Presbyopic lens exchange (PRELEX) is a procedure that encompasses both a refractive lens exchange and the implantation of a multifocal IOL. When introduced more than 15 years ago, multifocal IOLs did not produce very good results. Back then, surgical techniques were not suited to the requirements of these IOLs. For example, extracapsular cataract extraction was still the procedure of choice and mainly done without a capsulorhexis, which led to IOL decentration and a loss of multifocality. Additionally, the large incision caused significant astigmatism and reduced UCVA.
Today, multifocal IOLs are a refractive surgery tool designed to free patients from their spectacles. However, patients must desire this independence and be willing to accept the visual side effects that may occur with multifocal IOLs, such as halos and glare, especially at night. Because these lenses are designed to provide good UCVA at all distances, they are superior to monofocal IOLs or an aged, presbyopic human lens.
SIMULTANEOUS VERSUS ALTERNATING VISION
Multifocal IOLs represent a compromise based upon the simultaneous vision principle. With simultaneous vision, two or more images are formed on the retina at the same time, one image at near and the other at distant focus. The brain then selects the image it wants to see. Simultaneous vision involves some loss of image contrast. Spectacles or the crystalline lens in young adults provides alternating vision, meaning that only the object fixated upon is in focus and all incoming light is directed to this focus. In alternating vision, the lens accommodates with a change in focus or a change of spectacles. Contrast sensitivity is high with alternating vision. This basic difference between monofocal and multifocal IOLs explains why the latter are preferable for bilateral implantaion. The brain uses image data from both eyes to create a summation of image contrast. Bilateral implantation and image processing will therefore recover some of the loss of contrast inherent to simultaneous vision.
THE REZOOM IOL
The Rezoom IOL (Advanced Medical Optics, Inc., Santa Ana, CA), a three-piece acrylic multifocal IOL (Figure 1), was developed as a successor to the Array IOL (Advanced Medical Optics, Inc.), which was the first FDA-approved multifocal IOL. Recently approved by the FDA, the Rezoom features five optical zones devoted to distance and near vision: (1) a central distance zone, (2) a paracentral near-dominant zone, (3) another distance zone, (4) another near-dominant zone, and (5) a peripheral distance zone (Figure 1). The zones provide good distance vision under all light levels (pupil sizes).
The Rezoom IOL is a distance-vision–dominant multifocal IOL; the distant focus has a higher image contrast than the near focus, because more incoming light is directed to distance versus near focus. With a small pupil, about 80% of light is directed to the distance focus, and approximately 20% to the near focus. With a 5-mm pupil, approximately 60% of light is directed to the distance, 30% to the near, and 10% to the intermediate focus. This light-distribution advantage is that distance vision is not compromised even in low-light conditions, such as night driving. Reading vision is sufficient, because individual light levels can be adjusted easily.
Like the Acrysof Restor IOL (Alcon Laboratories, Inc., Fort Worth, TX), the Rezoom features a sharp edge to inhibit posterior capsular opacification. The Rezoom IOL is a refractive multifocal IOL, whereas the Acrysof Restor IOL is a diffractive multifocal IOL. Another difference between the designs is that the near add is 3.50D in the Rezoom IOL, resulting in about 2.80D at the spectacle plane, versus 4.00D in the Restor IOL. This means that the optimal reading distance is closer with the Restor than with the Rezoom IOL.
The ideal patient for PRELEX is of presbyopic or pre-presbyopic age (eg, 40 years or older). The best candidates are hyperopes, second-best are high myopes (more than -6.00D), third-best are emmetropic presbyopes, and the least likely candidates are low myopes. The Rezoom IOL's availability in a wide range of powers allows its use for most refractive errors.
Patients should be willing to undergo bilateral implantation, with the second lens procedure performed usually 3 to 8 days after the first. Visual function should be about the same in both eyes; there should be no amblyopia, maculopathy, diabetic retinopathy, glaucomatous damage to the optic nerve, or corneal diseases such as keratoconus. Pupil size should be normal. Patients should be informed about the potential risks of intraocular surgery as compared with other alternatives (eg, LASIK). In addition, they must understand that they may not achieve independence from their glasses on the first try. Other procedures, such as astigmatic keratotomy or LASIK, may be needed to correct residual refractive errors.
I prefer the Rezoom IOL because of its lower near add versus that of the Restor and its distance dominance. Most patients undergoing PRELEX are still working and, in my experience, prefer a somewhat greater reading and working (computer) distance than that achieved with the Restor IOL. The distance dominance provides excellent twilight vision while maintaining sufficient near vision.
The lens power calculation is critical. I recommend using optical axial length measurements such as those obtained with the IOLMaster (Carl Zeiss Meditec Inc., Dublin, CA), because they are more accurate in ametropic eyes. I use a 3.2-mm posterior limbal incision centered on the steepest corneal meridian (which equals the axis of the plus-cylinder). If that patient has more than 1.00D of astigmatism, I also perform limbal relaxing incisions. I use standard phacoemulsification. The size of the capsulorhexis should be slightly smaller than the IOL's optic to ensure overlap of the anterior capsule and avoid decentration by capsular contraction. I use the injector system from Advanced Medical Optics, Inc., to implant the Rezoom lens.
Patients' distance vision is usually good as early as
1 day postoperatively. Near vision, however, is more likely to need a couple of weeks to improve. Most patients will report some halos or ghosting, especially in dim lighting and when looking at lights. These phenomena are caused by the multifocality of the lens and basically represent the second image. These occurrences lead to a slight reduction in contrast vision and may cause some night-driving difficulties. Usually, these incidents will lessen within a few weeks. Very few patients will experience persistent ghosting.
As with other refractive surgery procedures, it is important to counsel the patient preoperatively and perform examinations on unhappy patients regularly. In most individuals, results will continue to improve in a couple of weeks. Any residual refractive error can easily be corrected with LASIK approximately 3 months after PRELEX surgery. I inform all patients about this option prior to surgery.
UNIQUE FEATURES OF PRELEX
PRELEX, as a refractive surgical procedure, “sells” spectacle independence. Surgeons therefore must be ready to perform additional surgery, usually LASIK or Epi-LASIK, if emmetropia is not achieved. Multifocal IOLs require a far more accurate refractive result, because even small amounts of ametropia will affect image quality far more significantly than with a monofocal IOL (if a surgeon can imagine the two images of a multifocal IOL each blurred by 0.75D of astigmatism and superimpose the two blurred images, then he will appreciate why multifocal IOLs are more critical to defocus).
PRELEX also requires a clear posterior capsule. Due to the reasons mentioned earlier, multifocal IOLs are extremely sensitive to light's scattering due to even minute amounts of capsular opacification. One must therefore be ready to perform a YAG capsulotomy as soon as a patient complains about poor near or distance vision due to small amounts of opacification.
PRELEX with the Rezoom multifocal IOL corrects presbyopia and ametropia. The lens is a valuable addition to LASIK or phakic IOL treatments for presbyopic patients. Its predictability is comparable to LASIK. In the few patients who may have residual refractive errors, LASIK may be used as an enhancement to increase spectacle independence. PRELEX is also the only surgical procedure available today that can maintain stereopsis without glasses and correct presbyopia. Monovision is an alternative, but it will impair stereopsis.
Michael C. Knorz, MD, is Medical Director of the FreeVis LASIK Center Mannheim and Professor of Ophthalmology at the Faculty of Clinical Medicine Mannheim of the University of Heidelberg, Germany. He is a consultant to Advanced Medical Optics, Inc. Dr. Knorz may be reached at +49 621 383 3410; firstname.lastname@example.org.