HO RMS showed significant variation across the inferior visual field for both groups. The old group had significantly higher values than the young group at 10° and 20° eccentricity (p < 0.05) (Fig. 6).
This study shows that peripheral ocular aberrations increase with age across the horizontal and inferior visual fields. The result of our study is in agreement with the earlier study done by Mathur et al.33 We have also extended the findings along the horizontal visual field up to ± 40° in a larger sample of subjects. The changes in refraction components and aberration coefficients with age in the periphery are discussed below.
The peripheral refraction pattern was similar in both age groups along the horizontal and inferior visual field, which is in agreement with the earlier studies30–33 (Figs. 1 and 4). Age had a significant effect on J45 astigmatism. However, the amount of J45 astigmatism was only 0.22 D higher in the younger group (at nasal 40° and inferior 20°) than in the older group, which is a relatively small difference in the periphery.
As expected, RPRE showed myopic shift with no significant difference in both groups along the horizontal and inferior visual field. This finding is in line with earlier studies,38,39 which have shown emmetropes become relatively myopic in the periphery. The older subjects had slightly higher negative values of J180 astigmatism in the periphery than the younger group (−1.4 vs. −1.0 D at temporal 40°) along the horizontal visual field. However, there were no significant differences in J180 astigmatism between the groups in the inferior visual field. J180 astigmatism showed a significant quadratic increase with eccentricity in both groups along the horizontal visual field. Our results regarding J180 astigmatism agree with the results of Charman and Jennings30 study in which they reported slightly higher negative values in older eyes along the horizontal visual field. We also observed temporal-nasal asymmetry for RPRE and J180 along the horizontal visual field with greater changes in refraction for the nasal than for the temporal visual field in both groups.31 The nasal-temporal asymmetry is most probably attributable to the difference between the measurement axis (line of sight) and the eye's optical axis.40
In summary, the increases in peripheral refraction (lower order) with age across the horizontal and inferior visual field are relatively small. Even though we find no difference in peripheral refraction pattern between the two groups, it should be noted that refractive error in the periphery increases with eccentricity. Furthermore, it might be important to correct these peripheral refractive errors to achieve a better visual function in central visual field loss subjects.23–25
The dominant higher order aberration in the periphery was coma, and this varied significantly between the groups and across the field. The horizontal coma C3 1 along the horizontal visual field and vertical coma C3 −1 along the inferior visual field increased linearly in both groups in agreement with Seidel theory. Furthermore, the horizontal and vertical coma slopes were twice as large in the older group than the younger group. This finding is consistent with the previous study by Mathur et al.33 who also reported a rapid increase in coma slopes for older emmetropes. However, the reason for this rapid increase of coma slopes in older emmetropes is not clearly understood, and currently available eye models cannot fully reproduce the observed age-related changes in coma.33
What is intriguing is why coma should increase with age—is this because of corneal, lenticular, or other changes? A recent axial study by Berrio et al.41 had found that in young eyes, the positive corneal coma is balanced by the negative internal (lenticular) coma. However, this balance is partly lost in the older eyes leading to increase of ocular (whole eye) horizontal coma. In addition, Atchison42 measured peripheral corneal and ocular aberrations in a middle-aged group and found that the corneal third-order coefficients were higher than the ocular third-order coefficients, indicating that the internal third-order coefficients provided a degree of balance to the corneal third-order coefficients. The studies by Bierro et al.41 and Atchison42 show that the balance between the corneal and internal third-order coefficients is present both axially and peripherally for the young and middle-aged groups. Furthermore, on-axis studies have shown that this balance is lost with aging thereby increasing the magnitude of ocular aberrations in old eyes.15 We could assume that the balance between corneal and lenticular coma is lost with aging also in the periphery. The loss of this balance could lead to both cornea and lens contributing to the increase of peripheral ocular coma in old eyes. This could be a reason for the increase in coma slopes that we observe in our study. Moreover, the loss of balance may not only affect coma but also other aberration coefficients. However, this assumption of loss of balance between the corneal and lenticular aberrations, and the consequent increase in total ocular aberrations in the periphery with aging needs to be investigated.
Among the fourth-order coefficients, the peripheral spherical aberration was significantly higher in the old group than the young group along the horizontal and inferior visual fields. However, in our study, the on-axis spherical aberration between the young (0.009 ± 0.020 μm) and old (0.012 ± 0.038 μm) group for a 4 mm pupil showed no significant age dependence in agreement with previous studies with emmetropic subjects.32,43 Studies that reported increase in spherical aberration with age included large refraction ranges, which could have influenced their results.44,45 The remaining coefficients of the third- and fourth-order aberrations did not show any major variation between the groups.
HO RMS showed quadratic field dependence in both groups along the horizontal visual field. The young and old group showed on average 1.5 and 2.1 times increase in HO RMS from center to ± 20° eccentricity (horizontal and inferior). This shows that the older group had higher rate of change in HO RMS than the younger group across the horizontal and inferior visual field. These results are in agreement with the previous study by Mathur et al.,33 who also reported a similar increase of HO RMS in their old group for the 20° visual field. However, we have extended the horizontal visual field to ± 40° and therefore considering a larger part of the visual field, the young and old group showed on average 3.1 and 5.4 times increase in HO RMS from center to periphery. This suggests that the rate of change in HO RMS from ± 20° to ± 40° is much more rapid in both groups with greater increase in the older group than the young group. This increase in HO RMS across the horizontal visual field is mainly because of the increase in horizontal coma (compare Fig. 2C with Fig. 3).
Does the increase in higher order aberrations influence visual performance in the periphery for old subjects? A previous study has shown that this amount of increase in higher order aberrations degrades the peripheral image quality.33 In addition, there are also other optical factors such as chromatic aberration,46 pupillary miosis,7 and intraocular light scattering,11 which could add to or partly mask the effect of increased monochromatic higher order aberrations. Nevertheless, some of the earlier studies47,48 have concluded that poor peripheral visual performance in older subjects is primarily neural in origin and optical factors play a minor role. However, a recent study by Rosen et al.22 has demonstrated that low contrast resolution acuity declines with optical defocus in the periphery of healthy young subjects. Further studies are needed to ascertain whether the increased higher order aberrations in combination with lower order aberrations reduce peripheral visual performance in older healthy subjects. In particular, this amount of increase in higher order aberrations could further degrade the peripheral vision in older central visual field loss subjects, who use their preferred retinal locus for reading or other resolution tasks. To understand this better, correction of higher order aberrations and evaluation of the visual function should be performed in older subjects with central visual field loss.
In conclusion, the peripheral higher order aberrations increase with age particularly coma and spherical aberration. However, the question of whether this increase in higher order aberrations and the corresponding decrease in retinal image quality have any impact on peripheral vision needs to be investigated further.
We thank Sebastian Carlström, Simon Dahlberg, and Roger Fransson for assistance with subject recruitment. We also thank Baskar Theagarayan, Robert Rosén, and Linda Lundström for their input and suggestions regarding the manuscript.
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