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Cataract Surgery | Apr 2010

Do Corneal Higher-Order Aberrations Increase Pseudoaccommodation?

The patient's innate level of apparent accommodation may influence the clinician's IOL recommendations.

Pseudoaccommodation, or apparent accommodation, has long been recognized as a phenomenon that occurs in some monofocal pseudophakic patients who are able to maintain good uncorrected distance and near vision with a single-focus IOL.1 Pseudoaccommodation is not pseudophakic accommodation of the IOL; as various studies have shown, there is little movement of conventional monofocal IOLs or change in anterior chamber depth when the patient focuses between distance and near targets.2,3 Several smaller studies have demonstrated that more than 50% of patients implanted with the AcrySof IQ SN60WF (Alcon Laboratories, Inc., Fort Worth, Texas) achieved a distance UCVA of 20/30 or better and maintained decent near vision (ie, Jaeger 4 or better).4,5

The ability to maintain good near vision has been attributed to factors including smaller pupil size, against-the-rule corneal astigmatism, corneal multifocality, and depth of focus.6-8 Postrefractive–surgery patients have also demonstrated enhanced near visual acuity and greater measured accommodative range,9,10 further supporting a direct relationship between corneal multifocality and pseudoaccommodation.

Upon evaluation of other corneal factors contributing to pseudoaccommodation, Oshika and colleagues subjectively demonstrated that coma-like aberrations of the cornea significantly correlated with apparent accommodation. 11 To better delineate the relationship of corneal higher- order aberrations to depth of focus in pseudophakic patients, we prospectively calculated the accommodative amplitude and depth of focus in 25 eyes implanted with the monofocal SN60WF.

At 1 month postoperative, the mean UCVA was 20/25, the mean distance BCVA was 20/20, and the mean distance-corrected near visual acuity (DCNVA) at 40 cm was J7. However, the range for DCNVA was wide (J2 to J12). The average accommodative amplitude, as determined by a subjective push-down method, was 1.16 ±0.64 D (range, 0.49–2.37 D), and the mean pupil size on accommodation was 3.1 mm.

Third- to sixth-order aberrations of the central 4 mm of the cornea and their potential contribution to the overall depth of focus were calculated with a theoretical model (Zernike Tool Program; Abbott Medical Optics Inc., Santa Ana, California). First, the effect of corneal higher-order aberrations on corneal image quality was determined by the polychromatic modulation transfer function (PMTF). The depth of focus was then calculated by determining how adding defocus affected the PMTF.

Early results show that corneal higher-order aberrations potentially provide a depth of focus between 0.57 and 1.33 D. As noted earlier, the average accommodative amplitude is approximately 1.16 D. Although these results are promising, it is too soon and the sample size too small to determine if a correlation exists between accommodative amplitude and depth of focus from higher-order aberrations.

Ultimately, our goal is to determine the specific corneal higher-order aberrations, if any, that contribute to depth of focus and corneal pseudoaccommodation in the cataract population and in eyes following myopic and hyperopic corneal refractive surgery. We are currently evaluating the corneal topographies of more than 400 such eyes. Clinical investigation of pseudophakic accommodative amplitude and higher-order aberrations is still under way.

Knowing a patient's innate level of pseudoaccommodation could influence the clinician's recommendation for monofocal, accommodating, or multifocal IOLs. Given that current presbyopia-correcting technologies possess different limitations, the potentially useful information provided by these studies could help surgeons' efforts to better predict and guide patients toward different presbyopia-correcting options.

Douglas D. Koch, MD, is a Professor and the Allen, Mosbacher, and Law Chair in Ophthalmology at the Cullen Eye Institute of the Baylor College of Medicine, Houston. Dr. Koch states that he has no financial interest in the products or companies mentioned. Dr. Koch may be reached at tel: +1 713 798 6443; e-mail: dkoch@bcm.tmc.edu.

Li Wang, MD, PhD, is an Assistant Professor in Ophthalmology at the Cullen Eye Institute of the Baylor College of Medicine, Houston. Dr. Wang states that she has no financial interest in the products or companies mentioned. Dr. Wang may be reached at tel: +1 713 798 7946; e-mail: liw@bcm.tmc.edu.

Elizabeth Yeu, MD, is an Assistant Professor of Ophthalmology, Baylor College of Medicine, Houston. Dr. Yeu states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +1 713 798 5143; fax: +1 713 798 3027; e-mail: yeu@bcm.tmc.edu.