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What is EPIDEMIOLOGY, and what does it offer for understanding, treating, and preventing Alzheimer's?
Deborah Blacker, MD, ScD
Professor of Psychiatry, Mass General Hospital/Harvard Medical School
Professor and Deputy Chair in Epidemiology, Harvard T.H. Chan School of Public Health
OVERVIEW
What is Alzheimer's?
What is epidemiology?
Major functions
Assess and forecast the magnitude of the problem (brief intro to Dr. Langa's talk)
Understand risk and protective factors and their potential role in prevention (brief intro to Dr. Yaffe's talk)
Put clinical findings in a population context
Assess and improve methods for observational studies
WHAT IS ALZHEIMER'S?
Alzheimer's disease (AD) dementia is a type of dementia caused by a specific neuropathology--amyloid plaques and neurofibrillary tangles
Confusingly, AD is used both to refer to the pathology and the clinical syndrome caused by the pathology
Dementia and Mild Cognitive Impairment (MCI) are clinical diagnoses based on cognitive and functional status and can be caused by AD pathology or other pathologies
Also confusingly, AD and dementia are sometimes used interchangeably, in part because AD is the most common form of dementia
Clinical vs. pathological diagnosis
A definitive diagnosis of AD requires evidence of AD pathology from biomarkers or autopsy, but diagnosis in clinical and population studies is often based on symptoms alone
Whether this matters depends on the purpose
Fine for understanding disease burden or costs, but problematic for targeted clinical trials, and complicates the interpretation of epidemiologic and genetic studies
Additionally complicating the interpretation of research findings...
AD pathology often co-occurs with other pathologies, particularly cerebrovascular pathology
The effects of these different pathologies are at least additive and may interact
At later ages, mixed dementia is very common, and additional pathologies likely remain to be identified
Graphic courtesy of Dr. Jennifer Weuve
WHAT IS EPIDEMIOLOGY?
Initial focus on infectious disease-- surveillance, containment, etc.
Later parallel focus on chronic disease-- to understand disease burden, patterns over time and space, and understand risk factors
What do epidemiologists do?
Quantify disease in the population overall, according to demographic factors, and across time and space
Use patterns of association and careful statistical analysis to understand risk and protective factors, offering insight into mechanism and prevention
Assess whether findings from clinical studies hold up in the general population, with implications for screening and real world impact of interventions
Assess and develop optimal methods for observational studies, with implications for clinical trials, screening, and health services
DESCRIPTIVE EPIDEMIOLOGY: Magnitude and patterns
How many people have the disease now? (Prevalence)
How many new cases can be expected... (Incidence)
at a given age or in a given time window?
cumulatively until a certain time ?
over a lifetime?
How do the above numbers change across demographic factors, geographic regions, and birth cohorts?
Descriptive Epidemiology: Magnitude and patterns
Provides estimate of disease burden to inform care planning, policy, and research priorities
Big picture suggests rising rates overall despite apparent fall in age-specific incidence* --and that's why we are here today!
Stay tuned: Details from Dr. Langa
RISK AND PROTECTIVE FACTORS
Use associations to discover factors that increase or decrease disease risk
Risk and protective factors can be genes or diet , activities, medical conditions, or local environment
Can identify high risk populations or select intervention strategies for clinical trials
Can be translated into individual health recommendations and population health policy
Can help understand mechanism and drug development
At Baseline
10 years later
Spiky crown may be a risk factor for turning red after 10 years
Examples
Systematic Review of risk of dementia with diabetes
Cumulative incidence of dementia by APOE-4 dose: Framingham Study
PUT CLINICAL RESEARCH AND EXPERIENCE INTO A POPULATION CONTEXT
Patients are more impaired than average for the same diagnosis (e.g., MCI, because symptoms drive care-seeking), and also have more other illnesses that may unmask decline, increase contact with doctors, etc.
Patients are also more educated, more affluent, and have other attributes that increase access to care
However, screening programs, publicity, or the availability of early intervention can bring those with milder illness into care
Research volunteers are often highly educated, and more likely to have family history, subclinical symptoms, etc.
