The evaluation of patients with thin corneas for the risk of glaucoma by posterior pole asymmetry analysis

Objectives: In this study, macular retinal and ganglion cell thickness values were compared between patients diagnosed with primary open-angle glaucoma suspicion, who have thin central corneal thickness and healthy individuals without any eye diseases. The aim of this study was to investigate the role of retinal thickness measurements by posterior pole asymmetry analysis (PPAA) using spectral-domain optical coherence tomography (SD-OCT) (Spectralis; Heidelberg Engineering) in early detection of glaucoma in patients with thin corneas. Materials and Methods: In this study, a total of 119 eyes were included for evaluation of retinal and ganglion cell complex thickness using the posterior pole asymmetry analysis (PPAA) method. These eyes comprised 59 eyes from 31 patients with suspected primary open-angle glaucoma, who had central corneal thickness (CCT) of 500µm or lower, and 60 eyes from 30 healthy individuals, who underwent routine ophthalmologic examinations at our hospital's ophthalmology clinic between August 2022 and March 2023. The patient's age, gender, intraocular pressure (IOP), macular thickness (MT), retinal nerve fiber layer thickness (RNFL), ganglion cell layer thickness (GCL), inner plexiform layer (IPL), and corneal thickness values were recorded. The included cases were divided into two groups: one comprised patients with suspected primary open-angle glaucoma and thin central corneal thickness, while the other consisted of individuals with healthy eye findings. The variable parameters were compared between the groups. Results: The average age of the included patients in the study was 55.36 ± 13.36 years (range: 18-71). Among the participants 51 (83.6%) were female and 10 (16.4%) were male. Among the participants 31 individuals (50.8%) showed signs of suspected glaucoma with thin corneas, while 30 individuals (49.2%) had normal eye findings. There was no statistically significant difference observed between patients with suspected glaucoma and individuals with normal eye findings in terms of age and gender values. In the group with suspected glaucoma, the total macular thickness (T-MT), superior hemifield macular thickness (S-MT), inferior hemifield macular thickness (İ-MT), as well as the segmented values including zone 1 total macular thickness (Z1 T-MT), zone 1 superior hemifield macular thickness (Z1 S-MT), zone 1 inferior hemifield macular thickness (Z1 İ-MT), and zone 2 superior hemifield macular thickness (Z2 S-MT) were found to be significantly higher compared to the values of the control group. In the group with suspected glaucoma, the superior peripapillary retinal nerve fiber layer thickness (S-ppRNFL) and inferior peripapillary retinal nerve fiber layer thickness (I-ppRNFL) values were found to be significantly lower compared to the values of the control group. No statistically significant difference was observed between the groups in terms of ppRNFL, temporal ppRNFL, nasal ppRNFL, Z2 MT, Z2 İ-MT, Z3 MT, Z3 S-MT, Z3 İ-MT and ganglion cell complex (GCC) thickness values. Statistically significant negative correlations were found between age and retinal thickness, as well as age and GCC thickness, based on correlation analysis. Statistically significant negative correlations were found between the optic disc cup-to-disc ratio and ppRNFL based on correlation analysis. Statistically significant positive correlations were found between ppRNFL and MT, as well as GCC thickness based on correlation analysis. When investigating the difference between upper and lower hemispheres in individuals without eye diseases, statistically significant higher average thickness values were observed in Z2, Z3 MT and Z1, Z2, Z3 GCC thickness in the lower hemisphere compared to the upper hemisphere. However, there was no statistically significant difference in the average upper-lower hemisphere thickness values for Z1 MT. In patients with suspected glaucoma, statistically significant higher average thickness values were found in the lower hemisphere compared to the upper hemisphere for Z1, Z2, Z3 ganglion cell complex (GCC) thickness. However, there was no statistically significant difference observed in the lower-upper hemisphere thickness values for Z1, Z2, Z3 macular thickness (MT). No statistically significant difference was detected when comparing the differences between S-MT and I-MT, as well as S-GCC and I-GCC thickness, in patients with suspected glaucoma and individuals with normal eye findings. Conclusion: As far as we know, our study is the first to investigate the structural retinal thickness changes in the posterior pole using the PPAA method in patients with suspected glaucoma and thin corneas. In our study, it was determined that macular thickness values were higher in patients with thin corneas compared to normal individuals, and as a result, it was concluded that ppRSLT is more valuable in the early diagnosis of glaucoma in patients with thin corneas. Keywords: Suspected glaucoma, posterior pole asymmetry analysis, thin cornea, glaucoma, retina.