This cross-sectional study was conducted at Al-Zahra Eye Hospital, Zahedan, eastern Iran. All participants in this study were residents of Zahedan with the same ethnicity.
The study was approved by the Institutional Review Board/Ethics Committee of Zahedan University of Medical Sciences (Code ID: IR.ZAUMS.REC.1398.431). The procedures performed in this study were based on the principles of the Declaration of Helsinki. The participants received information about the study and written informed consent forms were signed by them.
Initially, Meth and Crystal Meth addicts were identified from the camp protected by Zahedan Health Center. The participants received a comprehensive ophthalmic examination, including full patient history, slit-lamp biomicroscopy, ophthalmoscopy, noncontact tonometry (Topcon CT-1/CT-1P, Tokyo, Japan), autorefraction (Topcon KR-1, Tokyo, Japan), and best-corrected distance visual acuity. A refractive error between − 3.00 to + 3.00 diopters with a best-corrected distance visual acuity of 0.2 Log MAR was added to the inclusion criteria to avoid the manifestations of high refractive errors on the retina [18]. The exclusion criteria for both study groups involved any systemic or ocular disease, history of ocular surgery, and consumption of a specific drug. Also, women who were currently pregnant were excluded.
Considering the mentioned criteria, 41 Meth and Crystal Meth abusers with at least 5 years of Meth and Crystal Meth abuse were enrolled in this study. Notably, the route of Meth and Crystal Meth usage was smoking with a mean daily dose of 0.0074 ± 0.0034 gr. Forty-two healthy individuals of matching ethnicity to the addicted group and without a history of smoking were included in the study as the control group.
RNFL, ONH, and macula images of the participants were obtained using a Fourier-domain OCT (Optovue, RTVue™, Inc., Fremont, CA), which is the first United States Food and Drug Administration-approved FD-OCT system. Optovue Fourier-domain OCT system can take 26,000 A-scans per second with an axial resolution of 5 microns using a high wavelength light source. This technique utilizes a superluminescent diode that is compact, reliable, and more economical [13]. Repeatability of RNFL and macular thickness measurements have been studied in previous studies using Optovue Fourier-domain OCT [19, 20]. This device allows for automatic and manual imaging by the clinician. In this study, the manual imaging technique was used to examine RNFL, ONH, and macular thickness. The same examiner (M.M) obtained all OCT measurements based on the manufacturers’ user guide and previous studies [21]. Measurements based on the quality state provided by the instrument were accepted and erroneous captures were repeated after five minutes. It should be mentioned that the mechanism of the RTVue OCT has been described in previous studies [21, 22]. The manufacturer’s representative checked the calibration of OCT before the study. Lastly, only the right eye per subject was selected for analysis.
RNFL Measurements
The ONH protocol was used to obtain RNFL measurements. This protocol generates an RNFL thickness map measured along a circle of 3.45 mm in diameter centered at the ONH. The overall average, superior hemisphere, inferior hemisphere, temporal quadrant, superior quadrant, nasal quadrant, and inferior quadrant are provided [21, 22].
ONH Measurements
The ONH protocol was used to obtain ONH measurements. It consists of 12 radial scans of 3.4 mm in length (455 A-scans each) and 13 concentric circular scans ranging from 1.3 to 4.9 mm in diameter (425–965 A-scans each) centered at the ONH. The ONH parameters measured by the software involved the rim area, vertical cup-to-disc ratio, horizontal cup-to-disc ratio, cup area, cup-to-disc area ratio, and disc area [22].
Macular Measurements: The ganglion cell complex (GCC) protocol was used to obtain macular measurements. This protocol consists of one horizontal line scan of 7 mm in length (467 A-scans) followed by 15 vertical line scans of 7 mm in length (400 A-scans each) at 0.55-mm intervals. This protocol provides 14,810 A-scans in 0.58 seconds of a rectangular area. The GCC protocol provides a segmentation of macular B-scans in two layers: GCC layer and outer retinal layer. The GCC layer is composed of the ganglion cell layer, the nerve fiber layer, and the inner plexiform layer [22].
Statistical analysis was performed using the SPSS program (SPSS version 19 for Windows; SPSS Inc., Chicago, IL, USA). The normality of data distribution was assessed using the Kolmogorov-Smirnov test. The independent T-test was utilized to compare the meant of the studied variables in addict and healthy groups. The descriptive results of comparisons are presented using the mean and 95% confidence interval of differences. In this study, P-values smaller than 0.05 were considered significant.