Bilateral Essential Tremor Treatment With Gamma Knife (BEST-GK)

Combined Phase II/III, multi-center, prospective, single-blinded trial. Ten (10) patients with essential tremor who previously underwent successful and uncomplicated GK thalamotomy for essential tremor will undergo a contralateral treatment. The incidence of side effects will be determined at 3 months postoperatively, graded per the CTCAE v5 and analyzed by a data safety monitoring board. Upon successful review, this Phase II trial will be converted to a Phase III trial of utility that will enrol 40 additional patients. The primary outcome will be the change in QUEST score at 12 months postoperatively, as well as a patient-reported assessment of Health Utility. Secondary outcomes will include objective tremor, gait and speech assessments (filmed and scored by blinded evaluators), as well as quality of life questionnaires and adverse events questionnaires. Outcomes will be assessed at baseline, as well as 3, 6, 12, 24 and 36 months post-operatively.

Gamma Knife Thalamotomy, Tremor, Essential Tremor, Dyskinesias, Neurologic Manifestations, Nervous System Diseases, Movement Disorders, Central Nervous System Diseases

Participants will undergo a thalamotomy contralateral to their previous treatment with Gamma Knife using a frame-based, Gamma Knife Perfexion or Icon unit (Elekta, Stockholm, Sweden).

Ablation of contralateral VIM nucleus of thalamus with GK using a frame-based, Gamma Knife Perfexion or Icon unit (Elekta, Stockholm, Sweden).

Difference in QUEST score between baseline and the 12-month follow-up, with statistical significance defined as p = 0.05 and a minimum clinically important difference (MCID) defined as 5

A patient-reported assessment of utility after 12 months using the question "Knowing what you know now, would you do this treatment again?".

Patient-reported and blinded observer rating of the patient's ability to walk (Numerical rating scale (NRS-11) from 0 to 10, with 10 being best)

Patient-reported and blinded observer rating of the patient's ability to speak (Numerical rating scale (NRS-11) from 0 to 10, with 10 being best)

Assessment of tremor using the Clinical Rating Scale for Tremor (CRST) parts A (0-88) and B (0-36), higher scores correspond to a worse tremor).

QUality of life in ESsential Tremor (QUEST) questionnaire (0-100 percent, higher scores correspond to a worse quality of life)

Part C of the Clinical Rating Scale for Tremor (CRST) (0-32, higher scores correspond to a worse quality of life)

Health Utility measured using the EQ-5D-5L and computed using the Canadian preference weights. Values range from 0 to 1, with 1 representing a perfect health.

Inclusion Criteria: Patients must be ≥ 18 years The patient has a primary diagnosis of ET or ET plus confirmed by a movement disorder neurologist; The patient previously underwent a GK thalamotomy (> 12 months ago); The tremor on the untreated side negatively impacts the patients' quality of life; The patient wants treatment of the contralateral side. Exclusion Criteria: Clinically relevant gait and/or balance impairment (e.g. wide-based, more than 2 falls/months, wheelchair, walker, cane use); Clinically relevant speech impairment (e.g. impairment of intelligibility); Inability to comply with the follow-up schedule; Refusal of the treating physician.