title: The effect of age of onset of PD on risk of dementia.
abstract: BACKGROUND
  Dementia occurs in the majority of patients with Parkinson's disease (PD). Late onset of PD has been reported to be associated with a higher risk for dementia. However, age at onset (AAO) and age at baseline assessment are often correlated. The aim of this study was to explore whether AAO of PD symptoms is a risk factor for dementia independent of the general effect of age.
  
  
  METHODS
  Two community-based studies of PD in New York (n=281) and Rogaland county, Norway (n=227) and two population-based groups of healthy elderly from New York (n=180) and Odense, Denmark (n=2414) were followed prospectively for 3-4 years and assessed for dementia according to DSM-IIIR. All PD and control cases underwent neurological examination and were followed with neurological and neuropsychological assessments. We used Cox proportional hazards regression based on three different time scales to explore the effect of AAO of PD on risk of dementia, adjusting for age at baseline and other demographic and clinical variables.
  
  
  RESULTS
  In both PD groups and in the pooled analyses, there was a significant effect of age at baseline assessment on the time to develop dementia, but there was no effect of AAO independent of age itself. Consistent with these results, there was no increased relative effect of age on the time to develop dementia in PD cases compared with controls.
  
  
  CONCLUSIONS
  This study shows that it is the general effect of age, rather than AAO that is associated with incident dementia in subjects with PD.
authors.affiliation: Norwegian Centre for Movement Disorders, Stavanger University Hospital, Arm Hansen v 20, N-4005, Stavanger, Norway. daa@sir.no
authors.forename: D
authors.initials: D
authors.lastname: Aarsland
authors.forename: J T
authors.initials: JT
authors.lastname: Kvaløy
authors.forename: K
authors.initials: K
authors.lastname: Andersen
authors.forename: J P
authors.initials: JP
authors.lastname: Larsen
authors.forename: M X
authors.initials: MX
authors.lastname: Tang
authors.forename: A
authors.initials: A
authors.lastname: Lolk
authors.forename: P
authors.initials: P
authors.lastname: Kragh-Sørensen
authors.forename: K
authors.initials: K
authors.lastname: Marder
mesh_terms: D017668:Age of Onset; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D003704:Dementia / Q000453:epidemiology* / Q000209:etiology*; D005260:Female; D006801:Humans; D008297:Male; D009483:Neuropsychological Tests; D010300:Parkinson Disease / Q000150:complications* / Q000453:epidemiology*; D016016:Proportional Hazards Models; D012306:Risk; D016019:Survival Analysis; D013997:Time Factors