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Overview of Refractive Lens Exchange

Kristin Neatrour, MD; Lisa Sitterson, MD; and George Waring IV, MD, FACS

Compared with corneal-based refractive surgery, lens-based surgery not only offers the advantages of correcting a patient’s refractive error, but it has the distinct advantages of improving image quality when lens opacity exists, addressing presbyopia at the source, and obviating the need for future cataract surgery. Presbyopia is caused by age-related thickening and stiffening of the crystalline lens with gradual loss of accommodative function over time. While corneal refractive surgery can correct a patient’s refractive error, it does not halt the progression of presbyopia. As lens changes progress with age, patients will have some degradation of visual quality and function despite spectacle independence postoperatively after corneal-based surgery. This chapter provides an overview of lens-based surgery, specifically refractive lens exchange (RLE), and describes the relevant surgical indications, techniques, and diagnostic and intraoperative technology. In RLE, the presbyopic, or dysfunctional, crystalline lens is removed either manually or by using femtosecond laser–assisted surgery and replaced with an intraocular lens (IOL) implant. RLE has also been referred to in the literature as dysfunctional lens replacement and clear lens extraction or exchange.

DYSFUNCTIONAL LENS SYNDROME

Loss of accommodative function of the crystalline lens typically occurs in the late 40s and early 50s. Visually and clinically significant opacification of the lens typically occurs later in life. By studying the anatomy and biomechanics of accommodation, we have a much better understanding of the anatomical and physiologic changes that occur. Ciliary muscle contraction results in anterior and centripetal displacement of the ciliary body and causes an increase in optical power of the lens due to curvature and thickness changes. There are also accommodative changes in the zonular fiber configuration and support, axial length, corneal higher order aberrations, scleral contour, choroid, retina, aqueous, and vitreous. With presbyopia, the lens and capsule thicken and stiffen, and the anterior curvature steepens. Further age-related changes in the other extralenticular structures also contribute to reduced accommodation.¹

The term dysfunctional lens syndrome (DLS) refers to a normal aging phenomenon and progressive constellation of a triad of signs and symptoms. The triad of the dysfunctional lens is:

1. The loss of accommodation from presbyopia
   
2. Early lens opacities
   
3. Increased higher order aberrations

Figure 8-1 depicts the 3 stages of DLS. With RLE, patients are able to optimize their long-term visual performance well before the onset of a visually and clinically significant cataract.¹

SURGICAL INDICATIONS FOR REFRACTIVE LENS EXCHANGE

RLE is indicated for patients stages 2 and 3 DLS, or stage 1 with moderate to high degrees of hyperopia, with the goal of achieving emmetropia (or near emmetropia based on the desired target) and reasonable spectacle independence. A successful outcome in RLE hinges on careful consideration of the indications, risk-benefit ratio, and appropriate patient selection. General indications for RLE include refractive error, varying degrees of presbyopia, lens opacification, higher order aberrations, and desire for spectacle-independence.

For patients with presbyopia, RLE offers the potential for desirable functional binocular vision at multiple focal points for improved visual performance at distance, intermediate, and near. Specific considerations need to be made based on a patient’s age, refractive error, degree of presbyopia and lens opacity, corneal topography, goals for spectacle independence and functionality, ocular history, dry eye status, and subjective complaints.

Hyperopia and Short Axial Length

Moderate to high degree of hyperopia is an indication for RLE, especially in patients who have a shallow anterior chamber that predisposes them to the risk of developing angle-closure glaucoma. Our group has previously reported anterior segment parameters of patients with at least 1.5 diopters (D) of hyperopia pre- and postoperatively after laser–assisted cataract surgery to determine the surgical effects. In this study, surgery significantly increased the iridocorneal angle and the anterior chamber volume.²

Myopia

The primary refractive surgery options for myopia include LASIK, SMILE, photorefractive keratectomy (PRK), phakic IOL implantation, and RLE. The optimal procedure depends on age, degree of refractive error, presbyopia status, and ocular anatomy. For a younger myope who is a poor candidate for corneal-based refractive surgery, phakic IOL implantation may be a better option as it will preserve accommodation before the progression of presbyopia.

