Introduction to Epidemiology – Assignment | Online Assignment

LO1. Understand key concepts and methods of epidemiology. LO2. Calculate and interpret basic epidemiology measures LO3. Draw appropriate inferences from epidemiologic data. LO4. Evaluate the strengths and weaknesses of data from epidemiologic studies. LO5. Apply evidence from epidemiologic investigations to public health policy and practice Assessment 2 – Examples of questions related to exam paper How do you think, why this study was necessary to conduct? What was the study design? How do you think if selected study design was appropriate for particular study? What was the research question of the study? What was the main exposure and outcome? What is the variables/variables of the study?Background Previous studies suggested a protective
effect of smoking on Alzheimer’s disease, but most were
case-control studies based on prevalent cases. The
findings of prospective studies on the association between
smoking and the risk of dementia are inconclusive.
Methods We did a population-based follow-up study of
elderly people who were initially free of dementia. 6870
people aged 55 years and older agreed to take part.
Smoking history was taken at baseline and participants
were classified as never smokers, former smokers, and
current smokers. During follow-up, we recorded all incident
cases of dementia. We used never smokers as the
reference category to calculate relative risks of dementia
and Alzheimer’s disease by Cox proportional hazards
regression, after adjustment for age, sex, education, and
alcohol intake. We also examined modification of risk by
age, sex, and the apolipoprotein E (APOE) genotype.
Findings During mean follow-up of 2·1 (range 1·5–3·4)
years, 146 incident cases of dementia were detected, of
which 105 were Alzheimer’s disease. Compared with never
smokers, smokers had an increased risk of dementia
(relative risk 2·2 [95% CI 1·3–3·6]) and Alzheimer’s
disease (2·3 [1·3–4·1]). Smoking was a strong risk factor
for Alzheimer’s disease in individuals without the APOE4
allele (4·6 [1·5–14·2]), but had no effect in participants
with this allele (0·6 [0·1–4·8]).
Interpretation Smoking was associated with a doubling of
the risk of dementia and Alzheimer’s disease. Our finding
that carriers of the APOE4 had no increased risk of
dementia suggests an interaction between smoking and
the APOE4 genotype in the aetiology of Alzheimer’s
disease.
Lancet 1998; 351: 1840–43
Introduction
The prevalence of dementia increases greatly with age: at
least 30% of people aged 85 years and older have the
disorder.1 Alzheimer’s disease and vascular dementia are
the main subtypes of dementia, and account for about
90% of affected patients.1 Smoking is a risk factor for
vascular disease, including atherosclerosis and
thrombosis, and increases the risk of vascular dementia.2,3
Reduction of cerebral perfusion is an acute effect of
cigarette smoking and may be harmful in elderly people
with a compromised cerebral circulation.4 Vascular
involvement is probably more important than previously
thought in the pathogenesis of Alzheimer’s disease.5–7
Nonetheless, an inverse association has been reported
between smoking and Alzheimer’s disease.8Van Duijn and
colleagues9 showed that in early-onset Alzheimer’s
disease, the possible protective effect of smoking was
limited to carriers of the apolipoprotein 4 (APOE4)
allele. Although there are several possible explanations for
a protective effect of smoking on Alzheimer’s disease,8,9
this finding in previous studies could reflect bias.10
Smoking also strongly increases risk of premature death,
and those smokers who survive to old age may be a more
selected group than non-smokers of similar age.11
In this prospective population-based study we
examined the association of cigarette smoking with
dementia and Alzheimer’s disease, and whether this
relation depended on age, sex, and the APOE genotype.
Methods
Study design and population
The Rotterdam Study is a community-based prospective cohort
study of individuals aged 55 years and older,12 which was
undertaken with the approval of the local medical ethics
committee. All 10 275 residents of a Rotterdam suburb who
were older than 55 years were eligible. 7983 (78%) residents
agreed to take part and gave their informed consent and
permission for us to obtain medical information from their
family doctor and medical specialists. Baseline examinations took
place between 1990 and 1993 and comprised a home interview
and medical examinations at the study centre or at their
institution for those living in residential homes. At baseline we
assessed 7528 (94%) individuals for dementia,1 474 of whom
were diagnosed with the disorder and excluded from the study.
Of the 7054 without dementia, 184 were excluded because of
missing data on smoking history. The 6870 participants were
followed-up for about 2 years, until the second round of
examinations. For individuals who were not examined in person,
follow-up was completed by assessment of their medical files.
