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