Corneal spherical aberration in Saudi population

Abstract

Purpose

To find out the mean corneal spherical aberration and its changes with age in Saudi population.

Setting

AlHokama Eye Specialist Center, Riyadh, Saudi Arabia.

Methods

Three hundred (300) eyes of 185 Saudi subjects (97 men and 88 women), whose age ranged from 15 to 85 years old, with matched refractive errors, were divided into three groups according to their age, 100 for each. All the subjects were included in measuring the spherical aberration (SA) using pentacam HR (OCULUS, Germany) at the 6-mm optical zone.

Results

The mean corneal spherical aberration (CSA) of the fourth order (Z₄⁰) of the whole groups was 0.252 ± 0.1154 μm. Patients from 15 to 35 years old have root mean square (RMS) of CSA of 0.2068 ± 0.07151 μm, 0.2370 ± 0.08023 μm was the RMS of CSA of the patients from 35 to 50 years old, while those from 50 to 85 years old have a CSA-RMS of 0.31511 ± 0.1503 μm (P < 0.0001). A positive correlation was found between the spherical aberration (Z₄⁰) and the progress of age (r = 0.3429, P < 0.0001). The high order aberration (HOA) presented 28.1% of the total corneal aberrations. While the fourth order corneal spherical aberration constituted 57% of the HOA and 16% of the total aberration. The pupil diameter shows a negative correlation with the increase in age (P = 0.0012).

Conclusion

Our results showed a CSA (Z₄⁰) that is varied among the population, comparable to other studies, and significantly correlates to the progress of age.

Introduction

In the current era of recent corneal and phaco-refractive surgeries, thorough understanding of the human corneal topography and its inter-individual variations is becoming an ultimate need to know how corneal shape may influence postoperative outcome, and the resulting visual performance. Wavefront analysis of the ocular optical system has increased our knowledge of the aberrations in the eye.¹ Using Zernike polynomials, the aberrations of the ocular system can be characterized.¹ The total higher-order aberrations of the phakic eye are composed of aberrations arising from the anterior and the posterior corneal surfaces, the crystalline lens and the retina.² In the aphakic eye, however, 98.2 percent of the aberrations arise from the anterior corneal surface.⁴

Among the large number of Zernike polynomials, Zernike coefficient for spherical aberration (SA), a symmetrical higher-order optical aberration has been found to be linked to contrast visual acuity; as this value increases, contrast sensitivity has been found to decrease,^5,8 and greater amounts of haloes and glare are induced, it has been identified as a key contributor to the deterioration of image quality, night myopia and photic complaints.⁹ The cornea has positive SA in 90% of the cases and negative spherical aberration in the remaining 10%.³ Corneal positive SA is reported to be approximately +0.27 μm for a diameter of 6 mm with a large standard deviation of 0.10 μm,^9,10 it is compensated mainly by the negative spherical aberration of the crystalline lens which shifts toward positive values with aging⁷, so no compensatory effect is expected to occur in the elderly; on the contrary, the adverse effects of an increased spherical aberration are expected to worsen with age,^9–11 as the best contrast sensitivity has been measured in young patients, aged 20–30 years, while it worsens thereafter.⁹

Time related changes in the aberration profile of the cornea plays a detrimental effect in post operative visual quality in refractive and cataract surgeries. Age changes may even affect post refractive patients as they are getting older. Earlier studies depending on calculation of aberrations from placido-based corneal videokeratographers suggest that unlike coma aberrations, SA does not vary significantly with aging.¹² However later studies using specialized aberrometers reported either a significant or non significant increase in 4th order ocular aberrations with age specially SA, yet they expected minor changes to occur on the cornea.^13,14 Others report that corneal spherical aberration becomes slightly larger with normal aging.^15–17

For practical purposes, spherical aberration comes into play when pupils are greater than 4 mm; thus, it has the most impact under mesopic or scotopic conditions and in younger patients.¹⁸

Ethnic variation is thought to play an important role in interpersonal variability of SA. Corneal and ocular spherical aberrations in South East Asian Chinese eyes were significantly greater than that those in other populations.¹⁹ However, Japanese patients showed a relatively smaller corneal spherical aberration than previous studies of Caucasians.²⁰ Some other studies found no clear variation between different ethnic groups.^21–23 A controversy is present whether SA is correlated to ocular biometric measures especially axial length based on ethnic variation.^21,24 Population differences in wavefront errors should be noted in patient management, and individual evaluation of spherical aberration should be done for each patient.

