Analysis of stimuli triggering attacks of paroxysmal dystonia induced by exertion

In a patient with a familial form of paroxysmal exertion induced dyskinesia (PED), the efficacy of different stimuli and manoeuvres in triggering dystonic attacks in the arm was studied. As a new approach, transcranial magnetic stimulation (TMS) of the motor cortex was used to trigger motor paroxysms and to monitor cortical excitability during attacks. Motor paroxysms could be provoked by muscle vibration, passive movements, TMS, magnetic stimulation of the brachial plexus, and electrical nerve stimulation. Sham stimulation over the motor cortex and thermal and tactile cutaneous stimuli were ineffective in triggering attacks. It is concluded that dystonic attacks are triggered by proprioceptive afferents rather than cutaneous stimuli or the descending motor command itself. Outside the attacks, motor cortical excitatory and inhibitory neuronal mechanisms as assessed by TMS (response threshold and amplitudes, duration of the contralateral and ipsilateral silent period, corticocortical inhibition, and facilitation) were normal, which underlines the paroxysmal character of the disorder.