Diurnal variation in corneal thickness

Corneal thickness was measured at three different times of day in rabbits. The rabbit cornea showed a significant diurnal variation regarding thickness. In most cases, the cornea was thickest in the forenoon, and then thinning followed. It usually became thinnest in the afternoon, but sometimes at about noon. The decrease in thickness was reversed during the night. The amount of the diurnal change was inconsistent. The average value was 0·014 mm.The maximum and minimum values of the corneal thickness that occurred in the course of long-term measurements of diurnal variation are considered to represent the upper and lower limits of normal thickness.No relation was found between the diurnal variation and the presence of dellen on the corneal surface.Diurnal variation in corneal thickness was also found when the eye lids were kept closed to prevent evaporation.Overnight experiments showed that, in general, the relative corneal dehydration occurred while the rabbit was active, and the corneal hydration occurred during periods of relaxation or drowsiness, independent of closure of the lids.In an extremely humid atmosphere, the rabbit showed a daytime corneal thinning of the same order as that in the normal environment.     

Diurnal variation of corneal sensitivity and thickness

PURPOSE: To measure the diurnal variation of central corneal sensitivity and thickness over 24 hours. METHODS: A noncontact pneumatic esthesiometer to measure central corneal sensitivity and an optical pachymeter to measure central corneal thickness were used on 20 noncontact lens wearers. These measurements were performed at 22:00 before 8 hours of sleep and on the following day on eye opening and at hourly intervals between 7:00 and 22:00. RESULTS: Central corneal sensitivity varied by 35% over the 24 hours. Sensitivity was significantly lower by 17.1 +/- 2.1% on eye opening and recovered to the level of the previous night after 4 hours. Corneal sensitivity continued to increase throughout the day but was not significantly higher. Central corneal thickness varied by 3.9% over the 24 hours. The mean overnight corneal swelling was 2.9 +/- 0.31%, and 2 hours after eye opening, the cornea had deswelled to the same thickness as the previous night. The cornea thinned further throughout the day, but there were no statistically significant differences between the values after 14:00. There was a high correlation between corneal sensitivity and thickness over time (r = 0.8; p < 0.05). DISCUSSION: Corneal sensitivity correlated with corneal thickness: both were higher on awakening and then decreased and remained below the levels measured the previous night. This may be due to physiological and/or environmental factors. The lack of significant differences between the values after 14:00 suggests that baseline may be measured at any time from 7 hours after eye opening.     

Diurnal variations in human corneal thickness.

AIM: To elucidate the diurnal variation in human corneal thickness over a 48 hour period. METHOD: Changes in central corneal thickness were monitored in eight healthy subjects (four male, four female) aged between 10 and 63 years using an ultrasonic pachymeter. Measurements were made over a 48 hour period-immediately before sleep, immediately upon waking and at 15, 30, 45 minutes, 1, 1.5, 2, 2.5, 3 hours, and at 2 hour intervals thereafter throughout the remainder of each day. RESULTS: The mean corneal thickness for the group (SD) was 546 (14) microns, with a mean overnight increase of 5.5% (2.9%) (range 1.9-12.6%) and a maximum diurnal increase of 7.2% (2.8%) (range 2.1-14.3%). Individual differences in the extent of diurnal and overnight variation occurred within the group. For three subjects, the first reading taken on waking was not the highest and corneal thickness continued to increase. CONCLUSION: These data confirm an increase of corneal thickness during sleep, but also reveal considerable variation during waking hours. Thus, the overnight changes in corneal thickness are not truly representative of diurnal variations in human corneal thickness and, in fact, much greater diurnal variation occurs than the 3.0-4.4% previously reported.     

