Comparison of corneal measurements in keratoconic eyes using rotating Scheimpflug camera and scanning-slit topography

AIM: To compare the anterior segment measurements obtained by rotating Scheimpflug camera (Pentacam) and Scanning-slit topography (Orbscan IIz) in keratoconic eyes. METHODS: A total of 121 patients, 71 males (58.7%) and 50 females (41.3%) (214 eyes) with the diagnosis of keratoconus (KC) were enrolled in this study. Following diagnosis of KC by slit-lamp biomicroscopic examination, central corneal thickness (CCT), thinnest corneal thickness (TCT), anterior chamber depth (ACD), and pupil diameter (PD) were measured by a single examiner using successive instrumentation by Pentacam and Orbscan. RESULTS: There was no significant difference between the two instruments for the measurement of CCT and TCT. In contrast, scanning-slit topography measured ACD (3.46±0.40 mm vs. 3.38±0.33 mm, P=0.019) and PD (4.97±1.26 mm vs 4.08±1.19 mm, P<0.001) significantly larger than rotating Scheimpflug camera. The two devices made similar measurements for CCT (95% CI: -2.94 to 5.06, P=0.602). However, the mean difference for TCT was -6.28 (95% CI: -10.51 to -2.06, P=0.004) showing a thinner measurement by Orbscan than by Pentacam. In terms of the ACD, the mean difference was 0.08 mm (95% CI: 0.04 to 0.12, P<0.001) with Orbscan giving a slightly larger value than Pentacam. Similarly, Orbscan measurement for PD was longer than Pentacam (95% CI: 0.68 to 1.08, P<0.001). CONCLUSION: A good agreement was found between Pentacam and Orbscan concerning CCT measurement while comparing scanning-slit topography and rotating Scheimpflug camera there was an underestimation for TCT and overestimation for ACD and PD.     

Eye rubbing-induced changes in intraocular pressure and corneal thickness measured at five locations, in subjects with ocular allergy

AIM: To assess the effects of eye rubbing on corneal thickness (CT) and intraocular pressure (IOP) measurements obtained 0-30min after habitual eye rubbing in symptomatic patients. METHODS: Measurements of IOP and CT were obtained at five locations (central, temporal, superior, nasal and inferior) before, and every 5min for 30min interval after 30s of eye rubbing, for 25 randomly selected eyes of 14 subjects with ocular allergy and 11 age-matched normals. Differences in measurements were calculated in each group [Baseline measurements minus measurements recorded at each time interval after eye rubbing (for IOP), and for each corneal location (for CT)] and comparison were then made between groups (allergic versus control) for differences in any observed effects. RESULTS: Within groups, baseline mean IOPs in the allergic patient-group (14.2±3.0 mm Hg) and in the control group (13.1±1.9 mm Hg) were similar at all times, after eye rubbing (P >0.05, for all). The maximum reduction in IOP was 0.8 mm Hg in the control subjects and the maximum increase was also 0.8 mm Hg in the allergic subjects. Between groups (allergic versus control), the changes in IOP remained under 1 mm Hg at all times (P=0.2) after 30min of eye rubbing. Between 0 and 30min of CT measurements after eye rubbing, the mean central CT (CCT), inferior CT (ICT), superior CT (SCT), temporal CT (TCT) and nasal CT (NCT) did not vary significantly from baseline values in the control and allergic-subject groups (P>0.05, for both). Between both groups, changes in CT were similar at all locations (P>0.05) except for the TC which was minimally thinner by about 4.4 μm (P=0.001) in the allergic subjects than in the control subjects, 30min following 30s of eye rubbing. CONCLUSION: IOP measured in allergic subjects after 30s of habitual eye rubbing was comparable with that obtained in normal subjects at all times between 0 and 30min. Although, CT in the allergic subjects were similar to those of the control subjects at all times, it varied between +10 and -7.5 μm following eye rubbing, with the temporal cornea showing consistent reductions in thickness in the subjects with allergy. However, this reduction was minimal and was considered to not be clinically relevant.     

