The factor structure of the UPDRS motor scores changes during early Parkinson's disease.

Background: Parkinson's disease is a progressive movement disorder recognized by motor symptoms which frequently are measured and longitudinally monitored using the Unified Parkinson Disease Rating Scale (UPDRS). The factor structure of the UPDRS has yet not been analyzed in early PD.

Methods: A population-based cohort of patients with PD was investigated at the time of diagnosis, one year and three years later. The factor structure of the motor section of the UPDRS was explored using Principal Component Analysis (PCA).

Results: We found that the factor structure, the interrelatedness among variables, is different in the first year from diagnosis compared to later stages, but evolves at three years to resemble the factor structure reported for more advanced PD. The only consistent factor throughout the observation period was an axial factor.

Conclusion: The factor structure differs in early versus more advanced PD, but resembles the factor structure of more advanced PD after three years. This is likely due to a higher proportion of subjects with unilateral motor symptoms in early stages of PD together with a changing distribution of motor symptoms with further disease progression. Initiation of PD medication did not alter the factor structure in early PD. The axial factor was the most consistent factor in early PD and could be the most appropriate factor to monitor progression across the earliest stages of PD.