Ahmad, Baseer U. MD*; Shah, Gaurav K. MD*; Hardten, David R. MD†
With advances in cataract and refractive surgery over the past decade, an ever-increasing segment of our patient population seeks to become independent of spectacles. Over the past 2 decades, non-presbyopic corneal refractive surgery has gained popularity and acceptance to the point that even the United States Air Force now offers laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) to its aviators.
More recently, presbyopia correction has become the new frontier for enhancing vision without the use of glasses. Current methods can be divided into two groups—those involving replacement of the crystalline lens with a presbyopia-correcting intraocular lens (IOL) and those involving refractive surgical procedures of the cornea or crystalline lens.
Presbyopia-correcting IOLs are divided into two categories—multifocal and accommodative. Multifocal IOLs can be either diffractive or refractive. Diffractive IOLs use microscopic ridges (diffractive zones) across the lens surface to direct light toward the near or distance focal points. These ridges may be of uniform height and spacing (nonapodized) or gradually decreasing in height and spacing toward the periphery (apodized). In contrast, refractive IOLs simply incorporate concentric zones of varying optical power within the lens optic. Refractive IOL designs have largely been replaced by diffractive IOLs.1
Currently available Food and Drug Administration (FDA)–approved multifocal IOLs in the United States include the apodized diffractive ReSTOR lens (Figure 1), the nonapodized diffractive Tecnis lens (Figure 2), and the refractive ReZoom lens (Figure 3).2
Accommodative IOLs generally fall into three categories: single-optic, dual-optic, and other. Single-optic IOLs alter their image focal point by anteroposterior movement of the IOL and changes in IOL architecture. Dual-optic IOL systems use two lenses, an anterior one of high plus power and a posterior one of negative power, and changes in effective optical power occur as the relative distance changes between the lenses. Other systems are in clinical trials or development, including those that change their curvature with accommodation or change optical power using electroactive optics when accommodation is sensed.1
The only currently available FDA-approved accommodative IOL is the hinge-based single-optic Crystalens (Figure 4), but the flexible nonhinged single-optic Tetraflex (Lenstec, St Petersburg, FL) is also currently pending FDA approval. The dual-optic Synchrony (Visiogen, Abbott Park, IL) IOL system did not gain FDA approval because of lack of clinical data. However, resubmission is planned by its manufacturer. A rechargeable electroactive system, Elenza (PixelOptics, Roanoke, VA), is currently under development.
More recently, corneal procedures for correction of presbyopia have also undergone development, although they remain much less common than IOLs for this purpose. Corneal presbyopic refractive techniques include presby-LASIK (asphericity modifications of the cornea), creation of intrastromal ring incisions using femtosecond laser, and implantation of corneal inlays. Intrastromal ring incisions and corneal inlays are not yet currently FDA approved, but investigational devices include INTRACOR (Technolas Perfect Vision, St. Louis, MO), ACI-7000 (Acufocus, Inc, Irvine, CA), Invue (Acufocus, Inc), and Presbylens (ReVision Optics, Lake Forest, CA).1 Phacophotomodulation, or femtosecond laser–based “softening” of the crystalline lens to restore accommodation, remains experimental.3
Traditional corneal refractive procedures include PRK, LASIK, placement of intrastromal corneal ring segments, and peripheral corneal incisions including astigmatic keratotomy and limbal relaxing incisions. Both PRK and LASIK achieve refractive correction by appropriate laser ablation of corneal stroma, but PRK does not involve a residual corneal flap. Radial keratotomy has fallen out of favor because of long-term refractive instability, lesser predictability than modern procedures, and hyperopic shift.
Intraocular Lens Specifications and Optics
Of the four available FDA-approved presbyopia-correcting IOLs in the United States, most have optic sizes of 6.0 mm and an overall diameter of 13 mm. This is similar to other popular nonpremium IOLs, such as the 1-piece acrylic SN60WF (Alcon), 3-piece acrylic MA60AC (Alcon), and 1-piece L122UV (Bausch & Lomb) (Table 1).
Intraocular Lens Cost Comparison
The previously mentioned nonpremium monofocal IOLs (SN60WF, MA60AC, L122UV) have an average wholesale cost of approximately $100 to $150. In comparison, the average wholesale price of the 4 premium IOLs mentioned in Table 1 is approximately $900 to $1,000.
