Corneal optical density: Structural basis, measurements, influencing factors, and roles in refractive surgery.

The cornea is the main refractive medium of the human eye, and its clarity is critical to visual acuity. Corneal optical density (COD) is an important index to describe corneal transparency. Intact corneal epithelial and endothelial cells, regular arrangement of collagen fibers in the stroma, and normal substance metabolism are all integral for the cornea to maintain its transparency. In the last two decades, the Pentacam Scheimpflug imaging system has emerged as a breakthrough for the measurement of COD (also called corneal densitometry). It has been found that a wide variety of factors such as age, refractive status, and corneal diseases can affect COD. Different corneal refractive surgery methods also change COD in different corneal regions and layers and affect visual acuity following the surgery. Thus, COD has gradually become a significant indicator to evaluate corneal health, one on which the attention of clinicians has been increasingly focused.

Posterior staphyloma is associated with the microvasculature and microstructure of myopic eyes.

OBJECTIVE: To investigate the microvasculature and structural characteristics of the eyes of myopic patients and their association with posterior staphyloma (PS). METHODS: This was a retrospective, case-control study comprising of 106 eyes from 72 individuals. Using 1:1 matching of axial length (AL) of their eyes, patients were allocated into a PS group or no posterior staphyloma (NPS) group. All patients were examined using ultra-widefield fundus imaging, optical coherence tomography angiography, and ocular biometry to acquire microvasculature and microstructure parameters. RESULTS: The anterior chamber depth (ACD) of the PS group was significantly different from that of the NPS group (3.56 mm vs 3.76 mm, P < 0.001), as was 1ens thickness (3.72 mm vs 3.57 mm, P = 0.005) and spherical equivalent (SE)(-10.11D vs -8.80D, P = 0.014). The PS group had reduced choriocapillaris flow, subfoveal choroidal thickness (SFCT), and a thinner retinal layer compared with the NPS group. No difference in retinal blood flow between the two groups was observed. The PS group exhibited a smaller disc area (15082.89 vs 17,043.32, P = 0.003) and angle α between temporal retinal arterial vascular arcades (113.29°vs 128.39°, P = 0.003), a larger disc tilt ratio (1.41 vs 1.24, P < 0.001) and parapapillary atrophy (PPA) area (13840.98 vs 8753.86, P = 0.020), compared with the NPS group. Multivariate regression analysis indicated that disc tilt ratio (P = 0.031) and SFCT (P = 0.015) were significant predictors of PS. In addition, PS (P = 0.049), AL (P = 0.003), corneal refractive power (P < 0.001), ACD (P = 0.022), relative lens position (P = 0.045), and disc area (P = 0.011) were significant predictors of SE. CONCLUSIONS: PS was found to be closely linked to a reduction in choriocapillaris perfusion and anatomical abnormalities including posterior and anterior segments. Furthermore, PS exacerbated the progression of myopia.

A Multicenter Study of the Prevalence of Dry Eye Disease in Chinese Refractive Surgery Candidates.

OBJECTIVE: The aim of this study was to determine the prevalence of preoperative dry eye disease and evaluate tear film function in refractive surgery candidates in China. METHODS: In this prospective noninterventional cross-sectional study, refractive surgical candidates from 13 preselected eye hospitals in China were recruited from July 2015 to February 2016. Patient histories, subjective symptoms, tear film breakup time (TBUT), ocular surface fluorescein staining, and Schirmer I tests (SIT), were assessed to conduct subgroup analysis. RESULTS: A total of 1,849 patients were recruited, 41.4% were diagnosed with dry eye disease (766/1,849) and 44.9% (830/1,849) of subjects had a positive history of contact lens (CL) wear. The overall mean TBUT and SIT values were 7.3 ± 3.7 s and 15.2 ± 8.8 mm, respectively. The total prevalence of ocular surface fluorescein staining was 23.46% (422/1,849); 44.62% of patients had TBUT <5 s and 23.20% of patients had SIT <5 mm. CL wearers were observed to have a higher prevalence of dry eye than non-CL wearers (54.1 vs. 35.2%, OR = 2.17, 95% CI: 1.77-2.65). CONCLUSIONS: In this study, the most common abnormal finding in dry eye disease was tear film instability. A high proportion of refractive surgery candidates have preexisting dry eye disease and a history of CL wear prior to surgery. Careful attention should be given to the evaluation of preoperative dry eye in refractive surgery candidates.