Young-onset Parkinson's disease (YOPD) is defined as Parkinson's disease (PD) with an onset before age 40. It features a focal dystonia as a common early symptom and slower disease progression than older-onset PD. YOPD also sees hyperkinetic dyskinesia and dose-related motor fluctuations relatively early in treatment. Genetics play a stronger role in YOPD than older-onset PD, but the underlying Lewy body pathology is the same.