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Acute primary angle closure (APAC) is a potentially blinding ocular condition. The incidence of APAC is especially high in east Asia, with Singaporeans having the highest reported incidence rate in the world of 12.2 per 100 000 per year in those aged 30 years and older. 1 Early biometric research by Lowe 2 and Alsbirk, 3 and confirmed by Friedman et al 4 has shown that APAC eyes are smaller in axial length, have flatter corneas, shallower anterior chambers, and thicker lenses. Clearly, smaller ocular biometry is a risk factor for the condition. However, there could be other physiological factors which are equally important, especially with regard to triggering of the acute attack.
One mechanism proposed for APAC in anatomically predisposed eyes has been choroidal expansion causing a forward movement of the lens and greater iris convexity. 5 Normally, an increase in volume posteriorly in the choroid would be accommodated by fluid exit from the anterior chamber without any shift in iris or lens position. However, the vitreous has a finite ability to transmit fluid because of its chemical structure. When transvitreous flow is insufficient to equalise the pressure differential, the result is anterior movement of the compressed vitreous humour, iris, and lens. With time, the vitreous compresses more, further decreasing its fluid conductivity and establishing a vicious cycle. In small eyes that are predisposed to PACG, forward lens movement of even a modest amount is believed to increase iris convexity and make acute angle closure more likely. 5
The aim of this study was to investigate the evidence for choroidal expansion as a mechanism for APAC. Specifically we compared the biometric parameters, anterior chamber depth, lens thickness, and lens position within 24 hours of presentation and again after 2 weeks in eyes with APAC.
METHODS
This was a prospective observational case series of Asian subjects who presented with APAC. Written informed consent was obtained from all subjects and the study had the approval of the ethics committee of the Singapore National Eye Centre, and was performed according to the tenets of the Declaration of Helsinki. Demographic characteristics and ophthalmic data related to the APAC episode (such as laterality of affected eye, duration of acute symptoms and presenting IOP) were recorded for each subject....