Much like refractive surgery, there is considerable variation in the actual out-of-pocket cost for patients; cataract surgery packages, including premium IOLs, often average between $2,500 and $4,000. Some surgeons will also bundle femtosecond laser–based capsulorhexis and/or any further corneal refractive surgery into these types of surgical packages.
Issues with Intraocular Lenses for the Vitreoretinal Surgeon
Macula and Peripheral Visualization
An often-raised question about presbyopia-correcting IOLs is whether they impede visualization for retinal work. In general, the lenses that may cause issues are of the multifocal variety because they have optical or diffractive zones of varying power. However, several studies have suggested that visualization of the posterior pole is comparable with standard monofocal IOLs4,5 while others have contested this.6 An informal survey of nine retina surgeons in the first author's practice indicated that few have had macular visualization issues with modern multifocal lenses. The periphery can be more challenging in the case of smaller optic designs, such as Crystalens, because of the sudden change in optical power encountered once the edge of the lens optic is crossed.
Surgery for epiretinal membrane, macular hole, and diabetic macular edema requires delicate removal of epiretinal tissue and internal limiting membrane. This is typically performed with the use of a contact lens and intraocular forceps. With multifocal lenses that have multiple refractive or diffractive zones, one's stereopsis and sense of location in space may be altered as these zone edges are crossed. Hence, there is potential increased risk for iatrogenic retinal trauma from the intraocular forceps tips if extra caution is not taken.
Realizing that depth perception and focus may be altered, one should perform membrane peeling more slowly than usual and in smaller-diameter circular movements to minimize visualization changes as the various IOL zones are traversed.7 If an iatrogenic retinal break occurs, judicious use of elevated intraocular pressure tamponade can prevent bleeding into the fovea.
As in the case of monofocal IOLs, posterior capsule opacification can pose a challenge. Despite this issue, it is best to avoid intraoperative opening of the posterior capsule in the setting of a presbyopia-correcting lens for the reasons mentioned below. When collaborating with an anterior segment colleague regarding a patient with both a retinal issue, such as epiretinal membrane and posterior capsular opacification, it is often a good idea to advise delay of posterior capsulotomy until the retinal issue has been observed and its significance has been characterized. Jumping immediately to posterior capsulotomy often does not achieve the visual expectation of the patient and can complicate further surgical interventions by the retinal specialist and/or anterior segment specialist if vitrectomy or IOL exchange becomes necessary.
When air–fluid exchange is performed in a pseudophakic eye with a posterior capsulotomy, there is a potential for condensation to occur on the posterior surface of the IOL. This can occur on all IOL types but is particularly likely on silicone–polymer–based lenses. This is a well-known challenge in traditional pseudophakic patients and applies in the same way to patients with presbyopia-correcting IOLs. The presbyopia-correcting IOLs of most concern are the Crystalens and older generation Tecnis IOLs because they are silicone-based lenses.
Such condensation can severely limit the vitreoretinal surgeon's view of the posterior segment and can be particularly troublesome if it occurs before the addition of laser treatment for a retinal detachment. To minimize visualization problems when IOL condensation occurs during air–fluid exchange in retinal detachment repair, adding supplemental laser treatment to attached retina should be considered before converting to an air infusion. This may allow some treatment to be completed before any potential condensation issues.
If condensation occurs, sometimes surgical maneuvers may still be completed by visualizing the retina through the peripheral portion of the IOL where the posterior capsule is still intact because this will be the last portion where liquid will condense. If visualization is too poor, coating the posterior surface of the IOL with viscoelastic material using a soft tip cannula can be attempted. This should be done very gently to avoid altering the lens position. As an alternative method to reduce condensation, the air tubing that connects to the infusion line may be placed on a bed of ice to allow condensation to occur within the tubing itself; this may result in more desiccated air entering the eye and minimization of condensation on the IOL.7 However, if this strategy is to be used, it should be anticipated early and the ice bed should be placed at the start of the case because cooling of the line can take time.
