Introduction
Although the achievement of uncorrected 20/20 visual acuity remains a laudable target for any cataract or refractive surgeon, the goal of high-quality vision increasingly reflects our understanding of the visual system as a whole. In fact, Snellen acuity represents only a small portion of functional vision. A comparison of vision and hearing highlights the limitations of standard visual acuity tests: the auditory equivalent of a standard high-contrast Snellen eye chart would be a hearing test with only one high level of loudness for all sound frequencies. Today, contrast sensitivity testing is emerging as a more comprehensive measure of vision that will probably replace Snellen letter acuity testing, just as audiometric testing replaced the "click" and spoken-word tests used prior to the 1940s (Slide 1).1
Engineers understand that Fourier analysis allows the representation of any visual object as a composite of sinewaves of various frequencies, amplitudes and orientations. In fact, visual processing in the human nervous system works like Fourier analysis in reverse, with functionally independent neural channels filtering images to create what we see.2 Thus, sinewave gratings are the building blocks of vision, just as pure tones are the building blocks of audition.
Ophthalmologists realize that patients may complain about haziness, glare and poor night vision despite 20/20 Snellen acuity. This anomaly can be understood when one realizes that the Snellen acuity letter recognition test involves very high contrast. The jet black letters on the bright white background have a great deal of reserve contrast, so that even a patient with severely reduced contrast sensitivity can still read the chart. That patient perceives the letters as gray on white rather than black on white, but is still able to recognize them. The examiner has no way of knowing how gray the letters look to any particular patient. Thus, Snellen acuity is a relatively insensitive test of visual function.
Contrast sensitivity testing has the ability to detect differences in functional vision when Snellen visual acuity measurements cannot.3 For example, a patient with loss of low frequency contrast sensitivity may be able to read 20/20 but may be unable to see a truck in the fog. Although blur due to refractive error alone affects only the higher spatial frequencies, scatter of light due to corneal or lenticular opacities causes loss at all frequencies. Glaucoma and other optic neuropathies generally produce loss in the middle and low frequencies. Contrast sensitivity testing thus offers critical information to help explain patients’ complaints.
Numerous studies have demonstrated the relationship of contrast sensitivity and visual performance. Contrast sensitivity has consistently been found to be significantly correlated with visual performance including driving difficulty4 and crash involvement,5 falls6 and postural stability in the elderly,7 activities of daily living and visual impairment,8 and the performance of pilots in aircraft simulators.9
Contrast Sensitivity Declines with Age
It is unfortunate that contrast sensitivity declines with age, even in patients who do not have ocular pathology such as cataract, glaucoma or macular degeneration (Slide 2). The pathogenesis of this decline in vision likely involves changes in the spherical aberration of the crystalline lens.
Spherical aberration is a property of spherical lenses. A spherical lens does not refract all parallel rays of incoming light to a single focal point. The lens bends peripheral rays more strongly so that these rays cross the optical axis in front of the paraxial rays. As the aperture of the lens increases the average focal point moves towards the lens, so that a larger pupil produces greater spherical aberration.
Spherical aberration of the cornea changes little with age. However, total wavefront aberration of the eye increases more than threefold between 20 and 70 years of age.10 Wavefront aberration measurements combined with data from corneal topography demonstrate that the optical characteristics of the youthful crystalline lens compensate for aberrations in the cornea, reducing total aberration in younger people (Slide 3, Slide 4, Slide 5). It is unfortunate that the aging lens loses its balance with the cornea, as both the magnitude and the sign of its spherical aberration change significantly (Slide 6).11 Thus, a loss of balance between corneal and lenticular spherical aberration causes the degradation of optical quality in an aging eye (Slide 7, Slide 8, Slide 9).
The sinewave grating contrast sensitivity of a pseudophakic patient is no better than that of a phakic patient of a similar age who has no cataract. When a 65-year-old patient with cataracts undergoes cataract extraction and is implanted with spherical IOLs, the resulting visual outcome is no better than the visual quality of a 65-year-old patient without cataracts (Slide 2). The fact that the visual quality of the patients with IOLs is no better than that of their same-age counterparts may seem surprising because an IOL is optically superior to the natural crystalline lens. However, this paradox is explained when one realizes that the IOL has positive spherical aberration like the aging lens. It is not the optical quality of the IOL in isolation that creates the image, but the optical quality of the IOL in conjunction with the optical quality of the cornea.
The spherical aberration of a manufactured spherical IOL is in no better balance with the cornea than the spherical aberration of the aging crystalline lens (Slide 10). Aberrations cause incoming light that would otherwise be focused to a point to be blurred, which in turn causes a reduction in visual quality. This reduction in quality is more severe under low luminance conditions because ocular aberrations increase when the pupil size gets larger.
New Technology
The Tecnis Z9000 IOL (Pfizer, New York, NY), which is currently undergoing FDA-monitored clinical trials in the United States, has been designed with a modified prolate anterior surface to compensate for the spherical aberration of the cornea (Slide 11). I (MP) am the principal investigator for this multicenter, randomized, evaluator-masked clinical investigation now enrolling subjects at five sites. The Tecnis Z9000 shares basic design features with the CeeOn Edge 911 (Pfizer), including a 6-mm biconvex square-edge silicone optic and angulated cap C polyvinylidene fluoride (PVDF) haptics. The essential new feature of the Tecnis IOL, the modified prolate anterior surface, acts like the youthful crystalline lens and compensates for corneal spherical aberration. The new concept of the Z9000 has the potential for restoration of youthful optical quality and improvement of functional vision.