Dementia diagnosis and assessment
For the assessment of dementia, the same protocol was used at
baseline and at follow-up which we have described in more detail
elsewhere.13 Briefly, all participants were screened with a brief
test of cognition (mini mental state examination14 and the
geriatric mental state schedule, organic level15); those with
positive results underwent further cognitive testing, and a carer
or friend was interviewed about the daily funtioning of

Individuals in whom dementia was suspected were
examined by a behavioural neurologist, underwent neuropsychological
testing and, if possible, had a brain scan by
magnetic-resonance imaging.13 For participants who could not
be re-examined in person, information was obtained from
general practitioners and the regional institute for outpatient
mental health care (RIAGG), which covers the entire study
population. In the Netherlands, the RIAGG is responsible for
dementia-care-facility indications. Examinations by the RIAGG
include (carer) interviews, neurological and neuropsychological
testing, blood biochemistry, and syphilis serology. To avoid
misdiagnosis of terminal decline as dementia, we did not
diagnose dementia in individuals with unexplained mental
deterioration within 6 months before death.
The diagnosis of dementia was made on the basis of Diagnostic
and Statistical Manual of Mental Disorders (DSM-III-R) criteria16
by a panel of study physicians, a neurologist, and a
neuropsychologist, who reviewed all existing information. We
made a diagnosis of Alzheimer’s disease and vascular dementia
according to internationally accepted criteria.17,18 Patients with
Alzheimer’s disease were further classified as those with and
without cerebrovascular disease.17 For other dementias, we used
DSM-III-R criteria.16 Cerebrovascular disease was identified by a
history of stroke or transient ischaemic attack, or by evidence of
cerebrovascular lesions on brain scans.
At baseline, we asked participants about their current and past
smoking habits; those who smoked cigarettes were asked their
age at first smoking, the duration of time without smoking, and
the average number of cigarettes smoked. Former smokers were
asked about starting age, time without smoking, age when they
stopped smoking, and average daily number of cigarettes
smoked. We also asked participants about attained level of
education and, as part of a detailed questionnaire on food
frequency, about their average daily alcohol consumption.19
APOE genotyping was done under masked conditions on
coded DNA samples from 122 patients with dementia and a
random sample of 1428 non-demented participants. The PCR
product was digested with the restriction enzyme HhaI, and
fragments were separated by electrophoresis.20 The results of the
APOE typing were read independently by three researchers; in
case of discrepancies, APOE genotyping was repeated.
Statistical analysis
We grouped individuals at baseline into never smokers, former
smokers, and current smokers.We calculated pack-year exposure
by the average daily number of cigarettes divided by 20 and
multiplied by the number of years smoked. In all analyses, never
smokers were used as the reference category. The relative risk of
dementia by smoking was calculated with Cox proportional
hazards regression with a 95% CI. Age at onset of dementia
was taken to be the midpoint between baseline age and age at
diagnosis. For individuals with dementia whom we were not able
to re-examine, we defined onset age as the midpoint between
baseline age and age at the end of follow-up; for those who
survived, the follow-up ended on Dec 31, 1994.13 Since age is a
major determinant of dementia and age-distributions differed by
smoking status, we adjusted for age by inclusion of linear and
squared baseline age in the models. Sex, education, and alcohol
consumption were judged to be possible confounders of the
association and were therefore added to all models. Lack of
education beyond primary level was associated with incident
dementia, so we grouped education into primary school or less,
and beyond primary school. Daily alcohol intake, expressed in
g of pure alcohol daily, was added as a continuous variable.
We also investigated whether the smoking-related risk of
dementia was modified by the APOE genotype in a nested casecontrol
design with logistic-regression analysis. Adjustment for
age, sex, education, and alcohol intake was made according to
the above models. Length of follow-up was judged to have a
possible effect on the association because the probability of
developing dementia increases with duration of follow-up,
whereas smoking increases risk of death. We, therefore, added
duration of follow-up as a continuous variable to the logistic
regression models.
Results
Table 1 shows the baseline characteristics of the study
population. The mean follow-up was 2·1 years (range
1·5–3·4), and included 14 761 person-years of
observation. During follow-up, 429 (6·2%) of the 6870
participants died and 962 (14·0%) were not re-examined
because of refusal or severe illness. We detected 146
incident cases of dementia, of which 105 (72%) were
Alzheimer’s disease and 19 (13%) were vascular
dementia. The incidence rates of dementia per 1000
person-years for the age-groups 55–64 years, 65–74 years,
75–84 years, and 85 years and older were 0·9, 2·7, 20·1,
and 52·0, respectively; the corresponding rates were 1·1,
2·5, 18·6, and 28·0 for men and 0·8, 2·7, 20·9, and 58·8,
for women.
