title: Identifying clinical measures that most accurately reflect the progression of disability in Parkinson disease.
abstract: BACKGROUND
  The temporal relationship between disease and disability progression in Parkinson disease (PD) is not well understood. Our objective was to describe the natural, multidimensional trajectory of disability in persons with PD over a two-year period.
  
  
  METHODS
  We conducted a multi-center, prospective cohort study involving four institutions. Data were collected at baseline and at 6-month intervals over 2 years using standardized clinical tests representing three World Health Organization defined disability domains: impairment, activity limitation, and participation restriction. Unadjusted mixed effects growth models characterized trajectories of disability in the three disability domains. The data set was analyzed using restricted maximum likelihood (REML) estimation. Standardized estimates of change were also computed using Cohen's d for each measure.
  
  
  RESULTS
  Of the 266 enrolled participants, we analysed data from individuals who participated in at least 3 assessments (n = 207, 79%). Rates of disability progression over the 2-year period differed across domains. Moderate effects were detected for motor impairment (d = .28) and walking-related activity limitation (gait-related balance (d = .31); gait speed (d = .30)). Marginal effects were noted for upper extremity-related activity limitation (d = .11) and health-related quality of life participation restriction (d = .08).
  
  
  CONCLUSIONS
  The natural trajectory of walking-related activity limitation was the most potent indicator of evolving disability, suggesting that routine assessment of walking and periodic rehabilitation is likely to be warranted for many persons with PD. Natural trajectories of disability provide important comparison data for future intervention studies.
authors.affiliation: Department of Physical Therapy & Athletic Training, Center for Neurorehabilitation, Boston University, Boston, MA, USA. Electronic address: tellis@bu.edu.
authors.forename: Terry D
authors.initials: TD
authors.lastname: Ellis
authors.affiliation: Department of Physical Therapy, University of New England, Portland, ME, USA.
authors.forename: James T
authors.initials: JT
authors.lastname: Cavanaugh
authors.affiliation: Program in Physical Therapy, Washington University School of Medicine, St. Louis, MO, USA.
authors.forename: Gammon M
authors.initials: GM
authors.lastname: Earhart
authors.affiliation: Department of Physical Therapy, School of Health Professions, Samford University, Birmingham, AL, USA.
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Ford
authors.affiliation: Department of Physical Therapy, University of Utah, SLC, UT, USA.
authors.forename: K Bo
authors.initials: KB
authors.lastname: Foreman
authors.affiliation: Department of Physical Therapy, University of Utah, SLC, UT, USA.
authors.forename: Anne
authors.initials: A
authors.lastname: Thackeray
authors.affiliation: Department of Family and Preventative Medicine, Rocky Mountain Center for Occupational and Environmental Health, University of Utah, SLC, UT, USA.
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Thiese
authors.affiliation: Department of Physical Therapy, University of Utah, SLC, UT, USA.
authors.forename: Leland E
authors.initials: LE
authors.lastname: Dibble
mesh_terms: D000368:Aged; D015331:Cohort Studies; D004185:Disability Evaluation; D018450:Disease Progression; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010300:Parkinson Disease / Q000150:complications*; D016138:Walking
keywords: Disability; Natural history; Parkinson disease; Progression