Surgical presbyopia-correction techniques can be divided into three broad categories: (1) treatments that induce multifocality, such as presby-LASIK, Intracor/Supracor, conductive keratoplasty, and clear lens extraction with implantation of a multifocal or accommodating IOL, (2) monovision LASIK, and (3) corneal inlays. Although each category has a unique mechanism of action, some of the same considerations can be applied when performing cataract surgery in a patient who has previously undergone one of these treatments.
As with any patient, selecting the appropriate IOL and target refraction is a key component of successful cataract surgery after presbyopia correction. Even before these considerations, however, the most important factor is to evaluate psychological factors and expectations including the patient’s personality traits and visual goals. Failing to uncover certain expectations could lead to patient dissatisfaction and a lengthy postoperative counseling process.
A REVIEW OF THE CATEGORIES
The surgeon must also consider the effects of the presbyopia-correcting surgery in his or her cataract surgery treatment plan. Because these effects are different depending on the presbyopia treatment, below is a review of special considerations for each category.
Multifocal treatments. These procedures, including presby-LASIK, Intracor/Supracor, and conductive keratoplasty, change the shape of the cornea and consequently create multifocality. However, when multifocality is produced, there is a directly proportional loss in contrast sensitivity—in other words, the greater the multifocality, the greater the contrast sensitivity loss. Therefore, a multifocal IOL is always contraindicated to avoid further loss in contrast sensitivity in these patients. However, the multifocality from the original procedure will still provide the same amount of presbyopic correction.
The best strategy for these procedures is to choose a monofocal aspheric lens that best matches the patient’s individual corneal spherical aberration. This can be done based on corneal topography. Using this strategy leaves the optics the same as they were after presbyopia correction, and, therefore, the patient should achieve a similar visual outcome. One thing that will change in the patient’s optical system, however, is that there is no longer any ability to accommodate since the crystalline lens is gone. Fortunately, by the time the patient needs cataract surgery (average age, 72 years), the crystalline lens has lost almost all of its ability to accommodate, so the difference from an IOL is minimal. I always explain to these patients that near vision will be closer to what it was prior to cataract surgery than what it was immediately after their presbyopia-correction procedure.
The target refraction in the distance-dominant eye should be from 0.00 to -0.25 D and approximately 0.25 D more myopic in the nondominant eye.
Monovision LASIK. If a patient underwent monovision LASIK for presbyopia correction 4 or 5 years ago or more, there is a good chance that significant positive spherical aberration still remains in the eye. Just as with patients who have had a corneal multifocal presbyopiacorrecting treatment, the best way to calculate the individual corneal spherical aberration is with corneal topography. The selected aspheric monofocal IOL should have enough negative spherical aberration to either match the patient’s individual corneal spherical aberration or leave the patient with a little bit of negative spherical aberration (no more than -0.30 μm) to produce the best visual outcome. The latter strategy can increase depth of focus and thereby provide patients with a slight increase in near visual acuity.
In the majority of patients, targeting the distance eye for -0.25 D and the other eye somewhere around -1.50 D provides the best chance for spectacle independence after cataract surgery. With an interocular difference greater than 1.50 D, however, binocular vision in terms of depth perception may be affected slightly. The worst case is that, in extremely low light with very small print (ie, candlelight dinner with a menu with J1++ print), the patient may need some weak readers.
Corneal inlays. Implanted inside a corneal pocket, the corneal inlay addresses presbyopia in one of three ways: (1) by changing the central refractive index, which is the mechanism of the Flexivue Microlens (Presbia), (2) by reshaping the central cornea, which is the mechanism of the Vue+ (Revision Optics), or (3) by increasing depth of focus using a pinhole effect, which is the mechanism of the Kamra corneal inlay (AcuFocus, Inc.).
The first two methods are similar to the multifocal treatments described above, because they create a multifocality in the cornea. Consequently, a monofocal aspheric IOL is the best choice here as well. Since the Vue+ changes the central shape of the cornea, however, using an IOL with no spherical aberration (positive or negative) is the best choice to have the least effect on the corneal performance. With the Kamra, the central aperture of 1.6 mm results in no significant difference between an aspheric and spherical IOL in optical performance. The best target refraction in eyes with a Kamra corneal inlay is -0.75 D in the in the eye with the inlay and about -0.25 D in the other. This provides the patient with the best binocular performance.
