rat model of repetition suggest that excessive repetition can lead to focal hand dystonia. In one case, monkeys were trained to repetitively perform a complex manual task, and symptoms similar to those of dystonia developed in some. This led to the proposed learning-based sensorimotor learning hypothesis. Somatosensory cortical mapping of the primate hand representation was disorganized, similar to findings in patients with focal hand dystonia. The hypothesis of aberrant sensorimotor learning as one etiologic factor for focal hand dystonia has been confirmed with electrical and magnetic neuroimaging techniques.
Goal-directed, repetitive movements are known to drive measurable change in structure, neuroenzymes, myelination, and function. Selective spatial and spectral cell assemblies have sharp segregation and result in more complex, efficient behaviors. These event-by-event complex signal representations are highly plastic. This positive adaptive learning can also be measured as an increase in the size of the cortical representation; smaller receptive fields; increased efficiency, amplitude, and density of evoked responses; and distinct, orderly representation. (See Box 135-1 for a summary of the principles of neuroplasticity.) Because fine-motor control of complex and simple finger movements requires accurate feedforward and feedback information from the primary sensory cortex and related pathways, the focal hand dystonia that develops in individuals involved in high levels of repetitive fine-motor work could represent a type of negative integrative neural adaptation.
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With learning, the distributed cortical representations of inputs and brain actions specialize in their representations of behaviorally important inputs and actions for skill learning.
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There are important behavioral conditions that must be met in the learning phase of plasticity:
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When behaviorally important stimuli repeatedly excite cortical neuron populations, the neurons will progressively grow in number.
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Repetitive, behaviorally important stimuli processed in skill learning lead to progressively greater specificity in spectral (spatial) and temporal dimensions.
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A growing number of selectively responding neurons discharge with progressively stronger temporal coordination (distributed synchronicity).
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With learning, the selection of behaviorally important inputs is a product of strengthening input-coincidence-based connections (synapses).
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Plasticity is constrained by the following:
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Anatomic sources and convergent-divergent spreads of inputs
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The time constants governing coincident input co-selection
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The time structures and potentially achievable coherences of extrinsic and intrinsic cortical input sources
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Top-down influences
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Cortical field-specific differences in input sources, distributions, and time-structured inputs create different representational structures.
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Temporal dimensions of behaviorally important inputs influence representational specialization.
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The integration time (processing time) in the cortex is itself subject to powerful learning-based plasticity.
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Plasticity processes are competitive.
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Learning is modulated as a function of behavioral state.
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The scale of plasticity in progressive skill learning is massive.
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Enduring cortical plasticity changes appear to be accounted for by local changes in neural anatomy.
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Cortical plasticity processes in child development represent progressive, multiple-stage skill learning.
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Although the observed problem is a disorder of movement, the initial electrophysiologic and magnetic resonance imaging studies highlighted the changes in the somatosensory cortex. Byl and colleagues reported significant dedifferentiation of the sensory representation of the hand in nonhuman primates in which motor dysfunction resulted from highly repetitive, stereotypical digital opening and closing movements of the hand. In these primates, the receptive fields increased in size, with the evoked neural response engaging a broadened neuronal network across adjacent digits and across dorsal and glabrous surfaces. This degradation in representation disrupted the accuracy of the feedback control and ultimately interfered with accurate and specific motor control (sensorimotor learning hypothesis) ( Figs. 135-2 and 135-3 ). More recently, in human studies, using magnetoencephalography, these same researchers not only confirmed the degradation in topography, but also documented problems in timing and spatial processing in both the somatosensory (S1 and S2), premotor, and motor cortices in both the ipsilateral and contralateral hemispheres of the affected and unaffected digits in patients with dystonia.
In comprehensive animal models of dystonia research, there is evidence of both peripheral and central changes occurring after repetitive overuse of the hand. For example, using a learning-based rat model to study peripheral inflammation associated with repetition under different force conditions, Barbe and colleagues, Elliot and colleagues, and Coq and colleagues documented a progressive model of dysfunction post-repetition. This dysfunction began by a cascade of peripheral inflammation that crossed over midline and then centralized. In a series of experiments, local inflammation was initially observed in the heavily used extremity. Then the inflammatory signs spread to the opposite side and then spread centrally. When repetitive movements continued, a noticeable, inefficient movement dysfunction in feeding was observed in the reaching hand. Clinically, the rat lost dexterity and a scooping strategy was noted.
Under blinded conditions, aberrant cortical changes were measured in the somatosensory, sensorimotor, and motor cortices in the rats that developed the inefficient hand strategy of scooping for food. Other researchers have also shown a change in somatosensory cortical topography after repetitive motor movements. As in the primate studies, the somatosensory and sensorimotor cortices lost accurate differentiation of the digits, and the motor cortex became excessively sensitive. A low current excessively excited the neurons, and now firing of agonists and antagonists was seen across the wrist and the digits.
