Global Blindness course: Understanding cataract output


In this session, we
look at cataract output. By the end of the
session, you should be able to understand the public health strategy for the prevention and control of cataract blindness, and understand the concepts
of cataract output, cataract surgical rate, and
cataract surgical coverage. What do we mean by
prevention in public health? The aim of public health
is to deliver interventions to avert human suffering. These control measures
are referred to as prevention
strategies, and they can be grouped into three levels. Primary prevention strategies
protect healthy people from even developing a disease. Secondary prevention,
aims to halt or slow the progression of disease
through early intervention or risk reduction. And tertiary prevention
targets people who already have the disease,
and stops deterioration impacting on patient’s
quality of life. When we consider prevention
strategies for cataract blindness, we can see that at
present, primary prevention to control cataract blindness
is just not possible. There is no clear
method of preventing the lens from clouding as
part of the ageing process. Secondly measures, aimed
to identify and treat cataract patients who
are not yet blind, that is less than 360 visual
acuity, and tertiary prevention efforts, are focused
on finding and treating the cataract blind, and
restoring their sight. The need for cataract
prevention strategies is huge. Globally, there are
more than 19 million blind, and over 81
million visually impaired people who cataract. Most of this is found amongst
people age 50 years and over, and living mainly in low
and middle income countries. The solution is straightforward. Cataract surgery
is very effective, and relatively simple. The challenge is– how to
make sure all the people who need surgery can get it. Cataract control strategies
examine service delivery from three key positions. How many surgeries
are we doing, and how we increase this number? This is known as
the cataract output. What are the results
from the surgery, and is it the best
quality at all times? This is known as
cataract outcome. And finally, what is the
cost of our cataract surgery, and how can we make eye
care services sustainable? This is known as
the cataract outlay. In this presentation,
we will be focusing on understanding the factors
that influence cataract output. We can use the analogy of a
leaky can to help us analyse the cataract output situation
for a population of one million. Within the can is
the cataract backlog. These are all the untreated
cases of cataract at present. This is known as the prevalence. There are two exits from the can
for individuals with cataract. One is to receive treatment. And the other option, sadly,
is to die blind without ever seeing an eye health worker. As health workers,
it is our ambition that no one has to
experience this fate. We need to remember that
the population is not static and that new cataract cases
will develop over time. These are constantly entering
into the backlog, shown here as new cases pouring
into the leaky can. This is known as the
incidence of cataract. To reduce the cataract
backlog, the number of cataract operations that
need to be performed each year must be at least equal to
the number of new cases, or incidence of cataract. The definition of
a cataract case, also known as an
operable cataract, varies in different countries. In many low and middle
income countries, it is a visual acuity
of less than 6/60. In high income countries, it
can be a visual acuity of 6/24, 6/18, or sometimes even lower. Cataract surgical rate
refers to the number of surgeries carried out per
million population per year. To calculate it, we divide the
number of cataract operations carried out in a year by
the population in millions. So for example, in a population
of two million people, where 1,200 cataract surgeries
were carried out last year, the cataract surgical rate
would be 1,200 divided by 2. This gives us a rate
of 600 operations per million population. The minimum cataract
surgical rate needs to be equal to the
incidence, or new cases, for it to begin to have any impact
on the cataract backlog. Let’s put some numbers
into our leaky can. A one million population with a
prevalence of blindness of 1%, means there are 10,000 people
with a visual acuity of less than 3/60 in the better eye. If cataract is the main cause of
blindness in half these people, then the cataract
backlog can be calculated to be 5,000 people,
or 10,000 eyes. It has been calculated that
cataract incidence is about 20% of the backlog. So we can expect about
another 1,000 people to have cataract by next year. If no treatment is carried
out, and there is no mortality, by next year, there will be
6,000 people in the backlog. So if we are to have
any cataract control, the minimum number
of surgeries that must be done in the
next year is 1,000, to keep the backlog in check. The global initiative
VISION 2020 The Right to Sight set
targets for cataract surgical rate to help address
and reduce the backlog. For Africa, the rate is
2,000, and for Asia, the rate is 3,000. These targets are based on the
availability of human resources to facilitate surgery. When we look at
the data from 2011 on cataract surgical
rates across Africa, we find that only two
countries– The Gambia and Sudan– are reaching
this minimum target. Comparing cataract surgical
outputs across the world, we find that over 80%
of eye units in Africa do less than 1,000
surgeries a year. In South-East Asia, by
comparison, 50% of units are doing more than
1,000 surgeries a year, and 20% are even doing more
than two and a half thousand. Cataract surgical
coverage helps to answer the question of how much of
the need for cataract surgery has been met in a population. It can be calculated by
dividing the number of people of a defined visual acuity
who have had cataract surgery, by the
number of people who have had surgery, plus those
people who are still waiting for surgery. To obtain this information,
a rapid assessment of avoidable blindness, or
RAAB, survey can be carried out. Cataract surgical coverage
eyes, or CSC eyes, is the proportion of eyes in a
population of a defined vision acuity who’ve had
cataract surgery. And CSC persons
is the proportion of people in the population
who have had cataract surgery. These figures can be
divided by gender, to work out the proportion
of women needing cataract surgery who have been treated. If CSC eyes is greater
than CSC persons, than a large number of
bilateral operations have been carried out. And if CSC persons is
greater than CSC eyes, then single eyes have
been done, mainly, and this will reduce the
prevalence of blindness, as per the World Health
Organization classification. Let’s apply this to an example. In a region of Zanzibar,
the CSC persons for a visual acuity of less
than 6/60, is found to be 45%. This means that only 45% of
the people who need cataract surgery have been treated. The CSC eyes for
the same region, again, for a visual acuity
of less than 6/60, is 20%. Now CSC persons is
greater than CSC eyes. This means that most of the
cataract surgeries carried out have been unilateral
operations, rather than bilateral operations. Eye care planners would find
this information very useful to bring about
changes in the way cataract services are
carried out in this region. In conclusion,
cataract surgery is an effective secondary
and tertiary public health strategy to
prevent blindness. The cataract surgical
rate, CSR, is the number of cataract surgeries performed
per million population, per year. The minimum CSR has to be equal
to the incidence of cataract. For Africa, the minimum
CSR target is 2,000. And finally, cataract
surgical coverage measures how much of the need
for cataract surgery has been met in a population.

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