This study will be a single-centre, prospective, single-arm, open-label pilot trial assessing the feasibility and safety of unilateral MR-guided focused ultrasound (MRgFUS) thalamotomy for refractory hand tremor in up to 12 patients with multiple sclerosis (relapsing-remitting, primary progressive or secondary progressive MS). This study will be conducted at the Focused Ultrasound Centre of Excellence and MS Clinic located at Sunnybrook Health Sciences Centre/ University of Toronto. Patients with stable MS and refractory hand tremor providing informed consent will receive MRgFUS thermal ablation of the Vim thalamus contralateral to the most affected side of the body (frequently this will be the dominant hand).

Inclusion Criteria:

• Male or Female aged 18-80 years
• Willing and able to give consent and attend all study visits
• A confirmed diagnosis of medication-refractory, MS-related hand tremor
• No clinical evidence of relapse over 12 months or more before enrollment
• No MRI activity over 3 months or more before enrollment
• Presence of disabling postural or kinetic tremor
• Unsatisfactory tremor response to adequate trials of at least two medications
• Able to communicate sensations during the treatment
• Stable doses of all medications for 30 days prior to and during study

Exclusion Criteria:

• Severe cerebellar dysfunction measured by Scale for the Assessment and Rating of Ataxia (>35 out of 40)
• Severe weakness or sensory loss in arm contralateral to the side of the brain considered for MRgFUS
• Evidence of a superimposed or atypical movement disorder
• Unstable cardiac status such as angina pectoris, congestive heart failure, etc.
• Severe hypertension
• Patients with standard contraindications for MR imaging
• History of abnormal bleeding and/or coagulopathy
• Ischemic or hemorrhagic stroke within 6 months
• Symptoms and signs of increased intracranial pressure (e.g. headache, nausea, vomiting, lethargy, and papilledema)
• Untreated, uncontrolled sleep apnea
• Active or suspected acute or chronic uncontrolled infection
• Receiving anticoagulant or antiplatelet therapy within one week of focused ultrasound procedure or drugs known to increase risk or hemorrhage within one month of MRgFUS surgery
• Not able or willing to tolerate the required prolonged stationary supine position during treatment
• Participating or have participated in another clinical trial in the last 30 days
• Unable to communicate with the investigator and staff
• Presence of neurodegenerative disease or significant cognitive impairment
• Presence of significant cognitive impairment (≤24 on MMSE)
• Uncontrolled major psychiatric disorder or suicidal ideation
• Risk factors for intraoperative or postoperative bleeding or documented coagulopathy
• Presence of brain tumours
• Any illness that in the investigator's opinion preclude participation in this study
• Pregnancy or lactation
• Legal incapacity or limited legal capacity
• Deep Brain Stimulation or a prior stereotactic ablation of the basal ganglia
• A history of seizures within the past year