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|Year : 2017 | Volume
| Issue : 3 | Page : 537-538
Sensory tricks in dystonia: Phenomenology and mechanisms
Pooja Mailankody, Pramod Kumar Pal
Department of Neurology, National Institute of Mental Health and Neurosciences (NIMHANS), Bangalore, Karnataka, India
|Date of Web Publication||9-May-2017|
Pramod Kumar Pal
Department of Neurology, National Institute of Mental Health and Neurosciences (NIMHANS), Hosur Road, Bangalore - 560 029, Karnataka
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Mailankody P, Pal PK. Sensory tricks in dystonia: Phenomenology and mechanisms. Neurol India 2017;65:537-8
Dystonia is a hyperkinetic movement disorder characterized by co-contraction of agonists and antagonists resulting in abnormal twisting and repetitive movements or postures, or both. Cervical dystonia (CD) and blepharospasm (BSP) are examples of primary focal adult onset dystonias. Loss of reciprocal inhibition, abnormalities of the sensory input, changes in cortical excitability and neuroplasticity, and dysfunction of the basal ganglia inhibitory control are the various mechanisms implicated in the pathogenesis of dystonia.
Sensory trick (ST) or geste antagoniste, a hallmark of primary dystonia, is an internally generated, specific voluntary movement aimed at ameliorating the dystonia. It was recently renamed as 'alleviating maneuver' by Patel et al. The phenomenology is varied and the exact mechanism remains elusive. Nevertheless, ST has diagnostic and therapeutic implications. It can help in the differentiation of dystonic tremor from essential tremor.
ST could be of a 'classic' variety wherein a gentle touch on the chin or neck could reduce the dystonia, or a 'forcible' one where certain amount of pressure would be required to improve the dystonia. ST was found in 84% and 71% of patients with CD and BSP, respectively, in a study by Martino et al. The various phenomenology described in CD are touching the cheek, chin, mastoid or skull, and pushing the cheek, chin, mastoid or back of the forehead or head., Stretching the eyebrows or the eyelids is the ST that has been most frequently seen in BSP. Majority of the patients reported partial relief with ST in both the studies whereas only 20-25% of the patients reported complete relief., Application of a forcible trick by the examiner resulted in improvement in all the patients. However, with regard to the classic ST, application by the examiner was not effective in the majority of patients. The benefit of ST is usually transient and lasts for a few seconds. However, it can last for even hours, especially when the ST is forcible. Filip et al., noted that the sequence of sensory and motor inputs was important and any deviation from the sequence led to loss of effectiveness of the ST.
Proprioceptive stimulus is the most important ST for CD, whereas tactile stimulus works best for BSP. Kagi et al., demonstrated that patients with more effective ST had better visuotactile discrimination and shorter duration of disease compared to those patients with less effective ST. Very often, the effect of the trick is seen even before the actual contact and this has been confirmed by electromyographic experiments. Another intriguing feature is that imagination of the trick can produce the benefit in some patients.
Electromyographic (EMG) activity in the dystonic muscles is decreased during the ST., Studies using transcranial magnetic stimulation (TMS) have shown that ST produces decrease in the intracortical facilitation, which is usually abnormal in patients with dystonia. An elegant study by Schramm et al., suggested that the initial head position determined the efficacy of the ST. Using surface EMG, they demonstrated that the ST was most effective when it was applied with the head in neutral or slightly overcorrected position. The information regarding the head position that reaches the parietal cortex gets modified when the ST is applied, thereby modifying the sensory motor integration at the cortical level resulting in amelioration of dystonia. With this study, Schramm et al., showed that both dystonia and the benefit of ST have central as well as peripheral mechanisms. Cortical EEG and globus pallidus field potentials were evaluated in 4 patients with CD and effective ST. Desynchronization in the 6-8 Hz range was found to accompany clinical improvement with ST. The changes in the local field potential as well as neck EMG were noted even before the contact with the face. This study suggests that ST need not be necessarily triggered by the sensory stimulus.
With the help of blink reflex studies, Gomez- Wong et al., demonstrated that patients with BSP have decreased R2 response compared to subjects without BSP during application of ST. Transient diminution of the gain of trigeminofacial reflexes during the ST was the mechanism hypothesized by the authors.
Imaging study by Naumann et al., showed that the performance of ST is accompanied by increased activation in the parietal and occipital lobes and decreased activation in the supplementary motor area and the primary sensorimotor cortex.
Amelioration of dystonia with ST suggests that the disorder is amenable to modulation, and hence, this maneuver may be considered as a predictor of response to treatment especially with botulinum toxin. Filip et al., found that patients with effective ST responded better to treatment than those without. However, the presence of reverse sensory geste wherein the so called 'trick' can worsen the dystonia is a reminder that the mechanism of sensory-motor integration can turn out to be a disadvantage. Among the 47 consecutive patients with CD evaluated by Asmus et al., 12.8% had reverse sensory geste. EMG recording revealed an increased activity in the corresponding neck muscles. Patients with reverse geste also showed an initial worsening with treatment.
Pandey et al., have explored the phenomenology of ST in CD and BSP. Their study found that there is no correlation between the sensory trick and the duration or severity of dystonia reiterating the finding by Patel et al. However other studies., have found that the effectiveness of ST can decrease with an increasing severity of dystonia. Pandey et al., postulated that bulkier muscles and requirement of complex movements for the production of dystonia might be responsible for the lower frequency of ST in their patients with CD.
To summarize, sensory trick in dystonia is a specific maneuver that is generated by the patient to relieve dystonia. The effectiveness of ST has been found to be independent of the duration or severity of the disease in most of the studies. Electrophysiological and imaging studies show that the mechanism of the benefit seen with ST has both central and peripheral components. The benefit seen with ST, though transient, is nearly universal and has therapeutic implications. Better understanding of the various mechanisms can lead to the development of newer tools for the treatment of dystonia.
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