Steeper Corneal Curvature Research Articles

Intraocular pressure in keratoconus

This study aimed to investigate intraocular pressure (IOP) measurements taken using the dynamic contour tonometer (DCT) and the non-contact tonometer (NCT) in subjects with keratoconus. Intraocular pressure was measured in 20 keratoconus subjects and 20 age-matched control subjects using the DCT and NCT instruments. Central and off-centre measures were taken with the DCT in order to highlight any systematic errors associated with corneal biomechanical factors. Measures of anterior and posterior corneal topography and thickness were also taken in each subject. No significant difference was found between the central and off-centre DCT IOP readings for the keratoconus subjects and age-matched controls (p > 0.05). The average DCT IOP was 14.2 ± 1.4 mmHg in the keratoconus subjects and 14.2 ± 1.6 mmHg in the controls. However, the average NCT readings differed significantly (p < 0.001) between the keratoconus subjects (9.2 ± 1.5 mmHg) and the controls (12.9 ± 2.4 mmHg). Intraocular pressure measurements taken with the DCT showed no significant (p > 0.05) correlation with the severity of keratoconus as determined through measures of corneal topography and thickness. Equivalent IOP measurements taken with the NCT correlated significantly with certain measures of corneal curvature and thickness in the keratoconus population. The difference between DCT and NCT IOP was strongly correlated with measures of corneal topography and thickness, with differences increasing in more advanced keratoconus. The measurements obtained with the DCT do not appear to be dependent upon corneal factors, unlike those taken with the NCT. The presence or severity of keratoconus was not correlated with DCT IOP values.

Intraocular Pressure and Related Systemic and Ocular Biometric Factors in a Population-Based Study in Japan: The Kumejima Study

To examine the distribution of intraocular pressure (IOP) and its related systemic and ocular biometric factors in a population-based study in a southwestern island of Japan. Cross-sectional, population-based study. All residents of Kumejima Island, Japan, located in southwestern Japan (eastern longitude, 126 degrees 48 minutes and northern latitude 26 degrees 20 minutes), 40 years of age and older were asked to undergo a comprehensive questionnaire and ocular examination, including measurement of IOP with Goldmann applanation tonometry. Of the 4632 eligible residents, 3762 (81.2%) underwent the examination. In 2838 non-glaucomatous subjects from whom reliable measurements of IOP were obtained, the mean IOPs +/- standard deviation in all, men, and women were 15.1 +/- 3.1 mm Hg (n = 2838), 15.2 +/- 3.1 mm Hg (n = 1450), and 15.1 +/- 3.0 mm Hg (n = 1388), respectively, with no significant intersex difference (P = .63). Multivariate regression analysis demonstrated that higher IOP was significantly correlated with younger age (P < .001), higher body mass index (P < .001), higher systolic blood pressure (P < .001), history of diabetes mellitus (P = .001), thicker central corneal thickness (P < .001), steeper corneal curvature (P < .001), and longer axial length (P < .018), but not with anterior chamber depth and the Shaffer angle width grade. Younger age, higher body mass index, higher systolic blood pressure, diabetes, thicker central corneal thickness, and steeper corneal curvature were significantly correlated with higher IOP. The present results confirm that IOP is associated with systemic and ocular biometric factors and may define specific subgroups most likely to have an elevated IOP.

