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Investigating the plastic effects of repetitive paired associative stimulation (rPAS) in Dystonia

Monica Villegas Photo

Skidmore College (New York)

Summer Students: Monica Villegas & Alina Esquenazi

Mentor: Vesper Ramos, MD 

PI: Mark Hallett, MD


Dystonia is a syndrome characterized by sustained muscle contractions resulting in abnormal movements or postures. Currently, no physiologic test is available to distinguish psychogenic and organic dystonia; instead the diagnosis is made using the Fahn and William’s criteria, which defines psychogenic dystonia as inconsistent over time, with give way weakness, and obvious psychiatric comorbidities. The purpose of this project is to gather data to develop a neurophysiologic test to differentiate organic and psychogenic dystonia patients. First, the differences in temporal discrimination threshold (TDT) between healthy controls, psychogenic, and organic dystonia patients will be investigated. TDT is the shortest interstimulus interval at which the subject is able to perceive two consecutive tactile stimuli as occurring separately. Studies have shown that patients with dystonia have higher TDTs than healthy controls. In order to obtain normative data to compare the TDTs of dystonia patients, a substudy involves the measurement of TDTs for 100 normal volunteers from ages 18-79. Second, we will investigate the plastic effect of repetitive paired associative stimulation (rPAS) among the patient groups. rPAS is an established experimental technique to study sensorimotor integration and plasticity in dystonia patients through a timed interaction between two stimuli: transcranial magnetic stimulation (TMS) over the primary motor cortex and peripheral electrical stimulation of the contralateral median nerve. A previous study showed that organic dystonia patients have abnormally plastic brains and show a loss of topographical specificity, marked by increased responsiveness to repetitive TMS compared to psychogenic dystonia patients and healthy volunteers. The loss of specificity can be demonstrated by comparing the change in motor

evoked potential (MEP) amplitude of the flexor digitum indicis, innervated by the ulnar nerve, and the abductor pollicis brevis (APB), innervated by the median nerve, after rPAS of the APB hotspot. For this pilot study, 6 organic dystonia patients, 6 psychogenic dystonia patients, and 6 age-matched healthy volunteers will be recruited. Outcomes will be the change in MEP amplitude between pre-rPAS and 30 minutes post, the changes in motor recruitment curve parameters between pre-rPAS up to 60 minutes post, and the difference in TDTs among groups.

Last Modified November 27, 2013