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Coma found to be predominant aberration in eyes with nuclear cataract
J Cataract Refract Surg. 2008;34(12):2104-2109.
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Most higher-order aberrations in eyes with nuclear cataracts were attributed to internal optics of the lens, with coma emerging as the prevalent aberration.
Investigators examined visual acuity, non-contact tonometry, slit lamp biomicroscopy, dilated fundus images and wavefront analysis of 33 eyes of 20 patients. The Lens Opacities Classification System III was used to grade lens opacity.
Data showed no significant correlation between grade of cataract and average root mean square of higher-order aberrations, coma and trefoil. However, spherical aberration in the internal optics shifted negatively as the grade of nuclear cataract increased.
"In nuclear cataract, the nuclear or central portion of the lens becomes selectively denser," the study authors said. "Therefore, the refractive index increases more at the center of the lens. This causes the central rays to focus more anteriorly and the peripheral rays to focus more posteriorly. For this reason, although there are reports that spherical aberration is positive in normal lenses, spherical aberration may exhibit a negative shift as a nuclear cataract progresses."
Negative spherical aberration of internal optics of the eye and the whole eye increases with the grade of nuclear cataract in dilated eyes. The authors suggest that the increase in negative spherical aberration could be due to the increased density and therefore increased refractive index of the cataractous nucleus. Another possibility that the authors do not mention is that negative spherical aberration could also be due to changes in the lens surface asphericity as a secondary consequence of the cataract.
In my opinion, it is unlikely that this would affect clinical practice. The measurements were done in dilated eyes. When the eyes are not dilated and the pupils are of a natural size, in this population of older subjects who generally have small pupils, the extent of the aberrations reported will be considerably less than is measured with dilated pupils. Therefore, through natural pupils, there the negative spherical aberration reported would likely be substantially reduced and therefore of little practical consequence for everyday vision.
– Adrian Glasser, PhD
OSN Optics Board Member