Sanger and Merzenich translated the learning-based sensorimotor hypothesis into a computational model to explain the changes in topography and the abnormal motor output in individuals in whom focal hand dystonia develops ( Fig. 135-4 ). This model explains several features of focal dystonia: (1) symptoms develop in otherwise healthy individuals in response to highly attended repetitive movements, (2) evolution of symptoms is variable in terms of time, (3) symptoms appear only during the performance of a target-specific task, (4) dystonic movements persist despite stopping the task, (5) symptoms can be decreased but not remediated with dopamine-depleting drugs or botulinum toxin, and (6) evidence of abnormalities in motor and sensory cortical representations of the dystonic limb.
In this model, gain can be increased by expanding or even shrinking the sensory cortical representation of a limb as a result of adaptation to repetitive use or specific increased, simultaneous firing; coupling of multiple sensory signals; and voluntary coactivation of muscles. The loop through the deep nuclei, including the cortex, basal ganglia, and thalamus, combined with the sensorimotor loop gain, contributes to instability. If only certain mechanical models of the sensorimotor loop are unstable, a focal dystonia, rather than a generalized dystonia, can develop.
Based on this computational model of focal hand dystonia, treatment must decrease the imbalance in the loop gain. A permanent solution requires a redifferentiation of cortical and subcortical representations to release excessive gain. Retraining may not be possible in the context of severe dystonia without temporarily breaking the cycle. This might include botulinum toxin injections to stop the abnormal muscle firing at the periphery. Another approach would be to increase the variability of practiced movements so that there are many uncorrelated movement components, each with only a few relevant sensory neurons. This is comparable with behaviors uncoupling the pathologically coupled modes.
Trauma
Focal dystonia can develop after an insult such as trauma, disease, vascular insufficiency, or anatomic restriction. For example, dystonic movements have been reported in patients after a fall on an outstretched arm, a cervical neck injury or a degenerative condition with radiculopathy, a neurovascular entrapment in the thoracic outlet, or an entrapped ulnar nerve at the elbow. In some of these cases, surgical intervention has resolved the dystonia. Unusual hand movements that sometimes are referred to as dystonic in character have also been described in patients with severe chronic pain (e.g., complex regional pain syndrome with and without sympathetic signs). In addition, dystonic hand movements can emerge after a central nervous system insult such as a head injury or a stroke.
Psychological Risk Factors
For years, psychological factors were believed to be the underlying cause of focal dystonia. These occupational neuroses were emphasized until around 1982. At that time, Sheehy and colleagues suggested that the problem of dystonia had a neurologic pathomechanism.
Recently, Altenmuller and Jabusch compared the personality characteristics of musicians with and without dystonia with musicians with chronic pain. These researchers studied a variety of personality inventories and questionnaires regarding competence and control orientation, anxiety disorders (stage fright, panic attacks, free-floating anxiety), phobias (agoraphobia, social phobia, and specific phobias such as acrophobia, claustrophobia), life satisfaction, perfectionism, and social orientation. In addition, they asked patients about their anxieties before and after the onset of dystonia or pain. They also asked patients whether the specific anxieties had been present or absent before the onset of pain or dystonia and whether their attitudes concerning perfectionism were the same or different before and after onset of dystonic symptoms.
Anxieties were significantly more common in patients with dystonia and chronic pain than in normal healthy musician controls. Those with dystonia admitted the anxieties had been present before the onset of the disorder. Musicians with dystonia had more problems with social phobias than healthy musicians, while musicians with chronic pain did not have social phobias. Musicians with focal hand dystonia had significantly greater perfectionism than controls or those with chronic pain. The patients also noted they had problems with perfectionism even before the onset of dystonia. This suggests the psychological features contributed to the development of the dystonia rather than representing a psychoreactive response to the disorder. However, dystonia does not necessarily develop in all patients with these psychological disorders.
In conclusion, the etiology of focal hand dystonia is still considered idiopathic. There is agreement the problem is multifactorial. It is not yet clear whether endophenotypic factors cause focal dystonia or simply increase the risk of its development under stressful exophenotypic conditions such as high levels of repetitive hand use. To determine whether an imbalance in neuroanatomic and neurophysiologic excitation and inhibition, aberrant homeostatic plasticity, anatomic restrictions, degraded cortical topography, and/or abnormal sensorimotor processing put patients at risk for the development of focal hand dystonia under environmental stress (e.g., repetition, trauma, behavioral characteristics), preplanned longitudinal studies are needed across a heterogeneous population of patients who work in highly repetitive jobs. This research will take years to conduct.
In the meantime, researchers need to clarify some important factors that could assist in developing more effective remediation strategies. For example, basic, clinical, and translational research is needed to determine whether topographic degradation occurs in the somatosensory, sensorimotor, and the motor and premotor domains, whether cortical topography interferes with both spatial and temporal processing of sensory input and motor output, whether there are problems in motor preparation for a learned task as well as problems in processing during skill acquisition, and whether variations in clinical patient presentations are uniquely correlated with severity or with different types of aberrant neurophysiologic processing. This information could help guide the next iteration of research regarding the effectiveness of intervention strategies.