Diurnal variation in human tear enzymes

A fluorometric technique for measuring the activities of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) in human tear fluid is described. Measurement of separate LDH and MDH activities during the day resulted in fluctuating values with no definite pattern. However, a consistent diurnal pattern emerged when the tear $\text{LDH}\text{MDH}$ ratio was measured for individual tear sample. The tear $\text{LDH}\text{MDH}$ ratio was highest upon waking, declining soon after to a stable level for the remainder of the day.Evidence for a corneal epithelial origin of tear LDH and MDH is discussed, as well as the ability of the technique to detect differences in the metabolic status of the epithelium between the open-eye and closed-eye environments. Possible causes of the elevation of the tear $\text{LDH}\text{MDH}$ ratio following lid closure are unbinding of intracellular M-type LDH within epithelial cells and increased cell membrane permeability, both processes occurring in response to reduced cellular energy content under the more hypoxic conditions of lid closure. Although the nature of the present experiment did not allow any conclusive statement, de novo LDH synthesis may also be contributing to the increased tear $\text{LDH}\text{MDH}$ ratio. Deliberate corneal epithelial surface disruption was found to have no quantifiable effect on the tear $\text{LDH}\text{MDH}$ ratio.     

Diurnal variation of corneal and corneal epithelial thickness measured using optical coherence tomography.

PURPOSE: To determine the feasibility of optical coherence tomography (OCT) measurement of overnight corneal swelling and de-swelling and, specifically, to examine overnight changes in the corneal epithelium and recovery during the day. METHODS: Ten healthy volunteers were recruited. Corneal OCT imaging was performed at 10:00 PM (baseline), 8:30 AM, and every 2 hours thereafter until 4:30 PM. Either the right or left eye was taped shut (using a gauze pad and surgical tape) until the 8:30 AM session of the next day. Measurements were made along a 1-mm strip centered on the visual axis. RESULTS: At baseline, there were no differences in corneal or epithelial thickness between control and experimental eyes (all p > 0.05). Immediately after patch removal, corneal and epithelial thicknesses of patched eyes were higher than the baseline (all p < 0.05). In patched eyes, the cornea and epithelium swelled 5.5% and 8.1% overnight, respectively (p > 0.05). Two hours later, the epithelial thickness of the experimental eyes recovered to baseline level (p > 0.05), but corneal thickness did not reach to baseline level until 4 hours after patch removal. For control eyes, there were no differences compared with baseline (all p > 0.05). CONCLUSION: Both cornea and corneal epithelium experience proportionately similar amounts of overnight swelling. Recovery of overnight swelling may be slower for the cornea than for the epithelium. OCT provides valuable information about anterior segment morphometry.     

Measurement of corneal curvature in young and older normal subjects.

PURPOSE: A decrease in the vertex radius occurs with aging. The aim of this study was to investigate whether the degree of flattening from the vertex to the periphery of the cornea, ie, the surface profile, also changes with age. METHODS: A "p" value (a parameter that denotes the rate of flattening from the apex to the limbus) and vertex radius were measured in the horizontal meridian in a group of young and older subjects using the EyeSys videokeratoscope and the Topcon autokeratometer. RESULTS: With EyeSys data, horizontal vertex radius and p value were computed by means of plotting a graph of r2 (sagittal radius) versus y2 (perpendicular distance from the instrument optical axis). For Topcon data, previously derived equations were used. Mean vertex radii in the younger Topcon autokeratometer group was 7.91+/-0.31 and for the EyeSys videokeratoscope was 7.98+/-0.31. For the older group, Topcon mean vertex radii was 7.68+/-0.22 and EyeSys mean was 7.74+/-0.24. Mean p value in the younger group was 0.66+/-0.09 for the Topcon and 0.78+/-0.07 for the EyeSys. In the older group, mean vertex radii was 0.74+/-0.07 for the Topcon and 0.86+/-0.07 for the EyeSys. CONCLUSION: Vertex radius decreased with age, demonstrating a steepening of the cornea, and confirming previous results. The p value increases with age, indicating a shift toward a more spherical surface. The rate of change in the vertex radius and p value with age are predicted.     

