This study will help determine the safety and effectiveness of BIIB122, compared to placebo (an inactive substance), in people with early-stage Parkinson's disease. The use of BIIB122 in this study is investigational. "Investigational" means that the study drug is currently being tested and is not approved by the U.S. Food and Drug Administration (FDA) or any other health authorities around the world for treating people with PD. The study is expected to last a minimum of 1 year to a maximum of 3 years. You may or may not receive direct medical benefit from participating in this study. Your condition may get better, worse, or stay the same. The information obtained from this study, however, could help other patients with your disease in the future.

The purpose of this study is to see if the study drug BOTOX®, is safe and helps to reduce upper limb essential tremor in adults who experience persistent tremor during movement. The study is 38 weeks long and includes 12 clinic visits. Participants will be injected with the study drug or placebo 3 times over the 38 week period. The participant will have to complete a set of questionnaires and assessments at each visit.