Table 2 shows that male and female smokers were at
increased risk of dementia. In men, the small number of
cases in the reference category of never smokers resulted
in high relative risks, but the 95% CI were wide. Smoking
increased the risk of Alzheimer’s disease and vascular
dementia (table 3). At baseline, the relative risk of
dementia for current smokers was higher in participants
who died or were not re-examined than in those whom
we re-examined (2·7 [95% CI 1·2–6·0] vs 2·0 [1·0–3·7]).
THE LANCET • Vol 351 • June 20, 1998 1841
ARTICLES
Study population (n=6870) Men (n=2776) Women (n=4094)
Never Former Current Never Former Current Never Former Current
smokers smokers smokers smokers smokers smokers smokers smokers smokers
Number (% of total) 2686 (39·1%) 2755 (40·1%) 1422 (20·7%) 455 (16·4%) 1627 (58·6%) 694 (25·0%) 2235 (54·6%) 1130 (27·6%) 729 (17·8%)
Mean (SD) age in years 71·9 (9·9) 68·3 (8·0) 66·4 (7·6) 69·8 (9·8) 68·2 (7·7) 67·1 (7·9) 72·3 (9·9) 68·5 (8·5) 65·7 (7·3)
Primary education only 803 (29·9%) 534 (19·4%) 319 (22·4%) 68 (15·0%) 252 (15·5%) 128 (18·5%) 735 (32·9%) 283 (25·0%) 190(26·1%)
(% of group)
Mean (SD) alcohol intake 5·6 (9·8) 12·2 (15·3) 15·1 (19·8) 12·3 (15·2) 15·8 (17·2) 21·0 (23·6) 4·2 (7·6) 7·1 (10·2) 9·9 (13·7)
(g/daily)
Table

ARTICLES
The mean age at onset of dementia for never smokers was
85·5 (SD 6·4) years. Former and current smokers were
younger at dementia onset than never smokers (mean
difference in age 4·1 years [95% CI 1·7–6·6]; and 8·6
years [CI 5·2–12·1]).
The total amount of pack-years smoked was available
for 1368 (96%) of the 1422 current smokers and for
2527 (92%) of the 2755 former smokers. For current
smokers at baseline, the relative risk of dementia was
2·5 (1·1–5·5) for those with exposure of less than 20
pack-years and 3·0 (1·6–5·4) for those with exposure of
20 pack-years or more. The corresponding relative risks
for former smokers were 1·5 (1·0–2·5) and 2·1 (1·2–3·7).
The APOE allele modified the association between
smoking and dementia (table 4). In participants without
the APOE4 allele, smoking substantially increased the risk
of dementia (relative risk 3·2 [1·2–8·5]). Among carriers
of APOE4, however, there was no association between
smoking and risk of dementia (1·4 [0·3–5·6]). The effect
of APOE4 was most pronounced in patients with
Alzheimer’s disease: those without APOE4 had an
increased relative risk of 4·6 (1·5–14·2) and those with
APOE4 had a non-significantly decreased relative risk of
0·6 (0·1–4·8).
Discussion
In this large prospective population-based study, people
who smoked cigarettes were at increased risk of dementia
and Alzheimer’s disease. This increased relative risk of
dementia was greatest in smokers without the APOE4
allele. By contrast, smokers with this allele did not have
an increased risk of dementia.
Our study has several advantages over previous studies
on smoking and dementia.8 We avoided selection bias
because the study was prospective and population based
with a high response rate and complete follow-up. Since
smoking habits were assessed before onset of dementia,
major recall bias was avoided. However, because the
follow-up was fairly short, some participants who
developed clinical dementia during the study might have
had slight impairment of memory or judgment at
baseline. This effect may have resulted in some
misclassification of exposure status among cases, but if
such misclassification occurred, it would probably have
led to under-reporting rather than over-reporting of
smoking habits. Thus, any recall bias would have resulted
in an underestimation of the true effect of smoking.
Survival bias was avoided by the inclusion of
individuals who died during follow-up. Case-finding
methods, however, differed for participants who could be
re-examined and those who could not, so we may have
missed some very early cases in the latter group. Relative
risk by smoking status was slightly higher in individuals
who died, refused, or were too ill to be re-examined than
in those who we re-examined (2·7 vs 2·0). The actual
difference could be even greater because we may have
underdiagnosed dementia in those who were not reexamined.
The increased relative risk of dementia in
participants who died or could not be re-examined
suggests that survival bias may have weakened the
association of smoking and dementia in previous crosssectional
studies.
Educational attainment and alcohol intake are wellknown
correlates of smoking habits, and both have also
been linked to dementia.21 When we adjusted for these
variables in the analyses, the smoking-associated risk of
dementia did not change.
The main disadvantage of the fairly short follow-up in
our study was the limited number of incident dementia
cases, which restricted the power in some of our
subanalyses. On the other hand, we were able to achieve
complete follow-up of the cohort and to use identical
screening and diagnostic procedures at baseline and
follow-up.