Alternatively, if condensation is anticipated, one may simply plan to use perfluoro-n-octane (PFO) to flatten the retina and allow for laser treatment before air–fluid exchange. If PFO usage was not planned and the surgeon faces significant condensation issues while flattening the retina with air, he/she may also return to fluid infusion, followed by PFO flattening of the retina to apply laser treatment. It should be noted that even when using PFO, condensation issues will reoccur at the point when air–PFO exchange is performed.
Others have also proposed using an irrigating contact lens system in conjunction with warmed saline solution to warm the anterior segment and IOL surfaces, which may prevent condensation.8,9
Aside from condensation issues mentioned above, silicone oil adherence and bonding of oil droplets to the IOL surface can be a particular concern for silicone-based IOLs.10 Of the presbyopia-correcting IOLs available in the United States, Crystalens and older generation Tecnis lenses were the only silicone-based ones. The others (ReSTOR, ReZoom, and current-generation Tecnis) are composed of acrylic polymers. In addition, fibrosis between Crystalens and the lens capsule can occur after silicone oil placement. This fibrosis can be significant enough that removal of both the IOL and the capsular bag may be necessary.
Because of these issues, silicone lenses should be avoided in diabetic patients because they may need vitrectomy at some point, potentially with silicone oil placement. A similar case can be made for highly myopic patients with family history of retinal detachment or those with previous history of contralateral macular hole, macular pucker, retinal detachment, or tears—essentially any eyes suspected to have an increased chance of needing future or further vitrectomy procedures. If referring this type of patient back to an anterior segment colleague for cataract surgery, it is a good practice to remind him or her to avoid silicone-based IOLs.
Most cataract and refractive surgeons center the IOL at the end of a cataract procedure to minimize undesired optical aberrations. In the case of premium IOLs, this is usually performed even more meticulously, given the heightened need for optical zones of the IOL to be aligned with the visual axis.
In pseudophakic patients, particularly those who have had recent cataract surgery without adequate fibrosis of the lens into the capsular bag, the position of the IOL can be shifted during several common steps involved in posterior segment work. Mechanisms include anterior movement of the lens by an air bubble during air–fluid exchange, displacement of the lens during gas lavage, especially in nonvalved systems where myopic eyes may experience some level of transient collapse, and slight shifts in lens position as supporting vitreous is removed during anterior vitrectomy. Gas or air-induced shifts can be particular worrisome in cases where laser posterior capsulotomy has previously been performed.
Particular care should be given to air–fluid exchanges because these are the most likely to cause lens shifts and are commonly performed in cases of macular holes, retinal detachment, and in checking the adequacy of self-sealing sclerotomies in small-gauge vitrectomy. When air fills the posterior segment, it can push the IOL forward, causing vaulting or anterior displacement of the lens optic. This is of particular issue with Crystalens, where proper positioning within the bag ensures appropriate functioning of the IOL. In general, the Crystalens will normally fall back to its usual position as the gas or air bubble subsides over time.
Decentration and IOL shift complications are best avoided by doing air–fluid exchanges slowly without tilting the eye and by opting to use valved, small-gauge trocar systems. Other measures that may be helpful include injection of viscoelastic into the anterior chamber to prevent shallowing of the anterior chamber and to enhance stability of the intraocular pressure during air–fluid exchange. Any viscoelastic addition should be performed cautiously and slowly because an aggressive injection may displace the IOL. Another older technique that can be helpful in preventing anterior shift of the IOL is the placement of a temporary 9-0 or 10-0 nylon suture in a parallel fashion through the limbus into the anterior chamber and on top of the iris, resulting in a rectangular-shaped barrier (“Holekamp suture” first advocated by Timothy Holekamp, MD), which can later be removed at the slit lamp after surgery.
A rare idiosyncrasy of the Crystalens IOL (primarily the first-generation model) is asymmetric folding at the haptic–optic junction, dubbed “Z-syndrome” because of the shape of the distorted IOL. It generally occurs outside of the setting of vitreoretinal work when asymmetric forces or capsular contraction cause buckling of the lens into an asymmetric position. This tilted position of the IOL results in astigmatism, myopia, and coma aberrations.11
Z-syndrome is usually addressed with subsequent YAG laser procedures to relieve tensile forces of the capsular bag upon the IOL, manual repositioning of the lens, haptic amputation (if fibrosed into the capsular fornices), or even lens explanation with or without the capsular bag (Figure 5).
