Relationship between Corneal Thickness and Radius to Body Height.

PURPOSE: To investigate the possible association between body stature (height) and corneal thickness and radius in younger-adult Caucasians, especially within the context of previously published literature. METHODS: Body height and weight were measured in 109 healthy subjects, with an average age of 24 ± 6 years (mean ± SD). Subjects underwent an ophthalmic assessment including anterior segment imaging by Scheimpflug topography and specular microscopy. Central and peripheral corneal thickness and corneal radius were analyzed. The relationship between body stature and corneal parameters was assessed using simple and multiple regression analysis. Effect size was determined by generating regression and correlation coefficients. RESULTS: Body height ranged from 1.54 to 1.86 m (mean ± SD 1.67 ± 0.08 m), central corneal thickness from 465 to 629 µm (554 ± 33 µm), whereas corneal radius measured between 7.16 and 8.49 mm (7.75 ± 0.24 mm). Body height was weakly associated with central corneal thickness and peripheral corneal thickness (r ≥ -0.180), and moderately with corneal radius (r = 0.351). Based on the regression equations, central corneal thickness decreases by 8 µm, whereas corneal radius increases by 0.11 mm for each 0.1-m difference in body height. No significant correlations were found for similar assessments using body weight or body mass index. CONCLUSIONS: Differences in corneal radius and corneal thickness can be linked to body stature. However, effect sizes were consistently small and no more than 13% of the variability in corneal curvature could be explained by variations in body stature.

Repeatability of Pentacam peripheral corneal thickness measurements.

PURPOSE: To assess the repeatability of corneal thickness (CT) measurements, along the horizontal meridian up to 5mm from centre, in healthy eyes with the Pentacam instrument. METHODS: CT was measured in 82 right eyes (82 healthy subjects) at 11 corneal locations nominally 1mm apart along the horizontal meridian with Scheimpflug topography Pentacam. Two consecutive scans were performed in quick succession. The repeatability of CT was determined by assessing differences between measurements and calculating the coefficient of variation (CV). The relative repeatability (difference) was calculated as the ratio of the absolute peripheral CT difference to the central thickness. A variant of Bland-Altman analysis was carried out to determine the effect of the overall magnitude of CT on the absolute and relative differences. RESULTS: Mean CT (µm) at the temporal 5mm location was 770 ± 51 and 823 ± 56 µm at the corresponding nasal location; central CT was 554 ± 36 µm. Good repeatability (CV < 1.2%) was found at all corneal locations. Differences (arithmetic; absolute and relative) were 3 ± 17 µm; 13 ± 11 µm; 2 ± 2%; respectively temporally and -1 ± 15 µm; 13 ± 8 µm; 2 ± 1% nasally, both at 5mm from the corneal apex. These values decreased for central CT (0 ± 6 µm; 4 ± 4 µm; 1 ± 1%). Differences between both measurements in all corneal locations were not statistically significant (p = 0.308 ANOVA Games-Howell). CONCLUSIONS: Pentacam shows good repeatability for pachymetry measurements up to 5 mm away from the corneal apex across the horizontal meridian, even though repeatability decreases slightly towards the periphery. A single Pentacam scan will be sufficient for most clinical applications.

Peripheral nasal-temporal corneal asymmetry in relation to corneal thickness: a Scheimpflug imaging study.

PURPOSE: To investigate the asymmetry of the peripheral cornea up to 5 mm nasally and temporally from the centre and to assess correlations with regional peripheral corneal thickness. METHODS: Central and peripheral corneal thickness was measured by Scheimpflug imaging (Pentacam) in 113 eyes of 113 healthy, pre-presbyopic Caucasian subjects. Absolute and relative corneal thickness were analysed in 1 mm steps up to 5 mm to the nasal and temporal sides with the corneal apex as the central reference point. Nasal-temporal asymmetry was calculated as the thickness ratio between corresponding off-centre thickness measurements. RESULTS: The mean (±SD) central corneal thickness was 552 ± 36 µm. CT increased by 22% at 4 mm temporally to 672 ± 44 µm, and 32% at 4 mm nasally to 731 ± 45 µm. The nasal-temporal asymmetry became greater with increasing distance from the corneal centre, with a mean difference of 59 ± 22 µm at 4 mm from the apex. The nasal-temporal thickness ratio, based on this difference, was significantly related to the relative temporal (r = -0.41, p < 0.001, simple linear regression) and nasal corneal thickness (r = 0.61, p < 0.001). CONCLUSIONS: A substantial and progressively increasing nasal-temporal asymmetry in corneal thickness has been confirmed by Scheimpflug imaging, which is related to the magnitude of corneal thickness at peripheral locations. Pachymetry output data and models, including volume calculations, that assume symmetry to the corneal thickness profile may not provide optimum metrics for planning and predicting the outcome of corneal refractive surgery procedures.

Corneal assessment technologies: current status.

