Preventing blindness from glaucoma

The detection and management of primary open angle glaucoma is a major healthcare issue. It is the second largest cause of blindness in the world and affects some 66.8 million people, leaving 6.7 million with bilateral blindness.¹ In the United Kingdom, the ageing of the population means that the number of cases is expected to increase by 30% in the next 20 years.²

In primary open angle glaucoma, the retinal ganglion cells—the nerves that carry the visual stimulus from the retina to the brain—undergo apoptosis after insult at the head of the optic nerve. The progressive loss of ganglion cells leads to characteristic structural changes at the head of the optic nerve and functional loss to the visual field. Glaucoma is often, but not necessarily, associated with raised intraocular pressure. A paper in this week's BMJ shows that, in general, treatment to reduce intraocular pressure leads to delayed progression of visual field loss in patients with manifest open angle glaucoma.³ More research is needed in the subgroup of patients without increased intraocular pressure, to determine which patients with normal tension glaucoma will benefit most, since this meta-analysis was unable to show a consistent benefit in these patients.³ In 1982 Grant and Burke wrote a paper intriguingly titled “Why do some people go blind from glaucoma?”⁴ From a sample based in the United States, they found that some 30% of people who go blind from this disease are blind, in both eyes, at presentation. Most of the blind patients were aware of their decreasing vision for months, or even years, before they sought medical advice. Blindness was defined as an acuity of less than 20/200 (< 6/60 metric Snellen) in the better eye, or a residual visual field of less than 10 degrees. In a more recent report by Sinclair,⁵ who investigated registrations for blindness due to glaucoma in Fife between 1990 and 1999, a considerable number of patients were found to have moderate to advanced visual field loss at their first appointment, with 23% being eligible for registration as blind.

We recently reviewed all referrals for glaucoma and registrations for blindness or partial sight at Manchester Royal Eye Hospital during 2003. We found that 28% of patients with glaucoma were registered blind within three years of first presentation and that relatively few of those with blindness or partial sight were referred initially by optometrists: 42% compared with 90% nationally for all people with suspected glaucoma (unpublished data).This indicates that there may be barriers to access, such as the perceived costs associated with getting an eye examination. Laidlaw has already shown that the imposition of fees for sight tests had a negative effect on the number of referrals to Bristol Eye Hospital for glaucoma.⁶ New technologies, such as optic nerve and nerve fibre layer imaging devices, are promoted on the basis of being able to detect glaucoma before the patient has a reproducible visual field defect (because, unsurprisingly, those with more rapidly progressing disease leading to blindness are more likely to present with marked visual field loss⁷,⁸). But new technologies are not required to detect the extensive visual field loss that many of those who progress to blindness have at first presentation: the tests we already have are capable enough if used appropriately.

A series of epidemiological studies has shown that the more widespread use of existing technologies will improve early detection. In the north London trial,⁹ 75% of cases with “definite” glaucoma were new cases, and these were detected with a simple combination of tests—suprathreshold perimetry, tonometry, and slit lamp examination of the anterior eye and optic nerve head—that are readily available at most optometric practices.

The problem is not lack of suitably sensitive technologies but the infrequent use of existing technologies. Breaking down barriers to access, targeted screening, and a campaign to inform patients about the importance of regular eye examinations might have much more effect on the number of patients going blind from this disease than the current concentration of effort into the development of more sensitive technologies.