Diurnal intraocular pressure in young adults with central retinal vein occlusion

In the older population, there is a well-known relationship between central retinal vein occlusion (CRVO) and glaucoma and ocular hypertension. In young adults, CRVO is a rare occurrence, the cause of which is not well understood. Seven patients under the age of 36 years with CRVO and no associated systemic disease underwent modified diurnal intraocular pressure (IOP) measurements (8:00 AM to 11:00 PM). Abnormal IOPs were found in the affected and/or the unaffected fellow eyes. To the authors' knowledge, this is the first report implicating abnormal IOP as an etiologic factor in the development of CRVO in young adults. Only with diurnal IOP measurements were the elevated swings and peak IOPs detected. This finding suggests that abnormal IOP may be an important factor in the development of CRVO in young adults.     

Circadian variation of aqueous dynamics in young healthy adults

PURPOSE: Recent research indicates that intraocular pressure (IOP) does not decrease significantly during the nocturnal period, although aqueous humor flow decreases by 50% or more at night. This study was undertaken to investigate whether changes in outflow facility, episcleral venous pressure, or uveoscleral flow at night could account for the nocturnal IOP. METHODS: Sixty-eight eyes of 34 healthy subjects (age, 18-44 years; mean, 29) were studied. Aqueous humor flow rate, IOP, and outflow facility were measured with pneumatonometry, anterior chamber fluorophotometry, and Schiotz tonography respectively, in each eye during the mid-diurnal (2-4 PM) and mid-nocturnal (2-4 AM) periods. Nocturnal IOP, flow rate, and outflow facility were compared to the same variables during the diurnal period. Mathematical models based on the modified Goldmann equation were used to assess the conditions under which these results could be reconciled. RESULTS: Supine IOP decreased slightly from 18.9 +/- 2.7 mm Hg in the mid-diurnal period to 17.8 +/- 2.5 mm Hg in the mid-nocturnal period (mean +/- SD, P = 0.001). Aqueous flow rate decreased from 2.26 +/- 0.73 to 1.12 +/- 0.75 microL/min (mean +/- SD, P < 0.001). There was a nonsignificant trend toward a nocturnal decrease of outflow facility (diurnal, 0.27 +/- 0.11 microL/min/mm Hg; nocturnal, 0.25 +/- 0.08 microL/min/mm Hg; mean +/- SD, P = 0.13). CONCLUSIONS: Outflow facility measured by tonography does not decrease enough during the nocturnal period to compensate for the decreased aqueous humor flow rate. Modeling results indicate that the experimental results could be reconciled only if nocturnal changes in episcleral venous pressure and/or uveoscleral flow occurred.     

Meridional corneal components of myopia progression in young adults and children

Records were selected from a data pool of myopic private practice patients to study the relation of refractive error change and keratometer power change in young adulthood. Selection was on the basis of three or more refractions and keratometer readings at and/or after 18 years of age. There were significant correlations between rate of myopia progression and rate of keratometer power change. Linear regression slopes of rate of refractive error change on rate of keratometer power change were in the neighborhood of 0.7. These findings were in contrast to the lack of correlation between myopia increase and corneal steepening in childhood myopia progression. Young adulthood myopia progression also appeared to be accompanied by a slight tendency toward a with-the-rule astigmatic shift.     

Diurnal variation of central corneal thickness and Goldmann applanation tonometry estimates of intraocular pressure

PURPOSE: To determine whether there is a temporal relationship between the diurnal variation of central corneal thickness (CCT) and intraocular pressure (IOP) by Goldmann applanation tonometry in young normal human participants in vivo. METHODS: Twenty-five eyes of 25 young healthy normal participants were examined in a prospective observational cross-sectional study. IOP, CCT and corneal curvature were measured using standard clinical techniques over a 24-hour period, and the temporal interrelationships between these parameters were examined. RESULTS: The overnight change in IOP measured by Goldmann tonometry was 3.1+/-2.4 mm Hg (P<0.001), CCT was 20.1+/-10.9 mum (P=0.016), with no statistical change in central corneal curvature (0.05 mm, P=0.477, paired t test with Bonferroni correction). Both IOP and CCT were highest on awakening at 7:00 then dropped rapidly to baseline levels by 9:00 (linear mixed models), and these two parameters were highly correlated (r=0.978, P<0.001). After 9:00, there was no correlation between these parameters (r=-0.453, P=0.260). CONCLUSIONS: The results of this study have highlighted a potential link between the diurnal variation of CCT and the accuracy of Goldmann tonometry estimates of IOP during the first 2 hours after awakening. Clinicians should be wary of using Goldmann tonometry to estimate IOP until the overnight increase in CCT has resolved.     

