This document provides an overview of key concepts in community ophthalmology and epidemiology. It defines epidemiology as the quantitative study of disease distribution, determinants, and control in populations. It describes measures of disease frequency including prevalence, cumulative incidence, and incidence rate. Prevalence is the proportion of people with a disease at a point in time, while cumulative incidence is the proportion who develop a disease over time. Incidence rate expresses the speed at which disease develops. Factors that influence prevalence like disease duration and migration are also discussed. The document emphasizes the importance of clearly defining exposures, outcomes, and potential confounding factors in epidemiological studies.

1. Community Ophthalmology
J. B. Chand, DCEH
Outreach Coordinator
Himalaya Eye Hospital
Pokhara, Nepal

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. Definition
• Epidemiology can be defined as the
Quantitative study of the Distribution,
Determinants and Control of the
diseases in populations.

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Thank You