Epidemiology of exfoliation syndrome in the Reykjavik Eye Study

Exfoliation syndrome (XFS) is a major risk factor for glaucoma. It is characterized by a pathological accumulation of polymorphic fibrillar material in the anterior segment of the eye. It is likely that the increase in intraocular pressure (IOP) seen in XFS patients is at least in part because of flakes of material clogging up the trabecular meshwork, and thereby increasing the resistance to outflow and increasing IOP. XFS glaucoma progresses more rapidly, is more resistant to medical treatment and has worse prognosis than other glaucomas. The prevalence of XFS has been found to vary greatly between different studies, raising the possibility of racial and/or environmental modulators. XFS has also been linked to other changes in ophthalmological structures such as; changes in central corneal thickness (CCT), steeper corneal curvature (CC) and nuclear lens opacifications. Some studies have found XFS to be associated with systemic diseases, mostly cardiovascular and cerebrovascular. Exposure to ultra-violet (UV) light has also been investigated as a possible culprit, along with several other plausible factors. The aim of the present study was to determine the prevalence and 5-year incidence of XFS, to establish possible risk factors and/or concomitant symptoms and finally to investigate the relationship between XFS and glaucomatous changes. The Reykjavík Eye Study (RES) is a prospective study based on a random sample from the Icelandic national population sample. The baseline examination was performed in the autumn of 1996, when 1045 persons older than 50 years participated. Of these, 846 (88.2% of survivors) participated in a follow-up 5 years later. All participants went through a standard examination protocol, and answered a comprehensive questionnaire on health and life style. In the prevalence study, XFS was found in 10.7% of subjects, more frequently in women and older persons. Five years later, a further 5.2% of those that participated in the follow-up study and had no signs of XFS at baseline were diagnosed having XFS. We found a strong correlation between IOP and XFS. No difference was found in the anterior segment parameters measured, but there was a significant loss of neural tissue in the XFS as demonstrated by measurements of cup/disk ratio. In conclusion, we find XFS to be frequent among Icelanders, increasing with age and more in women. Our diagnostic criteria are reliable over time. We have also identified possible risk factors that point to a role of antioxidants in the development of XFS. We find changes in corneal curvature and thickness more related to age than XFS.

Ocular biometry and determinants of refractive error in rural Myanmar: the Meiktila Eye Study

Objective:To describe the ocular biometry and determinants of refractive error in an adult population in Myanmar.Methods:A cross-sectional, population-based survey of the inhabitants 40 years of age and over from villages...

Ocular and systemic factors related to intraocular pressure in Japanese adults: the Tajimi study

Background:As intraocular pressure (IOP) and age are consistent risk factors of glaucoma, it is of special interest to know the association between IOP and possibly relating factors including age in...

A Randomized Trial of the Effect of Soft Contact Lenses on Myopia Progression in Children

Soft contact lenses have been reported to increase the progression of myopia. The purpose of this study was to determine whether soft contact lenses affect the progression of myopia in children. Children between the ages of 8 and 11 years with -1.00 to -6.00 D myopia and less than 1.00 D astigmatism were randomly assigned to wear soft contact lenses (n = 247) or spectacles (n = 237) for 3 years. Refractive error and corneal curvatures were measured annually by cycloplegic autorefraction, and axial length was measured annually by A-scan ultrasound. Multilevel modeling was used to compare the rate of change of refractive error, corneal curvature, and axial length between spectacle and contact lens wearers. There was a statistically significant interaction between time and treatment for myopia progression (P = 0.002); the average rate of change was 0.06 D per year greater for contact lens wearers than spectacle wearers. After 3 years, the adjusted difference between contact lens wearers and spectacle wearers was not statistically significant (95% confidence interval [CI] = -0.46 to 0.02). There was no difference between the two treatment groups with respect to change in axial length (ANCOVA, P = 0.37) or change in the steepest corneal curvature (ANCOVA, P = 0.72). These data provide reassurance to eye care practitioners concerned with the phenomenon of "myopic creep." Soft contact lens wear by children does not cause a clinically relevant increase in axial length, corneal curvature, or myopia relative to spectacle lens wear. (ClinicalTrials.gov, NCT00522288.).

Risk factors for early angle-closure disease in a Burmese population: the Meiktila Eye Study

To determine risk factors for early angle-closure disease (AD) in a Burmese population. A cross-sectional, population-based survey of the inhabitants, 40 years of age and over, from villages in the Meiktila District was performed; 2481 eligible participants were identified, 2076 participated in the study, and 2050 could be categorized as having early AD (defined by the presence of so-called occludable angles (<90 degrees of posterior trabecular meshwork visible on gonioscopy), but without glaucomatous optic neuropathy). The ophthalmic examination included Snellen visual acuity, slit-lamp examination, tonometry, gonioscopy, biometry, and dilated stereoscopic fundus examination. The mean axial length (AL) and anterior chamber depth (ACD) in those with occludable angles were 21.9 and 2.60 mm, respectively; in those with non-occludable angles, the corresponding lengths were 22.74 and 2.84 mm, respectively (P<0.001 for both comparisons). In the univariate analyses, age, female gender, nuclear and cortical cataract, steeper corneal curvature, more anterior lens position, and myopia were also significantly associated with occludability. In the multivariate analysis, age, AL, ACD, and nuclear cataract were significantly associated with occludability. In this Burmese population, those with occludable angles had significantly shorter ALs, ACDs, and thicker lenses than those without occludable angles. In multivariate analysis, increasing age, decreasing AL, decreasing ACD, and nuclear cataract were significant predictors of early AD. The presence of nuclear cataract per se should raise clinical suspicion of the possibility of AD in this population.