NACC vs. population samples
Study
NACC
FHS
Rotterdam
Description
US volunteer, 80% white
US pop, 1°ly white
European pop, white
N
5073
4078
6399
mean (sd) age (yrs)
68.7 (4.30)
62.0 (1.71)
65.4 (4.18)
% male
33.6%
43.2%
45.2%
mean (sd) educ (yrs)
15.79 (2.99)
13.20 (*)
12.94 (*)
E-4 allele freq
0.178
0.117
0.150
% w/ family history
58.3%
N/A
21.7%
% w/ memory concerns
24.9%
N/A
43.1%
mean (sd) MMSE
29.0 (1.3)
28.5 (1.0)
28.8 (1.4)
Cumulative Incidence of MCI/dementia by APOE dose
Alzheimer Centers (NACC)
Rotterdam Study
Framingham Heart Study
Qian et al, PLoS Medicine, 2017
Example: 23andMe APOE Testing
Sample report courtesy of Robert Green and Debby Tsuang
Importance of Population Context
23andMe's risk estimates are too high (likely because they're based on data from clinical/volunteer samples)
Similar issues apply for imaging, biomarkers, and clinical symptoms tested in clinical/volunteer settings
Critical issue whenever findings from one setting are applied in another; examples:
The MCI trial of donepezil turned to community recruiting when clinical recruiting ran short, but lower onset rate of dementia in community participants reduced the study's power
Expected rates of developing dementia based on biomarker data from clinical samples of MCI will be lower if a new treatment becomes available, as that would drive less impaired individuals to seek care
ASSESS AND IMPROVE METHODS FOR OBSERVATIONAL AND OTHER CLINICAL RESEARCH
Caveat Auditor
Epidemiology focuses on observational research, so has developed careful methods to guard against and quantify potential false or distorted conclusions
The importance of these details depends on the question
Causal questions are particularly vulnerable to biases, and are critically important to translational applications
EPIDEMIOLOGIST: noun. [ep-i-dee-mee-ol-uh-jist] someone who solves a problem you didn't know you had in a way you don't understand. See also wizard, magician.
Key issues that can lead to bias
Measurement error: over or under-calling dementia or MCI outcome, noise in measuring cognitive or functional status, mismeasurement of risk factors
Sampling issues: impact of entry criteria and operational definition of non-demented at baseline, age at entry and survival bias, participant burden, and autopsy consent
Confounding: something related to both your outcome and to your putative risk factor that creates a spurious association between the two-age and education are major confounders in AD research
Reverse causation: an effect masquerading as a cause
Will illustrate by raising some questions about whether cognitive activity really reduces risk of dementia
Measurement Error
A particular concern for cognitive testing, MCI, and any diagnosis in claims or health records; mostly adds noise and reduces power, but can lead to biased findings if differentially affects each side of the question at hand
For understanding the role of cognitive activity in risk for dementia, typical cognitive activity surveys focus on activities of the highly educated, which could make high levels of activities look protective when education is actually providing the benefit
Sampling bias
In the examination of cognitive activity and risk of dementia, because there is a broad perception that cognitive activities are desirable, those who are inactive may be reluctant to participate (or may over-report their activity--see measurement error), or those who are very active may be too busy
Survival bias could also distort the relationship if those with greater levels of cognitive activity live longer, as might be expected given the association of longevity and education
Confounding
Age and education are major confounders that affect measurement, care seeking, and multiple risk factors
For cognitive activity and risk of dementia, the big concern is education, which is associated with greater cognitive activity, but contributes to dementia risk in multiple other ways
Or, early life cognitive activity might build protective "brain reserve," and later effects might be observed simply because those habits persist over time
Reverse causation
risk factor → dementia VS. dementia → "risk" factor
Given insidious onset of dementia, incipient cognitive decline can lead to changes in physiologic state (weight, BP) or lifestyle, which can in turn be mistaken for causes
Decline in cognitive activity in the face of early cognitive decline is highly plausible, and careful bias analysis cannot rule out reverse causation driving the apparent lower risk of dementia
Graphic courtesy of Dr. Jennifer Weuve
Parting words on the value of epidemiological methods