Caution should be exercised with RLE in high myopes. In the preoperative evaluation of high myopes, a retina evaluation is recommended. The major complication concern in RLE is the risk of pseudophakic retinal detachments (RD). Younger age (less than 50 years old), male sex, family history of RD, personal history of RD, high axial length, history of ocular trauma, predisposing retinal lesions, and posterior capsular tear are all associated with a higher risk of pseudophakic RD. The risk of RD after RLE or cataract surgery in high myopes varies from 0% to 8% based on multiple studies. In one study, the incidence of RD (2.7%) was twice that in unoperated myopic eyes and in nonmyopic eyes undergoing standard phacoemulsification. The surgical options should be considered within the context of the patient’s age, axial length, and vitreoretinal interface status. According to the American Academy of Ophthalmology’s preferred practice guidelines, prophylactic laser retinopexy is indicated for acute symptomatic horseshoe tears or dialysis and traumatic holes. Myopic patients who have not yet developed a posterior vitreous detachment may be at increased risk for RD.

Astigmatism

In patients with regular astigmatism, the primary methods of treating astigmatism at the time of RLE include performing limbal relaxing incisions (LRIs), astigmatic keratotomy, or implanting a toric IOL. The procedure of choice depends largely on the magnitude and location of the steep axis. Technological considerations for astigmatism management are discussed later in this chapter.

Irregular Astigmatism

Irregular astigmatism is approached differently and on a case-by-case basis. In patients with keratoconus, RLE and implantation of a toric IOL may be appropriate if the corneal topography displays radial symmetry in the effective optical zone, which is the central 3.0-mm zone. If there is radial asymmetry within the effective optical zone, then we recommend conservative astigmatic treatment targeting the lowest astigmatism difference between the keratometric axes in this zone. With this method, the total astigmatism may be undertreated, but it will provide the most optimal optical quality. A toric IOL is not indicated for patients with keratoconus who plan to continue wearing rigid gas-permeable contact lenses or who plan to undergo a corneal transplant or other corneal procedure in the future. In patients who have undergone corneal crosslinking, the ongoing keratometric changes are often unpredictable. For this reason, we recommend caution with performing LRIs or using toric IOLs to treat the astigmatism in patients who have undergone corneal crosslinking.

During cataract surgery for patients with Fuchs’ dystrophy, minimal phaco energy should be used to avoid further endothelial cell loss. RLE may be of benefit if done earlier in the disease process before more significant endothelial loss occurs because when a true cataract develops, higher amounts of phacoemulsification energy and surgical manipulation will be required. Our group has previously reported a case series of patients with mild to moderate Fuchs’ dystrophy undergoing femtosecond laser–assisted cataract surgery (FLACS) that were evaluated pre- and postoperatively for changes in corneal thickness and endothelial cell count. Mean corneal thickness measurements prior to and after surgery were not statistically different. In the subset of patients that had endothelial cell counts collected, the mean cell count preoperative and postoperative was not significantly different. Based on this analysis, FLACS may protect endothelial cell function to a greater extent than traditional phacoemulsification.3 LASIK is relatively contraindicated in Fuchs’ patients because the compromised endothelial pump system may not sufficiently hold the LASIK flap in place.

PREOPERATIVE CONSIDERATIONS

The preoperative evaluation for patients undergoing RLE is similar to that of refractive cataract surgery. Emphasis is placed on a specific and separate informed consent process, IOL selection based on the patient’s goals, determination of ocular dominance, and IOL calculations.