中青年人正常眼角膜最佳拟合形态的研究

目的 分析和判断中青年人正常眼角膜的最佳拟合形态.方法 随机选择30名中青年人60只正常眼.ORBSCANIIz系统获取角膜非锥面、椭球面、球面、球柱面拟合形态表面高度计算其表面像差,Zywave像差仪获取眼像差,采用线性回归分析其相关关系.结果 (1)非锥面拟合形态角膜像差(0.0653±0.8503)μm与眼像差(0.8640±0.1424)μm呈正相关(P=0.000)与眼高阶像差(0.6840±0.0688)μm呈负相关(P=0.015);(2)椭球面拟合形态角膜像差(-0.0492±0.3818)μm与眼像差、眼高阶像差均呈负相关(P=0.001);(3)球面拟合形态角膜像差(0.0770±1.2517)μm、球柱面拟合形态角膜像差(0.1134±1.2247)μm与眼高阶像差均呈负相关(P=0.001).结论 同一角膜不同拟合形态的角膜表面高度与像差不同;中青年人正常眼角膜非锥面拟合形态与椭球面拟合形态趋向于角膜最佳拟合形态.     

Changes in corneal curvature in accommodation

PURPOSE: To determine the changes in corneal curvature in accommodation. SETTING: Department of Ophthalmology, St. Luke's International Hospital, Tokyo, Japan. METHODS: Twenty-eight eyes of 14 healthy volunteers with a mean age of 38.9 years (range 28 to 65 years) were enrolled in this prospective study. Corneal topographies were taken before and during accommodation. Subjects achieved maximal accommodation by gazing at a target letter "A" in front of the eye. Keratometric values (K-values) obtained by topographies before and during accommodation were compared. RESULTS: Steepened corneal curvatures during accommodation were observed with the difference-plot view. Maximum K-values for the central 3.0 mm, 5.0 mm, and 7.0 mm corneal diameters significantly increased by a mean of 0.62 diopters (D) +/- 0.83 (SD) (P =.0005, paired t test), 0.60 +/- 0.90 D (P =.0015), and 0.72 +/- 0.65 D (P<.0001), respectively. Respective minimum K-values significantly increased by a mean of 0.62 +/- 0.83 D (P=.0005), 0.67 +/- 0.68 D (P<.0001), and 0.64 +/- 0.62 D (P<.0001). No statistically significant relationship was found between age and change in K-values. CONCLUSION: The results suggest that changes in corneal curvature in accommodation participate in the mechanism of accommodation.     

近视眼患者角膜直径和角膜曲率的研究

目的探讨近视眼患者角膜直径和角膜曲率的正常值及二者之间的关系。方法对220例(440眼)近视眼患者(年龄17～41岁,男118例236眼,女102例204眼)采用美国Lasersight公司的AstraMax角膜地形图进行角膜前表面摄像,测量3次并获取角膜水平直径(白到白)和角膜前表面平均曲率的平均值,并对这2项参数进行相关的统计学分析。结果本组中角膜直径平均值为(12.00±0.40)mm,男性为(12.12±0.36)mm,女性为(11.86±0.40)mm。角膜曲率平均值为(43.51±1.37)D,男性为(43.10±1.29)D,女性为(43.99±1.31)D。角膜直径和角膜曲率的回归方程为Y=19.060-0.162X(Y代表角膜直径,X代表角膜曲率)。结论男女之间角膜直径存在着统计学差别,男性大于女性;角膜曲率之间亦有统计学意义,女性大于男性。近视眼患者角膜直径和角膜曲率之间存在统计学负相关性,即角膜曲率越高,角膜直径越小。