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Editors Selection IGR 7-3

Epidemiology

Comment by Paul Foster on:

13137 The Tajimi Study report 2: prevalence of primary angle closure and secondary glaucoma in a Japanese population, Yamamoto T; Iwase A; Araie M et al., Ophthalmology, 2005; 112: 1661-1669

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Data on the prevalence, specific clinical characteristics and risk factors for Asian people remain relatively rare. Consequently, the paper by Yamamoto et al. (746) on the Prevalence of Primary Angle Closure -0.6% - and Secondary Glaucoma - 0.5% - in a Japanese Population is a welcome addition to the portfolio. This is especially so, as the authors have used standardised methods of classifying and reporting cases. This is important in epidemiological studies where comparison of rates of disease between groups within the population, or with other populations studied previously, is carried out. The authors follow the broad classification standards proposed in the ISGEO scheme. This suggested that the convention established for primary open angle glaucoma (POAG) - that the term glaucoma denotes the presence of specific structural and functional characteristics of an excavated optic neuropathy - be extended to the classification of secondary glaucoma and primary angle-closure glaucoma (PACG). This represents the final stage in the natural history that begins with an anatomically narrow angle, though pathological consequences of closure (including raised pressure and peripheral anterior synechiae) to glaucomatous optic neuropathy with the potential to cause significant visual loss, or even blindness. The ISGEO scheme is now widely accepted, and has been adopted by the American Academy of Ophthalmology in its Preferred Practice Patterns for Primary Angle-Closure.

All subjects in the Tajimi study underwent a preliminary screening examination that included optic disc photography, visual field assessment (frequency doubling technology) as well as Goldmann applanation tonometry. Limbal chamber depth assessment (LCD - the Van Herick test) was used as method of identifying subjects with anatomically narrow angles. The authors cite the high sensitivity of LCD in identifying people with very narrow angles. The omission of gonioscopy for all subjects was an understandable, pragmatic decision. However, it does mean that some people with peripheral anterior synechiae (PAS) in relatively deeper anterior chambers (probably in those with a non-pupil-block mechanism) may possibly have been missed. However, the authors rightly employ a more liberal definition of angle-closure than that used in other studies performed in Asia. The established 'epidemiological standard' definition of an occludable angle (the diagnostic sine qua non for angle-closure) requires more than three quarters of the posterior trabecular meshwork be hidden from view by the peripheral iris. This has been shown to be, at least in Asian people, inappropriately stringent. Around 50% of those people who are found to have primary" PAS, do not meet this criterion (Foster BJO 2004; 88: 486-90). In the Tajimi study, if subjects had an irido-trabecular angle of ≤20 degrees and elevated IOP, primary PAS or evidence if ischaemic sequelae, they were deemed to have primary angle-closure. This more inclusive diagnostic criterion for angle-closure largely redresses the weaknesses of the established convention requiring a effectively occluded angle before irido-trabecular is considered as an important component in the management of the glaucoma patient. With the growing availability of sophisticated imaging techniques such as ultrasound biomicroscopy and anterior segment OCT, there is an increasing recognition of the rapid, pronounced variation in angle anatomy in different lighting conditions. Preliminary evidence points towards a greater, undetected burden of angle-closure. Ultimately, a shift in mindset is needed, away from requiring that angle-closure is proven to exist, and towards proving that angle-closure is not occurring. This would probably improve the prognosis of many patients who have this disease which is preventable in most cases, but remains largely unrecognised in the asymptomatic majority.

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