Patients and methods

This cross sectional study included three hundred eyes of 185 Saudi subjects (97 men and 88 women), whose ages ranged from 15–85 years old, with comparable spherical equivalent of their refractive errors. The study was conducted according to the tenets of declarations of Helsinki in a central anterior segment referral clinic and received the approval of the institute. All the subjects signed a comprehensive written consent prior to participation in the study.

Subjects were divided into 3 groups according to their ages, 15 to <35, 35 to <50 and 50 to 85 years. Patients with history of contact lense wear, keratorefractive surgery, existing ocular or systemic pathologies were excluded. Otherwise, all patients with clear cornea, no scarring nor pigmentation were included in the study. Corneal aberrations of the 300 eyes were obtained with Pentacam HR (OCULUS, Germany), without dilatation in dark for 6 mm optical zone. Comparisons of corneal spherical aberrations (CSA) root mean square (RMS), total and high order corneal aberrations RMS up to 6th order were made between the 3 different age groups.

Statistical analysis

Statistical analysis of the results was done by Microsoft Excel, Graphpad prism, and Instat. P-value of <0.01 was considered extremely significant.

Results

Topographic and wavefront data from 300 eyes of 185 consecutive patients were obtained for analysis. Mean patient age was 42.873 ± 16.588 years (range: 17–85 years).

Our results demonstrate the RMS of CSA of the fourth order $\left(Z_4^0\right)$ in the three age groups at 6-mm optical zone. Patients from 15–35 years old have CSA-RMS of +0.2068 ± 0.07151 μm. 35–50 years old patients have RMS of CSA of +0.2370 ± 0.08023 μm, while those from 50 to 85 years old have a CSA-RMS of +0.31511 ± 0.1503 μm. (Fig. 1, Table 1)

Figure 1.

SA in Different age groups.

Table 1.

Range (minimum, maximum), and mean of the fourth, sixth order spherical aberration, HOA, and total RMS.

	(15–35) years	(35–50) years	(50–85) years	P-value
Z40 RMS (range)	0.053–0.388	0.054–0.433	0.017–0.708
(mean ± SD)	0.2068 ± 0.07151	0.23406 ± 0.07824	0.31511 ± 0.1503	P = 0.7238
Z60 RMS (range)	−0.06–0.07	−0.049–0.099	−0.2090–0.315
(mean ± SD)	0.01354 ± 0.01739	0.01259 ± 0.02308	−0.00183 ± 0.0669	P = 0.1075
HOA RMS (range)	0.2090–0.5980	0.248–1.137	0.269–2.513
(mean ± SD)	0.35603 ± 0.0866	0.39441 ± 0.1247	0.7102 ± 0.3883	P = 0.9994
Total RMS (range)	0.627–3.849	0.552–3.671	0.696–8.074
(mean ± SD)	1.47233 ± 0.749	1.65671 ± 0.7655	2.62003 ± 1.44	P = 0.3463

In all age groups, total CSA (4th and 6th) ranged from −0.091 to +0.733 μm, with a mean RMS of +0.2609 ± 0.1103 μm.

The overall mean corneal spherical aberration (Z₄⁰) of the whole 3 groups was +0.252 ± 0.1154 μm ranging from +0.017 to +0.708 μm, and it correlates positively with the progress of age (r = 0.3429, P value is <0.0001) (Fig. 2).

Figure 2.

Correlation of fourth order corneal spherical aberration with age.

On the other hand, the corneal spherical aberration of the sixth order (Z₆⁰) (Table 1) decreases very slightly with age (r = −0.1855, P value = 0.0012) (Fig. 3).

Figure 3.

Relationship between the sixth order corneal spherical aberration and age.

In our study, RMS of high order aberration (HOA) presented 28% of the total corneal aberrations while the fourth order CSA constituted 57% of the high order aberrations and 16% of the total (low and high order) aberrations of the cornea (Fig. 4).

Figure 4.

