Thirty-six healthy patients undergoing uncomplicated Phacoemulsification surgery were recruited for this self-controlled case series study. All the patients were recruited consecutively (from October 2016 to December 2016) from the cataract department of Guangzhou Aier Eye Hospital and signed the consent form after a fullest explanation of the purpose and procedures of the study. The study was adhered to the provisions of the Declaration of Helsinki for research involving human subjects and was approved by the Ethical Review Committee of Guangzhou Aier Eye Hospital.
All the study participants were healthy individuals with no history of ocular disease or visual symptoms; aged at least 40 years; intraocular pressure (IOP) < 21 mmHg; normal appearance of optic nerve head; normal anterior chamber angles; Patients were excluded if they had glaucoma, high myopia or hyperopia (magnitude exceeding ±6 diopters (D) of spherical equivalent refraction), AMD, or other retinal diseases that could interfere the choroidal thickness. The diagnosis of glaucoma was based on the findings from gonioscopy, optic disc characteristics, and visual fields results. Patients with severe systemic diseases, such as diabetes mellitus, rheumatism, or malignant tumors, serious opacity of refractive media or unstable fixation that could prevent EDI-OCT measurement were also excluded.
All the patients underwent a comprehensive ophthalmologic examination, including IOP measurement using Goldmann applanation tonometry, autorefraction examination, measurement of visual acuity and a best-corrected visual acuity (BCVA), axial length using ocular biometry (IOL Master, Zeiss, Germany), fundus examination and EDI-OCT measurement (Spectralis, Heidelberg Engineering, Heidelberg, Germany) before surgery and postoperatively at 7 days (D7), month 1 (M1) and months 3 (M3).
All patients received standard phacoemulsification surgery through clear corneal incisions under superficial anesthesia (0.5% Proparacaine hydrochloride Eye Drops, Alcon, Fort Worth, TX). All phacoemulsification surgeries was performed by the same experienced surgeon (HSC) using the Infiniti system® (Alcon Labs Inc). In all cases, after removal of the lens cortex, a foldable intraocular lens was implanted uneventfully in the capsular bag. Within 1 month after surgery, Tobradex (0.3% tobramycin and 1% dexamethasone, Alcon, Fort Worth, TX) eye drops were applied four times a day, a non-steroidal anti-inflammatory eye drops (Pranoprofen Eye Drops, Senju Pharmaceutical Co.,Ltd. Osaka, Japan) were applied four times a day, and TobraDex eye ointment (Alcon, Fort Worth, TX) was applied once every evening before bed.
Image acquisition
EDI-OCT scans of the macular were performed for the operated eye using the EDI mode of SD-OCT (Spectralis, Heidelberg Engineering, Heidelberg, Germany). Horizontal 6-mm line scans centred on the fovea were acquired. Due to the diurnal variation of choroidal thickness, all the measurements were performed at the same time of the day (08:00 AM~ 12:00 AM) and accomplished in triplicate by two independent examiners. The sections going directly through the center of the fovea were selected for further analysis. The subfoveal choroidal thickness (SFCT) was measured using the in-built calipers tool. SFCT was defined as the vertical distance between the outer surface of the retinal pigment epithelium and the choroidal–sclera interface [6].
Procedures of image binarization
Image binarization of the subfoveal choroidal area was performed by one public domain software, Image J (version 1.47, provided in the public domain by the National Institutes of Health, Bethesda, MD, USA; http://imagej.nih.gov/ij/) [7, 8]. In brief, the images with one central scan passing through the fovea were chosen. The region of interest was manually selected using the polygon tool and added to ROI manager. After measuring the brightness of the selected luminal areas of the original OCT, the average brightness was set at the minimum value to minimize the noise in the OCT image. Then the original images were converted to 8 bits and adjusted by the Niblack Auto Local Threshold. The binarized image was converted to RGB (red, green, blue) image again, and the luminal area was determined using the Threshold Tool. After the image binarization, the total circumscribed area (TCA) and area of dark pixels were calculated. The dark pixels represent the luminal or vascular area (LA) and stromal or interstitial area (SA) was defined as the area of light pixels (Fig. 1). CVI was defined as the proportion of LA to TCA.
Inter-rater and intra-rater agreement
All the preoperative images were initially segmented by two graders to evaluate the inter-rater agreement (HSC and JWW). The same amount of images was segmented by one grader (JWW) after an interval of one week to determine intra-rater reliability. Absolute agreement model of the intra-class correlation coefficient (ICC) was used for the intra- and inter-rater reliability for the image binarization. ICC value of 0.81–1.00 indicates good agreement. The mean difference between the measurements was calculated by Bland-Altman plot analysis, which was constructed using MedCalc version 17.5.3 (Medcalc Statistical Software, Ostend, Belgium) software. After obtaining good inter-rater and intra-rater agreement, all the image binarization was performed by single author (JWW).
Statistical analysis
All statistical analyses were performed using SPSS software version 20.0 (IBM-SPSS, Chicago, Illinois, USA). Normally distributed data were expressed as mean ± standard deviation (SD). Each postoperative value was compared with baseline by paired t-test. Univariate and multiple linear regression analyses were performed to determine the associations between CVI and other factors. Values of p < 0.05 were considered to be statistically significant.