the cornea is part of the optical system of the eye , and its condition is directly related to the quality of eyesight .
measurement of central corneal thickness ( cct ) and endothelial cell parameters is important when undertaking a functional and morphologic evaluation of the cornea for diagnostic purposes or before various surgical interventions . due to the increasing popularity of correction of refractive defects by excimer laser ,
cct has come to have higher prognostic significance for determination of the success of surgery and probable post - surgical complications .
surgery in the case of a cornea that is too thin may result in a very serious postoperative complication , ie , corneal ectasia .
it has been assumed that the optimal cct for refractive surgery is more than 500 m ; however , opinions regarding the impact of cct in anticipating treatment of refractive defects ( ie , which cct value is too low to perform surgery safely ) are not consistent and studies of patients with relatively thin cct indicate no excess risk for postoperative complications.1,2 it should be remembered that intraocular pressure depends on corneal thickness ; a thick cornea may explain a falsely elevated intraocular pressure reading and , conversely , falsely low intraocular pressure is measured in eyes with thin corneas.35 this should be taken into consideration in order to avoid both overdiagnosis and underdiagnosis of glaucoma and ocular hypertension .
factors thought to affect cct include race , age , sex , anthropometric parameters , drugs , time of day , blink rate , and type of measuring equipment used .
, none of these factors alone is able to predict cct . standards must be established in order to compare results obtained in different population groups . since cct varies in these populations , it is very important to verify the results and determine the limits of normal values.68 further , corneal parameters differ according to patient age.613 awareness of these ethnicity - related and age - related physiologic changes enables us to assess the influence of disease and surgical procedures more accurately .
the purpose of this study was to estimate the mean cct in the lithuanian population and compare cct between subjects according to age group and identify correlations between cct , age , and sex .
this was a prospective study performed between march 2009 and december 2010 , and enrolled lithuanian citizens visiting primary health care institutions in the cities and regions of vilnius , kaunas , klaipda , iauliai , panevys , teliai , taurag , utena , alytus , and marijampol .
all measurements were taken by the same examiner , who visited the above - mentioned health care institutions during the study period . after obtaining informed consent ,
1,650 adult caucasians comprising 688 ( 41.7% ) men and 962 ( 58.3% ) women of lithuanian origin were examined .
participants had a standardized interview about any previous or current ocular disease , previous eye trauma or surgery , wearing of contact lenses , and comorbidity , eg , diabetes or other chronic disease that may influence cct .
cct was measured using an ultrasound contact pachymeter ( quantel medical , france ) , applying one drop of 0.5% proxymetacaine ( alcon - couvreur , puurs , belgium ) for local anesthesia .
the pachymeter automatically recorded five cct measurements and the mean value was taken as the result of the test .
exclusion criteria were history of corneal degeneration or ulceration , anterior segment surgical procedures , glaucoma , diabetes mellitus , current conjunctivitis , or wearing of contact lenses .
the subjects were stratified into seven groups on the basis of age as follows : 1829 years , 3039 years , 4049 years , 5059 years , 6069 years , 7079 years , and 8089 years ( table 1 ) .
the sex distribution of the participants was similar ; however , there were more women in the age groups of 1829 , 4049 , 5059 , and 6069 years .
the study protocol followed the guidelines of the declaration of helsinki and was approved by the local ethics committee .
continuous variables were evaluated using the mean and standard deviation . when the distribution was normal , the mean differences between independent samples for the two groups were assessed using the student s two - sided t - test , and the paired student s t - test was used to compare means of dependent samples .
means of more than two groups were compared using one - way analysis of variance . a p - value less than 0.05 was considered to be statistically significant .
continuous variables were evaluated using the mean and standard deviation . when the distribution was normal , the mean differences between independent samples for the two groups were assessed using the student s two - sided t - test , and the paired student s t - test was used to compare means of dependent samples .
means of more than two groups were compared using one - way analysis of variance . a p - value less than 0.05 was considered to be statistically significant .
the average age of the study population was 55.419.2 years ( 53.618.8 years for women and 57.319.5 years for men ) .
mean cct for the overall patient population was 544.630.5 ( range 654449 ) m in the right eye ( figure 1 ) , 545.230.5 ( range 650451 ) m in the left eye ( figure 2 ) , and 544.630.5 m in both eyes .
the average cct for both eyes was 545.026.4 m in men and 544.433.2 m in women .
there was no statistically significant difference in cct between the eyes ( p>0.05 ) , so the right eye was selected for further investigation .
the average cct for the right eye in men was 545.025.6 ( range 468645 ) m and for women was 544.433.5 ( range 449654 ) m .
