Dystonia is a movement disorder characterized by involuntary, sustained, or repetitive muscle contractions that cause twisting or abnormal postures of body parts. These conditions are marked by co-contraction of agonist and antagonist muscles, and overflow phenomena, typically categorized into focal, segmental, multifocal, generalized, and hemidystonia. Sensory tricks (sensory trick / geste antagoniste) refer to voluntary actions that can temporarily reduce the severity of abnormal postures and involuntary movements, often by applying stimuli, including tactile stimulation, to adjacent body parts affected by the disease to improve symptoms. Recent research has found that passive contact or subjective imagination of certain trick actions by patients can also elicit trick effects. Sensory tricks are classic clinical features of focal dystonia, especially common in cervical dystonia (CD), and also present in other focal dystonias such as blepharospasm (BSP), oromandibular dystonia (OMD), and writer’s cramp (WC). Additionally, hemifacial spasm (HFS), which shares similar clinical manifestations with BSP, is considered a peripheral neuropathic myoclonus rather than a basal ganglia dysfunction, and some patients also exhibit sensory tricks. Recently, sensory tricks have been discovered in non-focal dystonia or other movement disorders. Kojovic et al. reported a generalized dystonia patient who showed significant symptom improvement while playing the piano; Lewitt and Gostkowski reported sensory tricks in patients with chorea and Parkinson’s disease; Garcia-Ruiz and del Val reported that patients with Tourette syndrome used sweeping or household chores as sensory tricks; Shprecher reported that applying pressure to the nape of the neck or wearing a neck brace improved tremor symptoms in patients with tardive tremors related to gastric motility disorders.
Case 1: Primary Meige syndrome: cranial dystonia
(Video 1: A 43-year-old woman presents with myoclonic and dystonic eyelid spasms, oral-facial movement disorder, and tongue dystonia. She uses tactile contact with her eyelids and face to perform sensory tricks.)
Case 2: Primary Meige syndrome: cranio-cervical dystonia with tremors related to dystonia
(Video 2: A 40-year-old male who has been on olanzapine for 6 months presents with myoclonic and dystonic eyelid spasms, oral-facial movement disorder, tongue dystonia, and left torticollis. He performs sensory tricks by placing his hand on the back of his neck.)
Currently, there is no consensus on the potential mechanisms underlying sensory tricks in domestic and international research. This article reviews the basic situation and research status of sensory tricks reported both domestically and internationally.
1.Types of Sensory Tricks
The clinical manifestations of sensory tricks are diverse, with the same patient potentially exhibiting different forms of trick actions, and the effectiveness of different tricks on symptom improvement varies. Traditionally, sensory tricks were thought to be a form of psychological suggestion or to correct abnormal movements by applying reactive force to specific body parts; thus, Stejskal early on considered sensory tricks to be reactive force. However, recent studies have shown that there are sensory tricks beyond tactile or reactive force, challenging the notion that sensory tricks are solely psychological suggestion or reactive force.
1. Classic Sensory Tricks: In focal dystonia, most sensory tricks work by applying mild tactile afferent information to specific body parts to correct abnormal postures, known as classic sensory tricks. CD patients commonly use classic sensory tricks by lightly touching their cheeks, jaw, behind the ear, top of the head, or occiput, with some patients finding that touching areas on both the contralateral and ipsilateral sides of head rotation improves symptoms. BSP patients adjust their symptoms by touching specific areas of the face, such as the thumb or index finger on the upper eyelid, adjusting their glasses, or covering their eyes. WC patients use their healthy hand to touch specific areas of their affected hand or arm while writing as a form of sensory trick. Lightly touching the affected side of the face is the primary classic sensory trick used by HFS patients.
2. Strength Sensory Tricks: Patients with more severe conditions or longer disease duration often use strength sensory tricks. Patients with longer disease duration or more severe CD typically need to apply reactive force to resist head rotation to correct abnormal postures. BSP patients may rub their eyelids, and HFS patients may massage the affected side of their face, both of which fall under strength sensory tricks. As the disease progresses, greater sensory stimulation may be needed to improve symptoms.