Repeatability and agreement of CCT measurement in myopia using entacam and ultrasound pachymetry

AIM: To evaluate the repeatability of central corneal thickness (CCT) measurement by entacam, and agreement of CCT measured by Pentacam and ultrasound pachymetry (USP) in Chinese myopia. Thereby investigate the possibility of Pentacam as a substitute for USP in CCT measurement before refractive surgery. The effects of corneal curvature measured by Pentacam on CCT were also evaluated. METHODS: One hundred and forty-eight right eyes of 148 individual with myopia were included in this study. Three successive Pentacam CCT measurements followed by 10 successive ultrasound pachymetry were carried out in the 148 eyes. Mean of CCT taken by each device was calculated for comparison. According to the CCT measured by USP, all the 148 eyes were divided into 3 groups: <520μm, 520-560μm, >560μm. For all eyes and each group the CCT obtained by Pentacam and USP were compared. Anterior corneal curvature of the 148 eyes was also adopted for correlation analysis with CCT obtained by ultrasound pachymetry. In addition, CCT measurement using 60 random selected Scheimpflug images was performed by 3 skilled investigators at different time, and this was repeated for 3 times by a forth investigator to assess repeatability of Pentacam CCT measurement using Scheimpflug images. RESULTS: Intraclass correlation coefficient (ICC) analysis revealed high intraobserver repeatability (ICC=0.994, F=158.60, P<0.001) for CCT measurement by Pentacam. The interobserver (ICC=0.998, F=494.73, P<0.001) and intraobserver (ICC=0.997, F=383.98, P<0.001) repeatability for Pentacam CCT measurements using Scheimpflug images were also excellent. There was high positive correlation between the CCT values measured by Pentacam and ultrasound pachymetry (r=0.963, P<0.001). Bland-altman plots showed that the Pentacam underestimate the CCT by 8.02μm compared with ultrasouond pachymetry. The differences between Pentacam and USP increased as the CCT readings by USP increased (Pentacam vs USP: slope=-0.04, P<0.05). The 95% upper and lower limits of agreement between CCT values obtained from the two devices were +9.33μm and -25.37μm. No significant association could be found between CCT and anterior corneal curvature. CONCLUSION: Inter- and intraobserver variability for CCT measurements by Pentacam was considerably below clinically significant levels. CCT of myopia obtained by Scheimpflug camera, Pentacam, were highly correlated to that by ultrasound pachymetry. However, the values obtained are not directly interchangeable between Pentacam and ultrasound pachymetry as the 95% limits of agreement are relatively wide. Pentacam can be a useful instrument for measuring CCT in candidates to refractive surgery in clinic.     

Corneal elevation indices and pachymetry values of Saudi myopes using scheimpflug imaging

Objectives:To report the corneal elevation and thickness values for Saudi myopes and to evaluate the differences between these parameters in subgroups of this target population.Methods:Pentacam corneal topographic maps of the right eyes of patients visiting Al-Hokama Eye Clinic, Riyadh, Saudi Arabia, a tertiary eye center between January 2009 and December 2015 were retrospectively analyzed in this cross-sectional study. The patients were grouped into 3 categories based on their spherical readings: mild (-0.25 to -2.75D), moderate (-3.00 to -5.75D), and severe (≥-6.00D). Furthermore, patients with cylindrical readings of ≥-1.00 diopter were categorized as having myopic astigmatism, whereas those with less than -1.00 cylindrical diopter were categorized as having simple myopia.Results:Our sample was comprised of 1,276 patients; 838 (65.7%) had simple myopia and 438 (34.3%) had myopic astigmatism. The values for the whole myopic group were as follows: anterior corneal elevation (AE) at the apex= 2.60±1.48 (standard deviation), thinnest AE= 2.56±1.68, posterior elevation (PE) at the apex= 3.67±3.58, thinnest PE= 4.92±3.81, central pachymetry= 550.09±34.29, apical pachymetry=550.73±34.64, and thinnest pachymetry= 546.30±34.61. All of the measurements, except the apical PE and thinnest PE, were statistically significant across the simple and myopic astigmatism groups (p<0.05). Comparing the mild to moderate myopia groups revealed a significant difference in the apical AE (p=0.037). Moreover, the comparison between the mild and severe myopia groups revealed that the apical PE and the thinnest PE, as well as the central, apical, and thinnest pachymetry values were statistically significantly different (p<0.05).Conclusion:The corneal elevation indices and thicknesses specific to the Saudi myopes were found to be comparable to the international databases in terms of the elevation and thickness in some of the parameters.     

