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Editors Selection IGR 11-2

Epidemiology

Comment by Ravi Thomas on:

15134 Gonioscopy in adult Chinese: the Liwan Eye Study, He M; Foster PJ; Ge J et al., Investigative Ophthalmology and Visual Science, 2006; 47: 4772-4779

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The report on gonioscopy in an adult Chinese population is another welcome addition to the much needed research on angle closure that He et al. (970) have already contributed so much to. This well-designed, population-based study confirms that narrow angles (primary angle closure suspect; PACS) and primary angle closure (PAC) have a high prevalence in Chinese.

This study confirms that narrow angles (primary angle closure suspect; PACS) and primary angle closure (PAC) have a high prevalence in Chinese

Two minor issues: the authors have attempted to explain peripheral anterior synechiae (PAS) detected in open angles. 'Pseudo' PAS may not be 'manipulated' open with a Goldmann type lens; the correct diagnosis requires true indentation and it is not clear if this was performed in all such cases.

Secondly, the weighted 'kappa' for gonioscopy increased from the initial 0.63 to 0.82 when, it seems, more narrow angles were included. This probably reflects the fact that kappa is maximized when the prevalence of the outcome studied is around 50%. The kappa we consider should be that determined with the same prevalence of PACS as in the population being studied; such a kappa has to be lower. A kappa of 0.61 is just 'substantial', but for a weighted positive or negative decision for PACS, I would be more comfortable around the 0.9 in the 'almost perfect' category. However, I do not think this would change the study conclusions.

The frequency of a narrow angle in each quadrant has been documented. More importantly, the prevalence of PACS in those with the 270 degrees of posterior trabecular meshwork non-visibility was 11% percent, versus 17.5% in those with 180 degrees non-visibility; as suspected, the latter definition is indeed more sensitive for PACS. The 270 degree criteria is more specific; those who fit this definition had a 3.5 times higher prevalence of peripheral anterior synechia (PAS).

Currently there are major differences in indications for performing a laser iridotomy for PACS. Follow up of the quadrant wise data should clarify the issue of what degree of 'narrow angle' leads to a high enough incidence of PAC&G to justify intervention. I do hope that such a follow up study is planned.

The authors identify "the designing of optimal screening strategies and assessing the viability of prophylactic iridotomy for the large proportion of PACS in this population" as the major challenge. This challenge should only really arise once the follow up study reports on the number of patients who progressed to PACG and or developed some condition that affected the quality of life.

I gather from the data that approximately 3% of this population had undergone cataract surgery; an intervention would be expected to resolve the risk of progression in PACS. A percentage that low probably reflects the low cataract surgical rate in China; the percentage of aphakes / pseudophakes in an equivalent population in India is about 14%.

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