<qa>  What is the purpose of the study?Deep brain stimulation is a FDA approved method in treating patients with advanced Parkinsons disease when the best medical treatment is insufficient. In this study the aim is to investigate the outcome of deep brain stimulation treated patients in Helsinki and Uusimaa University hospital between 2006 and 2014. This study is a retrospective study based on existing patients medical records and does not include an intervention in the treatment of an individual patient. 