124 S.A. Perera and T. Aung

factors (e.g. corneal curvature or lenticular changes) may be the main biometric constituent that underlies the risk for POAG.21

Myopia in other situations

Myopia and ocular hypertension

It is not surprising that some weaker associations have been found between myopia and ocular hypertension, as raised intraocular pressure (IOP) is an accepted major risk factor for the development of POAG.

The BMES study, whilst showing a strong relationship between myopia and glaucoma, only showed a borderline relationship with ocular hypertension with an odds ratio of 1.8 CI (1.2–2.9).

In one cohort study of ocular hypertensives, followed up for a mean of 7.3 years, axial myopia was found to be a significant risk factor for the development of glaucoma.22 This was in direct contrast to the much larger OHTS which showed that myopia was not a predictive factor for the development of glaucoma.3

There is also documented interplay between some glaucoma risk factors and myopia. The risk of steroid induced ocular hypertension is higher in patients with severe myopia, alongside other risk factors such as a history of open angle glaucoma and diabetes mellitus.23

Myopia in angle closure

Generally, it is hyperopic eyes, with short axial lengths and shallow anterior chamber depths that are associated with angle closure. However, angle closure can occur in myopic eyes. In a retrospective chart review of 322 cases of primary angle closure, six cases occurred in myopic eyes.24 This shows that eyes with a myopic refraction are not immune from changes that predominate in the angle recess and anterior chamber, which may predispose to angle closure.

Myopia in Pigment Dispersion Syndrome (PDS)

It is well documented that PDS patients are frequently myopes, and that this could play a role in the pathogenesis of reverse pupil block. It is claimed that higher degrees of myopia lead to glaucomatous optic

125 Myopia and Glaucoma

neuropathy earlier,25 and more often in PDS. This was established in a study of 18 patients with PDS and 93 with pigmentary glaucoma, who were analyzed for risk factors.26

Limitations of cross-sectional studies:

Methodologic issues

There are many difficulties in performing a good population-based study, including achieving a high participation rate and ensuring little missing data. It is important to standardize diagnostic criteria where possible, e.g. by using the International Society for Geographical and Epidemiological Ophthalmology (ISGEO) criteria for the diagnosis of glaucoma based on optic nerve changes and perimetric findings. Unfortunately, where IOP was used to define glaucoma, such as in the Beaver Dam Eye Study, its results are not so comparable. A standardized assessment of refraction, IOP measurement, and glaucoma definitions strengthen the validity of any conclusions. The measurement of AL by optical methods instead of ultrasound methods and the use of subjective refractions by trained optometrists instead of autorefraction are the gold standard methods of each measurement, however, they have rarely been implemented in large studies. A population-based design also minimizes selection bias. Some clinic-based studies are biased by the fact that more myopes attend, thereby increasing the likelihood of detecting POAG in these studies. Although many studies have shown the correlation of central corneal thickness (CCT) with optic disc parameters,27,28 and thinner CCT has been identified as a risk factor for the development of POAG in eyes with ocular hypertension,3 some studies have not controlled for the influence of CCT on myopia and glaucoma development.

The potential influence of myopia on POAG has not been derived from longitudinal data. Only a cohort study will elucidate if myopia is associated with a risk of developing POAG, but it may take many years of follow-up, especially considering the onset of POAG is variable and delayed compared to the onset and stabilization of myopia. Much of this population data has been based on single measurements (of refraction, IOP, optic disc, and visual fields) during the course of the study. In addition, many cross-sectional studies were underpowered to elicit any associations with myopia, especially once subdivided into relatively arbitrary myopic categories.