title: Subtypes of Parkinson's disease: state of the field and future directions.
abstract: OBJECTIVE
  Previously, outstanding questions have been identified including the relationship of proposed subtypes to etiology, underlying biology, and prognosis. This situation presents an opportunity for major developments in the field. The review summarizes the progress made over the past 1-2 years.
  
  
  RESULTS
  The etiologic, physiological, and clinical differences between tremor dominant, postural instability gait disorder, and indeterminate phenotypes have been further explored, finding genetic influences, functional imaging and clinical differences. New cluster analyses suggest that nonmotor features are important aspects of Parkinson's disease subtypes, but there was little association found between tremor-dominant /postural instability gait disorder phenotype and nonmotor symptoms. In the cognitive realm, empirically derived subtypes of PD-MCI did not map well onto cognitive subtypes derived using a data-driven approach. In data-driven subtype research, important survival differences between subtypes were identified within the PROPARK database.
  
  
  CONCLUSIONS
  It will be important to revisit PD-MCI classification to consider subtyping based upon data that relate cognitive phenotype to prognosis. Given the traction that traditional motor subtyping has had in the field it would be of value to consider how nonmotor symptom clusters can be used with or alongside the motor subtypes. Finally, incorporating subtypes into clinical trials remains a significant gap in Parkinson's disease research.
authors.affiliation: Morton and Gloria Shulman Movement Disorders Centre and the Edmond J Safra Program in Parkinson's Disease, Toronto Western Hospital, Toronto, Ontario, Canada.
authors.forename: Connie
authors.initials: C
authors.lastname: Marras
mesh_terms: D018450:Disease Progression; D006801:Humans; D010300:Parkinson Disease / Q000145:classification* / Q000175:diagnosis / Q000209:etiology; D012720:Severity of Illness Index