Statistical analysis was performed using SPSS Version 21.0 statistic software package (SPSS Inc., Chicago, IL). The Kolmogorov-Smirnov test determined normalization for each continuous variable. Data were expressed as means ± Standard Deviation (SD). Comparisons between groups were performed with analysis of the parametric test. A chi-square test was used to compare categorical data. An independent t-test was used for intergroup comparisons for standard distributed continuous variables. One way ANOVA test was used to compare clinical results between the three groups. Correlations between variables were determined using the Pearson correlation coefficient. A value of p <0.05 was considered statistically significant.

The corneal endothelium is a cell layer that cannot renew itself, has metabolic activity and protects corneal transparency and has a hexagonal structure. The density and morphology of these cells can be assessed by specular microscopy. Some researchers examined changes in this cell layer associated with pathological myopia. Delshad et al. [5] reported that there was a significant decrease in ECD and hexagonal structure in high myopic eyes. Similar results were also found by Urban et al. [6] and Chang et al. [7]. However, we did not find any association between AL and ECD.

CCT can be especially important in myopic eyes in cases where it is necessary, such as follow-up of glaucoma and ocular laser surgery to correct refractive errors. The effects of refractive changes on CCT in myopic patients have demonstrated conflicting results. While some studies [8] have reported no association between CCT and myopia, other studies [7, 8] have been demonstrated increased corneal scarring as myopia progresses. In a study comparing pathologic myopia and emmetropic eyes, no difference was found regarding as CCT between two groups [9]. Pedersen et al. [10] compared 57 emmetrops with 48 high myopia patients. They found that there was no correlation between CCT and AL. Similarly, we did not find a significant correlation between AL and CCT. On the other hand, Wang et al. [11] reported that CCT was thicker in patients with high myopia.

Previous studies investigating corneal curvature in high myopic patients have reported a positive correlation between corneal curvature and AL, which is similar to our findings [12, 13]. ACD is defined as the distance between the posterior aspect of the cornea and the anterior aspect of the lens and has great importance for phacoemulsification surgery. In myopic eyes, the changes in AL can affect the ACD. Hosny et al. [14] stated that there was an important relationship between ACD and AL in healthy eyes. Chung et al. [15] found a positive correlation between ACD and AL in mild myopic eyes, but did not observe in high myopic eyes. In contrast with these studies, we did not find any correlation between AL and ACD.

Several vitreoretinal disorders including vitreous degeneration, posterior vitreous detachment, peripapillary atrophy, lattice degeneration, tilted disc, retinal pigment epithelium (RPE) atrophy, posterior staphyloma, Lacquer cracks and Fuch’s spot can be seen in pathological myopia. Chang et al. [16] reported that posterior staphyloma (23%), chorioretinal atrophy (19.3%), and lacquer cracks (1.8%) were more common myopia-related macular findings. They also reported that the most common optic disc findings were peripapillary atrophy (81.2%), and tilted disc (57.4%). In contrast with this study, Chen et al. [17] showed that the most common myopic maculopathy finding was lacquer cracks (29.1%). Koh et al. [18] reported that the most common retinal findings were peripapillary atrophy (98.3%), tilted disc (22%), posterior staphyloma (32%) and chorioretinal atrophy (8.3%). Different studies have been indicated that the frequency of posterior staphyloma, chorioretinal atrophy, CNV and tilted disc were 90.0%, 60.9%, 11.3% and 57.4%, respectively [19-21]. Likewise, in our study, the rate of peripapillary atrophy was similar to literature. However, the rate of posterior staphyloma was higher than the previous literature.

Macular degeneration in patients with pathological myopia typically progresses slowly over the years, but the loss of visual acuity may begin earlier than expected. Pathological myopia is the second common cause of secondary CNV. It is usually seen in adult patients younger than 50 years and consists of approximately 62% of CNV cases caused by pathological myopia [22]. Myopic CNV usually appears small circular and grayish color and settles near the macula or optic disc. More than 70% of these lesions are classical type and 65% are subfoveal. However, occult type and pigment epithelial detachment are rarely seen [23]. Fundus fluorescein angiography (FFA) and OCT are complementary ancillary tests in both diagnosis and classification. OCT demonstrates the presence of subretinal fluid in the active phase.

On the other hand, FFA presents hyperfluorescence in early periods but shows leakage in late periods. Myopia-associated CNV rate reported to be 5-11% in different studies [2, 24, 25]. In contrast to previous studies, a higher CNV ratio (27.9%) was found in our research.

As is known, one of the earliest changes in pathological myopia occurs in the choroidal layer. With Enhanced Depth Imaging-OCT (EDI-OCT), choroidal tissue can be displayed more effectively and the changes that may occur can be interpreted in detailed. In a study conducted by Gupta et al. [26] who compared pathological myopia and emmetropic eyes, the SFCT was found significantly thinner in pathological myopic eyes. Similarly, Wang et al. [27] reported thinner SFCT in myopic eyes. In subgroup analysis, they also reported that the choroidal thickness in eyes with CNV was thinner than eyes without CNV. [27]. Similarly, in a study conducted by El Matri et al. [28], high myopia eyes with and without CNV were compared and the mean SFCT was found to be significantly thinner in the eyes with CNV. In addition, a negative correlation was demonstrated between AL and SFCT. [28] Also, they found that every 1 mm increase in AL, which is corresponding an 8.4 μm decrease in SFCT. However, in another study conducted on 120 eyes, this rate was found as 25.9 μm [29]. Hsu et al. [30] reported that the SFCT was thinner in patients with myopic maculopathy than in non-maculopathic patients. Similar to previous studies, we showed thinner SFCT in patients with myopic maculopathy.