There are now many devices that acquire data about the cornea: shape, power, pachymetry at any desired point of the cornea, corneal hysteresis, flap thickness (in LASIK procedures), endothelial cell count and morphology, and so forth. We review the literature on corneal assessment techniques and devices available in clinical practice. Specifically, we discuss slit lamp biomicroscopy, specular microscopy, ultrasound pachymetry, confocal microscopy, very-high-frequency digital ultrasound biomicroscopy, optical coherence tomography, Placido disk-based keratoscopy, slit-scanning elevation topography, Scheimpflug imaging, and dynamic applanation procedures-all of which can be used to assess the morphology of the cornea. In addition, we present a critical analysis of the instrumentation described and discuss the necessity of developing new technology for assessing both the morphology and the physiology of the cornea.

White-to-white corneal diameter differences in moderately and highly myopic eyes: partial coherence interferometry versus scanning-slit topography.

PURPOSE: To evaluate the differences between Orbscan scanning-slit topography and IOLMaster partial coherence interferometry (PCI) white-to-white (WTW) measurements in moderately and highly myopic eyes. SETTING: IOBA-Eye Institute, University of Valladolid, Spain. DESIGN: Comparative case series. METHODS: Myopic eyes were divided according to the degree of myopia as follows: Group 1 (<6.00 diopters [D]), Group 2 (between 6.00 D and 12.00 D), and Group 3 (>12.00 D). The WTW distance was measured with the scanning-slit topography and PCI devices. RESULTS: The study enrolled 328 eyes (64 subjects). The mean WTW in all eyes was 0.50 mm ± 0.26 (SD), lower with scanning-slit topography (11.69 ± 0.37 mm) than with PCI (12.19 ± 0.40 mm) (P<.01, paired t test). A low mean WTW was found in Group 2 (11.65 ± 0.34 and 12.15 ± 0.36 mm, scanning-slit topography and PCI, respectively) and Group 3 (11.51 ± 0.36 and 12.05 ± 0.46 mm, respectively) compared with Group 1 (11.79 ± 0.38 and 12.26 ± 0.40 mm, respectively) (P<.03, analysis of variance with Games-Howell correction). There was a low statistically significant relationship between WTW and spherical equivalent (SE) with both devices. CONCLUSIONS: Eyes with moderate and high degrees of myopia had lower WTW diameters than eyes with low spherical equivalent myopia measured with both devices. Scanning-slit topography provided less WTW distance than PCI in myopic eyes; thus, the devices are not clinically interchangeable.

Refractive stabilization and corneal swelling after cataract surgery.

PURPOSE: To determine the earliest time after cataract surgery when corneal swelling and automated refraction are stabilized sufficiently to allow reliable refractive prescription. METHODS: This was a prospective, nonrandomized, case series study of 124 consecutive eyes undergoing uneventful cataract surgery. Automated refraction and central corneal thickness (CCT) measurements were made at baseline before cataract surgery and on follow-up visits 1 day and weekly for 4 weeks afterward. Corneal swelling was determined as the percentage change in CCT after surgery. To determine the validity of automated refraction, it was compared with subjective refraction at the final visit. RESULTS: Corneal swelling decreased between the first postsurgical day and the first week and again between postsurgical weeks 1 and 2 (p < 0.01 each). Although thinning continued through the fourth postsurgical week, the changes were not significant. Both spherical and cylindrical refraction were stable 1 week after surgery, with the greatest change between the first postsurgical day and 1 week (p < 0.01). In replicate measurements of automated spherical refraction taken on the last visit, the intraclass correlation coefficient was 0.93. For automated cylindrical refraction, it was 0.81. CONCLUSIONS: Automated refraction is highly repeatable and can be used to monitor postsurgical refractive changes. Whereas corneal swelling becomes stable 2 weeks after cataract surgery, automated refraction becomes stable after 1 week and can be used to accurately prescribe corrective lenses at that time.

Repeatability and agreement of ARK-30 autorefraction after cataract surgery.

BACKGROUND: To evaluate the intra-test variability of ARK-30 handheld autorefractor and the agreement with subjective refraction and retinoscopy after uneventful cataract surgery. DESIGN: Prospective and non-randomized study that included 6 visits by patients undergoing uneventful cataract surgery at IOBA (Instituto de Oftalmobiología Aplicada) Eye Institute (University of Valladolid). PARTICIPANTS: The mean age of the 79 patients was 66.5years (range 23-90years). For the 124 eyes, the mean spherical equivalent of the sample at baseline visit was -3.59±6.28D (range -21.00D to +4.44D). METHODS: Automated refraction was performed on follow-up visits 1day and weekly for 4weeks. Retinoscopy and subjective refraction were conducted at the Week 4 follow up. MAIN OUTCOME MEASURES: Automated refraction. RESULTS: Sphere, cylinder and mean spherical equivalent, J(0) and J(45) coefficient variabilities were low in all visits. Standard deviations and the limits of agreement were smallest for the last visit. Subjective refraction sphere and cylinder values were more positive than autorefraction by 0.12±0.53D (P=0.031) and 0.23±0.42D (P<0.001), respectively. Comparison between autorefraction and retinoscopy showed a similar trend with the sphere and cylinder differences, 0.32±0.77D and 0.38±0.43D (P<0.05), respectively. CONCLUSIONS: The ARK-30 is sufficiently accurate and repeatable for automated refraction after uneventful cataract surgery. This instrument may be useful for monitoring refractive outcome in these patients.