Geometric ray tracing analysis of visual acuity after laser in situ keratomileusis

PURPOSE: Using a geometric ray tracing model, we explain the increase in visual acuity observed in myopic patients after laser in situ keratomileusis (LASIK). METHODS: This study included 37 eyes of 23 patients who underwent LASIK. All patients had myopia and a spectacle-corrected visual acuity of 0.95 or worse. Clinical tests included biometry, corneal topography, pachymetry, and refraction (with and without cycloplegia). Calculations were made by tracing rays through all the refractive surfaces of the eye based on a Le Grand-type theoretical model of the whole eye. RESULTS: Comparison of spectacle-corrected visual acuity of the eye before surgery, the size of the blur circle calculated by ray tracing, and the magnification for the ocular system facilitated a numerical criterion to assess visual acuity by geometric calculation. This criterion was applied to myopic eyes that underwent LASIK, and the maximum increase in spectacle-corrected visual acuity was predicted. An actual increase in visual acuity of approximately 40% of the predicted maximum was observed in patients. CONCLUSIONS: With geometric ray tracing, it was possible not only to obtain an estimate of the visual acuity before LASIK but also to assess the value of the maximum and probable increases in visual acuity after LASIK.     

Longitudinal changes in visual acuity in keratoconus

PURPOSE: The present investigation aimed to identify factors that predict reduced visual acuity in keratoconus from a prospective, longitudinal study. METHODS: This report from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study used 7 years of follow-up data from 953 CLEK subjects who did not have penetrating keratoplasty in either eye at baseline and who provided enough data to compute the slope of the change over time in high- or low-contrast best-corrected visual acuity (BCVA). Outcome measures included these slopes and whether the number of letters correctly read decreased by 10 letters or more in at least one eye in 7 years. RESULTS: Mean age of the subjects at the first follow-up visit was 40.2 +/- 11.0 years (mean +/- SD). Overall, 44.4% were female, and 71.9% were white. The slope of the change in high- and low-contrast BCVA (-0.29 +/- 1.5 and -0.58 +/- 1.7 letters correct/year, respectively) translated into expected 7-year decreases of 2.03 high- and 4.06 low-contrast letters correct. High- and low-contrast visual acuity decreases of 10 or more letters correct occurred in 19.0% and 30.8% of subjects, respectively. Independent predictors of reduced high- and low-contrast BCVA included better baseline acuity, steeper first definite apical clearance lens (FDACL), and fundus abnormalities. Each diopter of steeper baseline FDACL predicted an increased deterioration of 0.49 high- and 0.63 low-contrast letters correct. CONCLUSIONS: CLEK Study subjects with keratoconus exhibited a slow but clear decrease in BCVA during follow-up, with low-contrast acuity deteriorating more rapidly than high-contrast. Better baseline BCVA, steeper FDACL, and fundus abnormalities were predictive of greater acuity loss with time.     

Improvement of visual acuity and corneal physiology in keratoconus by fitting aspherical,high oxygen-permeable contact lenses

Keratoconus is a condition in which the cornea assumes a complex irregular curvature caused by central corneal thinning. The abnormal topography of the cornea in combination with central corneal scarring results in an impaired visual acuity. Even in mild cases spectacles do not correct vision adequately. The use of hard contact lenses with a spherical geometry in the past has already given a marked therapeutic improvement. The use of these lenses however, is complicated by hypoxia and mechanical trauma of the cornea. These complications could theoretically be avoided by fitting elliptical lenses with a high oxygen transmission. To investigate this hypothesis we compared low oxygen-permeable spherical lenses with high oxygen-permeable elliptical contact lenses in a group of twenty patients with mild keratoconus. Our results showed both a marked subjective and objective visual improvement after fitting elliptical lenses with a low incidence of complications.     

Contact lens fitting relation and visual acuity in keratoconus

The presence of visual decrement in keratoconus is widely recognized, but little is known about the effect of different contact lens fitting philosophies on visual acuity. We studied 10 eyes with keratoconus, each of which was tested with an automated visual acuity device while wearing rigid contact lenses of varying base curves and diameters. Lenses used were large, flat lenses and small, steep lenses, fitted from 0.4 mm flatter to 0.4 mm steeper than the average keratometric measurement in 0.1-mm increments, including parallel to the average corneal curvature. A small improvement in visual acuity was noted with the flat lenses, equalling approximately one-half line of visual acuity, which, although statistically significant, was not deemed to be clinically significant. In addition, a decrement in visual acuity of keratoconus patients with centrally placed cones was noted and is discussed.     

