Parkinson's disease and aging: Same or different process?

Parkinson's disease (PD), which probably commences in the olfactory bulb and nuclei of IX and X, may represent accelerated normal aging or a disease specific process. The olfactory and motor disorders are associated in PD, so that smell loss should reflect the underlying disease course. Information on age-associated decline can be obtained by single measurement from cross-sectional analysis of patients and controls of different ages, because the observed between-subject picture may reflect a similar within-subject pattern. University of Pennsylvania Smell Identification Test (UPSIT) scores were examined for aging effects in 263 controls and 266 PD patients. Three models were applied that are (a) simple linear regression and (b) quadratic term in age: (i) assuming two parallel curves and (ii) allowing for two separate curves. Both groups declined with age. All three models were fitted successfully to the cross-sectional data and none suggested that the lower mean UPSIT score in patients compared to controls was a premature ageing effect. The mean PD-UPSIT score even at the age of 40, was lower than the control mean at the upper life-span limit, implied that the decline in olfaction in PD, is faster than simple aging. Extrapolation back in time suggested onset before birth, which is unlikely and implies that at some point there is a more rapid decline than observed in this sample. On the basis of olfactory measurement, our data are consistent with the proposal that PD starts as an acute event, followed by further disease progression more rapid than simple aging. (c) 2007 Movement Disorder Society.