Percents of different aberration components in the study group to high order and other aberrations.

Fourth order CSA decreases slightly with increasing pupil diameter, (r = −0.1890, P = 0.0010 in all groups together) (Fig. 5), which in turn decreases with increase in age (r = −0.1858, P = 0.0012) (Fig. 6).The mean pupil diameter for the whole group was 2.9 ± 0.50 mm under mesopic conditions. There was no correlation between SA and pupil diameter in the younger age group of 15–35 years (r = 0.0013), and a negligible negative correlation was found in the two other groups (r = −0.1799 for the middle, −r = 0.1035 for the elderly age groups, p < 0.001).

Figure 5.

Low negative relationship between the pupil diameter and the (Z₄⁰) spherical aberration (r = −0.1890, P = 0.001).

Figure 6.

Variation of pupil diameter among age groups. (P = 0.0012).

Total and high ordered aberrations were highly positively correlated with the progress of age (r = 0.4352, r = 0.5596, respectively) with p value of <0.0001 for both (Figs. 7 and 8).

Figure 7.

High positive correlation between total corneal aberrations and the progress of age (r = 0.4352, P < 0.0001).

Figure 8.

High positive correlation between the high ordered aberrations and the progress of age (r = 0.5596, P < 0.0001).

Discussion

Since the principle of correcting spherical aberration was advocated in 2002,²⁵ researchers were trying to find the best methods for application of this concept in optical dispensing aids to improve contrast sensitivity and vision quality especially postoperatively. It is well accepted that aberrations change with age,^6,9 Another concern is that corneal spherical aberrations vary considerably among populations.

Further changes occur with the replacement of the crystalline lens by an artificial IOL during cataract surgery.^10,12 Postoperative cataract patients who benefited from the recent advances in instrumentations and operative techniques failed to attain good contrast sensitivity despite 20/20 vision when compared to young phakics unless proper manipulation of spherical aberration is taken into account intra-operatively ²⁶. Thus, for cataract patients, it is possible to measure the corneal SA preoperatively and then use this data to manipulate the outcome of cataract surgery by implantation of an aspheric intraocular lens, with the goal of achieving an optimum SA for the eye and maximum contrast sensitivity.²⁷ On the other hand, refractive surgery also modifies the aberration pattern.^13,16

McLellan et al. and Artal et al’s^13,14 cross-sectional studies on normal subjects revealed a considerable increase in total ocular third order and spherical aberrations with aging, yet they did not expect great corneal contribution in this. Oshika and colleagues¹² found a significant increase in 3rd order corneal aberration terms only, while Guirao and colleagues¹⁶ found that spherical aberration was also slightly larger for older corneas.

The 300 eyes of the current study having forth order corneal spherical aberration ranged from +0.0170 to +0.708 μm which was quite close to Wang et al.,¹¹ and Shimozono M.,²⁰ who found individual variability in corneal spherical aberration that ranged from +0.069 to +0.511 μm and −0.103 to +0.497 μm, respectively.

In our study, the mean RMS of CSA of the fourth order for the three groups collectively was +0.252 ± 0.1154 μm, which correlated positively with the progress of age (r = 0.3429, P value is <0.0001). This agrees strongly with most of the previously reported studies like that of Holladay et al.,¹⁰ whose mean corneal spherical aberration measured with Orbscan unit, was +0.27 ± 0.02 μm, and Packer M,^26,27 who found mean CSA RMS of +0.26 ± 0.089 μm for patients ranged from 62 to 86 years. This value is also consistent with Beiko et al.⁹ who evaluated the corneal spherical aberration in 708 patients (39–92 years), with the Easygraph, and found CSA RMS of +0.274 ± 0.095 μm, and Solomon JD’s ²⁸ study of Forty patients (67–82 years) whose 6.0-mm pupil CSA RMS was +0.260 ± 0.084 μm.

Slightly higher values were detected by Beiko et al. ¹⁸ and Li Z.H. ²⁹ who found average corneal (Z₄⁰) of +0.32 ± 0.075 μm and +0.294 ± 0.138 μm respectively for their older groups of patients ranging from 50–90 years.

