Department of Psychiatry, University of Leipzig, Germany
Correspondence: Steffi G. Riedel-Heller, Department of Psychiatry, University of Leipzig, Johannisallee 20, D-04317 Leipzig, Germany. Tel: +49-341-97 24 530; fax: +49-341-97 24 539; e-mail: ries{at}medizin.uni-leipzig.de
Declaration of interest Supported by Interdisziplinaeres Zentrum für Klinische Forschung (IZKF), University of Leipzig (01KS9504, project C7 79934700).
See part I, pp.
250254, this issue.
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ABSTRACT |
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Aims To compare incidence rates of dementia according to DSM-III-R and ICD-10.
Method A two-wave community study was conducted (n=1692, age 75+ years follow-up period 1.6 years). Cognitive function was assessed by the Structured Interview for Diagnosis of Dementia of Alzheimer Type, Multiinfarct Dementia and Dementia of other Aetiology according to ICD-10 and DSM-III-R (SIDAM).
Results The annual incidence rate for dementia by applying different case definitions was found to be quite similar (DSM-III-R: 47.4 (95% CI=36.1-61.2) per 1000 person-years; ICD-10: 45.8 (95% CI=35.0-59.0) per 1000 person-years). Age-specific incidence rates increase steeply with age.
Conclusions The impact of different case definitions on incidence rates of dementia appears limited if case definitions and case-finding procedures at baseline and follow-up are applied consistently.
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INTRODUCTION |
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METHOD |
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Instruments
The same assessment procedure as in the baseline assessment was used in the
follow-up (see Part 1: Riedel-Heller
et al, 2001, this issue). A fully structured face to face
interview was administered to the study subjects by trained female physicians
and psychologists. The core component assessing the cognitive function was the
Structured Interview for Diagnosis of Dementia of Alzheimer Type, Multiinfarct
Dementia and Dementia of other Aetiology according to ICD-10 and DSM-III-R
(SIDAM; Zaudig et al,
1991a,b).
The SIDAM comprises a test performance part (55 questions, including all items
of the Mini-Mental State Examination, MMSE;
Folstein et al,
1975), a section for clinical judgement and third-party
information used to determine if psychosocial impairment is present. If an
individual scored below 24 on the MMSE or reported impairment of the
activities of daily living that was not caused by physical or sensory
deficits, a comprehensive informant interview was conducted. The SIDAM
diagnostic algorithms were available to derive ICD-10 and DSM-III-R diagnoses
of dementia. Consensus conferences were held on each potential case to discuss
the psychosocial impairment.
If relatives of study subjects definitely refused participation on behalf of the elderly person cared for, or the study participant died between baseline assessment and follow-up, we offered the option of a fully structured proxy interview. Instead of cognitive testing, the Clinical Dementia Rating Scale (CDR) was used for assessment of cognitive function (Hughes et al, 1982). Concerning terminal decline of cognitive function, assessment focus in deceased individuals was based on the individuals' cognitive status 3 months prior to death.
The results presented are based primarily on those individuals interviewed face to face. This decision was made because determination of mild dementia solely by proxy interview harbours problems and may differ according to closeness of relationship, education and stereotypes held by caregivers. Despite this limitation, combined incidence rates of those interviewed face to face and by proxy will be given and discussed.
Analysis
For analysis, the person-years at risk method was used.
Incidence rates were estimated as the number of new cases of dementia divided
by the person-years at risk. The at-risk population were those without any
diagnosis of dementia at baseline. Age bands were based on the age at
prevalence wave. Person-years for those without dementia were calculated as
the time between prevalence and follow-up examination or, in the case of
death, the date of death. For individuals with dementia, the time of
occurrence of the disease was assumed to be the midpoint between baseline and
follow-up interview or date of death. Person-years then were calculated
accordingly. Study subjects who refused the incidence wave or could not be
traced were excluded. In order to analyse possible non-response bias, the
t-test was applied.
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RESULTS |
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A total of 787 (70%) subjects of the population at risk were examined face to face at baseline and follow-up; 59 of the 787 individuals (1244.32 person-years) developed dementia according to DSM-III-R. Thus the annual incidence rate for dementia for those individuals aged 75+ years is 47.4 (95% CI=36.1-61.2) per 1000 person-years. Age-specific incidence rates per 1000 person-years increase steeply with age: 15.8, 51.7, 105.3 and 165.7 for those aged 75-79, 80-84, 85-89 and 90+ years, respectively. Incidence rates do not differ significantly between men and women. Age- and gender-specific incidence rates are summarised in Table 1.
