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Natural History of Disease: Prevention and Prognosis. Dr. Namvar Zohoori Epidemiology Research Unit. Learning Objectives. Understand different stages of disease development. Relate above stages to phases of prevention.

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natural history of disease prevention and prognosis

Natural History of Disease:Prevention and Prognosis

Dr. Namvar Zohoori

Epidemiology Research Unit

learning objectives
Learning Objectives
  • Understand different stages of disease development.
  • Relate above stages to phases of prevention.
  • Describe advantages and disadvantages of population and high-risk prevention strategies.
  • Define and list methods of quantifying prognosis.
epidemiology
Epidemiology

‘Epidemiology: the study of the occurrence of illness’

Gaylord Anderson

epidemiology1
Epidemiology

‘study of the distribution and

determinants of health related states or events in specified populations and the application of this study to the control of health problems’

John Last, 2001

some important roles
Some Important Roles

Prevention

Detection

Prognosis

disease process
Disease Process
  • Diseases and other phenomena of interest in epidemiology are processes, not events.
  • Example of bronchogenic carcinoma:-
    • Several grades of abnormality (metaplasia, mild dysplasia, mod dysplasia, severse dysplasia).
    • Most can regress spontaneously
    • Some can progress to Ca in situ and then invasive carcinoma.
    • Every disease has a natural course of progression.
disease process1
Disease Process
  • Therefore, defining, observing and measuring health and disease require understanding of concept of “natural history”:-

“the evolution of a pathophysiologic process”

slide8

Pre-clinical

Pre-symptomatic

Clinical

Post-morbid

Susceptibility

Detection possible

Care

Dx

Rx

Biologic onset

Signs and symptoms

Primordial & Primary

Prevention

Secondary

Prevention

Tertiary

Prevention

Natural History of Disease

Outcome

prevention paradox
Prevention Paradox

“A preventive measure which brings much benefit to the population often offers little to each participating individual.”

primordial prevention
Primordial Prevention

“Prevention of the emergence of living patterns that contribute to increased risk of disease (e.g. the maintenance of low-fat diets in traditional societies)”

primordial prevention1
Primordial Prevention
  • Understanding of CVD epidemiology.
  • Dietary patterns in China and Japan
  • Socioeconomic development -> more widespread risk factors.
  • Have we missed the boat?
primary prevention
Primary Prevention

“Prevention of disease by controlling risk factors (e.g. non-smoking promotion)”

Two strategies:-

Population

High-risk

primary prevention1
Primary Prevention
  • The Population Strategy
    • Advantages:-
      • Radical
      • Large potential for population
      • Behaviourally appropriate
primary prevention2
Primary Prevention
  • The Population Strategy
    • Disadvantages:-
      • Small benefits to individuals
      • Poor motivation of subject
      • Poor motivation of physician
      • Benefit-to-risk ratio may be low
primary prevention3
Primary Prevention
  • The High-risk Strategy
    • Advantages:-
      • Appropriate to individuals
      • Subject motivation
      • Physician motivation
      • Benefit-to-risk ratio is favourable
primary prevention4
Primary Prevention
  • The High-risk Strategy
    • Disadvantages:-
      • High screening costs
      • Temporary effects
      • Limited effect
      • Behaviourally inappropriate
secondary prevention
Secondary Prevention

“Reduction in consequences of disease by early detection, diagnosis and treatment (e.g. cervical cancer screening)”

slide18

Natural History of Disease

Pre-clinical

Pre-symptomatic

Clinical

Post-morbid

Susceptibility

Outcome

Detection possible

Care

Dx

Rx

Biologic onset

Signs and symptoms

Premordial & Primary

Prevention

Secondary

Prevention

Tertiary

Prevention

tertiary prevention
Tertiary Prevention

“Reduction of complications of disease (e.g. role of ICU in MVA’s)”

prognosis
Prognosis

“A quantitative expression of the likelihood of a specific outcome (survival)”

General issues:-

  • At what point to begin counting survival?
  • How is diagnosis made?
slide22

Natural History of Disease

Pre-clinical

Pre-symptomatic

Clinical

Post-morbid

Susceptibility

Outcome

Detection possible

Care

Dx

Rx

Biologic onset

Signs and symptoms

Premordial & Primary

Prevention

Secondary

Prevention

Tertiary

Prevention

slide23

Prognosis

5-year

survival

Survival

Death

Case-fatality

Rate

Observed

survival

Relative

survival

Median

survival

prognosis1
Prognosis
  • Case-fatality rate
    • DFN: # who die of dis./# who have dis.
    • No explicit time frame.
    • Ideally suited for diseases that are short-term, in which death occurs soon after diagnosis.
    • With chronic diseases of long duration, case-fatality rate becomes meaningless.
prognosis2
Prognosis
  • 5-Year survival
    • DFN: % of patients still alive 5 years after diagnosis or treatment begins.
    • Used most in cancer treatment.
    • Note problem with ‘lead time’.
slide26

Natural History of Disease

Pre-clinical

Pre-symptomatic

Clinical

Post-morbid

Susceptibility

Death

Dx & Rx

Biologic onset

Signs and symptoms

1991

1995

Survival 4 years

slide27

Natural History of Disease

Pre-clinical

Pre-symptomatic

Clinical

Post-morbid

Susceptibility

Death

Detected by screening

Biologic onset

Signs and symptoms

1989

1995

Survival 6 years

prognosis3
Prognosis
  • Observed survival
    • DFN: probability of surviving x number of years.
    • Use of life-table analysis
    • Advantage of using data on all patients, regardless of how long they survive.
    • 2 assumptions:-
      • No temporal change in Rx efficacy
      • Those lost to follow-up have similar experience to those followed up.
prognosis4
Prognosis
  • Median survival time
    • DFN: length of time that half of the study population survives.
    • Advantages of median versus mean
      • Effect of extremes
      • “Sample size”
prognosis5
Prognosis
  • Relative survival rate
    • DFN: ratio of observed survival rate to expected survival rate if no disease
    • That is, compares survival to the survival one would expect in the given age group