Patients with high myopia has a sixfold increased odds to develop glaucomatous disease (20), and in this case the early diagnosis is mandatory and needs tests with high sensitivity and specificity (21).
The evaluation of peripapillary RNFL is used in common clinical practice to detect the presence of glaucomatous damage (22), but in high myopia its interpretation is made difficult by the frequent presence of optic nerve tilt. Shin et al (23) showed that optic disc tilt reduce RNFL diagnostic ability in detecting glaucoma, while it doesn’t influence ganglion cell-inner plexiform layer (GCIPL) thickness, which is more reliable in the evaluation of glaucoma in high myopia.
In our study all the OCT parameters and RGCs count are correlated to the increase in axial length with a moderate significant negative correlation, except GCC thicknesses that seem to be the less correlated to axial length increase because they have the weakest and less significant correlation in all groups (Table 3). This is this is in agreement with the results of many studies: Shoji et al (24) have shown that GCC parameters are not significantly affected by high myopia, while RNFL measurements have a decreased ability to detect glaucoma in myopic subjects;
Considering the results of the comparisons among subgroups (Tables 4.a-b-c and 5) there are statistically significant results for almost all the parameters in comparison between healthy and PPG, both among the myopes and emmetropes. So we could support, according to the studies of Tan et al (25) and Kim et al (26), that the RNFL and GCC parameters are complementary in the evaluation of glaucomatous damage also in myopic eyes.
As regards diagnostic ability of OCT parameters, in our study all parameters has an AUROC > 0.5 (Table 6), and all curves are statistical significant, with high values of sensibility and specificity. While RNFLinf and FLV showed the best AUROCs in emmetropes, GCCinf and GLV showed the best AUROCs in all myopic group. This is in agreement with many studies that demonstrated that GCC thickness have a glaucoma detection ability as effective as that of RNFL parameters (24–30).
In the comparison between GCCinf and GLV and all other parameters, there was a statistical difference with almost all RNFL parameters, especially in high myopes (Table 7). The study of Seol et al (31) showed that inferotemporal GCIPL has a significantly better diagnostic ability than RNFL parameters, and this is in agreement with our results, and also than average GCIPL parameters, while in our study GCCinf and GLV have a better AUROC than other GCC parameters, but non statistically significant.
In Table 8 the results of the comparison of AUROC of every parameters between different subgroups are illustrated: in all the comparisons with emmetropes there is a statistical significant difference between emmetropes and myopes for GCC parameters, which show better diagnostic abilities in myopes. This means that GCC parameters are more useful to differentiate glaucomatous from healthy eyes in myopic than emmetropic eyes.
However we must take into account some limitations of GCC in the evaluation of glaucoma in myopic eyes. In the eyes in which the head of the optic nerve is deformed and therefore difficult to evaluate, we may assume that the macular region is less distorted, but this is not always true. The studies by Kim et al (32, 33) have suggested that the outline of the entire posterior pole determines the possible configuration of the optic nerve head. So, the presence of irregularities in macular region could invalidate the evaluation of GCC in myopia. Another bias is due to the high axial length which causes a false positive GCC thinning (34). This is because a greater axial length determines a streching of the globe with an increase in the distance between the optic nerve and the macula and consequent false thinning of macular region(35, 36). Furthermore, the presence of macular degeneration can cause GCC thinning (for retinal atrophy) or thickening (for intraretinal fluid due to myopic CNV or to macular retinoschisis) that are independent from glaucoma (37).
To our knowledge, no previous studies have reported on use of RGCs count in identify glaucoma in myopic eyes and its diagnostic ability. There are many studies about RGCcount in non-myopic eyes demonstrating that a combined measure of structural and functional parameters performs better than the single OCT and perimetry parameters (13, 15). We wanted to evaluate if the diagnostic ability of the RGC was superior to those of the single parameters used in its calculation formula (MD and RNFLavg) also in myopic groups: both in mild and high myopes, AUROCs of RGCcount are significantly better than those of MD, and approximately similar to those of RNFLavg without statistically significant differences (Table 9). In all groups, both myopes and emmetropes, RGCcount shows very good AUROCs (between 0.873 in emmetropes and 0.929 in mild myopes), with sensitivity > 70% and specificity > 90%.
Based on the results of our study, RGCs count seems to be complementary to OCT parameters in the detection of glaucomatous damage in the myopia, also if GCC parameters show better diagnostic ability. Since the glaucomatous damage in the myopia is more early to be detected at macular level, it could be useful to evaluate the number of macular ganglion cells, as already done by Rolle et al (16), also in myopic subjects. This could be analysed in a further study.
The limitation of this study is that, despite of a good number of the total sample, the subdivision in different subgroups makes the number of each subgroup reduced. However, this is also found in other studies (31, 38, 39). To validate the results obtained it would be indicated to use an even larger cohort of investigation.
Another limitation is related to the fact that the sample of myopic eyes does not perfectly correspond to what we find in clinical practice, because it does not include all the eyes with perimetric alterations (enlarged blind spot, general reduction of sensitivity and superotemporal peripheral defects), which also are very frequent in myopic eyes.
A strength is represented by the use of preperimetric eyes, since comparing only eyes diagnosed with both functionally and structurally established glaucoma with healthy would lead to overestimate the performance of the test, as reported by Medeiros et al (40).