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  1. 1. Community Ophthalmology J. B. Chand, DCEH Outreach Coordinator Himalaya Eye Hospital Pokhara, Nepal
  2. 2. Epidemiology • It was in 400 B.C, Hippocrates (A Greek physician) used the term “EPIDEMEION’’ to describe the disease that visit the community. • Described distribution by season, age, climate, body build and habit.
  3. 3. Definition • Epidemiology can be defined as the Quantitative study of the Distribution, Determinants and Control of the diseases in populations.
  4. 4. Definition.. • Quantitative: – Occurrence of disease (Disease frequency: Prevalence, Cumulative incidence and Incidence rate). The disease has to be carefully defined. • Distribution: – Who is affected in the population? Male or female? • Determinants: – What is the underlying cause of the disease? • Control: – How can disease be controlled and the quality of life of communities be improved?
  5. 5. Disease Frequency • The fundamental task in any epidemiological study is to measure the occurrence of disease in population (Disease frequency) • There are 3 basic related measures of disease frequency: – Prevalence - How many NOW % (New & Old cases) – Cumulative Incidence % (Also known as risk) – Incident Rate % - Also known as rate (New case only)
  6. 6. Prevalence • “The proportion of people who have the disease at the specified point in time” • Prevalence is usually measured in cross- sectional studies (Surveys). • Prevalence is usually presented as a percentage (number per 100). Very rare disease it may be presented per 1000.
  7. 7. Prevalence.. • Prevalence is calculated as: a/a+b*100% • Where, a =Number of diseased persons (numerator) b =Number of none diseased person a+b =Total number of person examined (denominator) Blind Not Blind Total 12 1800 1812 Prevalence of blindness:12/1812*100 =0.66%
  8. 8. Cumulative Incidence • “The proportion of people who develop a particular condition or disease in a specified period of time” • Who were disease free at the beginning but were at risk of the disease. • It can be measured in longitudinal cohort studies.
  9. 9. Cumulative Incidence… • Calculated as: a/a+b*100% per year or month • Where, a =Number of persons who develop the disease (numerator) b =Number of person who do not develop disease a+b =Total number of person initially at risk (denominator)
  10. 10. Cumulative Incidence… – A longitudinal study with a sample of 620 people aged 40-45 were examined for glaucoma and found none of them have glaucoma – The same people were examined 2 year later and 5 were found to have developed glaucoma. None of original cohort had died or migrated. • The 2 year cumulative incidence is, 5/620*100% =0.8% • The 1 year cumulative incidence is, =0.4% per year
  11. 11. Incident Rate • This is “Measure of the speed at which a disease develops in a population” • It is a rate and when expressed should always include the “person time at risk” • Incident rate is true rate since other measure are only proportion • Calculated as: Number of new cases occurring during the follow up period Total person time (years, months) at risk
  12. 12. Prevalence: Increase & Decrease • Increase Prevalence – Long duration of disease – Prolong treatment – High incidence – In migration of ill cases – Out migration of healthy – In migration of susceptible – Improved diagnosis • Decrease Prevalence – Short duration of disease – High case fatality rate – Low incidence – In migration of healthy – Out migration of cases – Improved cure rate of cases
  13. 13. Exposure and Outcome • In any epidemiological study, there are 3 factors which needs to be measured: – Primary exposure – Outcome – Other exposure which may influence the outcome (Confounders)
  14. 14. Exposure • Exposure encompass factors that may be associated with condition or disease of interest or health • Common Exposures are: – Chemical (Dyes, pesticides, Industrial chemical) – Genetic (HLA type) – Medical (Previous immunization) – Behavioral (Smoking, Alcoholic, Exercise, Diet) – Age and Sex • Important to identify the main exposure
  15. 15. Outcome • The outcome measured in a study may be, Disease, Disability, Health and Death • Outcome should be carefully defined and measurable • Out of many outcome, identify the main outcome
  16. 16. Confounder • A confounder is a risk factor for the outcome of interest and can give totally misleading result in the studies • Example: – Smoking ALCOHOL Stomach ulcer – Malnutrition MEASLES Vitamin A deficiency CONFOUNDERS
  17. 17. Thank You