Factors Determining the Efficacy of Botulinum Toxin for Arm Tremor in Dystonia

Tremor occurs in up to 55% of dystonia patients, which is known as dystonic tremor syndrome (DTS). Tremor can be present in the body part affected by dystonia (dystonic tremor, DT), or an unaffected body part (tremor associated with dystonia, TAWD). DTS can be treated with botulinum neurotoxin (BoNT) injections, but BoNT is effective in only about 60-70% of patients. It is unknown which patients benefit most from BoNT treatment. The investigators aim to explore the associations between clinical and pathophysiological tremor characteristics and BoNT efficacy. To do so, the investigatorswill measure clinical, electrophysiological, ultrasonographic and (functional) magnetic resonance imaging ((f)MRI) characteristics before the start of BoNT treatment and measure BoNT efficacy after three three-monthly BoNT sessions.

Inclusion Criteria: * Clinical diagnosis of dystonic tremor or tremor associated with dystonia according to the 2018 consensus statement on the classification of tremors * Tremor of one or both upper extremities * Starting botulinum toxin injections as part of normal clinical practice * Age ≥ 18 years Exclusion Criteria: * Acquired aetiology of dystonic tremor syndrome * Previous botulinum toxin treatment of the to be treated upper extremity for ≥ 4 consecutive sessions * In case of previous botulinum toxin treatment of the to be treated upper extremity for ≤3 consecutive sessions: the last botulinum toxin injections ≤ 6 months before study enrolment * Unstable dose medications for dystonia and tremor ≤ 1 month before study enrolment * Deep brain stimulation implantation ≤ 6 months before study enrolment * Unstable deep brain stimulation variables ≤ 1 month before study enrolment * Comorbidity interfering with study participation * Known hypersensitivity for components of Dysport * Infection at the upper extremity * Pregnancy, trying to conceive and breastfeeding * Insufficient knowledge of the Dutch or English language Exclusion criteria for MRI scanning: * Contraindications for MRI (e.g. previous brain surgery, claustrophobia, active implant, epilepsy, metal objects in the upper body that are incompatible with MRI) * Moderate to severe head tremor while lying supine (to avoid artefacts caused by extensive head motion during scanning). * Inability to provoke postural tremor while lying supine.