Natural History of Disease: Prevention and Prognosis

1. Natural History of Disease:Prevention and Prognosis Dr. Namvar Zohoori Epidemiology Research Unit

2. 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.

3. Epidemiology ‘Epidemiology: the study of the occurrence of illness’ Gaylord Anderson

4. 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

5. Some Important Roles Prevention Detection Prognosis

6. 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.

7. Disease Process • Therefore, defining, observing and measuring health and disease require understanding of concept of “natural history”:- “the evolution of a pathophysiologic process”

8. 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

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

10. 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)”

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

12. Primary Prevention “Prevention of disease by controlling risk factors (e.g. non-smoking promotion)” Two strategies:- Population High-risk

13. Primary Prevention • The Population Strategy • Advantages:- • Radical • Large potential for population • Behaviourally appropriate

14. 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

15. Primary Prevention • The High-risk Strategy • Advantages:- • Appropriate to individuals • Subject motivation • Physician motivation • Benefit-to-risk ratio is favourable

16. Primary Prevention • The High-risk Strategy • Disadvantages:- • High screening costs • Temporary effects • Limited effect • Behaviourally inappropriate

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

18. 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

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

20. Levels of Prevention

21. 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?

22. 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

23. Prognosis 5-year survival Survival Death Case-fatality Rate Observed survival Relative survival Median survival

24. 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.

25. 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’.

26. 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

27. 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

28. 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.

29. 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”

30. 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

31. 5-Year Observed and Relative Survival Ratesby age for Colon Cancer