Calculating the localization and dimension of the real pupil in keratoconus with ray tracing of corneal topography data]

BACKGROUND: It is crucial to center surgical procedures for optical indications on the pupil or the optical axis of the eye. In keratoconus the pupil appears to be dislocated due to optical aberrations of corneal topography. The purpose of this study was to evaluate the real pupil structure from the virtual image using exact raytracing techniques. PATIENTS AND METHODS: Eighty-eight patients with keratoconus (46 with mild and 42 with severe clinical signs) and a control group of 40 normal subjects were included in this study. Topographic height data were calculated from refraction data of a commercially available topographer (TMS-1) using a local approximation algorithm and a convex surface was modelled using a subdivision scheme. For the posterior corneal surface we postulated an aspherical surface with a central radius of curvature of 6.5 mm using Navarro's model eye. At the virtual pupil outline a bundle of parallel rays were intersected with the anterior and posterior corneal surface and refracted into the anterior chamber. The intersections of these rays with the pupil plane was defined as the real pupil outline. We assessed the amount and direction of pupil dislocation, the ratio between the virtual and real pupil size for each group and correlated these parameters with the central corneal power. RESULTS: The size of the virtual pupil exceeded the reference value of the real pupil in the normal group by 11%, in the group with mild keratoconus by 19% and in the group with severe keratoconus by 35%. The center of the virtual pupil was decentered 0.06 mm in the normal group, 0.49 in the group with mild keratoconus and 1.24 mm in the group with severe keratoconus. Whereas the direction of decentration was randomly in the normal group, we measured a preferred decentration to the inferior quadrants in mild keratoconus and a systematic decentration to the temporal inferior quadrant in severe keratoconus. Correlation of the optical dislocation did not correlate with central corneal power in any group. CONCLUSIONS: In keratoconic eyes the pupil outline is distorted and dislocated due to optical aberrations of the cornea. Exact raytracing technique allows the calculation of the real pupil outline from the virtual image and the topographic height of both corneal surfaces. Knowledge about the real pupil position may have an impact on adequate centration of keratorefractive surgery and penetrating keratoplasty.     

IOL Master引导下白内障术后散光和视力影响

Objective To study and analyze the impact on corneal astigmatism and visual acuity during individualized phaeoemulsification with intraocular lens implantation guided by IOL Master. Methods Thirty-eight patients (50 eyes) were treated with phacoemulsification through 3.0mm clear corneal incision with the steepest meridian according to IOL Master (group A) or auto-refractor/A-scan (group B). All patients were divided randomly and equally. The corneal astigmatism and visual acuity between group A and B were compared after the operation. Results The astigmatism of group A, B were (1.01± 0.10)D and (0.95 ± 0.13)D before operation, (1.33± 0.13)D and (1. 25 ± 0.15)D 1 day after operation, (1.15± 0.14)D and (1.07 ± 0.13) D after 1 week, (0.90 ± 0.13)D and (0.87 ± 0.12)D after 1 month, (0.89± 0.12)D and (0.80± 0.11)D after 3 months, respectively. There was no significant difference in the estimated values of astigmatism before and after operation (P ＞0.05), so between the methods of IOL Master and auto- refractor (P ＞0.05). In group A the naked visual acuity above 0.8 were 8 eyes 1 month, and 10 eyes 3 months after operation, better than group B with 2 eyes and 3 eyes. There was significant difference in the estimated values of visual acuity between the methods of IOL Master and A-scan (P ＜0.05). Conclusions Individualized phaeoemulsification with intraocular lens implantation guided by IOL Master can reduce corneal astigmatism and improve the visual impact.     

ZyQv视力表在角膜屈光手术中的应用分析

目的 介绍一种新型的不同对比度视力表(ZyQv视力表,美国Baush&Lomb公司)及其临床应用.方法 以准分子激光原位角膜磨镶术治疗近视为例,使用ZyQv视力表测量54例近视患者手术前后不同对比度和不同照明环境下的视力,并与标准对数视力表的测量结果进行对比.结果 标准对数视力表测得的术前BCVA和术后不同时间点UCVA之间均无显著差异;使用ZyQv视力表发现:术后1周时,三组患者明暗环境中90%对比度视力均有显著下降,术后1月时均恢复至术前水平;三组患者明暗环境中10%对比度视力均于术后1周显著下降,其中低度近视组于术后1月恢复至术前水平,而中度和高度近视组在术后3月时仍低于术前水平(明环境中10%对比度视力除外).结论 ZyQv视力表操作便捷,在角膜屈光手术中可用于完善术前检查,综合评价术后视觉质量,动态观察术后视力恢复,协助临床实验和相关研究.     

