127 Myopia and Glaucoma
Chang et al. reported that the CCT was thinner in the more myopic eyes, Fam et al.36 found no correlation with the degree of myopia with CCT in Singaporean Chinese. Among normal populations, a significant correlation between CCT and refraction was demonstrated in the Japanese.37 However, for the majority of studies in Chinese and Malays, CCT was not correlated with refraction.38–40
Optic disc assessment in myopic eyes
The clinical diagnosis of glaucoma in myopic eyes may be difficult. The optic discs of myopes are notoriously difficult to assess, especially those with coexistent tilted discs.41 The discs frequently appear glaucomatous with larger diameters, greater cup disc ratios, and larger and shallower optic cups.42–44 Myopic discs are often obliquely inserted and tilted.45 A possible source of bias for graders in studies is that highly myopic eyes usually show a characteristic appearance of the optic nerve head with an enlarged disc, shallow cupping, a large myopic crescent, and fundus hypopigmentation (Figs. 1 and 2).
Figure 1
Figure 2
128 S.A. Perera and T. Aung
Visual fields in myopic eyes
In addition, myopic retinal degeneration, which is common in high myopes, may cause visual field defects that mimic glaucomatous visual field defects. Non-glaucomatous visual field abnormalities, e.g. enlarged blind spots and superotemporal defects, have been reported in both highly myopic eyes and those with tilted discs.46–48 It is important not to overstate the effect of simple myopia on visual field testing. In one study, there appeared to be a low prevalence of visual field defects amongst a population of young healthy myopic males with no myopic degeneration. The observation that myopia affected threshold sensitivity, especially in higher degrees of myopia48 and in relation to AL,49 may be explained by microscopic structural changes in the retina and choroid, axial elongation of the eye with increased spacing of retinal photoreceptors, or distortion of the stimulus by the negative prescription of the lenses. It is possible that such cases of high myopia may have been misclassified as POAG in studies, leading to a spurious association between myopia and POAG. In younger myopic individuals with tilted optic discs, significant focal visual field defects were rare, suggesting that these defects probably do not develop until their later years.50 Abnormal visual fields in myopic eyes have also been reported when tested with other forms of perimetry. Myopic optical defocus has a significant effect on the Humphrey Matrix 30–2 test results, impacting more so in the moderate-myopic group than in low-myopic eyes.51
Imaging tests and variations with myopia
Recently, there has been an explosion in new technologies, which aim to give quantitative and objective measurements of optic nerve head (ONH) and retinal nerve fibre layer (RNFL) parameters. Although they have reasonable sensitivity and specificity for detecting glaucoma, each technology has some challenges when assessing myopic eyes. A study using the Stratus-optical coherence tomography (OCT) revealed that overall RNFL thickness decreased 7 microns per 1 mm of axial length, and 3 microns per 1 D sphere. Moderately myopic subjects tended to have thinner peripapillary RNFL, mainly at the superior and inferior poles, which would make judging the RNFL thickness at these critical points less straightforward. This, together with the likelihood of having a thinner RNFL, should be considered when interpreting a glaucoma suspect’s OCT measurements
129 Myopia and Glaucoma
compared with the normative database.52 For eyes with a spherical equivalent of <−10.0 D, it has been shown that the average RNFL thickness is 80 µm, as opposed to 100 µm in emmetropes.53 Myopic eyes were more likely to develop abnormal birefringence patterns in both types of scanning laser polarimetry (GDxTM VCC and ECC, Carl Zeiss Meditec, Jena, Germany).54 This could stem from a more spread out RNFL and may lead to anomalous diagnoses if not used together with clinical assessment. In contrast, Heidelberg retinal tomography (Heidelberg Engineering, Heidelberg, Germany) tests do not seem to be as affected by refractive error as they are by optic disc tilt.55
ONH susceptibility to damage
Jonas and Budde56 showed that for a given intraocular pressure (IOP) level in eyes with POAG, optic nerve damage appears to be more pronounced in highly myopic eyes with large optic discs than in non-highly myopic eyes. The optic nerve head in myopic eyes may be more susceptible to glaucomatous damage from elevated or normal IOP57,58 than in nonmyopic eyes. Also, the cup-disc-ratio is higher in myopes,60 which may predispose more nerve fibers to damage at any level of IOP60–62 as shearing forces exerted by scleral tension across the lamina cribrosa are heightened in myopia for the same IOP.63 It has been proposed that similar connective tissue changes may also occur in glaucoma and myopia.64,65 The finding that AL largely explains the association between myopia and POAG also supports a theory involving the connective tissue changes associated with longer axial dimensions as a potential mechanism for POAG. It may fit with the finding of a thinner lamina cribrosa in combination with a secondary enlargement of the optic nerve head in highly myopic eyes.66–70 In two cadaveric studies, it was found that in highly myopic eyes, the lamina cribrosa was significantly thinner than in non-highly myopic eyes.71,72 This was postulated to steepen the translaminar pressure gradient for a given intraocular pressure and cause increased susceptibility to glaucoma in highly myopic eyes.
A trial combing A-scan ultrasonography with confocal scanning laser ophthalmoscope images of the optic disc showed that increased disc area is associated with longer axial length measurements (and African ancestry) in normal individuals. This may have implications for pathophysiology and risk assessment of glaucoma, as these normal eyes may be misclassified as glaucomatous.73