The large standard deviation of CSA in our study; ±0.1154 μm agrees with that of Wang et al., ¹¹ and Beiko⁹ who stated that although the average corneal SA for the population is +0.27 μm, the standard deviation is large and approaches 0.10 μm, or one-third of the value. The implication of this wide spread of the value of the SA is that it cannot be assumed that the individual patient undergoing surgery has the “average” values.

Older researches like Oshika et al’s¹² used calculation methods to detect corneal aberrations from Placido-based videokeratography. They stated that spherical-like aberrations did not show any age-related changes for both 3- and 7-mm pupils, implying that the central-to-peripheral balance of the corneal curvature is not significantly affected by increasing age. However, recent advances in corneal photography revealed that the radius of the cornea changes with time to render the anterior cornea more spherical than ellipsoid, adding more spherical aberrations to the anterior corneal system.¹⁶ In the current study a statistically significant increase in fourth order CSA was noted as the age group of the patients increases, and a positive correlation was found between CSA and age (r = 0.3429, P value is <0.0001). This was also proven in a recent study by Li Z.H.,²⁹ who found a linear positive correlation between the mean CSA (Z₄⁰) and age (r = 0.203, P < 0.003) in their 93 patients aging from 50 to 89 years. Our slightly higher positive correlation is due to inclusion of younger age group (<35 years) that constituted 33.3% of the study patients as the increase in positive corneal aberration in elderly is attributed to increase in oblateness of the eyes where the central cornea tends to be flatter in relation to the periphery with time.

Spherical aberration is known to increase in magnitude as the effective pupil diameter increases, this correlation seems to be masked by the narrow pupils and/or senile miosis as found in the presented study. For practical purposes, spherical aberration comes into play when pupils are larger than 4 mm; thus, it has the most impact under scotopic conditions and in younger patients.^4,9,30,31 Randazzo and colleagues have estimated that SA increases by 100% for every 1 mm of increased pupil diameter above 5 mm.³² Mrochen et al. showed that even in young phakic eyes the critical pupil size at which the optical aberrations of the eye deteriorate the quality of the retinal image, is 3.3 mm for 50% and 4.5 mm for 89% of the eyes.³⁴ In the current study 66% of the patients had a mesopic pupil diameter of less than 3 mm due to the illumination recording conditions of the pentacam machine,³⁵ that masked the direct relation of increasing SA with pupil diameter increase. Certainly, this acquires its clinical importance in aspheric IOL practice where miotic pupil states (e.g. senile) might limit the beneficial effect of aspheric IOLs.^36,37

In fact, there is further evidence of a racial difference in ocular and corneal SA; in our study of Saudi population it was found to be 0.252 ± 0.1154 μm, where HOA constituted 28% of the total RMS which is higher than the conventionally known ratio of 10% ³⁸ and positively correlates to age (r = 0.5596, p < 0.0001) which was proven by many previous studies.^13,14,33,40 In other studies, Asian eyes have been measured to have an average corneal SA of +0.37 μm,³⁹ the same for South East Asian Chinese eye whose corneal and ocular aberrations were significantly greater than that reported in other populations, corneal spherical aberration was greater than ocular spherical aberration (mean 0.312 ± 0.114 μm versus 0.200 ± 0.170 μm),¹⁹ Cervino et al.,²¹ concluded that ethnicity is strongly linked to ocular (Z₄⁰), with British Asian subjects having less primary spherical aberration than Caucasians, although there were no significant differences in HO RMS between the two ethnic groups. On the other hand Nakano et al.,²³ found great differences in total and individual wavefront aberrations between the Asian and non-Asian patients from Brazil and those measured in US studies⁴⁰ where Asian people have considerably higher more than 1.4X the values of US students, with no significant differences between the two Asian groups. Although the differences were attributed partly to differences in measuring techniques, it still indicates that various ethnic and regional populations may have differing levels of higher order aberrations that must be considered during patient screening and surgical treatment planning.

In our sample of patients, we provided normative values for corneal wavefront error with special emphasis on spherical aberrations in a subset of the population that present a wide range of ages. This study was the first in our knowledge to document the CSA and its changes with age in Saudi population. The values are especially pertinent for refractive surgery centers, and will help us in improving the cornea and cataract practice in the future.

Conflict of interest

The authors declared that there is no conflict of interest.