Clinical Presentation: Focal Hand Dystonia
The onset of the signs and symptoms of focal hand dystonia is variable. The problem may initially manifest as an abnormality in the quality of sound produced by a musical instrument (e.g., a deterioration of vibration in a violinist), increasing errors in task performance, unusual fatigue or sense of weakness, or involuntary and/or excessive movement of a single digit or multiple digits). Initially, the symptoms are subtle and virtually indistinguishable from the normal variations that may be seen in the execution experienced by all musicians studying technically demanding music or a software engineer who is spending excessive hours at the computer. Clinicians must always be alert to the possibility that a musician, a keyboard user, a professional dentist or dental hygienist performing repetitive fine-motor techniques on the teeth, or a worker performing repetitive fine-motor assembly tasks who presents with minimal pain but vague motor control problems and/or somatosensory dysfunction may be manifesting early signs of focal dystonia.
Although co-contractions of flexors and extensors can be observed while an individual with hand dystonia performs the target task, at rest and during the performance of nontarget tasks, the hand appears to function normally. Some patients demonstrate a variety of subtle abnormalities such as a reduced arm swing; loss of smooth, controlled grasping; a physiologic tremor; hypermobility of the interphalangeal joints; decreased range of motion in some upper limb joints (e.g., shoulder abduction, external rotation, finger abduction, forearm pronation); neurovascular entrapment; compression neuropathy; and/or poor posture. Patients with hand dystonia also usually describe a sensory trick to minimize the dystonia, but do not necessarily describe specific awareness of dysfunction in sensory discrimination.
The initial presentation often varies by the type of hand dystonia. A good example is a recent review of the clinical features of nearly 1000 musicians with focal, target-specific musician’s dystonia. The patients had a mean age of 35.7 years (standard deviation, 10.6) at onset with a male predominance (80%). Although both hands were affected in 4% of the patients, more commonly the problem was unilateral with the right hand affected in 64% of cases and the left in 32%. When the dystonia primarily affected one finger, the third finger was the most common, followed by the index and ring fingers. In cases with multiple affected fingers, there were four frequent patterns: (1) combination 4, 5 (32%); (2) combination 3, 4 (17%); (3) combination 3, 4, 5 (17%); and (4) combination 2, 3 (10%). Flexion of one or more fingers was the most common phenotype (54%). Isolated extension was less commonly reported (13%), and a combination of flexion and extension was seen in 17%. Side of involvement varied by type of musician. For example, although the right hand was predominantly affected in keyboard players (77%) and plucked string players (guitar, lute, 78%), the left hand was predominantly affected in bowed string players (68%) and flutists (81%). Either hand was affected in woodwind, percussion, and brass players.
Frucht also observed that task-specific hand dystonia seemed to begin after motor skills had been acquired rather than during skill acquisition. Thus, focal hand dystonia in a musician is probably not a disorder of motor learning, but rather a corruption of acquired, complex, motor programs. The data also suggested that peripheral environmental influences seemed to play an important role in molding the dystonic phenotype. For example, the hand performing the more complex musical tasks (e.g., right hand in pianists and guitarists, left hand in violinists) seemed to be more predisposed to the development of dystonia. In addition, the dystonia usually began in one finger and spread to adjacent fingers, rarely skipping a finger. Further, the ulnar side of the hand (fingers 4 and 5) was disproportionately affected, potentially because of the challenging ergonomics of the musical instrument as well as technical burdens required for this part of the hand in terms of gripping and activation of individual finger movements.
There also seemed to be a correlation between physiologic patterns, dystonic phenotype, and the musical instrument. For example, flexion of the right fourth and fifth fingers was common in pianists, but flexion of the left fourth and fifth fingers was common in violinists. Flexion of the right third through fifth fingers was commonly reported in guitarists, whereas isolated extension of digit 3 was common in woodwind players. Frucht also noted that once a phenotype was established in a given patient, the pattern rarely varied, even if the patient took an extended break from playing his or her instrument. Thus, patterns seem to result from the technical requirements of the instrument, coupled with the technical burden of the hand. However, the observations also reinforce the possibility that with continued repetition of an abnormal, dystonic sensorimotor network, the network actually becomes entrained and learned.
In other studies, different patterns of dystonic presentations have been documented for patients with different types of hand dystonia. In a study by McKenzie and colleagues, there were significant differences in multifactorial measures of musculoskeletal, sensory, and motor performance for patients with writer’s cramp compared with patients with musician’s cramp. Patients with musician’s cramp had a higher level ( P < 0.05) of functional independence and better range of motion, but more neural tension and less strength in the affected upper limb than patients with writer’s cramp. Compared with subjects with writer’s cramp, on the affected side, subjects with musician’s cramp demonstrated greater ( P < 0.05) accuracy on graphesthesia, kinesthesia, and localization, but less accuracy and speed in stereognosis. It also seems that patients with writer’s cramp have a reduced response to vibration or oscillation. No between-group differences were noted in motor performance.
