126 S.A. Perera and T. Aung

Theories for a Link Between Myopia and POAG

The association between myopia and glaucoma:

How is it mediated?

It is unclear if the relationship between myopia (as measured by spherical equivalent) and POAG is mediated by AL or other factors (corneal curvature or lenticular changes with age) although there is some supporting evidence from both the Meiktila eye study and the SiMES data that AL is the mediator. Comparatively, the SiMES data is more robust as many studies have not controlled for the effects of central corneal thickness, which is now known to strongly influence the measurement of IOP and is a risk factor for POAG.3

Several theories have been put forward to explain a true link between myopia and POAG. Myopia has been found to influence IOP, with myopes associated with having higher IOP than emmetropes and hyperopes. The Blue Mountains study found 0.45-mmHg IOP difference between myopic and emmetropic eyes.13 This was slightly lower than in some previous reports, which have reported differences ranging from 0.75 to more than 1.0 mmHg.8,29,30

This has failed to be replicated in the Beijing Eye Study, where the mean IOP was not significantly different between the eyes with marked and high myopia compared to the remaining eyes, despite having a significantly higher frequency of glaucoma.31 Other studies have reported higher applanation pressures in children,32 or in subjects with increased axial length.33 This may shed some light on the variation in findings of the populationbased studies.

Glaucoma Assessment in Myopic Eyes

Biometric differences

Axial length and CCT

Recently, there has been some interest in CCT, as it was revealed as a significant risk factor for the development of glaucoma in ocular hypertensives,3 and as its role in accurate IOP measurement became known. Longer AL, however, does not correlate with CCT in myopes34 or in populations of normal or glaucoma patients.35 The relationship between CCT and refractive error favors no correlation. Although in myopic populations,