3. Other Forms of Sensory Tricks: Some patients can elicit sensory trick effects through indirect contact with specific body parts using another person’s hand or objects. BSP patients can improve symptoms through actions such as writing, humming, whistling, yawning, coughing, chewing gum, or flossing. Lee et al. reported a CD patient who used visual stimuli as sensory tricks: this patient could improve symptoms not only through classic sensory tricks but also through visual activities such as looking in the mirror or observing patterns on a flat surface, and symptoms could also be alleviated by focusing on stationary objects while walking. A Parkinson’s disease patient had a habit of tying a ribbon to their glasses; the sensory input from the ribbon could induce segmental dystonia of the cranio-cervical area, which was termed “negative sensory trick.”
Hertie Clinical Brain Research Association investigated 47 patients with sensory tricks, of which 41 patients showed significant symptom improvement when applying sensory tricks, while the remaining 6 patients (12.8%) exhibited negative sensory tricks, where additional tactile afferent information applied to the neck or occiput worsened symptoms.
2.Frequency and Effect Evaluation of Sensory Tricks in Movement Disorders
Studies investigating the prevalence of sensory tricks in CD reported rates of 70%, 71.3%, 81%, and 84.4% by Hallet, Schramm, Loyola, and Martino, respectively. Loyola and Martino found that the prevalence of sensory tricks in BSP patients was 55% and 71.2%, respectively. Martin reported that among patients with sensory tricks, 69% of BSP patients and 48.1% of CD patients used strength sensory tricks. Loyola reported that 20% of WC patients exhibited sensory tricks. More than one-third of HFS patients improved their symptoms through tactile stimulation. Loyola categorized the effects of sensory tricks into high, medium, and low efficacy groups and found that most patients experienced a medium or low degree of improvement in involuntary movements using sensory tricks.
3.Research on the Mechanisms of Sensory Tricks
Movement disorders are believed to be caused by abnormalities in the cortical-basal ganglia-thalamocortical loop. It has been hypothesized that the existence of sensory tricks requires the anatomical integrity of the basal ganglia connectivity network, but secondary dystonia often accompanies lesions in these network structures, which may explain why sensory tricks are more frequently observed in primary dystonia compared to secondary dystonia, and also reflects the significant research value of sensory tricks in primary dystonia. Abnormal sensory-motor integration is an important pathophysiological mechanism in dystonia, referring to the abnormal peripheral sensory afferent information or the brain’s abnormal integration of sensory input that may affect the execution process of motor programs in the motor cortex. Studies on focal hand dystonia (FHD) and CD have found that patients exhibit abnormal spatial discrimination thresholds (SDT), indicating a decrease in spatial acuity, supporting the view that sensory abnormalities, namely peripheral afferent information and central integration dysfunction, exist in dystonia.
Direct visual input under higher-level sensory-motor integration commands can compensate for impaired proprioceptive information from the head, thereby influencing the contraction of neck muscles. These findings suggest that vision may play a compensatory role for impaired vestibular and proprioceptive systems, similar to the effects of visual cues in Parkinson’s disease patients, where applying external visual cues or auditory input can facilitate the initiation and execution of their movements. Since the basal ganglia and motor areas are closely connected not only to the somatosensory area but also to visual and auditory areas, abnormalities in vision and hearing may exist in the pathogenesis of more severe primary dystonia.