Sirius眼前节分析系统与A型超声测厚仪对中央角膜厚度测量结果的对照分析

目的对照分析Sirius眼前节分析系统和A型超声测厚仪对中央角膜厚度(CCT)测量结果的差异,研究Sirius系统的准确性和可靠性。方法随机抽取屈光手术前175例(175只眼),分别应用Sirius(意大利科以康)和A超(DGH500)进行CCT测量,用SPSS13.0进行配对t检验。结果 Sirius系统测量的CCT平均值为(534.75±33.757)μm,A超所测CCT平均值为(550.70±35.037)μm。A超比Sirius测量的CCT厚15.95μm,有统计学差异(P<0.01),相关分析显示两种方法呈正相关(r=0.954,P=0.00)。因Sirius测量不受操作者影响且样本量较大,便以Sirius所测CCT分3组(1组CCT<500μm,2组CCT500～550μm,3组CCT>550μm),3组结果均为A超比Sirius测量的CCT厚,但都有很好的相关性。1组Sirius系统和A超的CCT分别为(478.96±13.408)μm和(496.70±20.003)μm,r 1=0.823,P=0.000;2组分别为(524.49±14.031)μm和(540.08±16.981)μm,r 2=0.794,P=0.000;3组分别为(571.08±15.769)μm和(586.90±18.952)μm,r 3=0.839,P=0.000。结论用Sirius三维地形图、眼前节分析系统进行屈光手术术前CCT测量,其结果可靠性好、影响因素少且操作简单、无创,具有很好的临床应用价值。     

角膜厚度对三种不同眼压计测量的影响

目的:比较采用Goldmann压平眼压计(Goldmann applanation tonometer,GAT)、非接触眼压计(non-contact tonometer,NCT)和Schiotz眼压计(Schiotz tonometer,ST)的眼压(intraoeular pressure,IOP)测量,评估角膜中央厚度(central corneal thickness,CCT)对读数的影响。方法:使用GAT、NCT和ST对所有患者的右眼进行眼压测量。超声角膜厚度测量法测定CCT。所有IOP及CCT测量由同一检查者进行。计算CCT25%(Q1)百分位数和75%(Q3)百分位数值,并通过这种方法将该组分为薄、中、厚角膜亚组。使用Statplus软件进行统计分析。结果:全系列144眼,GAT测量平均IOP为17.4±4.9mmHg,NCT为16.0±5.8mmHg,ST为14.0±4.0mmHg(Friedman方差分析P<0.01)。IOP水平和CCT之间的相关系数NCT为0.787(P<0.01),GAT为0.630(P<0.01),ST为0.565(P<0.01)。ST测量中,纠正的IOP误差和CCT之间的相关性在厚角膜明显弱(r=0.381,P=0.022)。结论:NCT是最易受不同CCT影响的设备。ST读数似乎比GAT和NCT读数受CCT的影响小。特别是在厚角膜,与NCT和GAT相比,ST可以被认为是一个更可靠的仪器。     

A new, pachymetry-based approach for diagnostic cutoffs for normal, suspect and keratoconic cornea

Purpose

To analyze whether an association exists between keratometric and pachymetric changes in the cornea, and whether it can be used to create pachymetric cutoff criteria secondary to keratometric criteria.

Methods

In this cross-sectional study, 1000 candidates presenting to the refractive surgery services of a tertiary care hospital underwent bilateral Orbscan IIz (Bausch and Lomb) assessment along with other ophthalmic evaluation.

