Discussion

Among the social, economic, systemic and ocular factors examined, the two factors associated with undiagnosed POAG in the community were lower pretreatment IOP and the participants reporting no eyesight problems. The first points to an over-reliance on IOP level to exclude glaucoma in the community, leading to patients with lower IOP to be missed. The National Institute for Health and Clinical Excellence guidelines in the UK recommend a referral to the Hospital Eye Service if IOP is greater than 24 mm Hg in the absence of other risk factors, as ocular hypertension on its own may warrant treatment.9 Nevertheless, IOP has been shown to be an ineffective tool for glaucoma case finding in the EPIC-Norfolk Eye Study, and no single IOP level provides both high sensitivity and specificity in glaucoma diagnosis.8 This study demonstrates that it is easy for eye care providers to be reassured by an IOP level <24 mm Hg, while other features of glaucoma are missed. It must be stressed therefore that among patients with non-elevated IOP, care should be taken to examine the optic disc carefully and with supportive disc imaging and visual field testing to improve the chances of identifying suspicious disc features.

In this study, other features of severity of glaucoma such as vertical CDR and visual field mean deviation were not associated with missed OAG cases. It could be because CDR does not adequately capture features of a glaucomatous disc, and visual fields may not be done routinely at the optician. Even with advanced field defects, many patients with glaucoma are asymptomatic, so field defects will not necessarily provide a reason to visit the optician.

Several factors were examined that could reflect a participant’s access or likelihood to seek eye care. An important limitation of the methodology is that specific questions about the participants’ history of visits to eye care professionals were not collected prospectively, and asking the questions retrospectively would be prone to recall bias and significant inaccuracies. Hence, there was the need to use proxy factors, such as absolute refractive error, glasses use, being pseudophakic and self-reported visual problems, which require careful interpretation. The strongest factor associated with undiagnosed POAG was answering no to the question ‘do you have problems with your eyesight’, while worse acuity of either eye was not related. A possible interpretation of this finding is that it was the self-perception of good eyesight, and by implication, a lesser likelihood to visit an optometrist, rather than actual visual function, that led the POAG to be undiagnosed. Nevertheless, the participants with existing glaucoma might have perceived their eyesight as being worse than those with undiagnosed glaucoma as a recall bias, as they had a known eye condition, or the use of eyedrops or having had eye surgery diminished their visual function. Wearing glasses or contact lenses was not a significant factor, most likely because 97% of the cohort wore glasses, and it was not effective in discriminating those with previously diagnosed and undiagnosed POAG.

Published studies have explored the question whether visits to eye care professionals is important in facilitating the discovery of glaucoma. The Thessaloniki Study found that previously undiagnosed patients were more likely to not have seen an eye doctor in the past year.4 Similarly, both the Barbados Eye Study3 and the Melbourne Visual Impairment Project5 reported that previously undiagnosed patients sought eye care less frequently in the past year, with the source of eye care more likely to be an optometrist rather than an ophthalmologist.3 5 However, these findings are potentially confounded by the fact that patients with diagnosed glaucoma would already be under the care of an ophthalmologist, so a prospective study is required to adequately answer that question.

Currently in the UK, POAG is diagnosed by opportunistic case finding, relying on patients presenting to an optometrist for an eye test, and referral made to the Hospital Eye Service under the National Health Service if glaucoma is suspected. Those most at risk of glaucoma—aged >60 and those aged >40 with a positive family history of glaucoma in a first degree relative—can get the optician’s eye test for free, and so there is no financial barrier to the diagnosis of glaucoma among those at risk. In EPIC-Norfolk, a greater proportion of all glaucoma cases (67%) were previously diagnosed, compared with the usual 50% reported in most western population studies. There are a few potential explanations. First, due to the method of recruitment and the low proportion of retained participants from the original cohort, the study population was not representative of the general population of Norfolk and UK. They were older, with an over-representation of those above age 60, the age from which eye tests are free at the optician, therefore biasing the likelihood of them visiting an optician. Another explanation could be recruitment bias, as health conscious individuals who were more likely to visit their optician were also more likely to participate in this study. It might also reflect more effective healthcare provision in Norfolk. However, one cannot discount the possibility that some participants who were known patients with glaucoma were motivated to take part by an enthusiasm to have additional eye testing, causing response bias.

In conclusion, the most important healthcare implication from this analysis is to avoid being falsely reassured by a lower level of IOP in glaucoma case finding. There is also a suggestion that raising public awareness of glaucoma and encouraging regular eye tests at the optician can help reduce undiagnosed glaucoma.