Regarding the potential mechanisms of sensory tricks, various studies have been conducted internationally, including neurophysiology and neuroimaging:(1) Neurophysiological Studies: A study using electromyography to evaluate the effects of sensory tricks in CD found that in patients with sensory trick effects, the activity of both agonist and antagonist muscles at all detection sites significantly decreased when using sensory tricks. Tinazzi et al. used paired sensory evoked potentials of the median and ulnar nerves to study 10 WC patients, finding that WC patients exhibited abnormal integration of sensory input signals from adjacent body parts, which led to abnormal motor output manifesting as hand muscle spasms during writing.(2) Voxel-Based Morphometry (VBM) Studies: A VBM study on BSP patients found that the cortical thickness of the right middle frontal gyrus was greater than that of the control group, while the thickness of the left postcentral gyrus and left superior temporal gyrus was lower than that of the control group. BSP patients exhibited only differences in cortical volume, indicating a strong correlation between the disease and sensory presentation and conscious regulation of motor behavior. Changes in gray matter in the primary sensory cortex may represent characteristics of primary dystonia. Another VBM study involving 36 unilateral FHD patients found an increase in the volume of the primary sensory cortex bilaterally.(3) Functional Magnetic Resonance Imaging (fMRI) Studies: An fMRI study on WC patients showed increased excitability in the basal ganglia and multiple cortical areas (bilateral visual areas, contralateral insula, and ipsilateral parietal cortex) when performing tactile stimulus discrimination tasks.(4) Positron Emission Tomography (PET) Studies: A PET study on primary CD patients showed increased metabolism in the motor cortex and supplementary motor area (SMA) contralateral to the direction of head rotation, accompanied by decreased metabolism in the primary sensory-motor cortex. Another PET study conducted simultaneously while applying sensory tricks in CD patients showed increased metabolism in the upper and lower parietal lobes on the same side as the direction of head rotation, while metabolism in the SMA and primary sensory-motor cortex contralateral to the direction of head rotation decreased.(5) Transcranial Magnetic Stimulation (TMS) Studies: TMS can excite or inhibit neural circuits. Bhidayasiri and Bronstein hypothesized that applying specific parameters of TMS to specific brain regions could simulate the effects of sensory tricks and theoretically improve symptoms in CD patients.
4.Clinical Application Value of Sensory Tricks
1. Providing Basis for Disease Diagnosis and Differentiation: The sensory trick phenomenon can help differentiate dystonic tremors from other diseases such as essential tremor. In patients exhibiting cervical tremor, the presence of sensory tricks indicates that the tremor originates from the dystonia itself, allowing differentiation from essential tremor. A study comparing tremor-type CD, dystonia-related head tremor, and essential head tremor found that if patients experience a decrease in tremor amplitude while performing sensory tricks, it aids in differentiating the former two types of tremors from essential tremor.
2. Guiding Rehabilitation as a Treatment Strategy: Sensory tricks can serve as a treatment strategy. Given that most patients with focal dystonia or even some movement disorders exhibit clear sensory tricks, sensory tricks can be utilized in the treatment and rehabilitation of related diseases. This includes designing various devices, such as glasses or helmets with specific supports for BSP patients, oral appliances for OMD patients, neck braces or supports for CD patients, and ergonomic writing correction devices or wrist supports for WC patients. Sensory tricks can also be used in the care of some generalized dystonia patients or even Parkinson’s disease patients, facilitating the patient’s comfort and avoiding mechanical injuries caused by excessive pulling.
3. Evaluating Disease Progression and Prognosis: Research indicates that the effects of sensory tricks gradually diminish as the disease progresses. Sensory tricks seem to occur more frequently in patients with an onset age below 32 and can persist throughout the disease course. A study on visual-tactile discrimination suggested that patients with lower visual-tactile discrimination thresholds experience better effects from sensory tricks. This may reflect a dynamic process of progressive loss of compensatory mechanisms for basal ganglia dysfunction. At the level of the parietal cortex, the integrity of the compensatory mechanisms for basal ganglia dysfunction may be a prerequisite for the good effects of sensory tricks. Furthermore, compared to patients without significant sensory trick effects, those with sensory trick effects tend to have a significantly younger onset age.
In summary, sensory tricks hold significant research value in dystonia, as they not only provide a basis for disease diagnosis and differentiation but also offer new ideas for disease rehabilitation. In-depth research on the potential mechanisms of sensory tricks will contribute to a more reasonable interpretation and discussion of the etiology and pathogenesis of dystonia and even movement disorders.
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