The principal outcome measurement for the clinical investigation consists of sinewave contrast sensitivity. The Hartmann-Shack wavefront sensor will provide information on optical aberrations, while driving simulation studies will examine the functional benefit of this new IOL technology. Theoretical calculations and optical bench measurements support the hypothesis of improved contrast sensitivity with the Tecnis IOL (Slide 12).
A study performed by Ulrich Mester, MD, that was reported at the American Society of Cataract and Refractive Surgery Symposium ("Improved Optical and Visual Quality with Aspheric IOL," Philadelphia, June 2, 2002), has compared the quality of vision obtained with the Tecnis IOL and the SI40 (AMO, Santa Ana, CA), which is a spherical silicone IOL. Forty-five patients were enrolled and randomized to receive the Tecnis IOL in one eye and the SI40 in the fellow eye. The average photopic contrast sensitivity values demonstrated a statistically significant advantage for the Tecnis IOL at all spatial frequencies (Slide 13). The contrast sensitivity curves show an even greater difference under mesopic conditions (Slide 14), which was an expected result due to the larger pupil size and consequent greater contribution from spherical aberration in dim light. A comparison of corneal and total ocular aberrations demonstrates the improved wavefront of the eye with the Tecnis IOL (Slide 15). This improvement in total aberrations demonstrates the critical compensatory relationship of cornea and lens in reducing spherical aberration.
We have compared peak contrast sensitivity in healthy, normal eyes, stratified by age of patient, with eyes implanted with either the Tecnis IOL or the AR40e (AMO), which is an acrylic spherical IOL, and presented the results at the European Society of Cataract and Refractive Surgery Symposium ("Quality of Vision with a Modified Anterior Prolate Aspheric Intraocular Lens," Nice, September 11, 2002). We have given special emphasis to the peak contrast sensitivity values because they have, in general, the highest degree of correlation with functional vision. For example, as mentioned above, peak contrast sensitivity correlates well with pilots’ detection and identification distances.
As expected, we found that mesopic contrast sensitivity declined with age. Among 69 eyes of 36 patients, ranging in age from 21 to 61 years, we found mean peak mesopic contrast sensitivity at 3 cycles per degree of 72.4 for the 20- to 30-year-old patients. Patients aged 30 to 50 years demonstrated mean peak mesopic contrast sensitivity of 51.9. Ten eyes implanted with the Tecnis IOL in patients of average age 69.5 years achieved a mean peak mesopic contrast sensitivity at 3 cycles per degree of 83.8, better than the 20 to 30 year old group. Meanwhile, 11 eyes implanted with the control IOL in patients with an average age of 69.4 years demonstrated a mean peak mesopic contrast sensitivity at 3 cycles per degree of 47.1, which was worse than the 30- to 50-year-old age group (Slide 16).
As advances in technology allow cataract and refractive surgeons to address higher order optical aberrations, the measurement of functional vision becomes increasingly critical as a gauge of our progress. Sinewave contrast sensitivity testing is assuming a prominent place in our evaluation of surgical modalities because it reflects functional vision, correlates with visual performance and provides a key to understanding optical and visual processing of images. The Tecnis Z9000 represents a first step towards the integration of wavefront technology and lens-based surgery.
Posted December 2003
References
- Ginsburg AP. The Evaluation of Contact Lenses and Refractive Surgery Using Contrast Sensitivity, in Contact Lenses: Update 2. Orlando, Fla: Grune & Stratton, Inc.; 1987:56.5.
- Spillman L, Wooten DR, eds. Visual form perception based on biological filtering.
In: Sensory Experience, Adaptation and Perception. Hillsdale, NJ: Lawrence Erlbaum Associates; 1984.
- Evans DW, Ginsburg AP. Contrast sensitivity predicts age-related differences in highway sign discriminability. Human Factors. 1985;27(12):637.
- McGwin G Jr, Chapman V, Owsley C. Visual risk factors for driving difficulty among older drivers. Accid Anal Prev. 2000;32(6):735-744.
- Owsley C, Stalvey BT, Wells J, Sloane ME, McGwin G Jr. Visual risk factors for crash involvement in older drivers with cataract. Arch Ophthalmol. 2001;119(6):881-887.
- Lord SR, Dayhew J. Visual risk factors for falls in older people. J Am Geriatr Soc. 2001;49(5):508-515.
- Lord SR, Menz HB. Visual contributions to postural stability in older adults. Gerontology. 2000;46(6):306-310.
- Rubin GS, Bandeen-Roche K, Huang GH, Munoz B, Schein OD, Fried LP, West SK. The association of multiple visual impairments with self-reported visual disability: SEE project. Invest Ophthalmol Vis Sci. 2001;42(1):64-72.
- Ginsburg AP, Evans DW, Sekule R, Harp SA. Contrast sensitivity predicts pilots’ performance in aircraft simulators. Am J Optom Physiol Opt. 1982;59(1):105-109.
- Artal P, Berrio E, Guirao A, Piers P. Contribution of the cornea and internal surfaces to the change of ocular aberrations with age. J Opt Soc Am A Opt Image Sci Vis. 2002;19(1):137-143.
- Glasser A, Campbell MC. Presbyopia and the optical changes in the human crystalline lens with age. Vision Res. 1998;38(2): 209-229.