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Higher-order ocular aberrations caused by crystalline lens waterclefts

Qu, Jingtao MD; Sasaki, Hiroshi MD; Sakamoto, Yasuo PhD; Kawakami, Yutaka MD; Sasaki, Kazuyuki MD; Jonasson, Fridbert MD

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Journal of Cataract & Refractive Surgery: May 2010 - Volume 36 - Issue 5 - p 799-805
doi: 10.1016/j.jcrs.2009.12.026
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Abstract

Opacification of the crystalline lens, generally defined as cataract, develops in different parts of the lens and can exist in various conformations.1 There are 3 main types of cataract: cortical, nuclear, and posterior subcapsular.2 Patients with cataract may report visual impairment, photophobia, or monocular diplopia. Several studies3–5 have established associations between impairment of visual function and increased nuclear cataract, cortical cataract, and posterior subcapsular cataract. However, in some patients, the symptoms of visual impairment do not parallel the expression of the 3 main types of cataract. Cataract subtypes, such as waterclefts and retrodots, may affect visual function.6

Waterclefts are fiber-based opacities that are usually located in the Y-sutural in the anterior superficial cortical zone or in both the anterior cortex and posterior cortex (Figure 1). The prevalence of waterclefts is fairly high in the elderly population. It has been suggested that waterclefts have the potential to cause visual impairment.6,7

Figure 1
Figure 1:
Waterclefts (arrows) in slit Scheimpflug (left) and retroillumination (right) photographs.

It remains difficult to differentiate cataracts from normal changes caused by aging. In addition, clinical practice varies in terms of which lens features ophthalmologists assess when deciding whether to offer cataract surgery. The introduction of wavefront analyzers into clinical practice has provided clinicians with effective, more definitive methods of assessing visual quality. In the present study, we analyzed higher-order aberrations (HOAs) in eyes with waterclefts.

SUBJECTS AND METHODS

A nested case-control study was performed. The host trial was the Reykjavik Eye Study (RES),8 a population-based prospective cohort survey. Baseline interviews and eye examinations occurred in September 1996. Follow-up examinations and interviews took place 5 years later, in September 2001, and 12 years later, in September 2008. Previous RES reports8–12 describe the general methods and examination protocols in detail and present the prevalence rates for some visual diseases that have an effect on vision.

The study population consisted of subjects from the host cohort with waterclefts in the right eye presenting for the third examination in the RES. To evaluate the relationship between wavefront aberration and waterclefts, eyes were excluded from the study if they had other types of lens opacity and coexisting ocular disease as follows: cortical, nuclear, or posterior subcapsular cataract grade 1 or higher (WHO Grading System13); retrodots; anterior subcapsular cataract; pterygium; pseudoexfoliation; suspected glaucoma; age-related maculopathy. A control group was used for comparision; criteria for inclusion in this group were a transparent lens in the right eye and no coexisting ocular disease.

Ocular Examination

All participants had complete ophthalmic examinations that included visual acuity, noncontact tonometry, slitlamp biomicroscopy, and dilated fundus evaluation. Pupils were dilated with tropicamide 0.5% and phenylephrine 0.5%. The same investigator diagnosed the types of lens opacification, including waterclefts, under the slitlamp with the pupil dilated. Wavefront analysis was performed with a KR9000PW wavefront analyzer (Topcon Corp.) with optical zones of 4.0 mm and 6.0 mm. Three types of HOA (coma, trefoil, spherical) and total ocular, corneal, and internal eye aberrations were calculated. All HOA values were determined from the component map installed in wavefront analyzer and were calculated from the corneal and ocular aberrations.

Statistical Analysis

Statistical analysis was performed using StatView software (version 5.0, SAS Institution, Inc.). The Mann-Whitney rank-sum test was used to compare age and corrected distance visual acuity (CDVA) between the watercleft group and the control group. Comparison of the magnitude of the total HOAs, trefoil, coma, and spherical aberrations between the 2 groups was by analysis of covariance (ANCOVA) adjusted for age as a continual variance. Analysis of covariance was used to assess the associations between age and HOAs in internal optics. A P value less than 0.05 was considered statistically significant.