Corneal curvature and central corneal thickness in eyes with pseudoexfoliation syndrome

Background: Previous studies have reported different central corneal thickness (CCT) values in eyes with pseudoexfoliation syndrome (PXS). There has been no report about corneal curvature (CC) in PXS eyes. The aim of this study was to investigate the CCT together with the CC in PXS eyes with and without glaucoma. Methods: The CC (simulated keratometry 1 [SimK1], SimK2, and mean K) and CCT were prospectively studied by rotating a Scheimpflug camera in 72 PXS eyes and comparing them with 65 normal eyes. Results: In all PXS eyes, the mean K was significantly steeper than in control eyes (44.1 [SD 1.5] diopters (D) vs. 43.2 [SD 2.0] D, p = 0.04). When PXS eyes were subcategorized, the mean K was significantly steeper in both normotensive PXS eyes (44.6 [SD 1.6] D) and pseudoexfoliation glaucoma (PEXG) eyes (44.0 [SD 1.2] D) than normal eyes (43.2 [SD 2.0] D) ( p = 0.001 and 0.02, respectively). In all PXS eyes, the mean CCT (546.4 [SD 39.6] μm) was not significantly different than the control eyes (542.9 [SD 32.2] μm) ( p = 0.56). When PXS eyes were subcategorized, however, the CCT was significantly thinner in normotensive PXS eyes and significantly thicker in PEXG eyes. Interpretation: It appears that PEXG eyes have a significantly steeper CC and higher CCT than normal eyes. This could be of clinical significance because overestimation of true intraocular pressure may then occur in these eyes.

Peripheral light focusing as a potential mechanism for phakic dysphotopsia and lens phototoxicity.

Our aim was to examine secondary image formation in the anterior segment caused by peripheral light focusing (PLF) in the human cornea, and in particular the crystalline lens. Non-sequential ray-tracing (OptiCAD) was applied to an anatomically based human eye model, which incorporates a gradient index crystalline lens. For analysis of the limbal effect, we varied the incident angle from 100 to 122 degrees, while for the crystalline lens effect, the incident angle was varied from 60 to 90 degrees. The corneal shapes studied included central radii from 7.4 to 8.2 mm with a range of shape factors. In each case, we computed the peak and average intensities, and the area of exposure at the limbus or lens periphery. The computation was repeated with a previous model eye for comparison. For the limbal effect, a peak intensity gain of x22.5 was found at an incident angle of 104 degrees which compares well with previous results. The average intensity gain at this angle was x7.5 over an area of 0.23 mm2. Steeper corneal curvature produced a greater PLF effect. For the crystalline lens effect, maximum UVA (365 nm) intensity gain peaked at x8.6 at 84 degrees with average intensity gain of x2.3. The area of UVA exposure peaked at 4.7 mm2 at 70 degrees. A relatively wide range (30 degrees ) of incident angles produced peak PLF gains of x3 or more in the lens. Significant focusing of light is directed to the nasal limbus, and to a lesser extent to the crystalline lens over a broad range of incident angles. PLF in the nasal cornea is reduced by an order of magnitude when a UV-blocking soft contact lens is used. The concentration levels and intraocular sites of PLF action on UV and visible light suggest a new mechanism of phakic dysphotopsia and lens phototoxicity.