A limitation of the study is the absence of necropsy
confirmation of our diagnosis of Alzheimer’s disease.
Another point is that Alzheimer’s disease and
cerebrovascular disease frequently coexist, which makes it
difficult, both clinically and pathologically, to assess their
contribution to the development of dementia.18 Although
our clinical work-up was extensive and we adhered to
standard reliable criteria,17,18 we may have misclassified
the dementia subtype in some cases.
Smoking increased the risk of dementia in men and
women. The relative risks of dementia were higher for
men than for women. However, the proportion of men
who had never smoked (the reference group) was small
and only a few of them developed dementia, which led to
greater variability in our estimates of relative risk in men
and, thus, the sex difference may reflect chance. Possible
effect modification between sex and smoking should be
looked at more closely in future studies.
Most previous studies on smoking and dementia used
prevalent cases and a case-control design.8 As reviewed by
Lee,8 four of 17 studies reported a significant inverse
relation between smoking and Alzheimer’s disease. Some
earlier results conflict with our findings because of
differing methods,10 particularly the definition of
Alzheimer’s disease. Patients with coexisting vascular
disease might have been classified as mixed dementia.
The exclusion of such patients from the Alzheimer’s
disease category in previous studies could have selectively
cleared cigarette smokers from the cases, since smokers
are more prone to vascular diseases than non-smokers.2,3
Nevertheless, we found that smokers also had an
increased risk of Alzheimer’s disease without
1842 THE LANCET • Vol 351 • June 20,

cerebrovascular disease. Previous studies of the relation
between smoking and vascular dementia found, as we
did, a positive association in smokers.22,23 A few studies
used incident cases to assess the risk of Alzheimer’s
disease by smoking, with varying results,24–27 but none
looked at a possible effect modification by APOE
genotype.
Van Duijn and colleagues9 reported that the inverse
association between smoking and Alzheimer’s disease was
limited to carriers of the APOE4 allele, which accords
with our finding that the increased risk of dementia and
Alzheimer’s disease associated with smoking was
restricted to individuals without this allele. Before
addressing possible explanations for this apparent
effect modification by APOE4, one should consider that
elderly smokers who are APOE4 carriers may be a
selected group. Since the APOE4 allele increases the
risk of cardiovascular disease,28 mortality may be
disproportionately high among smokers who are APOE4
carriers.10 If atherosclerosis is, indeed, the intermediate
factor in the association between smoking and dementia,
various explanations could be proposed for a modifying
effect of APOE4. First, the presence of this allele could
alter the association between smoking and atherosclerosis.
In this study, however, the strength of the relation
between smoking and various indicators of atherosclerosis
was similar for participants with or without this allele.
Second, the APOE4 allele may inhibit the association
between vascular damage and dementia, but such
inhibition contrasts with the synergistic effect between
APOE4 and atherosclerosis on the risk of dementia.6
Third, smoking may exert different and opposite effects
on the r isk of Alzheimer’s disease, being generally har mful
through, for example, a vascular mechanism, but also
partly beneficial in selected individuals, especially carriers
of an APOE4 allele. This hypothesis is supported
by Poirier and co-workers’finding 29 that Alzheimer’s
patients who carry an APOE4 allele have fewer nicotinic
receptor binding sites and lower activity of choline
acetyltransferase than non-carriers. Smoking could
have some effects on the cholinergic system that
might counterbalance the selective impairment associated
with APOE4, for example increasing the density of
nicotine receptors or facilitating the release of
acetylcholine.9,25
Contributors
A Ott examined participants for presence of dementia; he analysed the
data and prepared the paper. A J C Slooter assisted in examinations of
participants, was involved in apolipoprotein E research, and helped
prepare the paper. A Hofman was principal investigator of the Rotterdam
Study. F van Harskamp was the study neurologist and supervised the
team that diagnosed dementia. J C M Witteman was responsible for the
data on smoking history. C Van Broeckhoven and C M van Duijn were
responsible for APOE genotyping. M M B Breteler supervised dementia
research and the study.
Acknowledgments
We thank the general practitioners of Ommoord and the RIAGG,
Rijnmond Noord-Oost.This study was supported by the NESTOR
stimulation programme for geriatric research in the Netherlands (Ministry
of Health and Ministry of Education), the Netherlands Organisation for
Scientific Research (NWO), the Netherlands Prevention fund, and the
municipality of Rotterdam. CVB is a research associate of the Flemish
Biotechnology Programme Fund for Scientific Research, Flanders,
Belgium.The research of MMBB has been made possible by a fellowship
of the Royal Netherlands Academy of Arts and Sciences.
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