LENS POWER CALCULATION
Keeping in mind that the target refraction for the distance eye with previous presbyopia correction should be -0.25 D, several considerations come into play when selecting a lens power calculation.
Consideration No. 1: Measure the axial length. Optical coherence tomography (OCT) or optical biometry (IOLMaster [Carl Zeiss Meditec] and Lenstar [Haag-Streit]) should be used to measure the axial length. Several studies have confirmed that OCT is more accurate than ultrasound because it measures to the fovea and accounts for retinal thickness, two things that ultrasound cannot do.1,2
Consideration No. 2: Choose the best method for keratometry. Manual keratometry (K), autokeratometry, topography (Placido-disc imaging), and tomography (Scheimpflug imaging) provide similar results in a virgin cornea. However, after refractive surgery, the corneal profile changes—as much as 2.00 to 3.00 D across the corneal surface—and therefore K readings across these technologies vary. Autokeratometry is the least accurate when used in an irregular cornea, as it measures only one ring around the central 3 to 4 mm of the cornea. Topography and tomography are more accurate because they measure the total zonal power across the entire cornea. Recent studies have shown that the power of the posterior cornea contributes to the total corneal power.3-5
Consideration No. 3: Not all IOL power calculation formulas are equal. All contemporary IOL power calculations use a vergence formula, which was developed by Johann Karl Friedrich Gauss circa 1840. Unlike regression formulas, vergence formulas take into account the nonlinear relationship of axial length and corneal power to the power of the IOL. Two-variable predictor vergence formulas include the Holladay 1, Hoffer-Q, SRK/T, and Haigis. The first three of these use keratometry and axial length, and the Haigis formula uses axial length and anterior chamber depth.
Other variables that can help predict effective lens position more precisely include anterior chamber depth, lens thickness, corneal diameter, the patient’s age, and the patient’s refraction before cataract surgery.6-21 These seven variables are used in the Holladay 2 formula. Olsen’s recent formula also uses more than two variables, taking into account K readings, axial length, anterior chamber depth, lens thickness, and refraction.
The bottom line is that the more variables that are measured, the better the prediction of effective lens position will be.
Consideration No. 4: Optimize the surgeon constant. A-constants suggested by the manufacturer should be used only as a starting point. After the first 20 or 30 cases with a new IOL, the surgeon should optimize his or her personal constant depending on personal surgical results, as continued use of the suggested manufacturer’s global average A-constant will lead to suboptimal postoperative results.
Reasons for differences in the individual surgeon lens constants are many. Incision construction varies from surgeon to surgeon, and everyone has his or her own way of implanting the IOL and performing irrigation and aspiration. Additionally, some surgeons inject an ophthalmic viscosurgical device behind the lens while others do not. All of these subtle differences ultimately affect where the haptics end up in the bag and where the optic sits in the eye. Also, it is unlikely that the surgeon has all of the exact instruments used by the manufacturer to determine the global average lens constant. Therefore, every surgeon should optimize his or her lens constant, eliminating any errors that can result from surgical technique.
To review: Use OCT to measure axial length, measure all of the seven variables (axial length, K, anterior chamber depth, lens thickness, white-to-white, age, and refraction) for lens power calculation, and personalize your IOL constant. If you do these things, you will end up with the best possible postoperative outcomes in patients who have previously undergone presbyopia correction.
A final note is simply this: No matter how closely a surgeon follows the considerations outlined above, some of these postpresbyopia correction eyes will result in a larger refractive surprise after cataract surgery than is typically seen in a virgin cornea (same as postrefractive surgery). Therefore, it is crucial to prepare the patient for the potential of a postoperative touch-up to correct residual refractive error. For the monovision LASIK patient, it may be as simple as lifting the flap and performing an enhancement. For the presby-LASIK patient, however, it is harder to duplicate the original treatment. Secondary lens implantation (see Versatility and Safety of a Supplementary IOL, page 41, for more information) and lens exchange are possible options for enhancement.
Postoperative enhancements, whether they involve refractive surgery or a lens-based option, should be chosen considering the best option for the patient.
Jack T. Holladay, MD, MSEE, FACS, is a Clinical Professor of Ophthalmology, Baylor College of Medicine, in Houston. Dr. Holladay states that he is a consultant to Abbott Medical Optics Inc., Alcon Laboratories, Inc., AcuFocus, Inc., and Oculus Optikgeräte GmbH. He may be reached at fax: +1 713 669 9153; e-mail: email@example.com; website: www.hicsoap.com.
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