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Emphasis has been placed on the importance of making a positive diagnosis in functional (psychogenic) movement disorders. It has been suggested that ‘laboratory-supported criteria’ should be developed where electrophysiological and other tests can improve the level of certainty of diagnosis.1 Such criteria have been suggested for functional myoclonus1 and functional tremor.2 We report a preliminary study aimed at developing similar criteria for patients with ‘fixed’ dystonia (FD),3 a common presentation of functional dystonia.4
We performed surface electromyography (EMG) to assess the motor unit action potentials (MUAPs) in agonists and antagonists of the affected limb in patients with either functional FD (n=9, etable 1A), documented or clinically established following Fahn and Williams4 criteria, or acquired (n=9, etable 1B) dystonia (AD) due to brain lesions affecting an upper or lower limb. The inclusion criteria for the AD cases were the presence of brain lesions consistent with the clinical pattern of the dystonia.
We recorded EMG at rest and during a reaction time (RT) task where patients were asked to attempt to move in the opposite direction to the habitual limb posture after an auditory ‘go’ cue. Most participants had been receiving chronic treatment with botulinum toxin injections; however, they had not received the treatment for more than 3 months before the study. Dramatic immediate response to botulinum toxin is reported in similar patients. Participants were seated in a comfortable armchair. EMG was monitored with Ag/AgCl surface electrodes positioned on the agonist and antagonist muscles studied. The EMG activity was recorded at rest for 5 s, and then each trial included two auditory cues (100 ms, rise–fall time 20/20 ms, frequency 1000–5000 kHz). The initial sound was the warning stimulus and the second was the ‘go’ stimulus. The patients were asked to perform an isometric muscle contraction of muscles opposing …
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