Muscle Vibration May Retrain Abnormal Sensorimotor Organization in Focal Hand Dystonia

Pauline Anderson

January 22, 2008

January 22, 2008 – Muscle vibration applied to small hand muscles of patients with focal hand dystonia may help regain hand motor control, a new study suggests.

The study, published in the January 22 issue of Neurology, tested whether interventions with muscle vibration would change the pattern of sensorimotor organization (SMO) in focal hand dystonia among patients with writer's cramp and musician's dystonia.

"In focal hand dystonia, particularly in musician's dystonia, it is possible to retrain an abnormal SMO toward a more differential pattern, which has potential implications for therapy," the researchers, with lead author Karin Rosenkranz, MD, from the Institute of Neurology, in London, United Kingdom, conclude.

Disorganization in SMO

In healthy people, the interaction of proprioceptive input as induced by low-amplitude vibration of small hand muscles is integrated in the motor cortex in a spatially organized way: the representation of the vibrated hand muscle is focally activated, while the activation in the representation of neighboring nonvibrated hand muscles is suppressed. In people with focal hand dystonia, this pattern, referred to as the SMO, is disorganized.

The pathophysiology of focal hand dystonia is not entirely clear, the authors note; however, the repeated performance of skilled motor tasks ("overtraining") such as writing, or in particular playing a musical instrument, appears to play a role. The sensory feedback from these repetitive movements may cause reorganization in the sensory areas of the brain, consequently influencing the interaction of the sensory and motor areas.

For this study, researchers evaluated whether attended and unattended periods of behavioral proprioceptive training delivered over 15-minute intervals could promote reorganization of sensorimotor connections in 6 nonmusicians, 6 professional musicians, 6 patients with musician's dystonia, and 6 patients with writer's cramp.

Short-interval intracortical inhibition (SICI) was measured before and after hand muscle vibration with 2 training types: AttVIB, where subjects paid attention to stimulation of the abductor pollicis brevis (APB), and AttIndex, where subjects paid attention to stimulation of the index finger (FDI). Since musical training alone has an influence on the SMO, the study group included healthy musicians as controls.

Results showed that the SMO in focal hand dystonia, especially in musician's dystonia, could be modulated toward patterns seen in healthy control subjects spontaneously. "AttVIB tended to restore a more differential SMO in musician's dystonia but not in writer's cramp, while AttIndex failed to induce any changes in both groups," Dr. Rosenkranz said.

"Our results show that the SMO of hand muscle in the motor cortex is flexible not only in healthy subjects, but also in healthy musicians, patients with musician's dystonia, and patients with writer's cramp . . . [although] the way in which SMO responds to behavioral proprioceptive training is influenced by the baseline pattern of SMO," she said.

The study observations suggest that patients may retain the capacity to adapt their sensorimotor connections, which is an important prerequisite for the application of proprioceptive training in rehabilitation of focal hand dystonia.   

The results also suggest that the pathophysiology of musician's dystonia and writer's cramp may differ, since sensory discrimination during behavioral training and the patterns of SMO effects observed between the 2 groups differed, said Dr. Rosenkranz.

"The new findings in the present data are that the same interventions have a different effect on SMO in patients with musician's dystonia and patients with writer's cramp," she said. Importantly, she added, the dedifferentiated SMO state seen in subjects with musician'sdystonia was not fixed, but rather could be modified by vibration interventions.

Follow-up studies will test whether this reorganization of the SMO in focal hand dystonia goes along with an improvement of hand motor control. If that proves to be the case, prolonged application of such training may be applied in the treatment of musician's dystonia and writer's cramp.

Neurology. 2008;70:304-315. Abstract

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