Rosenkranz and colleagues also reported differences in pathophysiologic factors including sensorimotor reorganization in patients with musician’s dystonia compared with patients with writer’s cramp. Applying a vibration stimulus to measure effects on cortical excitability and short latency intracortical inhibition, these researchers noted that vibration increased the amplitude of excitability and decreased the short latency intracortical inhibition in healthy nonmusicians. In patients with writer’s cramp, vibration had no measurable effect on either excitability or inhibition. In patients with musician’s cramp, vibration strongly reduced the short latency intracortical inhibition. It is possible that repetition in the musician leads to significant reorganization in the motor cortex, which ultimately interferes with rather than assists controlled fine-motor movements. The lack of response to sensory vibration in patients with writer’s cramp may suggest that sensory processing plays less of a role in provoking pathologic, dystonic changes than in patients with musician’s cramp. In summary, despite common trends in clinical presentation, patients with focal hand dystonia not only demonstrate variability in the affected limb and fingers, but may also have different problems in the neuromusculoskeletal system.
Diagnosis
Signs and symptoms of focal dystonia usually begin slowly, with the hand initially feeling thick and then uncoordinated in a few specific movements (e.g., alternating movements) while retaining the ability to do other movements. Often, the problem develops in musicians at the time when they are attempting to play dynamically restrained passages. The diagnosis may be readily apparent from the patient history; however, it may be difficult to make a specific diagnosis until the patient literally cannot complete a target task without disruption from the writhing type of involuntary movements. The signs and symptoms occasionally appear more acutely, for example, after an acute trauma (e.g., after a motor vehicle accident or after a fall on an outstretched arm). There is significant subjectivity in the early symptoms, and the movements associated with a specific instrument are idiosyncratic. Wilson and colleagues and Wagner proposed that occupational hand cramp or focal dystonia be defined as sustained and functionally significant loss of a previously attained manual skill, satisfying all of the following conditions.
- 1
The affected skill is impaired by movements in which there are errors in timing, force, or trajectory associated with stereotypical tonic postures and/or cramping sensations that are absent at rest.
- 2
Abnormal movements are initiated by the attempt to exercise a specific motor skill within a characteristic context and may fail to develop except under those conditions.
- 3
Skill loss at the outset cannot be explained by diminished practiced efforts.
- 4
Degraded movements cause the individual to function at a reduced level of skill despite any masking strategies adopted to disguise or circumvent the problem.
- 5
The movement abnormality persists despite resolution of any and all antecedent inflammatory, toxic, traumatic, myopathic, and neuropathic abnormalities.
As part of the history, it is important to inquire about a family history of dystonia, previous injuries, chronic pain, or other medical problems such as a cardiovascular accident and head injury. As with many conditions in which the pathophysiology and the treatment are not well understood, it is assumed that the problems of repetitive strain injury originate as a result of biomechanical, anatomic, and psychological stress. Psychological stress is part of a high motivational drive and literally a basis of survival for most musicians and other successful professionals.
In addition, there seems to be an integral connection between sensory and motor function, performance anxiety, and the limbic system. When an individual begins to expect movement dysfunction when performing the target task, then the expectation could reinforce the outcome of poor motor control. Continued negative expectations could potentially contribute to abnormal learning. Thus, psychosocial issues and expectations should be thoughtfully reviewed as part of the patient history.
All patients need a complete neurologic examination. Some parts of the neurologic examination should be elaborated, such as the sensory and motor control components. Both hands should be evaluated carefully. Hochberg and colleagues suggested that it was important to carefully define the abnormal movement and which fingers are most involved. For example, one or more terminal phalanges may go into flexor spasm or the extensors of the fingers may contract simultaneously with the flexors. In other cases, the wrist turns inward with ulnar deviation or there may be irregular hyperpronation of the forearm with lifting at the elbow. It is important to assess whether contraction of the flexors drives the contraction of the extensors or vice versa. Biofeedback can be used to determine this. In addition, all patients should be videotaped while performing the target task. It is important to analyze which normal movements have been preserved while performing the target task. It is also important to apply some type of rating scale to score the severity of the movement dysfunction to serve as a baseline to measure change. Suggestions for grading these dystonic movements are summarized in Figures 135-5 through 135-8 (online).
It is important to carefully differentiate between the signs and symptoms of hand dystonia, peripheral neuropathy, and other central movement disorders. Some patients with hand dystonia have a peripheral neuropathy. This neuropathy may be a risk factor for hand dystonia, but usually it is not a sufficient cause. Dystonia also needs to be distinguished from other central, hypertonic movement disorders such as spasticity and rigidity.