no statistically significant difference was found between the sexes ( p>0.05 ) ; however , there was a significant sex difference in one age group , ie , men aged 1829 years had higher cct than women ( p<0.05 ) .
the average cct ( m ) according to age group is shown in table 2 .
a statistically significant weak negative correlation was found between cct and age ( r=-0.263 , p<0.05 ) .
this relationship was stronger in men ( a moderate negative correlation ; r=-0.406 , p<0.05 ) than in women ( a weak negative correlation ; r=-0.118 , p<0.05 ) .
taken overall , cct decreases over the lifetime in both sexes ( figures 35 ) , but the age - dependence of cct is stronger in men . percentiles were calculated for cct according to subject age group ( < 40 years , 4060 years , and > 60 years , see table 3 ) .
corneas were thinner in people over 60 years of age when compared with the other age groups ( p<0.05 ) .
mean cct values were classified into percentiles as follows : < 489 m ( percentile <3 , extremely thin cornea ) ; 489510 m ( percentile 310 , moderately thin cornea ) ; 510524 m ( percentile 1025 , mildly thin cornea ) ; 524564 m ( percentile 2575 , average corneal thickness ) ; 564586 m ( percentile 7590 , mildly thick cornea ) ; 586609 m ( percentile 9097 , moderately thick cornea ) ; and > 609 m ( percentile > 97 , extremely thick cornea ) . more details on percentiles for cct are shown in table 3 .
the purpose of this study was to estimate mean cct in adult lithuanians and identify any correlations between cct , age , and sex .
there are reasonable data showing differences in cct between various ethnic groups.14,15 the mean cct in our study ( 544.6 m ) was similar to that of 545.2 m reported for the caucasian population by nemesure et al and within the limits for mean cct reported by other studies , ie , 520579 m.6,1618 the corneas of african americans have been reported to be thinner and those of caucasians and mongoloids as thicker.6,9,14,15,1923 thin corneas were identified in mongoloids residing in mongolia ( 504.5 m ) and india ( 520.7 m ) , whereas mongoloids living in singapore and the people s republic of china had cct values similar to those of caucasians ( 541.2 m and 556.2 m , respectively).9,14,15,19 some of the available data show that cct correlates significantly with sex while others do not .
however , a statistically significant difference was found between young men and women aged 1829 years , with cct being higher in men .
leskul et al did not find any correlation between these parameters in their study in thailand , in which 467 subjects aged 1260 years were examined.24 hashemi et
al examined 800 eyes and also did not find any relationship between cct and sex.25 eballe et al examined 970 eyes in their study and reported finding no statistically significant difference in cct between men and women.8 findings of other studies indicate that corneas are thicker in men than in women . in the beijing eye study conducted in 2008 in the people
s republic of china , xu et al examined 3,251 participants and found that corneas were thicker in men than in women.26 suzuki et al examined 2,848 men and 4,465 women and came to the same conclusion ( finding a mean cct of 521 m in men and 514 m in women).27 similar results have been published elsewhere.2830 there is no clear answer as to why women have thinner corneas than men .
it has been shown established that cct is lowest in women at the beginning of the menstrual cycle and highest at the end and during ovulation.31 further , keskin et al determined that cct was significantly decreased in postmenopausal women compared with their premenopausal counterparts.32 we observed a weak correlation between cct and age .
some authors did not define any dependence between these parameters ; others , however , refer to a decrease in cct with age.7,9,33 kamiya et al examined 204 eyes from 204 healthy subjects , and reported that biomechanical data for the cornea change during the course of the lifetime , but could not identify significant changes in age - related cct or intraocular pressure.34 mercieca et al reported that the cornea becomes thinner with age and mean cct is lower in women than in men.10 the reikjavik eye study published in 2002 , in which 925 healthy people aged 50 years and older were examined , showed that cct does not depend on age.4 prasad et al reached the same conclusion after measuring corneal parameters in 5,158 subjects.35 thinning of the cornea at a rate of 37 m per decade has been observed in older age in some ethnic groups . a study in 1998 with 1,242 participants aged
1087 years showed that cct decreased by 56 m for each decade of life.9 eballe et al reported that cct decreases by 4.2 m for each 10 years of life , with the thickest corneas found in subjects younger than 20 years.8 aghaian et al indicated that cct decreased by 3 m over a decade , and brandt et al reported a 6.3 m decrease over the same period.14,36 our data show that cct is decreasing at a rate of 28 m per decade in lithuanians over 40 years of age .
