Global Blindness: Understanding cataract output

Editor's Notes

• #3: In this session we look at cataract output. By the end of this session you should be able to: Understand the public health strategy for the prevention and control of cataract blindness Understand the concepts of cataract output, cataract surgical rate and cataract surgical coverage
• #4: What do we mean by prevention in public health? The aim of public health is to deliver interventions to avert human suffering.
• #5: 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 the disease.
• #6: Secondary prevention aims to halt or slow the progression of disease through early intervention or risk reduction
• #7: Tertiary prevention targets people who already have the disease and stops deterioration impacting on patients’ quality of life
• #8: When we consider prevention strategies for cataract blindness we see that, at present, primary prevention to control cataract blindness is not possible. There is no clear method of preventing the lens from clouding as part of the ageing process. Secondary prevention measures aim to identify and treat cataract patients who are not yet blind (that is less than 3/60 visual acuity). Tertiary prevention is focused on finding and treating the cataract blind and restoring their sight.
• #9: The need for cataract prevention strategies is huge. Globally, there are more than 19 million blind and over 81 million visually impaired people from cataract. Image: Jock Anderson CC BY-NC 2.0
• #10: Most of this is amongst people aged 50 years and above, and living mainly in low and middle income countries. Image: Adriane Ohanesian/Sightsavers CC BY-NC 2.0 flic.kr/p/q29JqL
• #11: 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. Image: LAICO CC BY-NC 2.0 flic.kr/p/mRKoKy
• #12: Cataract control strategies examine service delivery from three key positions: How many surgeries are we doing and how can 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 the cataract outcome What is the cost of our cataract surgery and can we make eye care services sustainable? This is known as the cataract outlay In this presentation we focus on cataract output.
• #13: 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 (known as the prevalence). There are two exits from the can for the individuals with cataract. One is to receive treatment, 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.
• #14: The definition of a cataract case, also known as an operable cataract, varies across 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 or 6/18 or sometimes even lower.
• #15: Cataract surgical rate refers to the number of surgeries carried out per million population per year. To calculate this we divide the number of cataract operations carried out in a year, by the population in millions.
• #16: For example, in a population of 2 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.
• #17: 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. Image: Bawku Hospital Eye Unit CC BY-NC 2.0 https://flic.kr/p/9v2PYP
• #18: Let’s put some numbers into our leaky can! A 1 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 is 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.
• #19: 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.
• #20: The global initiative, VISION 2020: the Right to Sight, set targets for cataract surgical rates to help address and reduce the backlog. For Africa the rate is 2,000 For Asia the rate is 3,000 These targets are based on the availability of human resources to facilitate surgery.
• #21: When we look at data from 2011 on cataract surgical rates across Africa we find that only 2 countries (the Gambia and Sudan) are reaching this minimum target.
• #22: Comparing cataract surgical outputs across the world we find that over 80% of eye units in Africa do less than 1,000 surgeries per year. In South-East Asia by comparison, 50% of units are doing more than 1,000 surgeries per year and 20% are even doing more than 2,500.
• #23: 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, with 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 RAAB survey can be carried out.
• #24: Cataract surgical coverage for eyes (or CSC eyes) is the proportion of eyes in a population, with a defined visual acuity, who have had surgery 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.
• #25: If CSC eyes is greater than CSC persons then a large number of bilateral operations have been carried out. If CSC persons is greater than CSC eyes then single eyes have been done mainly. This will reduce the prevalence of blindness as per the World Health Organization classification.
• #26: 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%. As CSC persons is greater than CSC eyes this means that most of the cataract surgeries carried out were unilateral operations rather than bilateral operations. Eye care planners would find this information useful to bring about changes in the way cataract services are carried out in this region.
• #27: 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
• #28: The minimum CSR has to be equal to the incidence of cataract. For Africa the minimum CSR target is 2,000 Cataract surgical coverage measures how much of the need for cataract surgery has been met in a population