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Using this combined strategy (subjects who died and information on cognitive status+individuals alive who were classified solely based on information by proxy+individuals interviewed face to face), 1022 individuals were examined (1519.78 person-years). These comprise 90.9% of the population at risk and 86 cases of dementia were identified. This approach yields an annual incidence rate of 56.6 per 1000 person-years (95% CI=45.3-69.9), which is higher than the rate covering solely individuals interviewed face to face at baseline and follow-up. Age-specific incidence per 1000 person-years revealed 16.3 (95% CI=8.4-28.5), 70.8 (95% CI=47.8-101.1), 110.3 (95% CI=74.9-156.6) and 163.4 (95% CI=87.0-279.3) in the age groups 75-79, 80-84 and 90+ years, respectively.
Incidence of dementia according to ICD-10
Of the 1378 subjects available for follow-up, 191 participants were
identified as dementia cases according to ICD-10 at baseline. As a result,
1187 participants constitute the population at risk for follow-up.
A total of 827 (69.7%) participants of the population at risk were examined face to face at baseline and follow-up; 60 of the 827 individuals interviewed face to face (1309.75 person-years) developed dementia according to ICD-10. Thus, the incidence rate for individuals aged 75+ years per 1000 person-years is 45.8 (95% CI=35.0-59.0). Age-specific incidence rates are 15.6, 48.3, 90.8 and 174.6 per 1000 person-years for those aged 75-79, 80-84, 85-89 and 90+ years, respectively. Age- and gender-specific incidence rates are summarised in Table 2.
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Using the combined strategy, 102 dementia cases were found in 1078 individuals (=1593.42 person-years) examined. Information on 90.8% of the population at risk was gathered. This yields an annual incidence rate of 64.0 per 1000 person-years (95% CI=52.2-77.7), which is higher than the rate covering solely individuals interviewed face to face at baseline and follow-up. Age-specific incidence per 1000 person-years revealed 18.8 (95% CI=10.3-31.5), 73.3 (95% CI=50.5-102.9), 117.5 (95% CI=82.3-162.6) and 210.2 (95% CI=126.6-328.3) in the age groups 75-79, 80-84, 85-89 and 90+ years, respectively.
Figure 3 compares age-specific incidence rates of dementia according to DSM-III-R and ICD-10. Results indicate that dementia incidence increases steeply with age. No levelling off in the oldest of the elderly, at least up to age 90 years, is seen. Incidence rates according to DSM-III-R and ICD-10 appear to be quite similar and they do not differ significantly.
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The MMSE and age at baseline were used to study possible non-response bias. Calculations were performed based on the population at risk according to DSM-III-R. First, individuals interviewed face to face are compared with the rest of the incidence sample or population at risk (individuals alive interviewed by proxy, deceased individuals, study subjects who refused or could not be traced). Individuals interviewed face to face at follow-up differ significantly from the rest of the incidence sample: they were younger (mean age=81.2 (s.d.=4.6) v. 82.9 (s.d.=5.3) years, t=5.4, P=0.000) and had a higher MMSE (mean MMSE=27.1 (s.d.=2.1) v. 26.1 (s.d.=3.2), t=5.7, P=0.000) at baseline. Second, responders (regardless of how they were interviewed) are compared with non-responders (subjects who refused participation, could not be traced or died without further information). The compared groups do not differ significantly regarding age (mean age=81.8 (s.d.=4.9) v. 81.7 (s.d.=4.9) years, t=0.2, P=0.822) or MMSE (mean MMSE=26.4 (s.d.=2.2) v. 26.9 (s.d.=2.5), t=1.7, P=0.084).
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DISCUSSION |
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The 59 ICD-10 incidence cases comprise individuals who were diagnosed as having dementia according to both ICD-10 and DSM-III-R (n=38) and individuals having ICD-10 diagnosis only (n=21). The prevalence wave of LEILA75+ revealed that some individuals were judged to have mild dementia by DSM-III-R who were not considered using ICD-10 criteria. Although showing a considerable level of disturbance (enough to fulfil DSM-III-R criteria), these individuals entered the ICD-10 dementia-free risk population to be followed up. The latter finally were included in the follow-up, comprising 21 persons with solely ICD-10 diagnosis. The 60 DSM-III-R incidence cases comprise 38 individuals with both ICD-10 and DSM-III-R diagnosis and 22 individuals who were diagnosed solely by DSM-III-R. They were not caught by ICD-10 because these criteria set a higher threshold, especially requiring decline in emotional control or motivation or a change in social behaviour. Because the number of individuals solely diagnosed by ICD-10 and DSM-III-R are almost identical and the number of person-years for the ICD-10 and DSM-III-R risk population (1309.77 v. 1244.32) do not differ very much, similar incidence rates were found. The impact of different case definitions on incidence rates appears limited if case definition and case-finding procedures at baseline and follow-up are applied consistently.