Goldman and colleagues provide a thorough comparison of the signs and symptoms of a peripheral nerve compression in the upper limb and focal hand dystonia ( Table 135-1 ). They outlined and differentiated hand dystonia and compression neuropathy in terms of tetany (muscle hypercontractility), muscle spasm (cramps), fasciculations, pain, sensation, strength, limb posture, and movement disturbance. Compared with patients with a peripheral neuropathy, patients with focal hand dystonia usually do not have pain. However, if the muscle co-contractions are maintained over a long period of time, vasoconstriction may occur and the cramping is painful. In addition, patients with hand dystonia are likely to demonstrate tetany, disturbances in sensorimotor integration, strength deficits in wrist flexors and extensors, inappropriate sustained co-contractions of agonists and antagonists, and motor control problems at a target-specific task. On the other hand, compared with a patient with hand dystonia, the patient with a peripheral neuropathy is more likely to have painful migratory muscle cramps and fasciculations, nocturnal pain, sensory deficits in the distribution of the sensory nerve, and weakness in muscles innervated by the involved nerves without sustained postural alterations, with movement disturbance primarily due to weakness of the muscles. Muscle spasm is generally secondary to pain, trauma, nerve compression, a degenerative joint condition, or a neural degenerative condition of the dorsal horn or dorsal root. The muscles involved are often two-joint muscles, with the spasm more likely to occur when the muscles are too short or too long.
| Symptom | Focal Hand Dystonia | Compression Neuropathies |
|---|---|---|
| Tetany (a muscle hypercontractile state) | May occur when the task is attempted and persists as long as the task is continued | Typically not present |
| Muscle cramps, muscle spasms |
|
|
| Fasciculations | Few reports of fasciculations in patients with focal hand dystonia | Can be present with peripheral nerve motor axon compromise; however, fasciculations are also found in other conditions involving the central nervous system, such as amytrophic lateral sclerosis |
| Pain | Pain may be secondary with sustained muscle contraction, but is not the hallmark of the diagnosis |
|
| Sensation | May have disturbances in sensorimotor integration; patient may report increased sensitivity or hyposensitivity. Sensory deficits are not associated with the distribution of a peripheral nerve. | Primary sensory deficits isolated to the sensory distribution of the involved nerve |
| Strength |
| Strength deficits are primarily isolated to muscles innervated by the involved nerves |
| Limb posture | Altered due to inappropriate sustained contraction of antagonist muscles | No sustained postural alterations; however, atrophy of enervated muscles specific the involved nerve may be present with advanced motor involvement |
| Movement |
|
|
Based on muscular resistance, the speed of passive movement, the presence of a fixed posture, the pattern of muscle activation, and the impact of the emotional state, one can usually distinguish between the common central movement disorders of spasticity, rigidity, and dystonia ( Table 135-2 ). Although spasticity can be characterized by a velocity-dependent resistance to passive motion, rigidity appears to be independent of the speed of passive movement or position, and dystonia tends to involve a sustained or intermittent co-contraction of agonists and antagonists when performing a target task.
| Clinical Neuromuscular Findings | Dystonia | Spasticity | Rigidity |
|---|---|---|---|
| How does muscular resistance respond to PROM speed? | No effect |
| No effect |
| How quickly is resistance detected when PROM to the limb is rapidly reversed? | Immediate | Delayed | Immediate |
| Is there a presence of a fixed posture? | Yes | Only in severe cases | No |
| Does AROM influence the pattern of activated muscles? | Yes (especially a target-specific motion) | Minimal effect | Minimal effect |
| Do the behavioral task and emotional state, such as stress, influence the pattern of activated muscles? | Yes, definite increase | Minimal to mild increase | Minimal to mild increase |
| Summary | Sustained or intermittent involuntary muscle contractions | Velocity-dependent resistance | Independent of both speed and posture |
Spasticity represents an exaggerated stretch reflex usually secondary to a central condition involving cortical spinal pathways. Traditionally, spasticity is evaluated by moving the limb quickly through the range of motion and noting whether this is smooth and easy or whether there is resistance, clonus, or a sudden catch (stop to the motion) followed with a release after stopping. Usually the postural righting muscles are most frequently involved (e.g., in the upper limb: shoulder abductors, shoulder internal rotators, forearm pronators, and elbow, wrist, and finger flexors; in the lower limb: hip flexion, knee extension, ankle inversion, and plantar flexion). The stretch reflex is exaggerated. Slow positioning can inhibit the tone.
Rigidity involves a co-contraction of the flexors and extensors. The co-contraction of the muscles blocks voluntary movement. This co-contraction may not be equal, limiting movements like dorsiflexion and eversion of the ankle as well as wrist extension and finger spread. Usually patients have difficulty initiating movements, but once started, they can proceed, particularly if the movement is rhythmical. The co-contractions restrict not only movement range but also movement variability. There is some evidence that there is excessive inhibition in patients with rigidity. Positioning does not seem to help relieve the rigidity. The stretch reflex may be decreased.
Dystonia is also based on a co-contraction of the flexors and extensors, with either the flexors or the extensors dominating the movement, leading to end-range twisting motions. The observation that patients can use a sensory trick to inhibit the dystonic movements and that the dystonia is often target specific is unique to dystonia. In this case, there is inadequate inhibition of the antagonist muscle. The stretch reflex is generally normal.
