Why primary angle-closure glaucoma is declining in Scotland

The prevalence of acute primary angle-closure glaucoma (APACG) is declining in Scotland as cataract surgery becomes more common, researchers said.

“We propose that increasing rates of cataract surgery performed at an earlier age group in Scotland have played a significant role in reducing the rate of APACG by reducing the incidence of obstruction and the level of lens and papillary block mechanisms,” wrote Stewart N. Gillan from Ninewells Hospital and Medical School in Dundee, United Kingdom, and colleagues. 

They published their finding in Ophthalmic Epidemiology.

The researchers defined APACG as iridotrabecular contact resulting in peripheral anterior synechiae and/or raised IOP, causing glaucomatous optic neuropathy.

They enumerated four possible mechanisms for the condition: pupil block, obstruction at the level of the iris and/or ciliary body (plateau iris), obstruction at the level of the lens, and obstruction posterior to the lens (aqueous misdirection syndrome).

Six risk factors they reported include: older age, female sex, hypermetropia, lenticular changes, use of pharmaceutical products, and East Asian ethnicity.

Despite predictions that the prevalence of APACG, a leading cause of blindness, is set to rise worldwide, some authors have reported a decline in certain countries.

To situate Scotland in this context, Gillan and his colleagues analyzed national data from the Information Services Division (ISD) Scotland, which includes all activity carried out across the 14 Scottish health boards, including National Health Service (NHS) patients treated in private hospitals. (None of the private Scottish hospitals have emergency ophthalmic facilities.)

They counted the number of NHS patients diagnosed principally as having APACG, the number who underwent YAG laser peripheral iridotomy, and the number who had cataract surgery as an inpatient or day case, from 1 April 1998 to 31 March 2012.

They found that the number of patients coded for APACG dropped 46.4%, from 46.7 per million to 25 per million, a statistically significant change (P < 0.005).

Meanwhile, the rate of cataract surgery went up 73.4%, from 354.2 per 100,000 to 615.2 per 100,000, which was also significant (P < 0.005).

In addition, the rate of peripheral iridotomy spiralled 116.3%, from 38.0 per million to 82.2 per million. Even more striking, this increase took place in the most recent period; peripheral iridotomy actually decreased by 48.2% from1998 to 2008, before rocketing back up by 317.% between 2008 and 2012.

How to explain the trends

What factors could explain these trends? During this period, the mid-year estimates of the Scottish population increased by 4.6% The Scottish population under 50 years of age remained stable during this period while the population 50 years and older rose from 1.7 million to 2.1 million.

From the work of others, Gillan and colleagues said cataract surgery is being performed more frequently and at a younger age in the United Kingdom.

The beginning of the study period paralleled the kickoff of the U.K. Department of Health “Action on Cataracts” project. This initiative is designed to improve access to care for patients in need of cataract surgery, and to reduce variations in waiting times for cataract surgery.

The increased efficiency of the patient pathway succeeded in “massively increasing surgical throughput,” Gillan and colleagues wrote.

However, increasing rates of cataract surgery may not provide the only explanation for the decline in APACG, they wrote.

Work done by the International Society for Geographical and Epidemiological Ophthalmology has tightened the definition and classification of APACG. The European Glaucoma Society and the American Academy of Ophthalmology have endorsed this narrower definition. As a result, clinicians may be using the diagnosis less often, creating the perception of a decline in cases.

As for the peripheral iridotomy rates, increasing cataract rates could explain the decline in the first decade of the study period.

The authors attributed the subsequent increase to the introduction of the new General Ophthalmic Services contract in Scotland in 2006, providing free eye care services by optometrists.

Mandatory examinations

The mandatory, comprehensive ocular examinations introduced through this initiative included an assessment of the fundus following pupil dilation. Optometrists assessed anterior chamber depth and angle width using a slit lamp. National training included workshops on anterior segment assessment and gonioscopy.

As an effect of this, optometrists may be referring more patients to hospital eye services with suspected narrow angles, the researchers speculated.

The number of glaucoma specialist consultants in Scotland doubled from 2006 to 2012, and patients were streamlined into specialist glaucoma clinics. This, too, could have contributed to greater awareness of narrow angle disease and a higher rate of prophylactic iridotomy.

These findings in Scotland mostly parallel trends reported in England over the same time period, the researchers wrote. In the English study, overall PACG rates increased while acute angle-closure episodes decreased.

The authors speculated that the discrepancy might be attributable to the coding system in Scotland, which does not capture asymptomatic or chronic PACG that is managed in an outpatient clinic. They also said that not including these patients would not entirely account for the trends they documented.