Parkinson disease (PD) is one of the most common age-related neurodegenerative diseases, affecting 1% of the population older than 60 years of age and 0.3% of the general population. It is a chronic, progressive disorder whose spectrum of clinical features can affect an individual for more than 40 years of his or her natural life span. Idiopathic PD is clinically and pathologically a heterogeneous disorder that is variable in progression and phenotypic expression.
With no known biomarker for disease diagnosis or progression, clinicians and scientists have long recognized the importance of classifying patterns of PD into distinct subtypes in order to identify features that influence rate of progression and response to interventions and that correlate with neural and biochemical substrates of disease pathogenesis. To that end, Hoehn and Yahr in their landmark study of 1967 may have been among the earliest clinical scientists to recognize the heterogeneity of PD by describing that “the clinical picture of one may be dominated by tremor, of another by rigidity or akinesia,”1(p433) and subsequently developed a scale to rate how symptoms of PD progress. Disability staging by the Hoehn and Yahr scale continues to be used universally in rating disease and progression as it relates to walking and postural stability.1 More recently, the global Unified Parkinson’s Disease Rating Scale (with its subscales that assess mentation, mood and behavior, activities of daily living, motor and nonmotor symptoms, and complications of therapy) has introduced the importance of nonmotor symptoms in impacting quality of life. The Montreal Cognitive Assessment, which allows for a bedside evaluation of 8 domains of cognition, has been added to the repertoire of clinical measures that inform clinicians of the functional state of the patient with PD. Although powerfully informative, these measures, which are routinely performed by neurologists, and the standard of care for movement disorders fall short in determining prognosis or predicting rate of change for patients with PD.
Schiess MC, Suescun J. Clinical Determinants of Progression of Parkinson Disease: Predicting Prognosis by Subtype. JAMA Neurol. 2015;72(8):859–860. doi:10.1001/jamaneurol.2015.1067
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