A good sensory examination is important. Although two-point discrimination and differentiation of sharp and dull stimuli appear intact, cortical sensory problems are commonly reported (e.g., abnormal graphesthesia, stereognosis, kinesthesia). Poor temporal and spatial processing and poor somesthetic temporal discrimination have also been reported. These findings are consistent with reports that patients with idiopathic focal dystonia demonstrate increased abnormal dystonic posturing after the vibration of the tendon or muscle belly, abnormal perception of arm movements with tonic vibration, a reduction in dystonic posturing after cutaneous and proprioceptive stimuli, and anesthesia of the muscle spindle. Some report that patients with focal hand dystonia have decreased motor accuracy and decreased efficiency of fine-motor control of the hand. It is not clear whether this dysfunction is related to the sensory problems or directly related to the dystonia. All the measurements from the neurologic and physical examinations should be objectively recorded: (1) range of motion and strength, (2) neurovascular entrapment, (3) peripheral nerve entrapments, (4) somatosensory function, (5) sympathetic signs, and (6) pain. For the sensory examination, sharp/dull discrimination, detection of light touch threshold with the Semmes–Weinstein monofilament, and two-point discrimination should be performed. However, more time should be spent on cortical sensory discrimination testing such as localization, graphesthesia, stereognosis, and kinesthesia. Unfortunately, many of these latter tests are standardized for children, and therapists need to be certified to administer the Sensory Integration Praxis Tests. Box 135-2 provides some suggestions for clinical sensory discrimination testing. Because occupational hand cramps can develop in both hands, the examination and the measurements should be recorded bilaterally.
Draw an outline of a hand. Place points on this hand pattern that will serve as the sites for you to deliver stimuli to your patient. Put target points on each segment of each finger as well as on the palm of the hand. Make a few similar points on the dorsum. Give the patient a fine-tip marker pen. Ask the patient to hold the pen close to the tip. The examiner also needs a fine-tip marker pen that is a different color from the one used by the patient. The examiner explains the task to the patient: “I will ask you to close your eyes, and then I will be placing a mark on your hand. Please take the pen I gave you and mark where you felt me touch you. Once you place the pen on the skin, please do not move it around. Keep it at the location until I have time to measure it. Now, close your eyes and we will begin.” Using a marker pen, the examiner lightly touches on one of the points (do not depress the skin more than 1 mm). After the patient places a pen mark on the place that he or she felt the stimulus, the examiner must take the measurement between where the stimulus was delivered and where the patient thought it was delivered. Although there is some variability in performance, adult patients will be 0 to 1.5 cm from the point of contact. This test is designed after the localization test of the Sensory Integrative Praxis Test.
Kinesthesia
Take a piece of 11 × 14-inch paper and make lines of different lengths going in different directions. Have a clear starting place and a clear ending point. Mark each line from 1 to 10 and indicate right or left (five trials per hand). Place the piece of paper in front of the patient while you sit across from him or her. Ask the patient to close his or her eyes. Take the third finger of the patient’s right hand and place it on the start position (beginning of the line, right-1). Now say, “With your eyes closed, I am going to move your finger to another position, and then I will return your finger to where we started. Then, I want you to take your finger back to the place I took it. Please keep your finger there until I can take the measurement.” Normal adult subjects usually will be less than 1.5 cm off target. This recommendation for testing is designed after the kinesthesia test of the Sensory Integrative Praxis Test.
Graphesthesia
For testing graphesthesia, make a chart with 4 columns and 10 rows. Label one column Affected and another column Unaffected . Draw different simple designs approximately 0.5 inch in size on each row in the first of two columns for the affected and unaffected digits. Make the answer sheet large enough so that you have room to copy the drawing that the patient produces for you after the stimulus is delivered. Now explain to the patient, “I will ask you to close your eyes. I am going to draw a design on each of your finger pads. After I draw the design, I want you to try to draw it exactly like I drew it on your finger. Make the design similar in size and orientation to the one I drew.” After the subject draws the design, grade the design as follows: 2, completely correct; 1, partially correct; or 0, incorrect. Each hand will have a total of 10 tests, with a maximum score of 20. Determine the percentage of the designs that are correct for each side. This recommendation for testing is designed after the graphesthesia test from the Sensory Integration Praxis Test.
Stereognosis
Collect 12 keys that have the same base but different cuts. Make a photocopy of each key. Then put six of the keys on each of two separate key rings. Create an answer sheet that includes a photocopy of each key. Show the patient a photocopy of one key. Hand the patient a ring of keys that includes the photocopied key. Instruct the patient as follows: “Put the keys under the table where you cannot see them. Feel each key until you find the one that matches the one you see. I will note the time that it requires for you to find the key and your accuracy.” Using a stopwatch, record how long it takes the subject to match the key on the ring with the photocopy of the same key. Alternate between sides (i.e., have a ring of keys for the right and the left sides). Have an answer sheet where you can record both the time required and whether the match was correct. This test has been used in our practice setting for research purposes. The reliability is high on test–retest (>90%). It takes between 30 seconds and 2 minutes for the subject to evaluate each key. Thus, the test takes from 10 to 20 minutes to administer. We have been using a six-key set; however, we are in the process of designing a new Byl-Cheney-Bocsai Discriminator that can be used as an alternative to the key test. It will require less time to administer than the key test.
