This study is a single-center, parallel and double-blind study in Movement Disorders Clinic of Shohadaye Tajrish Hospital. Patients, researchers (physicians, outcome assessors) and data analysts are blinded. After assessing the inclusion and exclusion criteria's, patients who assigned the informed consent form, are randomly divided into control and treatment groups.Patients in treatment or placebo groups respectively receive mirtazapine (15 mg) or placebo, once a day for 12 weeks. Primary (anxiety) and secondary (depression, fatigue, sleep disorders, and quality of life) outcomes are evaluated at baseline, and after 4 and 12 weeks of treatments.