Assessing the quality of vision after cataract surgery is a topic of great interest. The time-honored method of assessing visual acuity using the Snellen chart has come into question, especially relative to objective ways of measuring true visual quality. In the heyday of intracapsular cataract surgery, before the advent of modern intraocular lenses (IOLs), many patients ended up with a visual acuity of 20/20 on the standard Snellen eye chart postoperatively yet had marked problems with activities of daily life from aberrations induced by the aphakic spectacle correction. Refinements to the basic Snellen acuity chart were made; eg, the Early Treatment of Diabetic Retinopathy Study charts, which were developed to evaluate diabetic study patients. Although this method provides more detailed information than Snellen acuity, it does not give an accurate measurement of the functional vision attained by patients.
Other ways of assessing visual function include contrast sensitivity charts, which provide a more realistic assessment of the patient's quality of vision. This is especially important with the advent of multifocal IOLs with near and distance corrections. Once again, patients with multifocal IOLs and 20/20 Snellen acuity were found to have problems with their functional vision, as demonstrated on contrast sensitivity charts.
Recently, aberrations within the visual system in general have been studied. Instruments that measure aberrations of the entire system—beyond simple spherical and cylindrical corrections—have been developed. Higher-order aberrations (HOAs) in the optical system have been found to increase with age.1,2 While the cornea has a positive spherical aberration that does not change significantly throughout life, the crystalline lens in a young person compensates for this with a negative spherical aberration. As the eye ages, the negative spherical aberration of the lens is reduced considerably by an increase in aberrations in the total visual system, which can lead to slow degradation of the optical quality.
There is great interest in combining visual-aberration-sensing equipment with the excimer laser to correct HOAs in patients having excimer laser treatment for refractive errors. This wavefront-guided laser surgery may improve the quality of vision following corneal laser surgery.
The issue of HOAs is also important when assessing quality of vision following phacoemulsification with IOL implantation. Although removal of the cloudy cataract with placement of an IOL can greatly improve the visual acuity measured by Snellen charts, most currently available IOLs have a surface contour that does not counteract the positive spherical aberration of the cornea. In this issue, Vilarrodona and coauthors (pages 571–575) present the results of a study analyzing HOAs in patients who had phacoemulsification with implantation of various standard IOLs. The study showed that HOAs significantly increased in patients with acrylic IOLs with various curvatures compared to patients with silicone and poly(methyl methacrylate) IOLs.
Findings of HOAs in phakic and pseudophakic eyes have stimulated the development of IOLs that will decrease HOAs following cataract surgery. One example (Tecnis [Pfizer]) is a silicone IOL with a modified “prolate” surface.3 Several studies show superior contrast sensitivity and low-contrast visual acuity with the aspheric lens.4,5 Wavefront measurements also show fewer aberrations in patients with a prolate lens than in those with a standard lens. Some studies have looked at retinal image contrast as well as functional vision, comparing aspheric IOLs to standard silicone and acrylic IOLs. Again, the aspheric IOL shows a significant improvement in contrast sensitivity, as well as significant improvement in retinal image contrast.6 Despite these results, many questions about the relationship between this type of IOL and the overall visual system, including the quality of vision, have to be answered.7
Future developments may help correct HOAs in the visual system in patients having cataract surgery as well as patients with previous corneal refractive surgery in addition to cataract. These may include the use of customized IOLs, which take into account aberrations within the patient's visual system. Another possible solution is the use of a light-adjustable IOL in which a wavefront pattern is placed on the IOL after implantation by a digitally driven laser light delivery system.
While the clinical significance of the correction of HOAs within the visual system must be established, this is a potential area for further refinement of the quality of vision after cataract extraction.
1. Kuroda T, Fujikado T, Ninomiya S, et al. Effect of aging on ocular light scatter and higher order aberrations. J Refract Surg 2002; 18:S598-S602
2. Kuroda T, Fujikado T, Maeda N, et al. Wavefront analysis in eyes with nuclear or cortical cataract. Am J Ophthalmol 2002; 134:1-9
3. Holladay JT, Piers PA, Koranyi G, et al. A new intraocular lens design to reduce spherical aberration of pseudophakic eyes. J Refract Surg 2002; 18:683-691
4. Packer M, Fine IH, Hoffman RS, Piers PA. Prospective randomized trial of an anterior surface modified prolate intraocular lens. J Refract Surg 2002; 18:692-696
5. Mester U, Dillinger P, Anterist N. Impact of a modified optic design on visual function: clinical comparative study. J Cataract Refract Surg 2003; 29:652-660
6. Kershner RM. Retinal image contrast and functional visual performance with aspheric, silicone, and acrylic intraocular lenses: prospective evaluation. J Cataract Refract Surg 2003; 29:1684-1694
7. Kohnen T. Aberration-correcting intraocular lenses [editorial]. J Cataract Refract Surg 2003; 29:627-628