Sensorimotor Accuracy
On an 11 × 14-inch piece of paper, start at the middle of the bottom of the paper and draw a wavy oval shape with a medium felt-tip pen. The oval should come back to the starting place at the middle, bottom of the sheet. Measure the entire length of the oval that you drew. Use this as the master from which to make copies. Give the patient a red pen to draw with using the right hand and a blue pen for the left hand. Instruct the patient as follows: “Start here and try to follow this line as carefully and as quickly as you can. I will be timing you, but I want you to try to be as accurate as possible.” A stopwatch must be used to time how long the subject requires to copy the line. Have the subject begin with the unaffected side. Then repeat the same test with the affected side. Again, time how long is required to complete the test. After the subject has completed the task, grade the test by measuring and then summing up the length of the areas where the subject was off the line. Subtract the total length of the line where the subject was off the line from the total line length. Divide this calculated length by the total length of the line to create a percentage. Normal subjects should be able to stay on the line approximately 80% to 90% of the time and complete the line tracing in approximately 30 seconds. This test is designed after the motor accuracy test of the Sensory Integration Praxis Test.
Dynamic EMG testing can be included as part of the diagnostic workup. It is not uncommon to detect a peripheral neuropathy of the ulnar, median, or musculocutaneous nerve in a patient with hand dystonia. EMG changes in conduction may be noted even before weakness or sensory changes are detected. In addition, the electromyogram can document classic co-contractions of antagonists and agonists as well as confirm that muscles are recruited with excessive force. In addition, the electromyogram can show that once the muscles are contracted, not only is it difficult to release the contraction, but it is difficult to hold a maximum contraction given the interference of the contractions of the antagonists. This inability to hold a consistent firing pattern can be used to determine whether a muscle should be injected with botulinum toxin.
Even though pain is not usually an issue with patients with focal hand dystonia, it is still important to evaluate pain as part of the initial workup. The most common self-rated measurement instrument is the visual analog scale. This scale includes either rating pain on an ordinal scale of 0 to 10 or drawing the severity on a 10-cm line.
Severity of Dystonia
There are a number of standard scales that can be used to quantify the severity of the dystonia. The common scales include Fahn-Marsden Scale (see Fig. 135-5 , online); the Global Rating Scale (see Fig. 135-6 , online); the Toronto Western Spasmodic Torticollis Rating Scale (see Fig. 135-7 , online); and the Unified Dystonia Rating Scale (see Fig. 135-8 , online). These are ordinal scales and may include an evaluation of the hand as part of the scoring, but do not uniquely target the hand.
In the case of task-specific hand dystonia, the general scales are not particularly detailed to measure objective changes in hand function. Further calculations of statistical significance are questionable based on small sample sizes and ordinal scales. The severity of dystonia in patients with hand/arm dystonia can be rated using the Arm Dystonia Disability Scale (ADDS). The ordinal scale for the ADDS is 0 = normal, 1 = mild difficulty in playing/writing, 2 = moderate difficulty in playing/writing, 3 = marked difficulty in playing/writing. It is difficult to objectively determine mild from moderate to marked difficulty. Technical performance is also rated on an ordinal scale according to the Tubiana and Chamagne Scale with the following ordinal assignment: 0 = unable to play, 1 = plays several notes but stops because of blockage or lack of facility, 2 = plays short sequences without rapidity and with unsteady fingering, 3 = plays easy pieces but avoids difficult passages for fear of motor problems, 5 = returns to concert performances. The Tubiana and Chamagne Scale more objectively defines the severity; however, with this scale, good performance is a high score and poor performance is a low score (just the opposite of the other scales). Patients may also be subjectively asked to assess the effects of treatment based on a percentage scale or a percentage scale can be created by subtracting their pretreatment motor performance from the post-treatment performance. If patients have writer’s cramp, a video should be made during writing, a force sensor placed on a pen, and an ordinal score used to grade the force of the grip, the force of the pressure down on the paper, the legibility of writing, and the amplitude of the letters.
Despite the shortcomings of the scales, dystonia severity ratings have been correlated with movement kinematics. Mean stroke frequency is significantly reduced in patients with dystonia. In patients with focal hand dystonia, drawing movements showed a greater decrease in stroke frequency than handwriting movements. During circle drawing, mean vertical peak velocity was more variable in patients compared with controls. This may indicate an impaired ability to maintain the reproduction of the same kinematic pattern over time. An increase in vertical writing pressure was only observed during handwriting, but not during circle drawing. It is possible that this increase in vertical writing pressure reflected a compensatory effort to stabilize the pencil. Kinematic measures and individual scores on the ADDS and Toronto Western Spasmodic Torticollis Rating Scale were not correlated. The lack of correlation should not be surprising given ADDS, the Toronto torticollis scale, and kinematic analyses probe different aspects of motor impairments. The ADDS characterizes how dystonia affects a set of fine manual tasks, whereas the torticollis scale scores the manifestation of dystonia during handwriting. Therefore, the clinical scores and kinematic analysis of handwriting provide complementary insights into the motor impairments of patients with hand dystonia. Future studies need to address which combinations of clinical scores and kinematic measures are needed to determine the most appropriate method to quantify impairments in patients with writer’s and musician’s cramp.