Although no cases have been reported in vitreoretinal literature, it is theoretically possible that such a “Z” configuration could occur from asymmetric forces exerted by an air or gas bubble if it is quickly introduced or while the eye is tilted.
During the preoperative visit, the IOL location (sulcus or capsular bag), type of IOL, centration of the IOL, and integrity of the posterior capsule should be recorded. Typically, patients who have received a premium IOL are fairly knowledgeable about their lens selection and will have their IOL identification card. This should be noted, and the vitreoretinal surgeon should take time before surgery to familiarize himself/herself with that lens model. If a referring physician plans to do a posterior capsulotomy, particularly in the setting of a silicone IOL, a small capsulotomy should be advised.
Intraoperatively, any contact with the IOL should be minimized to preserve its centration. Specifically with Crystalens, anteroposterior positioning of the implant is critical to its functioning. Multifocal lenses are more forgiving in this respect but require that the central zone remain centered in the visual axis. The necessity of performing air–fluid exchange should be evaluated and avoided if possible to avoid displacement of the lens. However, in most cases of retinal detachment repair, air–fluid exchange will be needed. As part of the informed consent process, patients should be informed that these lenses may become displaced after vitrectomy. Achieving and maintaining a refractive error close to emmetropia is also critical for the function of these lenses and it is helpful to discuss the possibility that even if good best-corrected vision is restored after the retinal surgical procedure, there may be induced astigmatism or spherical refractive error from a buckle or sclerotomies.
Issues with Corneal Procedures
Laser-Assisted In Situ Keratomileusis and Photorefractive Keratectomy
Of the various enhancing corneal procedures that a patient may have undergone before vitreoretinal surgery, LASIK remains the most common. The greatest concern for the retinal surgeon regarding the cornea is maintaining the integrity of the LASIK corneal flap. Although the LASIK flap is generally quite durable after it has fibrosed into place, there is a potential plane of weakness along the original incision and it can be dislodged. Extreme care should be taken to prevent maneuvers that may stress the original flap incision such as epithelial scraping. Therefore, it is important to keep the corneal surface constantly hydrated and to work efficiently so that epithelial decompensation can be prevented. If absolutely necessary, epithelial scraping should be performed with great care using central to peripheral strokes or superior to inferior strokes because the anchoring LASIK hinge is most commonly located in the superior peripheral cornea.
If a LASIK flap becomes dislodged, an attempt can be made to irrigate beneath it and to lay it back down in proper position. If a flap is completely sheared off, resulting in a free cap, it should be replaced in its original orientation as well as can be established. Intraoperatively, a small corneal suture such as 10-0 nylon and/or postoperative contact lens can be considered. On conclusion of the surgical case, the patient should be expediently referred to a corneal/refractive surgeon.
Because the corneal epithelium and anterior stroma have no cleavable planes after PRK, there should be no special or specific intraoperative concerns for the retinal surgeon beyond those of standard surgical technique. The epithelium may be more difficult to remove following PRK because it tends to adhere more strongly to the underlying stroma that has no Bowman membrane.
Intrastromal Corneal Ring Segments
Intracorneal ring segments may be indicated for patients with low myopia (−1 to −3 diopters), keratoconus, and post-LASIK keratectasia. The only FDA-approved intracorneal ring segment (for keratoconus and myopia) is called INTACS (Addition Technology, Inc, Sunnyvale, CA) and involves placement of thin polymethylmethacrylate inserts into the deep peripheral corneal stroma to modify the corneal curvature.
Although these segments are translucent, they do not allow for a completely unimpeded view through to the retina though. Therefore, the field of view through wide-angle viewing systems can be reduced depending on how far peripherally the segments have been placed.
Corneal inlays, such as Kamra (Acufocus, Inc), are currently in use outside the United States and are likely to gain FDA approval in the future as well. They are implanted in the nondominant eye through surgical creation of a flap or corneal pocket, and the procedures are potentially reversible.
These devices work by reducing the normal aperture for light entry into the eye, thereby providing a pinhole phenomenon and increasing depth of focus. The Kamra ACI-7000 inlay is 3.8 mm in diameter, 5 μm thick, and has a central 1.6-mm aperture with the space between the 2 rings occupied by a nearly opaque material called Kynar.