IOL Master引导下白内障术后散光和视力影响

目的 观察IOL Master引导下个性化角膜切口白内障超声乳化联合人工晶状体植入术术后散光和视力的变化.方法 选择行3.0mm透明角膜最大屈光轴切口方向白内障超声乳化术术共50只眼,随机分成IOL Master引导下(A组)和自动验光仪及A超引导下(B组)两组,每组25只眼.检测术前、术后1d、1周、1月、3月角膜散光和视力变化.结果 A组术前、术后1d、1周、1月、3月角膜散光度分别为(1.01±0.10)D、(1.33±0.13)D、(1.15±0.14)D、(0.90±0.13)D及(0.89±0.12)D;B组为(0.95±0.13)D、(1.25±0.15)D、(1.07±0.13)D、(0.87±0.12)D及(0.82±0.11)D.术后1d及3月与术前比散光度差异均无统计学意义(P＞0.05).A、B两组术后1d及3月散光度变化量相比差异无统计学意义(P＞0.05).A、B两组术后1d、1周、1月及3月顺规性散光百分率较术前均有所增多,但比较差异均无统计学意义(P＞0.05).A、B两组术后1月和3月裸眼视力＞0.8者分别为8只眼和10只眼、2只眼和3只眼,两者相比差异有统计学意义(P＜0.05).结论 IOL Master引导下角膜个性化切口白内障超声乳化联合人工晶状体植入术有助于患者减少术后角膜散光及视力恢复,且方便可行性高.     

不同位置角膜切口对白内障术后角膜散光和视力的影响

目的：探讨不同位置角膜切口对白内障术后角膜散光和视力的影响,提高白内障患者术后视觉质量。方法：选取行超声乳化白内障吸除联合人工晶状体植入术的患者96例96眼,随机分为两组,各48例48眼,使其有可比性。 A组患者角膜切口选择右眼在颞上、左眼在鼻上。 B组角膜切口根据曲率检查选择在角膜最大子午线轴向。结果：两组患者术后1wk,1、3mo视力比较差异无统计学意义(P>0.05)。术后1wk,1mo时A组患者角膜散光度分别为4.03±0.41、3.72±0.35D,B组分别为2.96±0.38、1.29±0.15D,均较治疗前明显增高,且A组明显高于B组,差异有统计学意义(P<0.05)。术后3mo时两组患者散光度均与术前差别不大,且两组比较无统计学意义(P>0.05)。术前两组患者散光轴向均以逆规最为常见,术后1 wk顺规性散光增多,明显多于术前,术前术后比较差异有统计学意义(P<0.05),至术后1、3mo顺规性散光逐渐减少,逆规性散光逐渐增加,接近术前。两组患者不同时间角膜散光轴向比较无统计学意义(P>0.05)。结论：白内障超声乳化术透明角膜切口选择在角膜最大散光子午线方向可以有效减少术后早期散光度数,但是对于远期角膜散光影响不大,而且对患者视力影响不大。在条件允许的情况下,可以通过该方式制作角膜切口,但是考虑到手术复杂性提高,意义不大。     

New perspectives on keratoconus as revealed by corneal confocal microscopy

Confocal microscopy (CM) of keratoconus is reviewed. In the Manchester Keratoconus Study (MKS), slit scanning CM was used to evaluate 29 keratoconic patients and light microscopy (LM) was performed on two of the keratoconic corneas post‐keratoplasty. The findings of the MKS are compared with other CM studies. Consideration of the differences between studies of cell counts is confounded by the use of different experimental controls. A consensus exists among studies with respect to qualitative observations. The epithelium appears more abnormal with increasing severity of keratoconus. In severe disease, the superficial epithelial cells are elongated and spindle shaped, epithelial wing cell nuclei are larger and more irregularly spaced and basal epithelial cells are flattened. Bowman’s layer is disrupted and split in the region of the cone and intermixed with epithelial cells and stromal keratocytes. Stromal haze and hyper‐reflectivity observed with CM correspond with apical scarring seen with the slitlamp biomicroscope (SLB). Hyper‐reflective keratocyte nuclei are thought to indicate the presence of fibroblastic cells. Increased haze detected with CM is found with LM to be due to fibroblastic accumulation and irregular collagen fibres. Dark stromal bands observed with CM correlate with the appearance of Vogt’s striae with SLB. Desçemet’s membrane appears normal with both CM and LM. Some evidence of endothelial cell elongation is observed with CM. The application of CM to ophthalmic practice has facilitated a greater understanding of medical and surgical approaches that are used to treat keratoconus. This review offers new perspectives on keratoconus and provides a framework, against which tissue changes in this visually debilitating condition can be studied in a clinical context in vivo using CM.     