Informed Consent Process

Once it is determined that a patient is an appropriate candidate for RLE, he or she should be counseled with an extensive discussion of the risks, benefits, and alternatives. The risk-benefit ratio is discussed in the context of the patient’s surgical and nonsurgical options, including glasses, contact lenses, and other refractive procedures. Patients need to understand the risks of intraocular and corneal-based surgery. This is an ideal time to assess and manage a patient’s expectations about RLE. Despite the advancing technology of IOLs, the functionality of the young crystalline lens cannot be replicated and the remaining natural accommodation is lost. The patient’s goals should be carefully examined to determine if spectacle independence outweighs the changes in visual quality that may occur with certain IOL choices.

The written consent for RLE is distinct from that for cataract surgery. The procedure should read “removal of crystalline lens and insertion of IOLs.” A written consent should be done for the femtosecond laser and any astigmatism correcting procedures as well.

Intraocular Lens Selection

IOL selection is based on the patient’s desired postoperative refraction and goals, as well as prior ocular surgeries. The effectiveness of a blended vision strategy has been demonstrated and has been used for years. Historically, the dominant eye is targeted for a plano target and the nondominant eye is typically targeted for a -1.5 D target with monofocal implants. It is advantageous when the candidate has previously worn contact lenses for blended vision.

Patients who desire binocular vision at various targets may be better candidates for multifocal, accommodating, or extended depth of focus IOLs. Preoperative counseling involves discussion regarding dysphotopsias of the diffractive presbyopia correcting IOLs and possible need for reading spectacles for reading fine print or other specific tasks. The indications for different IOLs are discussed later in this chapter and in more detail in the subsequent chapters. Patient discussion on the basic IOL types and their indications is important at this stage of the preoperative clinic visit.

Patients with a history of refractive corneal surgery or who have a highly aberrated cornea require unique consideration for IOL selection. The spherical aberration of the IOL is matched or balanced with the induced corneal spherical aberration from the prior refractive procedure. In post–myopic LASIK or PRK patients, whose corneas have positive spherical aberration, IOLs with negative spherical aberration are indicated as they balance the corneal positive spherical aberration with the IOL’s negative spherical aberration. In post–hyperopic LASIK or PRK patients, whose corneas have negative spherical aberration, IOLs with aspheric neutrality are preferred. In highly aberrated corneas, such as those with keratoconus, an aspherically neutral implant will provide the best refractive results.

Ocular Dominance Determination

Determination of ocular dominance is an important measurement in refractive lens surgery, particularly when a mix and match strategy is utilized or when patients opt for blended vision postoperatively. There are both motor and optical methods. Motor dominance is easier to determine and is more commonly used in the clinical setting. However, optical dominance is more accurate in determining the true dominant eye.

Optical dominance can be determined with the lens fogging technique. First, dial the patient’s refractive error into the phoropter to provide best-corrected visual acuity and ask the patient to fixate on an image on the Snellen chart. Next, add +1.5 D to +2.0 D of sphere power to each eye individually and ask the patient which eye has more apparent blur. The eye with more subjective blurriness is the dominant eye. Conversely, the eye that better tolerates optical blur is the nondominant eye. Agreement between these 2 methods varies.4

For some patients, the laterality of the dominant eye at distance does not correspond with the dominant eye for near work, considered near dominant. This subset of patients tends to have more difficulty adjusting to blended vision. Currently, we do not have a good screening tool to assess for this discrepancy.

Intraocular Lens Calculations

For RLE, it is essential to obtain precise preoperative measurements of various parameters, especially axial length and keratometry, for accurate IOL power determination. The formula(s) should be chosen based on axial length and post–refractive status. In eyes with prior corneal refractive surgery, the power calculation is more complicated and can be less predictable. Further specialized testing such as anterior segment optical coherence tomography can be done and intraoperative aberrometry is helpful in improving accuracy. For calculating astigmatic correction, in general, we recommend using a weighted mean of the total astigmatism and the astigmatism axis from multiple measurements based on the corneal topography/tomography with Scheimpflug and/or Placido disk-based technology and optical biometry. Various IOL calculation formulas from different generations can also be weighted and averaged to determine the appropriate power. Further discussion of these methods and techniques are beyond the scope of this chapter.