Psychosocial Assessment
Some studies report pathologic behaviors in patients with writer’s cramp. It has been reported that patients with focal dystonia have difficulty with shifting mindsets, increased perseveration, and obsessive compulsivity that reflect complex neurophysiologic dysfunction (dorsolateral, orbitofrontal, and motor frontostriatal).
In musicians, studies confirm the presence of anxiety, perfectionism, and phobias as risk factors for the development of hand dystonia. Thus, it may be helpful to administer a personality inventory, a questionnaire on anxiety, and perhaps some standard questioning on perfectionism. This may highlight some of the personality risk factors that could contribute to the development of dystonia but may also help guide comprehensive intervention to remediate the dystonia.
Radiographs and Neuroimaging
Neither a radiograph of the hand nor a magnetic resonance image (MRI) of the hand is generally ordered by the physician unless there is a history of an injury, a structural biomechanical problem, or signs and symptoms of some type of soft tissue lesion (e.g., ganglion, cyst, ligamentous tear, fibrosis of the retinaculum, trigger finger). However, a physician may request brain magnetic resonance imaging or a computed tomography scan to rule out a stroke, brain tumor, or structural anomaly that could explain the movement dysfunction. However, these expensive techniques may not necessarily rule in the diagnosis of focal hand dystonia. Thus, at this time, health insurance will usually not pay for expensive imaging tests unless the neurologist suspects pathology consistent with a brain tumor or a vascular event.
On the other hand, advances in noninvasive imaging have expanded the opportunities to understand the etiology of dystonia. Imaging is useful for clarifying what areas of the brain may be contributing to the motor deficits. Detailed brain anatomy and unusual deviations in volume, density, morphology, and temporal and spatial processing in cortical and subcortical structures may also be examined with MRI and functional MRI (fMRI). For example, with basic MRI and fMRI paired with TMS, several cortical abnormalities have been noted in patients with focal hand dystonia: (1) a reduction in cortical blood flow and abnormal transient asymmetry in movement-related cortical potentials, (2) asymmetrical muscle response to double-pulsed magnetic stimulation, and (3) asymmetry in the tonic vibration reflex after intramuscular lidocaine.
Diffusion tensor imaging can measure water diffusion across myelinated central nervous system structures to reconstruct the white matter tracts (e.g., the expansion of gray matter volume has been noted in the putamen of patients with dystonia). In highly active brain regions, positron emission tomography and fMRI provide indirect measures of neural activity through the consumption of oxygen. For example, compared with controls, patients with writer’s cramp demonstrate an increase in the amplitude of the blood oxygen level–dependent signal in the fMRI in the basal ganglia. On the other hand, using positron emission tomography, hypermetabolism has been noted in the frontal cortex and the basal ganglia in patients with dystonia. Spectroscopy (a special advanced application of MRI) can identify biochemical markers such as neurotransmitters (e.g., γ-aminobutyric acid and glutamate). Although proton magnetic resonance spectroscopy provides evidence of mitochondrial dysfunction as the pathophysiology of primary dystonia, in one study of 14 patients with primary focal hand dystonia, no statistically significant differences were found in any of the measured brain metabolites.
Functional imaging methods like electroencephalography (EEG) and magnetoencephalography (MEG) can record brain activity through changes in the electrical current or changes in the magnetic fields across the scalp. MEG studies are noninvasive and can be safely used to look at cortical organization in humans. MEG studies confirm sensory, sensorimotor, and motor degradation in patients with focal hand dystonia. For example, in patients with focal hand dystonia, the size of the sensory receptive fields is enlarged, the separation of the digit representations is reduced, and the digital receptive fields are altered in their sequential order. Further, MEG cutaneous stimuli delivered to the digits produce somatosensory evoked fields with decreased amplitude and longer duration in the contralateral hemisphere of the affected hand for both the early (S1) and late (S2) responses.
However, in S2, both earlier and higher amplitude responses have been measured in the ipsilateral and contralateral hemispheres of patients with hand dystonia.
An example of the variations in the somatosensory evoked responses can be seen in Figures 135-9 through 135-12 . After sensory stimuli were delivered to the digits, the somatosensory evoked potentials in patients with dystonia were disorganized, with a reduced amplitude, excessive oscillations, and early peak responses. In addition, although the poststimulus somatosensory responses for digits 1-5 (D1-D5) were progressively ordered medial to lateral and inferior to superior, there was significant overlap across dorsal and lateral surfaces, between digits, as well as across segments within digits. Sanger and colleagues also quantified the overlap in the somatosensory evoked responses after simultaneous stimulation of adjacent digits in normal subjects and in subjects with hand dystonia. The magnitude of response when stimulating D2 and D3 in normal subjects was equivalent to a serial addition of the response of the individual digits alone. In patients with focal hand dystonia, the response of simultaneous stimulation was less than the addition of each digit stimulated individually, suggesting overlap and redundancy between adjacent digits. This is similar to the overlap recorded electrophysiologically in a behavioral monkey model, supporting the sensorimotor learning hypothesis.