leskul et al examined 467 subjects aged 1260 years of age and found that young people had higher cct.24 a similar conclusion was reached in the liwan eye study reported by wang et al.11 wangsupadilok et al also identified corneal thinning over the lifetime after examining subjects aged 1896 years.37 pfeiffer et al , who reported the european glaucoma prevention study , found that younger persons had thicker corneas.12 narayanaswamy et al who examined 1,132 individuals , reported that biomechanical features of the cornea change with the age , ie , there is a reduction in corneal resistance factor and corneal hysteresis.13 the impact of age on corneal thickness can explained in a number of different ways . referring to theory based on histologic studies , the corneas of older people are thinner because of a reduction in keratocyte density and possible destruction of collagen fibers , and senior individuals are exposed to environmental factors for a longer period of time , which might influence corneal structure.38 hasemian
et al demonstrated that corneal endothelial cell density decreases in persons until 60 years of age ; however , the volume of these cells increases.39 as yet there is no consensus among scientists as to whether the corneal endothelium and cct are interdependent.40,41 it has been established that cct is lower in children of african american origin ( same as in adults ) in comparison with caucasian children.42,43 it has also been demonstrated that cct is lower in children born prematurely than in those born at term.42,44 lim et al examined 271 children of various ages and did not found any interdependence between cct and the axial length of the eye or refractive defects.45 hikoya et al examined 169 children aged 118 years and established that cct in japanese children is lower than in caucasian children , but higher than in children of african american origin.46 osmera et al who examined 124 subjects from 7 to 17 years of age , reported that cct and intraocular pressure values in czech children and adults are very similar to those in adults.47 in a study involving 106 children younger than 18 years , dai et al determined that corneas in children of african american origin were thinner than in caucasians.48 further , a survey conducted in singapore showed that chinese children had thicker corneas than malay or indian children.49 classification of cct according to our nomogram could be used in clinical practice to evaluate a patient s cct in context of the rest of the population , ie , percentile <3 , severely thin cornea ; percentile 310 , moderately thin ; percentile 1025 , mildly thin ; percentile 2575 average thickness ; percentile 7590 , mildly thick ; percentile 9097 , moderately thick ; and percentile > 97 , severely thick cornea .
our study shows that the mean cct of the adult lithuanian population is similar to the corneal thicknesses reported for other caucasian populations , and was within the limits of the mean cct defined in similar studies . a difference in cct between the sexes
was observed only in the youngest age group , indicating that young men have a higher cct than women . a weak negative relationship between cct and age
this age dependence was stronger in men ( a moderate negative correlation ) than in women ( a weak negative correlation ) .
these data will help to evaluate cct in our patients in a more practical and accurate manner .
 backgroundthe purpose of this study was to estimate mean central corneal thickness ( cct ) and determine whether there are any correlations between cct , age , and sex in the adult lithuanian population.methodsa total of 1,650 caucasians of lithuanian origin ( aged 1889 years ) comprising 688 ( 41.7% ) men and 962 ( 58.3% ) women were examined . 
 subjects were stratified by age into seven groups . 
 cct was measured using ultrasonic pachymetry . 
 correlations between cct , age , and sex were sought.resultsmean (  standard deviation ) cct for both eyes was 544.630.5 m . 
 mean cct was 545.230.5 m in the left eye and 544.630.5 m in the right eye , and was 545.025.6 m in men and 544.433.5 m in women . 
 mean cct was 550.835.7 m in subjects aged 1829 years , 557.527.6 m in those aged 3039 years , 551.331.4 m in those aged 5059 years , 544.031.4 m in those aged 5059 years , 544.231.6 m in those aged 6079 years , 535.127.8 m in those aged 7079 years , and 530.116.8 m in those aged 8089 years . 
 no statistically significant difference in cct was found between the sexes ( p>0.05 ) . however , there was a significant difference in subjects aged 1829 years ; men had higher cct than women ( p<0.05 ) . 
 a statistically significant negative correlation was found between cct and age ( r=-0.263 , p<0.05 ) that was stronger in men ( r=-0.406 , p<0.05 ) than in women ( r=-0.118 , p<0.05).conclusionthe mean cct in adult lithuanians was 544.630.5 m , of the left eye 545.230.5 m and of the right  544.630.5 m . 
 cct of the right eye was equal to the cct of both eyes . 
 mean cct was 545.025.6 m in men and 544.433.5 m in women . 
 young men tended to have higher cct than women . 
 cct decreases over the lifetime , meaning that older people have thinner corneas . 
 cct s dependence on age is stronger in men . 
Introduction
Materials and methods
Statistical analysis
Results
Discussion
Conclusion