What's in a number?

How do research and practise work together to identify and manage glaucoma?

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Author: Daniel Hardiman-McCartney MCOptom, Clinical Adviser and Martin Cordiner, Head of Research
Date: 8 January 2016

If you’re asked, ‘how common is glaucoma’, what answer do you give? And where does your answer come from? Putting aside the fact that there are many types of glaucoma and concentrating only on primary open angle glaucoma, it draws attention to how quickly the statistics can become confused. 

Firstly, are they asking about the prevalence or the incidence? In epidemiology 'prevalence' is the proportion of a population found to have a condition at a point or period in time in total, where as ‘incidence’ is defined as the number of new cases per population in a given time period. Put simply, prevalence is how many people have the disease right now, but incidence is how many people per year acquire the disease.

This means that big changes to the content of a population, including its age and its ethnicity, can have an impact on the prevalence and incidence of a disease in such a way as to suggest that a single individual is more likely to get it, where as in fact the population changing is the reason for the change in figures. A rise or fall in figures without this additional consideration could be meaningless.

And so to the actual figures available. NHS choices states prevalence figures of 2% for those over 40 and 5% for those over 80. Great. But are they right for the patient in front of you?

A meta-analysis published in Ophthalmology reviewed fifty population-based studies and concluded a global POAG prevalence of 3.05%. When broken down by geographic region, for a population aged 40-80 a prevalence of 2.51% was found in Europe, increasing to 4.20% for Africa.

And what about the effect of age? The Baltimore study examined prevalence in different ethnic populations (which they defined as white people and black people) and reported POAG rates for white people ranging from 0.92% in those aged 40 -49 and 2.16% in those aged 80 years or over. By contrast, rates for black people ranged from 1.23% to 11.26%, four to five times higher than white people (depending on the age group). The study also found an increase in the incidence of POAG with age. In another study published in July 2015 the population of Denmark was reviewed for 15 years. The prevalence of glaucoma was 3.76% of the population above the age of 50 and 10% of those above the age of 80, with an incidence around 1.25-1.55% each year. Interestingly, the Denmark study found an overall prevalence for the total population of around 2%, the same figure for only those over 40 in the UK.

While the widely quoted 2% and 5% figures may be a fair ballpark for an entire population and appropriate for the NHS Choices website, are they the right answer for the patient? When they ask how common glaucoma is, they are really asking how likely they are to get it, which means the prevalence for them based on their age and race, which also won’t include additional factors such as family history of the disease and any contributing lifestyle factors.  As many patients are now older and not Caucasian, using the 2% figure notably understates how likely they are to get glaucoma and we are well placed to be more accurate (see table below). Moreover, as practitioners it is clinically useful to be mindful of the variance in prevalence and incidence for different patients and to adjust our investigations accordingly. 

Data POAG - Baltimore Study and Melbourne Study

    Prevalence (%)   Incidence (%, annual)
     Black  White Black White (Melbourne)
Age 40-49 1.23 0.92 0.24 0
  50-59  4.05 0.41 0.40 0.02
  60-69  5.51 0.88 0.73 0.12
   70-79 9.15 2.89 0.87 0.28
   80+ 11.26 2.16   0.82
All age groups   4.74  1.29 0.55 0.1


To find out more about using the latest research to inform your glaucoma practice this year, there are a number of relevant sessions at Optometry Tomorrow 2016, from 'Glaucoma Conundrums' to 'Integrating OCT into glaucoma practice’. How will your patients benefit from the latest research? 

Further Reading:The Glaucoma handbook

Daniel Hardiman-McCartney FCOptom
Clinical Adviser, The College of Optometrists

Daniel graduated from Anglia Ruskin University, where he won the Haag Strait prize for best dissertation. Before joining the College, he was Managing Director of an independent practice in Cambridge and a visiting clinician at Anglia Ruskin University. He has also worked as a senior glaucoma optometrist with Addenbrooke’s Hospital in Cambridge, with Newmedica across East Anglia and as a diabetic retinopathy screening optometrist. Daniel was a member of Cambridgeshire LOC from 2007 to 2015 and a member of the College of Optometrists’ Council from 2009 to 2014, representing its Eastern region.  

He is Clinical Adviser to the College of Optometrists for four days each week, dividing the remainder of his time between primary care practice and glaucoma community clinics. Daniel is a passionate advocate of the profession of optometry, committed to supporting all members of the profession and ensuring patient care is always at the heart of optometry. He was awarded Fellowship by Portfolio in December 2018.

Martin Cordiner
Head of Research, College of Optometrists

Martin graduated with a Masters in Modern History from York University in 2005, having completed his BA there in 2003. Since then he has worked in project management in higher education before joining the College and its fledgling research department in 2009, where he now supports the Director of Research and manages the research team to implement all elements of the College’s Research Strategy. 

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