Introduction

With increasing age, human organs have some changes and begin to lose some balance, including eyes. Previous articles have reported that according to corneal microstructure, two changes were identified with age: (1) an increase in the cross-sectional area associated with each molecule in corneal collagen, which may be due to an increase in nonenzymatic cross-linking between collagen molecules, and (2) a decrease in the stroma interfibrillar spacing, which could be related to changes in the proteoglycan composition of the interfibrillar matrix. [1] These changes suggest that corneal elasticity and rigidity may change with aging, which may also cause refractive changes in the cornea. Ocular response analyzer and aberrometer are instruments used to observe corneal biomechanical properties and aberration changes; we used these instruments on people older than 65 years and younger than 35 years, aim in order to investigate corneal biomechanical and high order aberration changes with aging.

Material and Methods

This is a prospective observational cross-sectional study. 60 patients were recruited from Zhong Shan ophthalmic center, both eyes were included. All subjects were healthy people without systematic diseases such as hypertension, diabetes and hyperthyroidism. They were grouped based on age. 30 people older than 65 years were group one, 30 people younger than 35 years were group two. Patients from group one had no ophthalmic disease other than refractive error or cataract. Patients from group two had no ophthalmic disease other than refractive error, and their myopia was lower than -6.00D, hyperopia lower than +2.00D, astigmatism lower than ±2.00D. 
Ultrasound was used to obtain central corneal thickness(CCT). Reichert ocular response analyzer(ORA, Reichert Ophthalmic Instruments, Depew, NY) was used to obtain corneal resistance factor(CRF) and corneal hysteresis(CH), Goldmann-correlated intraocular pressure (IOPg) and corneal correlated intraocular pressure(IOPcc) were also recorded, IOPcc was used as intraocular pressure in our analysis. The reason why we used IOPcc but not IOPg for statistical analysis was that IOPg is considered to be influenced a lot by corneal properties such as corneal thickness, many studies showed that IOPcc is less affected by corneal characteristics and is a parameter which reflects the real IOP better. [2] Corneal topography (OPTICON 2000, KERATRON, ROMA-ITALY) was used to get third and fourth order aberrations of the anterior surface of cornea. All measurements were performed at the same time of day to decrease the effect of the diurnal curve. All measurements were performed by a single doctor. Central corneal thickness was measured 8 times per eye and averaged. ORA was performed several times and the best score value was chosen for analysis (waveform score at least 7 [3]). Aberrometry was performed three times and averaged, aberration at 3mm and 6mm was used in statistics. All statistical analyses were performed using SPSS software (SPSS 18.0), The normal distribution data results are expressed as mean±standard deviation, and p﹤0.05 is considered statistically significant.

Results

120 eyes (60 people) were analyzed. In group one there were 15 men and 15 women, in group two there were 14 men and 16 women. The mean ages of group one and two were 71.30±5.18 (ranging from 65 to 81years) and 24.60±4.13 (ranging from 18 to 34 years old) respectively. Demographic data like gender, IOPg, IOPcc, corneal curvature, pupil size, central corneal thickness are shown in Table 1.1 and Table 1.2. All data shows no significant differences between the two groups except IOPcc and central corneal thickness. Table 2 shows CH, CRF, 3mm and 6mm astigmatism, coma, trefoil, spherical aberration, secondary astigmatism, quatrefoil between the two groups. There are significant differences for CH and CRF between different age groups (p﹤0.05). There are no significant changes for 3mm and 6mm coma ,3mm spherical aberration between different age groups (p＞0.05), but significant differences for 6mm spherical aberration, 3mm and 6mm trefoil, secondary astigmatism and quatrefoil between different age groups (p﹤0.05). There are no significant differences for all parameters between left and right eyes, and between women and men.

Because our age data are not continuous variables, Pearson correlation is not used in the analysis between biomechanical parameters，aberration and age. According to the correlation between biomechanical parameters and IOPcc, central corneal thickness, the relation between CH, CRF and CCT was found to be linear, regression analysis result are shown in Figure 1 and Table 3, thinner cornea is correlated to lower CH (r=0.356, p﹤0.001) and CRF (r=0.5, p﹤0.0001), IOPcc is found to have negative correlation with CH (r=-0.651, p﹤0.001) but not correlated with CRF (r=-0.059, p=0.552) as shown in Figure 2. Multiple linear regression analysis results are shown in Table 4, The explanatory variables relevant to the CH are the CCT (Standardized Coefficients=0.39, p﹤0.001), and the IOPcc (Standardized Coefficients=-0.67, p﹤0.001 ).

