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Understanding cataract output

Understanding cataract output
SPEAKER: 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 contract 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 contract 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 versus 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.
Contract 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.

The aim of public health is to deliver interventions to avert human suffering. These interventions are also referred to as prevention strategies. The need for cataract prevention strategies is huge, given the tens of millions of people who are blind or visually impaired as the result of cataract. In this step we look at the public health strategies used to prevent and control cataract blindness.

Think about how many cataract surgeries are done in your setting each year. What is the level of visual impairment at which cataract surgery is offered to patients?

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Global Blindness: Planning and Managing Eye Care Services

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