Diagnostic Technology

Meticulous preoperative diagnostic testing is the foundation for accurate IOL calculations and optimal refractive outcomes. With the evolution and advancement of diagnostic technologies, we are now able to better objectively assess patients who are experiencing symptoms of DLS.

Helpful diagnostic technology includes Scheimpflug imaging, double-pass retinal imaging, and aberrometry analysis. Scheimpflug imaging technology can objectively measure lens density in patients that do not yet show a decrease in visual acuity testing with the Snellen chart or Brightness Acuity Tester (Marco). Images are generated by a slit illumination and a rotating Scheimpflug camera that rotates 180 degrees around the eye and captures sectional images of the anterior and posterior corneal surfaces as well as the anterior segment. The refraction of light at various ocular tissue interfaces delineates anatomic features while differences in brightness provide an indication of tissue density.⁵ Transparent layers appear black and tissues of increasing densities appear incrementally lighter based on gray-scale image analysis. It has been shown that in patients with DLS, the crystalline lens gray-scale units on Scheimpflug imaging show a significantly increased density over time as compared to phakic, healthy young eyes without visual complaints.⁶ The lenticular image captured by Scheimpflug imaging is an objective way of documenting lens density and anterior chamber depth (Figure 8-2). This imaging modality is very useful for patient education regarding lens status.

Double-pass wavefront technology is another emerging diagnostic tool used to assess ocular optical quality.⁷ The AcuTarget HD Analyzer (Visiometrics) is an Optical Quality Analysis System (OQAS); an infrared diode laser is collimated and passed through an entrance aperture before entering the eye. Laser light reflected from the retina is then reflected by a beam splitter and captured by a digital camera.⁸ The captured image is a representation of the amount of forward light scatter in the eye. This is quantified by an objective scatter index (OSI) score (Figure 8-3). The OSI is derived from the ratio of integrated light in the periphery to that in the central area of the image. OSI scores are around 1 in normal eyes and increase with greater degrees of ocular scatter. In this manner, lens opacities and cataracts can be graded objectively compared to the traditional methods of subjective interpretation by slit lamp examination and the use of clinical lens grading systems such as the Lens Opacities Classification System III, which are descriptive and physician subjective.^8–10 The OQAS also provides qualitative information with the point spread function, which simulates the pattern of light projected onto a patient’s retina (see Figure 8-3). This is particularly useful in understanding why some clinically pronounced lens opacities do not cause much subjective visual disability while other very small lens changes seem to cause significant visual impairment. Furthermore, the OQAS generates a defocus curve that demonstrates the degree of accommodative loss.

Multifunctional devices, such as the OPD-Scan II (Nidek) and iTrace (Tracey Technologies) have the ability to analyze whole eye and internal aberrations, aiding in the diagnosis of DLS. The OPD-Scan II uses both Placido disk-based technology and retinal reflection of infrared light to provide a variety of data maps including total refractive error, wavefront higher order aberrations, corneal topography, internal aberrations, and visual quality of the eye.11 The iTrace aberrometer and topographer projects 256 individual light rays through the pupil onto the retina to measure lower and higher order aberrations. The wavefront maps facilitate physician analysis of lenticular contributions to the total eye aberrations by separating maps of corneal and internal aberrations. In the setting of a normal retina and vitreous, increased contributions of internal aberrations to the total eye aberrations may be a sign of a dysfunctional lens. A noteworthy feature of the iTrace technology is the Dysfunctional Lens Index (DLI; Figure 8-4). A number from 0 to 10, the DLI calculation combines measures of internal higher order aberrations, analysis of contrast sensitivity, and pupil size dynamics to aid in the diagnosis of DLS.¹²

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