The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis

Objective To evaluate the relationship between predicted flap thickness and actual flap thickness and between predicted tissue ablation and actual tissue ablation. Design Prospective, nonrandomized comparative (self-controlled) trial. Participants A total of 60 patients (102 eyes) who underwent laser in situ keratomileusis (LASIK). Main outcome measures Subtraction pachymetry was used to determine actual corneal flap thickness and corneal tissue ablation depth. Other measurements included flap diameter and keratometry readings. Results Actual flap thickness was significantly different ( P < 0.0001) from predicted flap thickness. Fifteen eyes had a predicted flap thickness of 160 μm and a mean actual flap of 105 μm (standard deviation [SD], ±24.3 μm; range, 48–141 μm). Sixty-four had a predicted flap of 180 μm with an actual flap mean of 125 μm (SD, ±18.5 μm; range, 82–155 μm). Seventeen eyes had a predicted flap of 200 μm, with an actual flap mean of 144 μm (SD, ±19.3 μm; range, 108–187 μm). In addition, we found that significantly more tissue ( P < 0.0001) was ablated than predicted. Linear regression of the observed ablation on predicted ablation yielded the following relationship: actual ablation = 14.5 + 1.5 (predicted ablation). Neither flap diameter nor flap thickness were found to increase with respect to steeper corneal curvatures. Conclusions Actual corneal flap thickness was consistently less than predicted regardless of the depth plate used; actual tissue ablation was consistently greater than predicted tissue ablation for the laser used in this study.

Ocular parameters in the subgroups of angle closure glaucoma.

This study was conducted to compare anatomical parameters, thought to be responsible for causing angle closure glaucoma (ACG), among eyes having acute, subacute or chronic ACG. Ninety consecutive patients diagnosed with a subgroup of ACG, and 30 age, sex and refraction matched controls, provided a total of 240 eyes for a prospective study. The refractive error, corneal diameter, keratometry, pachymetry, lens thickness and axial length were measured and the relative lens position was calculated. The data were analysed by paired t-test, ANOVA, signed rank test and multivariate analysis. Acute ACG eyes were mildly hyperopic. All the ACG subgroups had similarly short eyeballs and a steeper corneal curvature compared to control eyes. Acute ACG lenses were thicker than all the other groups (P < 0.001), but all ACG eyes had thicker lenses than the controls. Corneal diameters and anterior chamber depths were decreased in acute and chronic ACG eyes compared with subacute ACG and controls (P < 0.001). The uninvolved fellow eyes in each subgroup differed from affected eyes only in having more posteriorly positioned lenses. There was a spectrum of anatomical variations seen in the subgroups of ACG. Acute ACG eyes expressed an extreme shift of anatomical features away from normal, especially, smaller corneal diameters, leading to a large mobile lens in an already crowded anterior segment. This predisposed them to a severe relative pupillary block, and to a form of ciliary block glaucoma. Chronic ACG eyes were less divergent from normal and therefore could have suffered a milder form of the same kind of angle closure, but over a more prolonged period. Subacute ACG eyes deviated least from controls, and therefore exhibited mild signs and spontaneous resolution. Further work is required to elucidate completely the pathophysiology that leads to ACG.

Effect of base curve, power, and center thickness on visual acuity with thin, low-water hydrogel lenses

The high- and low-contrast visual performance of patients with relatively flat (<43.50 D) and relatively steep (>45.00 D) central corneal curvatures was measured when they were fitted alternately with Polymacon ® lenses of 20, 26, 31, and 55-μm center thickness, powers of − 1.00 and − 5.00 D, and base curves of 8.4, 8.7, and 9.0 mm. Analysis of variance for repeated measures showed trends for poorer visual acuity under all testing conditions with thinner center-thickness lenses when fit with a steeper lens/cornea base curve fitting relationship. Interaction analysis indicated that thinner center-thickness lenses provide optimal visual acuity only when fit with the correct base curve, even though the actual on-eye fit may seem acceptable. The results suggest that visual acuity testing, particularly under conditions of low contrast or low illumination, is a sensitive measure of soft lens fitting performance and should be considered in addition to the more traditional indicators such as lens movement and decentration.