Results

Stepwise regression analysis-based models showed that simulated keratometry (simK) astigmatism was significantly predicted by the minimum corneal thickness (MCT) and difference between central and MCT (δCT), mean SimK by the MCT and δCT, and maximum keratometry in the central 10-mm zone by the MCT and δCT (P<0.001). The mean MCT values were 542.5±39.6, 539.9±39.2, 524.2±49.5, and 449.3±73.7 μm for flatter normal (<44 D), steeper normal (≥44 D), keratoconus suspect and keratoconic eyes, respectively (P<0.001). The mean differences between central corneal thickness and MCT (δCT) were 12.2±7.1 μm, 12.4±7.4 μm, 14.4±8.9 μm and 23.2±10.1 μm for the flatter normal, steeper normal, keratoconus suspect, and keratoconic eyes, respectively (P<0.001). Mean and 2SD cutoff were used to suggest that a cornea having MCT<461 μm or δCT>27 μm has only a 2.5% chance of being normal and not a keratoconus suspect or worse.

Conclusion

Pachymetric diagnostic cutoffs can be used as adjuncts to the existing topographic criteria to screen keratoconus suspect and keratoconic eyes.     

急性闭角型青光眼眼压下降后中央角膜厚度与慢性闭角型青光眼的比较

目的采用前瞻性非随机对照研究比较急性闭角型青光眼和慢性闭角型青光眼的中央角膜厚度和眼部其他参数的异同。方法观察75例急性闭角型青光眼和69例慢性闭角型青光眼患者的中央角膜厚度。眼压控制后每例患者接受A超检查,检查项目包括中央角膜厚度、中央前房深度、晶状体厚度和眼轴长度。统计分析采用t检验。结果急性闭角型青光眼患者的平均中央角膜厚度是(576.97±50.93)μm,慢性闭角型青光眼患者为(543.35±35.85)μm,两组之间中央角膜厚度差异有统计学意义(P<0.001)。急性闭角型青光眼对侧眼中央角膜厚度为(544.68±33.53)μm,比急性发作眼薄,对侧眼与慢性闭角型青光眼中央角膜厚度之间差异无统计学意义(P=0.818)。急性闭角型青光眼患者眼轴长度为(21.71±1.14)mm,慢性闭角型青光眼为(22.20±1.21)mm,两组之间的眼轴长度差异有统计学意义(P=0.014)。急性闭角型青光眼前房深度(2.42±0.39)mm,慢性闭角型青光眼为(2.45±0.31)mm,急性闭角型青光眼组晶状体厚度(4.84±0.39)mm,慢性闭角型青光眼组为(4.81±0.43)mm。两组之间的前房深度和晶状体厚度差异均无统计学意义(均为P>0.05)。结论与慢性闭角型青光眼相比,急性闭角型青光眼的中央角膜厚度较厚,可能是急性高眼压后角膜水肿造成,而前房浅、眼轴短可能是其急性发病的原因之一。     

Orbscan眼前节分析系统与超声测厚仪测量角膜厚度比较

目的比较Orbscan眼前节分析系统与超声测量角膜厚度的差异。方法对我院屈光手术中心术前近视患者68例（135只眼）,男46例（91只眼）,女22例（44只眼）,同时用两种方法测量角膜中央厚度,进行临床比较分析。结果 Orbscan眼前节分析系统、超声测量角膜中央平均厚度结果分别为（518.22±31.80）μm和（538.16±29.30）μm。超声测厚比Orbscan测量结果大19.94μm,差异有显著统计学意义（P〈0.01）。根据超声角膜厚度测量结果及等效球镜度分别分组分析,任一组两种方法测量结果差异均有显著统计学意义（P〈0.05）。不同屈光度数组间角膜厚度差异无统计学意义（P〉0.05）。结论采用系统默认校正系数,Orbscan眼前节分析系统与超声对近视患者角膜中央厚度检查结果存在一定差异,在屈光手术术前角膜厚度检查评估时应注意仪器间的差异。近视度数与角膜中央厚度无关。