RESULTS

In the 2008 RES study of 573 subjects, the prevalence of waterclefts in the 491 participants with phakic eyes was 21.8%. Table 1 shows the characteristics of the watercleft group and the control group in the present study.

Table 1
Table 1:
Subject demographics.

Table 2 shows the visual acuity and the root mean square (RMS) HOAs from the 3rd-order as well as coma, trefoil, spherical, and total aberrations with a 6.0 mm pupil. The mean CDVA was statistically significantly better in the control group than in the watercleft group (P = .002). Total HOAs were significantly different between the groups. Total ocular and internal trefoil aberrations were statistically significantly higher in the watercleft group, with differences in the RMS of 0.076 μm for total ocular and 0.066 μm for internal eye. Internal coma was statistically significantly higher in the watercleft group than in the control group. There were no statistically significant differences between the groups in total eye or internal eye spherical aberration.

Table 2
Table 2:
Visual acuity and wavefront analysis (6.0 mm pupil) by group.

Figure 2 shows the logMAR CDVA in both groups. The difference between the 2 groups was approximately 1 line.

Figure 2
Figure 2:
Box-and-whisker plot of logMAR CDVA in the watercleft group and the control group. Boxes are bounded by the 25th and 75th percentile values of the distribution of the results. The whiskers extend vertically from the end of the box to the 5th percentile and the 95th percentile.

Figure 3 shows the relationship between total ocular HOAs and age. In both groups, there was a statistically significant relationship between CDVA and total ocular HOAs (r = 0.405 and P = .026, watercleft group; r = 0.156 and P = .030, control group).

Figure 3
Figure 3:
Relationship between total ocular HOAs and CDVA (CDVA = corrected distance visual acuity; HOAs = higher-order aberrations).

Figure 4 shows the correlation between age and internal optic RMS HOAs in both groups. The internal total HOAs in the watercleft group increased significantly with age (r = 0.209, P = .04); no relationship with age was observed in the control group (P = .09). There was no significant relationship between age and trefoil or coma aberration in either group. However, the increase in total HOAs, coma, and trefoil aberration with aging was greater in the watercleft group than in the control group (P<.01). The internal spherical aberration in the watercleft group had no significant relationship with age; however, the increase with age in the control group was statistically significant (r = 0.189, P = .02). Despite these findings, there was no statistically significant difference between the 2 groups in the relationship between internal spherical aberration and age (P>.05).

Figure 4
Figure 4:
Graphs of the relationship between age and internal optical total HOA and each Zernike vector term.

DISCUSSION

In the 2008 RES study of 573 participants, the prevalence of waterclefts in the 491 participants with phakic eyes was 21.8%. This finding agrees with that of Deane et al.,7 who reported waterclefts in 17% of participants aged 55 to 74 years in an English population-based survey. Waterclefts are also age-related cataracts. In the present study, the mean age of the subjects was 66.8 years old (range 61 to 82 years), which is higher than that in Deane et al.'s study. Thus, the prevalence rate of waterclefts (ie, waterclefts combined with other types of lens opacity) was higher (72.0%) in our study. Durant et al.14 report that the body mass index, alcohol intake, vitamin status, sunlight, urea, creatinine, and uric acid are possible risk factors in the development of waterclefts.

Shun-Shin et al.15 suggest that changes at the membrane of the lens fiber tips may activate enzymes and inhibit the Na+/K+-ATPase pump (ie, sodium–potassium pump), resulting in waterclefts. Because some cortical cataracts and waterclefts may share an etiology and it has been suggested that cortical cataracts may be caused by disruption of the Ca2+ATPase pump (calcium pump), waterclefts may represent an early stage of cortical cataract development. Indeed, waterclefts can become opaque and appear similar to cortical cataracts.16

Cortical watercleft development occurs at double the frequency in the anterior lens cortex as in the posterior lens cortex. This may be due to the much greater flattening effect (with stronger shear force) on the anterior part of the lens than on the posterior part lens when going from the accommodated to the unaccommodated shape.17