Age-specific incidence rates
Regardless of which diagnostic classification system was used, age-specific
incidence rates increase steeply with age. No levelling off in the oldest of
the elderly was found. However, estimates, especially of those aged 90+ years,
are available on relatively small numbers of cases, resulting in large
confidence intervals. This notion is supported by the latest meta-analysis on
incidence data (Jorm & Jolley,
1998). Comparison with other studies reporting on the incidence of
dementia according to DSM-III-R, including mild, moderate and severe forms,
reveals the following results: for individuals aged 75-79 years our rates fall
within the range of values suggested in all other recent studies investigating
this age group (Paykel et al,
1994; Clarke et al,
1996; Fratiglioni et
al, 1997; Ott et
al, 1998; Andersen et
al, 1999). For the older age groups our rates exceed the
rates of some of the recent field studies
(Paykel et al, 1994;
Clarke et al, 1996;
Fratiglioni et al,
1997; Ott et al,
1998). However, they compare with the results reported by Andersen
et al (1999),
Aevarsson & Skoog (1996),
Johansson & Zarit (1995),
Fichter et al (1996)
and Copeland et al
(1999). Incidence rates mirror
those predicted by a recent meta-analysis conducted by Jorm & Jolley
(1998), which is shown in
Fig. 5.
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Gender-related pattern
Schröppel et al
(1996) reviewed gender-related
patterns in incidence studies. They suggested an increased incidence of
dementia for men up to the age of 70-80 years. In older age groups either the
same incidence for women and men or an increased incidence for women was
reported. In our study the short follow-up interval and high response rate may
have avoided the selective death of males with incidence of dementia in the
follow-up interim, which may have accounted for the apparently higher rates in
women in other studies. Although incidence rates showed no statistically
significant difference between men and women, our results mainly follow the
pattern described by Schröppel et al
(1996).
Potential bias owing to selective refusal or death
One of the major concerns of population-based longitudinal studies is the
potential bias owing to selective refusal or death. This holds especially true
in studies on dementia, because dementia is associated with an increased
mortality (Burns et al,
1991). According to the age span investigated and the length of
the follow-up intervals, death rates in recent studies on dementia vary
considerably, reaching up to half of the population at risk
(Gussekloo et al,
1995). Response rates of the survivors do vary from 69% to 86%
(Boothby et al, 1994;
Aevarsson & Skoog, 1996). Only a few studies assessed and included information on study subjects who
died or refused to participate (Bickel
& Cooper, 1994; Aevarsson
& Skoog, 1996; Clarke
et al, 1996;
Fratiglioni et al,
1997; Ott et al,
1998). Hospital documents, general practitioner records or death
certificates were used, despite their known unreliability regarding dementia
diagnosis (Teresi & Holmes,
1997; Losonczy et al,
1998). In our study, for those individuals not available for face
to face interviews, all effort was made to collect information through close
relatives. Using this combined strategy, information from almost all deceased
study subjects and a substantial number of the refusers (often fragile
individuals shielded by their caring relatives in order to avoid stress
related to the examination) was collected. This covered over 90% of the study
subjects. Incidence rates for those individuals interviewed face to face and
for the so-called combined strategy, including information on those who were
deceased or refusers, were reported separately. As expected, incidence rates
based solely on face to face interviews are lower than those calculated using
the combined strategy. Incidence rates based on face to face interviews only
are conservative estimates because it has been shown that drop-out was
selective in favour of younger and cognitively less-impaired study
participants. Given the short follow-up interval, selection occurred mainly
owing to selective refusal (in only 6/114 deceased individuals dementia was
diagnosed). Using the combined strategy, response bias is unlikely because
there was no difference found regarding age and MMSE between responder and
non-responder.
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Clinical Implications and Limitations |
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LIMITATIONS
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Received for publication September 21, 2000. Revision received February 16, 2001. Accepted for publication February 16, 2001.
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