PRK后角膜地形图类型与视力关系的临床观察

目的:分析PRK术后角膜地形图的类型及其动态变化,探讨地形图类型与视力的关系。方法:利用EYESYS角膜地形图仪分别于1、3、6个月对246眼PRK术后的地形图进行定性分类观察,并结合临床资料分析与视力的关系。结果:PRK术后的地形图可分为二类八型,即规则类(均匀型、顺轴圆枕型、逆轴圆枕型、局灶性地形图改变)和不规则类(中央岛型、半环型、钥匙孔型、不规则型),各型随时间而改变,且趋向于规则。不规则地形图(尤其是中央岛型)对术后最佳矫正视力的影响较大。结论:角膜地形图标准分类是一种理想的分类法,不规则地形图是导致术后最佳矫正视力下降的主要原因之一。     

硬性透气性角膜接触镜矫正圆锥角膜不规则散光后对比度视力的观察

目的 观察硬性透气性角膜接触镜（RGPCL）矫正圆锥角膜完成期患者不规则散光的临床效果.设计病例系列研究. 研究对象圆锥角膜完成期患者20例（22眼）. 方法采用多功能视力测量仪MFVA100分别测试20例（22眼）圆锥角膜患者裸眼（UCVA）、最佳框架矫正（BSCVA）和RGPCL矫正状态下100%、25%、10%和5%的4种对比度视力.角膜荧光染色法和相干光断层成像术（OCT）观察镜片下泪液的分布情况,判断RGPCL的静态配适状态.用单因素方差分析、LSD法两两比较相同对比度下3组视力之间的差别.主要指标对比度视力.结果 22眼在100%、25%、10%和5%对比度下平均UCVA分别为0.195、0.098、0.053、0.042（P均〈0.05）,平均BSCVA分别为0.421、0.205、0.116、0.064（P〈0.05）,平均RGPCL矫正视力分别为0.880、0.520、0.308、0.169（P均〈0.05）.在100%、25%、10%和5%四种对比度下RGPCL的矫正视力均优于UCVA和BSCVA（P均=0.000）,而在5%对比度下UCVA与BSCVA无显著差异（P=0.142）.22眼RGPCL矫正后荧光素染色裂隙灯钴蓝灯光下见呈稍平坦配适、大曲率子午线上泪液填充较多、边缘部泪液宽度适中.戴镜后用前节OCT扫描中央光学区角膜显示出大、小曲率子午线上镜片下泪液分布厚度不同.结论 RGPCL可矫正圆锥角膜完成期患者的不规则散光,明显提高圆锥角膜患者不同对比度下的矫正视力.     

应用角膜地形图对屈光不正儿童进行圆锥角膜的筛选

目的：探讨应用计算机辅助角膜地形图检查系统对屈光不正儿童进行圆锥角膜筛选的可行性。方法：应用计算机辅助角膜地形图检查系统对1822例3～14岁屈光不正儿童进行圆锥角膜的筛选。结果：共筛选出亚临床圆锥角膜2例，圆锥角膜2例。结论：应用角膜地形图对屈光不正儿童常规进行圆锥角膜的筛选，可以对圆锥角膜早期发现，早期治疗，避免误诊和漏诊。     

Contrast sensitivity and visual acuity of the pigmented rat determined electrophysiologically

The contrast sensitivity function of the rat was assessed by investigating the relationship between the amplitude of visually evoked cortical potentials (VECP) and the spatial frequency and contrast of grating stimuli. Pattern reversal VECPs were recorded in Area 17 in the region of representation of the central binocular visual field. Transient responses were obtained with a 1 Hz contrast reversal. The mean contralateral monocular CSF peaked around 0.1 c/deg, with a threshold sensitivity of 20–25, low frequency attenuation and a high frequency cut-off of 1.18 c/deg. The mean binocular CSF showed a cut-off of 1.20 c/deg, which matches several behavioural measurements of visual acuity. The greater binocular sensitivity in the low frequency range (0.04 c/deg) could be tentatively attributed to the greater influence of the population of large ganglion cells that reaches its maximal density in the ipsilateral projection