Table 1.1: Demographic data

Table 1.2: Demographic data
a Data are expressed as mean +- SD
b*The star symbol indicates a significant difference at p<0.05

Table 2: Comparaison of corneal biomechanical parameter, 3mm, 6mm corneal aberrations between two groups
a Data are expressed as mean +- SD
b*The star symbol indicates a significant difference at p<0.05

Fig. 1: The correlation between central corneal thickness and CH, CRF
 

Fig. 2: The correlation between IOPcc thickness and CH, CRF

Table 4: Results of multiple regression analysis to select variables relevant to the CH

Discussion

Corneal biomechanical changes with aging 

The Ocular Response Analyzer (ORA) was created in 2005 and this instrument provides a new diagnostic tool that enables noncontact measurement of corneal biomechanical properties like CH and CRH. [4] Corneal hysteresis (CH) is a dynamic measure of the viscous damping in corneal tissue, which represents the energy absorption capability of the cornea, and corneal resistance factor (CRF) is an indicator of the total corneal response, including the elastic resistance of corneal tissue. [5] 

As shown in our results, a significant difference of CH and CRF was found between young and old groups. CH and CRF seem to be lower in older eyes. This result is consistent with some earlier studies.[5, 6, 7] Experimental ex vivo and in vivo studies have shown an age-related change in corneal collagen fibril properties that may contribute to an increased stiffness but decreased viscoelastic properties of the cornea with age, [8, 9, 10, 11] This increase in stiffness could in part be related to the reported increase in collagen fibril diameter and cross-sectional area with age.[1] Simultaneous and gradual diminution of PGs and GAGs of the viscous ground substance occurs，resulting in decreased CH and CRF [5, 11, 12, 13]. This result can explain why corneal laceration occurs so easily in old people due to trauma. Many studies reported significant but weak linear correlations between age and CH, CRF. In their report, The CRF declined significantly with age at a rate of 0.29-0.31 mmHg/decade, as did CH by 0.24-0.34 mmHg/decade.[4] But we have to notice that in our data, IOP and CCT also play a part in biomechanical parameter’ change. Linear regression analysis showed that CH and CRF are positively dependent on corneal thickness. IOPcc was found to have a negative correlation with CH but to have no correlation with CRF in our results. It is a pity that our age data was not continuous so we could not explore the linear correlations, but as CCT and IOPcc can only explain 56.8% of CH changes, CCT can explain 25% of CRH changes, we believe age can also partially explain corneal biomechanical changes. 

It is interesting that in our result, corneal thickness and IOPcc was significantly different between young and old groups, old people seemed to have thinner corneas and higher intraocular pressure. Some studies have also reported such results, in Lekskul M’s report, an increase in age would decrease the CCT 0.28µm/decade.[14, 15, 16] But this problem is still controversial. Siu A reported the corneal thickness of 108 human subjects, and no significant differences were found in the thicknesses of the central, mid-peripheral or peripheral cornea with increasing age. [17] They considered other things than a gender unbalance may have resulted in an artificial increase in corneal thickness due to hormonal actions in the younger premenopausal female population, but this cannot explain our findings because gender between young and old groups in our study had no significant differences. They also speculated that decreases in hydration with aging may cause an increase of ultrasound velocity, which may give an artificially thin corneal thickness reading, but this speculation did not have enough evidence, so whether corneal thickness really decreases with age needs more rigorously designed experiments to confirm. Does IOP change with aging? This is also a controversial question. Positive [18, 19], negative [20, 21], or no correlation [22] has been shown in different studies. The ethnic group, age range and IOP measurement varied and Goldmann tonometry or non-contact tonometry were used in these studies. As IOP is a complicated parameter that can be affected by many properties like corneal thickness, blood pressure, age, body mass index (IMB) etc. It is hard to say if IOP and age have a certain correlation. In our study we found that unlike IOPcc, IOPg have no significant differences between two groups, Thin corneas can cause an underestimated IOPg, which may explain this difference. 

High order aberrations change with aging

The corneal anterior surface accounts for about two-thirds of the optical power of the eye. This important role can be defined in terms of corneal shape, regularity, and clarity and is a function of its refractive index.[1] Our study indicated that 3mm, 6mm coma, 3mm spherical aberration will not change with aging while 6mm spherical aberration, 3mm and 6mm trefoil, secondary astigmatism, quatrefoil decrease with aging. Aberration values had large inter-subject variability. Former studies showed some different results, Feng-ju Zhang et al found that only corneal horizontal coma increases with age, but the people they studied were younger than 50 years[23]. Antonio Guirao et al found a significant increase in coma, spherical aberration and other high order aberrations, which was almost the same as our results, the different results about coma may be due to different devices used to measure corneal aberration, they also reported a large variation of aberration values and the correlation between coma and age in their result was weak(r=0.26) [24]. 

The increase of high order aberration may be due to 3 factors: (1). Tear film changes. It has been proved that there is a decline in tear film function throughout life and tear film will became unstable with aging [25, 26]. Several clinical studies support the hypothesis that tear film disruption may bring on the irregularity of the cornea, increase corneal aberration and reduce retinal image quality.[27, 28, 29] (2). Lid pressure. There has been conjecture that lid pressure may cause corneal distortion,[30] blepharochalasis and age related ptosis are common in the elderly, which may induce an increased aberration in the cornea. (3). Corneal structure changes. As age increases, the cornea become more asymmetric and unsmooth, the collagen fibril and stroma interfibrillar spacing may become nonuniform, these will all cause scatter of light and lead to an increase in high order aberrations.[31, 32] 

In conclusion, compared with people younger than 35 years old, we found that IOP, 6mm spherical aberration, 3mm and 6mm trefoil, second astigmatism, quatrefoil increase in people older than 65 years, while CCT, CH and CRF decrease. Older people seemed to have more fragile corneas and a decrease of corneal image quality.