Influence of corneal topography on centration and movement of low water content soft contact lenses

Recent studies have determined that movement of mid- and high-water contact lenses is little influenced by the central cornea-lens-bearing relationship; however, the possible influence of variations in the peripheral corneal contour has not yet been established. We measured central and peripheral corneal power using the Topographic Modeling System videokeratoscope and used a video slit lamp to measure movement and centration of a soft lens after 15 minutes' wear in 44 subjects. The contact lens was a standard commercially available design with nominal 38% water content and 0.06 mm center thickness. The only aspect of corneal topography predictive of the lens fitting was a steep central corneal curvature (keratometry 44.00–47.20 D), which was associated with significantly poorer lens centration (chisquare, p = 0.05). Flattening of peripheral corneal power was not predictive of lens movement or of lens centration (ANOVA, p > 0. 05). Other factors such as characteristics of the tear film and blinking may be of greater significance in determining hydrogel lens movement than is corneal topography alone.

Inverse Arcuate Incision: A New Approach to the Correction of Astigmatism

Standard concentric arcuate transverse corneal relaxing incisions flatten the meridian incised and steepen the meridian 90 degrees away, a phenomenon designated "coupling". We have demonstrated a new method of reducing the steep corneal curvature without inducing coupling, by utilizing both transverse and radial components of an inverse arcuate incision. This technique has been found useful in cases of myopic astigmatism because it reduces the myopic spherical equivalent, which is left unchanged by standard arcuate incisions, without having to place radial incisions.

Corneal Refraction and Topography in School Myopia

Changes in corneal refraction and topography among 145 myopic children were monitored over a three-year period as part of a clinical trial of myopia treatment. The spherical equivalent of the right eye increased from -1.46 D to -3.13 D. Refractive astigmatism increased from 0.28 D to 0.46. Corneal astigmatism, horizontal and vertical corneal curvatures, as well as shape factors, did not change significantly. The mean deviation of the corneal apex from the visual axis increased from 0.48 mm to 0.67 mm. Girls showed steeper corneal curvatures than boys. For both sexes the vertical corneal curvatures were steeper, and shape factors were smaller than those along the horizontal plane. Axial elongation of the eye was the primary explanation for myopic progression.

Variable expressivity of autosomal dominant microcornea with cataract.

Autosomal dominant microcornea with a cataract, previously described in four families, was documented in a seven-generation family. Eighteen family members had microcornea and a cataract, and an additional six had sclerocornea or Peters' anomaly. Most individuals with microcornea had a corneal diameter of less than 11 mm in both meridians, with moderately steep corneal curvatures. The inherited cataract progressed to form a total cataract after visual maturity had been achieved. In the four affected children who had not undergone cataract extraction, the common abnormality was a posterior polar lens opacity. The variability of expressivity of the dominant gene would suggest that the embryological origins of microcornea and sclerocornea are similar.

Corneal iron lines after refractive keratoplasty.

Twenty-three of 55 patients who underwent refractive keratoplasty for the correction of aphakia were found to have a pigmented corneal epithelial line similar to the Fleischer ring. In 21 of 23 cases, the pigmented ring was adjacent to the superficial keratectomy scar that resulted from the keratorefractive operation. In two cases, the pigmented line was located central to the lamellar keratectomy scar. Histopathologic study demonstrated that the line was due to iron in the form of ferritin particles deposited within basal corneal epithelial cells. We believe that the steep anterior corneal curvature (average, 51.45 diopters) and changes in corneal topography at or central to the margin of the lamellar keratoplasty may lead to malapposition of the eyelid to the cornea and cause disturbances in the precorneal tear film. Iron from the tears may then be deposited in the corneal epithelium in areas of tear pooling.

Aphakic Refractive Penetrating Keratoplasty Using Newborn Donor Corneas: A Preliminary Report on an Alternative Approach to Refractive Correction

Eyes with corneal disease receiving transplants from premature, full gestational newborns or postnatal donors yield refractive results with a myopic shift. When the donor cornea is from a newborn (40–41 week gestation) this myopic shift was sufficient to correct 5 of 7 aphakic eyes to an average postsurgical refraction of −1.0 diopter (spher equiv). One premature cornea gave −6.0 diopters while six postnatal infant corneas gave an average correction of +8.6 diopters (spher equiv). The steep corneal curvature resulting from surgery explains the additional refractive power noted. The surgical difficulties encountered and the unknown long-term results auger for a slow and controlled introduction of this concept into our everyday practices.