"Air pollution linked to condition that causes blindness in major UK study," says The Independent, as a new study looking at links between exposure to air pollution and glaucoma has been published.
Glaucoma is an eye condition where the pressure in the eyeball builds up. This causes compression and damage to the optic nerve at the back of the eye, which carries signals to the brain.
In the most common type of glaucoma (called primary open angle), the pressure build-up happens very gradually over years. The condition is often hereditary, with other risk factors including increased age and being of Asian, African or Caribbean origin.
In this UK study, over 100,000 people completed questionnaires and had eye tests for glaucoma. The researchers linked this data with pollution levels at their home address in the same year. Around 2% of the sample self-reported being diagnosed with glaucoma. The researchers found a 6% increased chance of self-report in more-polluted areas.
They also found that higher pollution was linked with the thickness of nerve fibres at the back of the eye, which is another marker for glaucoma.
The links are worthy of further study, but they do not prove that pollution is responsible for these effects. Many unmeasured factors may be involved.
People should not be concerned by these findings. The most important thing with glaucoma – whatever the underlying cause may be – is to get it diagnosed and treated. This happens through regular eye checks.
This study was conducted by researchers from Moorfields Eye Hospital, University College London, Cardiff University and University of Bristol. Funding was provided by the National Institute for Health Research, Moorfields Eye Charity and the Department of Health. The study was published in the peer-reviewed journal Investigative Ophthalmology & Visual Science, and is freely available to access online.
The study was widely reported in the UK media. Most sources made clear that a definitive cause and effect relationship was unproven.
An exception to this was the Mail Online who led with a headline saying pollution "can trigger glaucoma" and "study finds chemicals in air narrow the blood vessels and raise the risk". The study has not proven that air pollution is a trigger for glaucoma, neither has it demonstrated that narrowing of the blood vessels due to exposure to air pollution could be a contributory factor.
This was a cohort study using data from the large ongoing UK Biobank study, where people received clinical assessments and completed health and lifestyle questionnaires. The researchers analysed this alongside air pollution measures taken at the same time.
The main limitation is that this study cannot prove direct cause and effect between air pollution and glaucoma.
The UK Biobank study recruited 502,656 adults aged 40 to 69 years. Between 2006 and 2010 they received clinical assessments at 22 centres across the country. They also completed health questionnaires that included the question "Has a doctor ever told you that you have glaucoma?"
In 9 centres participants also received an eye examination. This included measuring the pressure inside the eye (a standard test for glaucoma), and measuring the thickness of nerve fibres in the back of the eye (called the ganglion cell-inner plexiform layer or GCIPL). People with glaucoma have been shown to have thinner GCIPL.
Air pollution measures were provided by the Small Area Health Statistics Unit. The unit is based at Imperial College London and collects environmental data in collaboration with the Department for Environment, Food & Rural Affairs and the Environment Agency. The researchers looked at the average concentration of fine particulate matter (PM2.5) – tiny particles that can pass into the body via the lungs – in the year 2010. They modelled the average air pollution at each participant's home address.
The researchers analysed the link between glaucoma and PM2.5 taking into account potential confounders of age, gender, ethnicity, deprivation score, body mass index (BMI), smoking status and eyesight.
The final sample available for analysis was 111,370. This included those who completed the questionnaire and had eye examinations, and excluded those who had diabetic eye disease, eye surgery, or any other serious eye disease. People who did not have information available on all confounders were also excluded.
1.8% of the sample (2,040 adults) reported being diagnosed with glaucoma. Each quartile increase in PM2.5 was associated with a 6% increase in chance of self-report (odds ratio [OR] 1.06, 95% confidence interval [CI] 1.01 to 1.12).
The researchers also found a trend where higher PM2.5 was linked with a thinner nerve fibre layer (GCIPL).
There was no clear relationship between PM2.5 and pressure inside the eye.
The researchers conclude: "Greater exposure to PM2.5 is associated with both self-reported glaucoma and adverse structural characteristics of the disease. The absence of an association between PM2.5 and [intraocular pressure] suggests the relationship may occur through a non-pressure-dependent mechanism, possibly [toxic effects on the nerve] and/or vascular effects."
This is a novel study to have looked at the link between air pollution and glaucoma. The researchers say how the World Health Organization (WHO) ranks air pollution as one of the main contributors to diseases across the globe, notably respiratory disease, but also possibly cardiovascular and neurological conditions. Glaucoma affects the optic nerve, and as past research has observed that people with glaucoma more often live in towns and cities, it's been speculated whether pollution could be among the causes.
The researchers say their findings show "a meaningful association". Particularly given they did not find the increased pressure typically seen with glaucoma, they consider that pollution could be causing other damage to the nerve fibres in the eye.
But this study does not prove direct cause and effect. It looked at links between people who self-reported their eye measurements and having glaucoma, and the average pollution levels at their home address. This cannot prove that the pollution is responsible for these outcomes. Although the researchers have considered several confounders, they cannot account for all those that may be involved. Pollution levels at the home address might also not reflect the levels participants were actually exposed to.
Looking at self-report, the risk association was also very small at only 6% increased risk. There's also the potential for inaccurate answers using the broad question: "Has a doctor ever told you that you have glaucoma?" For example, some people may have regular eye checks for glaucoma because they have risk factors, and so have answered yes when they have not actually reached the diagnosis yet.
With the GCIPL, although there was a trend for decreased thickness with higher pollution, we do not know whether there was actually a link with clinically diagnosed glaucoma. Overall questions remain, and while this research provides a new avenue for further research it does not provide the whole answer.
Glaucoma can be effectively treated if it's picked up early. The most important thing is to ensure you have regular eye tests.