The tilted disc is a common finding in myopic population. The optic disc is seen as being tilted and rotated around its axes. It may cause myopia, astigmatism, visual field defect, and colored vision problem. Although the tilted disc is not progressive, it may be misdiagnosed as optic disc edema and glaucoma [31]. Two hypotheses regarding its formation have been proposed. First, it is associated with impaired closure of congenital embryonic optic fissure [31, 32]. Second is that it results from the progressive prolongation of the globe [33]. In a population-based SCORM (Singapore Cohort of Risk Factors for Myopia) study, tilted disc was detected in 37% of eyes. They also reported that eyes with tilted disc had higher myopic and astigmatic, and longer AL [34]. In our study, only high myopia patients were included and the rate of tilted disc was higher than SCORM study (63.6% vs. 37%). In addition, no correlation was found among tilted disc and AL, astigmatism and corneal curvature. Similarly, Moghadas et al. [35] did not find a difference in biometric parameters between eyes with and without tilted disc in the patients with pathological myopia.

Pathological myopia and prolonged AL are known to be closely related to peripheral retinal degenerations and retinal detachment [36]. Lattice degeneration, pavement degeneration, and white-without-pressure are the most common peripheral retinal degenerations associated with pathological myopia. Burton TC. [37] reported that this risk is increased 200-fold in cases with pathological myopia and lattice degeneration. In a study conducted on patients with asymptomatic pathologic myopia, peripheral retinal lesions were seen in 56.1% of the patients [38]. Similarly, in another study conducted by Cheng et al. [39] on pathologic myopia in pediatric patients, this rate was found to be 61.7%. Different studies have stated that the rate of lattice degeneration in pathological myopia varies between 3% and 10.7% [40]. However, this rate was higher in our review (42.1%) and Celorio et al's study [41] (33.0%) than in previous studies. Racial and environmental factors may be responsible for higher rates of the tilted disc, posterior staphyloma, lattice degeneration and CNV in the population with pathological myopia in our region.

Pathological myopia may also lead to many complications such as cataract, Rhegmatogenous Retinal Detachment (RRD), and secondary glaucoma [42-44]. The link between cataract formation and high myopia was investigated by the Blue Mountains Eye Study and the Beaver Dam study [43, 44]. In these studies, myopia was associated with posterior polar cataract [43], posterior subcapsular cataract [43] and nuclear cataract [44]. In addition, significant visual field defects were defined in highly myopic eyes [45]. High myopia is known to be a risk factor for RRD and half of the patients with RRD are estimated to be myopia patients [46]. Myopia has also been described in association with systemic disorders such as Marfan, Ehler Danlos, Stickler, and Wagner syndrome and diabetes mellitus (DM) [47, 48]. Several studies have reported the association of Type 2 DM with myopia. In a study conducted by Ganesan et al. [48] on patients with Type 2 DM, the prevalence of myopia and high myopia was reported as 19.9% and 1.9%, respectively.

Prevention of the onset of myopia or the progression of low myopia to high myopia is of extreme importance to prevent the ocular complications of myopia. Early detection and treatment of possible ocular complications associated with high myopia are crucial to reducing morbidity. Therefore, patients should be informed in detail about complications associated with high myopia. In addition, the ophthalmologist can advise that patients with high myopia can check their own visual acuity and visual field with the same target in the same distance, and wear protective glasses during the sports. An ophthalmologist may also facilitate the screening with the regular control of intraocular pressure, central and peripheral retina and optic nerve. Furthermore, detailed fundus examination before cataract or refractive surgery is of great importance in these patients. Treatment of risky lesions before surgery may prevent postoperative complications and legally protect the physician.

This study has several limitations. First, our study design was cross-sectional and we could only present our observations. However, we could not document the progression of the lesions. Second, our study had limited statistical power to detect small differences due to the relatively small sample size. Third, we did not include the control group in the study. Therefore, we could not compare the parameters such as AL, CCT, ECD, ACD, and CMT between myopic eyes and non-myopic eyes. Fourth, the calculation of average SFCT was carried out manually due to the lack of automatic software. The objective automated measurements were better to yield convincing results and conclusions. However, we were measured the SFCT by two independently two blinded clinicians to remove the bias. Finally, the adult-based design may not be truly representative of the entire population.

In conclusion, our study clearly shows that pathological myopia may affect both anterior and posterior ocular segments. However, posterior ocular segment changes are the most important cause of vision loss in high myopic patients. Therefore, patients should be informed about the potential risks associated with myopia. In addition, periodic follow-up is essential for both the early diagnosis and treatment of possible asymptomatic ocular findings. This Turkish community-based study has reported the clinical characteristics of patients with pathological myopia and compared with the previous studies. We consider that the data may be useful in assessing the global or local status of pathological myopia.