Some waterclefts are barely visible on retroillumination; therefore, they previously escaped attention as potential causes of visual impairment. In our study, the CDVA in the watercleft group was significantly lower than in the control group with transparent lenses (P = .002). The mean decimal-equivalent visual acuity was 0.79 (0.103 ± 0.086 logMAR) in the watercleft group and 0.94 (0.022 ± 0.042 logMAR) in the control group. The difference between the 2 groups may not be considered clinically significant. However, in our clinical experience, patients with waterclefts, even those with a good CDVA, have a tendency toward decreased contrast visual acuity. This concurs with a study by Frost et al.,6 who suggest that waterclefts have the potential to cause visual impairment, with symptoms including monocular diplopia. However, until our study, the effect of waterclefts on vision remained largely speculative.

Ocular, corneal, and internal optic aberrations are reported to be significantly correlated with visual function in phakic eyes and pseudophakic eyes of elderly patients.18,19 In our study, visual acuity was significant correlated with total ocular HOAs in the watercleft group and in the control group.

Several studies have evaluated the HOAs in eyes with 1 of the 3 main types of cataract. Sachdev et al.20 found that cortical opacification induced an increase in coma and that nuclear opacification produced a decrease in spherical aberration compared with aberrations in eyes without opacities. In a study by Rocha et al.,21 coma predominated in the cortical cataract and spherical aberration in the nuclear cataract. However, to our knowledge, there are no previous reports of HOAs in eyes with waterclefts.

In the present study, we compared coma, spherical, and trefoil aberrations because they are located in the more central and upper areas of the pyramidal table of Zernike basis functions and tend to have more significant effects on visual function.22 There was no significant difference in corneal total, coma, trefoil, or spherical aberration between the watercleft group and the control group. However, ocular HOAs in the watercleft group were significantly higher than in the control group after adjusting for age. These results indicate that the crystalline lens responds to the increase in ocular aberration in eyes with waterclefts.

In the present study, internal optical total HOAs in the watercleft group increased with age; however, no relationships with age were observed in the control group. Also, internal total HOAs, coma, and trefoil aberrations increased more with age in the watercleft group than in the control group. These observations indicate that if the crystalline lens remains transparent, internal ocular HOAs would not change much with aging. However, in eyes with waterclefts, there was an increase in internal optical HOAs, and the increase may have been mainly induced by increases in trefoil and coma aberrations. In the control group, only internal spherical aberration was significantly related to aging. This finding concurs with a report of Amano et al.23 that ocular coma increases with age mainly because of an increase in corneal coma and that ocular spherical aberration increases with age mainly because of an increase in spherical aberration in the internal optics.

The increases in trefoil and coma may partly explain the monocular triplopia in eyes with waterclefts. In these eyes, the increases in internal optical coma and trefoil aberrations, combined with the increases in spherical aberrations (corneal and internal) with aging, can induce monocular triplopia (Figure 5). In the watercleft lens, an incident ray of light can meet many interfaces between the normal crystalline tissue and waterclefts. At each interface, the potential for scattering of light can affect visual function. From a clinical viewpoint, this partly explains why patients with waterclefts report reduced visual function even though their visual acuity is not too bad. Understanding the wavefront optical changes in eyes with watercleft may help ophthalmologists decide whether early surgical intervention is necessary in these cases.

Figure 5
Figure 5:
Monocular triplopia caused by waterclefts. A: Total ocular HOAs. B: Retinal simulation at visual acuity 20/40. C1 and C2: Scheimpflug photographs at 0 degrees and 90 degrees, respectively. D: Retroillumination photograph. E: Corneal and internal ocular total HOAs and Zernike maps.

A weakness of our study was that the age of subjects was higher in the watercleft group than in the control group. It is difficult to identify an age-matched group of subjects who are approximately 70 years old without any form of lens opacity and ocular disease. However, the statistical significance of the differences was adjusted for age between the 2 groups using ANCOVA.

In conclusion, waterclefts have a significant influence on CDVA. Eyes with waterclefts have higher rates of coma and trefoil aberrations. The increase in HOAs may be responsible for the decrease in visual acuity in eyes with waterclefts. Intraocular lens surgery may be beneficial in some patients with waterclefts only, especially elderly patients.

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