Discussion
The relationship between myopia and the risk of glaucoma has been extensively investigated, though it remains controversial. This may be due to difficulties in accurately diagnosing myopia or because abnormal results may occur frequently in patients with high myopia. Few studies have used RNFL photography to study myopic eyes. This is at least in part because RNFL can be obscured by relatively small amounts of pigment in the retinal pigment epithelium (RPE) in patients with myopia. In Caucasian populations, the low amount of RPE pigment may exacerbate this phenomenon. In Asian eyes, visualization of the RNFL in myopic eyes is fairly easy due to the prominence of the RPE pigment.
Our results showed that RNFL defects were larger in size and closer to the macula in the high myopia group compared with the low to moderate myopia and emmetropia groups (Figure 3). Possible mechanisms by which this occurs include: (1) early RNFL defects occur near the macula, (2) RNFL defects progress more quickly, resulting in widening to the macula, or (3) glaucomatous damage may develop at a younger age in patients with high myopia compared to patients with low to moderate myopia or emmetropia, thereby allowing more time for progression. Subjects in the emmetropia group were significantly older than those included in the other 2 groups. The MD values were significantly reduced in the high myopia group compared to the low to moderate myopia and emmetropia groups. Our data indicate that all three mechanisms may play a role in the characteristics of glaucomatous optic nerve damage in myopia. In order to apply the perimetric results, however, prismatic deviation at extra-axial points must be taken into consideration, since some subjects in the high myopia group required use of high-power lenses to correct near vision for the VF examination.
Although the mechanisms linking glaucoma and myopia are poorly understood, some population-based studies have shown an increasing risk of glaucoma with increasing severity of myopia. A possible explanation is that in myopic eyes, the optic nerve head may be structurally more susceptible to glaucomatous damage because of weakness in the connective tissue structure and arrangement. Although this has not yet been shown at the histological level, the elongation of the globe in myopic eyes leads to mechanical stretching and thinning of the retina. This elongation of the eyeball is also associated with pathologic changes in the fundus. Structural changes associated with myopia, such as a longer axial length, a larger and/or tilted optic disc, and peripapillary atrophy may make the maculopapillary bundle more susceptible to glaucomatous injury. The reduction in RNFL thickness with increasing axial length is a potential source of vulnerability in highly myopic eyes. Some studies have suggested that optic nerve damage is more pronounced in highly myopic eyes, and that high myopes have a higher susceptibility to glaucomatous optic neuropathy for any given IOP.
The relationship between myopia and glaucomatous damage have been evaluated by several groups. Some studies have found that myopic eyes have higher IOPs and severe visual field defects compared with emmetropes. Mayama et al. reported that higher myopia threatens the remaining lower cecocentral VF, and that it may also negatively affect the quality of vision in advanced stage OAG eyes with high IOPs, but not eyes with low IOPs. Despite these data, however, few studies have considered the glaucoma characteristics specific to myopic NTG.
It has been reported that glaucomatous optic nerve damage in highly myopic eyes is more diffuse compared to that of emmetropic eyes. In highly myopic eyes, inferior RNFL defects, atypical fiber defects, and multiple RNFL defects are common. In the early to moderately advanced stages of glaucoma, myopia is positively associated with an abnormality in the cecocentral VF, especially in the lower cecocentral field. However, according to the results of this study, in eyes with high myopia RNFL defects occurred frequently in both the superior and inferior quadrants. Additionally, where a defect occurred in a single quadrant, it was often in the inferior quadrant. When defects occurred in both the superior and inferior quadrants, the inferior defects were wider than the superior defects. Because defects were found more frequently in the inferior quadrant than in the superior quadrant, in cases where defects are seen in both quadrants the inferior defect was likely to have occurred first.
In the current study there was a higher male to female ratio in the high myopia group compared to the emmetropia group. While this is an interesting finding, we are unable to draw definitive conclusions from this given the hospital-based, retrospective, cross sectional nature of the study and relatively small number of subjects. To further validate these findings, a longitudinal population-based study is needed. Some studies have failed to demonstrate any gender preference with respect to refractive error and glaucoma. However, in the Rotterdam eye study, an association between male gender and high myopia was made with the development of glaucomatous visual field loss. In another study, Wu et al. also found that the incidence of moderate-high myopia was related to the presence of glaucoma as well as male gender.
There were several limitations to our study. First, because of its retrospective nature, we cannot exclude the possibility of selection bias. Second, we did not adjust the angle for the axial length or refractive error, which may cause differences in magnification. However, we believe that these differences cannot account for the results of this study. Magnification effects should not have altered angular measurements, and the measured arterio-venous ratio in the retina did not change with refractive error. However, this does not apply to the cases in which there was irregular magnification, thus we cannot definitely determine whether this impacted our results. Additionally our study was limited by the exclusion of diffuse atrophy as well as a lack of age-matching across groups.