Parkinson’s disease beyond motor symptoms: a review of the validity and limitations of current classification of non-motor symptoms

Non-motor symptoms (NMS) in Parkinson's disease (PD) are highly prevalent and significantly impact quality of life. However, they are under-recognized and undertreated. Current classification systems of NMS inadequately reflect the phenomenological complexity and interrelationship of these symptoms. In this narrative review, we synthesized recent evidence about NMS in PD to critically appraise the current symptom-based classification. We also explore emerging classification models that may better capture the multifaceted nature of these symptoms.