Incidence

Most studies on the incidence of PD-D have been based on longitudinal studies of community- based cohorts. These studies have reported incidence rates per 1000 PD patients per year of 54.7 [9] in the UK, 95.3 [10] in Norway, and 112.5 [11] in the United States, indicating that about 10% of people with PD will develop dementia per year.

The relative risk for developing dementia in PD patients compared with people without PD has been reported to be 1.7 [11], 2.6 [9], 4.7 [7], 5.1 [4], and 5.9 [10]. There are several reasons for this variation, including case selection procedures, definitions of dementia, and the use of different estimates of risk.

Whereas most incidence studies have explored the probability of developing dementia in defined PD populations, the frequency of dementia in PD patients as part of a large, prospective, population-based cohort study of the general population has been recently reported in some studies. In the MRC Cognitive Function and Ageing Study [12], all subjects aged 65 years and over living in defined geographical regions of the UK were invited to participate, and more than 13 000 participants had a screening interview. Participants were assessed at baseline and in two follow-up waves. The proportion of PD patients among those with dementia was 2% at 2 years and 3% at 6 years after baseline, compared with 1% in those without dementia, and the total adjusted odds ratio for PD-D compared with non-PD dementia was 3.5 (1.3-9.3).

The Rotterdam study was based on a door-to-door survey of nearly 8000 people aged 55 years and above at baseline in 1990-3 [7]. Two follow-up visits were performed, and patients were diagnosed with prevalent PD at baseline (n = 99) and incident PD during follow-up (n = 67). The mean follow-up time was 6.9 years (4.3 years in the incident PD group). During follow-up, 15% of the prevalent PD group developed dementia compared with 4.9% of the control group, with a hazard ratio of 2.80 (1.79-4.38). In the incident cohort, the hazard ratio was 4.74 (2.49-9.02). The association of PD with dementia was more pronounced in those with at least one APOE e4 allele, compared with e3 carriers.

The majority of ‘incidence’ studies of PD-D have been longitudinal studies of prevalence cohorts, meaning that patients with a variety of disease durations have been followed. Since the risk for developing dementia depends on the duration of disease, variations among cohorts in the duration of PD will affect the incidence of dementia. Thus, following patients from the onset of their disease provides a more accurate and representative estimate of the incidence of PD-D. In the first study of PD-D based on an incident PD cohort, the CamPaIGN (Cambridgeshire Parkinson’s Incidence from GP to Neurologist) study, 180 PD patients were reexamined 3 and 5 years after baseline. The annual incidence of dementia was 30 (16-53) per 1000 person-years [13]. In addition to the shorter duration of disease, the lower incidence of dementia may also be related to the younger age at baseline in this cohort compared with most prevalence studies (Table 2.1). In a subsequent analysis of the CamPaIGN cohort after 10 years, the incidence of dementia in the PD cohort was 54.7 per 1000 person-years, which was 2.6 times higher than the estimated incidence of dementia in the general Cambridgeshire population aged over 65 years [9]. In a prospective population-based incidence study of 182 cases in Norway, the ParkWest study, the overall incidence of PD-D was 20.5 per 1000 person-years [14].

In an incidence study over 15 years in Olmsted, MN, United States, 542 incident cases of PD were identified, and the incidence of PD-D in the population was 2.5 per 100 000 person-years. The incidence of PD-D was similar in men and women, but increased with age and was 47.0 in people aged 80-99 years [15].

Table 2.1 Studies of the incidence and relative risk for dementia in patients with PD

Study

Population

No. of PD patients

Age at

baseline

(years)

Duration of PD at baseline assessment (years)

Rate/1000 per year

Relative risk of dementia in PD (95% CI)

Country

Mayeux, 1990 [16]

Incidenta

249

71.4

4.75

69

USA, New York

Marder, 1995 [17]

Communitya

140

71

7

113

1.7 (1.1-2.7)

USA, New York

Hughes, 2000 [18]

Hospital

83

64

4

43

-

UK

Aarsland, 2001 [10]

Communitya

130

70

8.5

95

5.9 (3.9-9.1)

Norway

Hobson, 2005 [19]

Communitya

86

74

7

107

5.1 (2.1-12.5)

North Wales

De Lau, 2005 [7]

Incidentb

67

-

-

-

4.7 (2.5-9.0)

Netherlands

Hely,

2005 [21], 2008 [20]

Incidenta

136

71.6

10.9

44

2.3

Australia

Buter, 2008 [22]

Incident

233

75

8

82

5.9

Norway

Williams- Gray, 2013 [9]

Incidenta

121

70.2

54.7

2.5

UK

Savica, 2013 [15]

Communitya

542c

80+

47

USA,

Minnesota

Pedersen, 2013 [14]

Incidenta

182

67.5

2.3

20.5

Norway

Perez, 2012 [23]

Communitya

44

82.4

6.8

74

2.5 (1.55-3.95)

France

a Aged 65+.

b Door-to-door survey of whole population. c Cases of parkinsonism, including DLB and PD-D.

 
Source
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