It remains to be seen how such inlays will affect vitreoretinal surgical work, but it is likely that the field of view through wide-angle viewing systems will be reduced and that stereopsis may also be adversely affected (Figure 6).
Macular Disorders and Contrast Sensitivity
Vitreoretinal specialists are well aware that patients with macular disorders, such as age-related macular degeneration or central serous chorioretinopathy, often complain not just about the level of visual acuity but also of the quality of that vision. One of the primary reasons for subjective poor vision despite otherwise reasonable acuity on a Snellen eye chart is decreased contrast sensitivity secondary to the macular disease process.
Multifocal IOLs split incoming light rays into variable focal points and create two or more coexisting retinal images in which the image corresponding to the distance or near focal point is sharp depending on factors such as pupil size due to accommodation. This concept is known as simultaneous vision and is actually a pseudoaccommodative strategy.12 Unfortunately, these pseudoaccommodative strategies can lower contrast sensitivity, and this can compound the frustration of patients who already have reduced contrast sensitivity resulting from macular pathology. In addition, photic phenomena, such as glare, flare, and halos, can also be encountered and are among the most frequent reasons for dissatisfaction after multifocal IOL implantation.13 Photic phenomena tend to occur more with refractive IOLs than in diffractive IOLs.14
Accommodative IOLs, such as the Crystalens, may not be associated with as significant contrast sensitivity issues because they focus much of the incoming light at a single focal point similar to monofocal IOLs.
If time and resources permit, contrast sensitivity with a Pelli-Robson or similar chart should be tested for patients with macular disease when subjective visual acuity is reduced out of proportion to Snellen visual acuity. In general, it is advisable to avoid presbyopia-correcting lenses in these patients, particularly of the multifocal variety.
Postoperative Retinal Image Displacement
Recent reports have described an interesting phenomenon of foveal migration or macular displacement after internal limiting membrane peeling for treatment of macular holes and diabetic macular edema.15,16 Another recent study found that although patients remained asymptomatic, mild inferior displacement of the fovea may occur after vitrectomy for rhegmatogenous retinal detachment.17 Anecdotal reports of such changes after retinal detachment surgery have also been encountered by the authors of this review. With the increasing use of high-resolution macular imaging using spectral-domain optical coherence tomography, it is likely that more support will be lent to this emerging concept.
Although such displacement was found to be clinically insignificant in the study by Shiragami et al,17 measurable extorsion of 1° to 5° was observed in 16 of 27 eyes. They theorized that gas tamponade may cause an inferior shift of the retina because of minimal subretinal fluid that remains after internal drainage when combined with any brief sitting position during the immediate postoperative recovery period.
It is possible that changes in the location of the visual axis may cause undesired visual changes if they result in relative decentration of a multifocal IOL. We expect that this would be a less troublesome phenomenon for accommodative IOLs, such as the Crystalens, which do not have discrete diffractive or refractive zones.
For the patient with a presbyopia-correcting IOL and extremely high expectations, it is wise to discuss the possibility that the optical benefits of the premium IOL may be adversely altered after retinal surgery.
Unhappy Premium Intraocular Lens and Refractive Surgery Patients
The vitreoretinal surgeon faces the same challenges as his/her anterior segment colleague when dealing with the premium IOL or refractive patient—those of managing expectations. These patients usually have high expectations of their visual outcomes given the expense they have incurred for previous corneal surgery or implantation of a premium IOL.
Often, vitreoretinal consults for dissatisfied patients are requested for the purpose of determining whether their symptoms are partially or completely related to subtle retinal disease, such as coexisting epiretinal membrane. Our anterior segment colleagues often counsel patients intent on IOL exchange that some of the multifocal glare or distortions will go away, but even monofocal IOLs are associated with dysphotopsias. In deciding whether to recommend IOL exchange, the relative risk and benefit should be carefully scrutinized and factors to consider may include difficulty of exchange particularly regarding posterior capsule status, preexisting macular edema that may predict postoperative cystoid edema, or progression of any epiretinal membrane. Mild dissatisfaction with a well-placed presbyopia-correcting IOL may be preferable to major dissatisfaction following a complex IOL exchange or subsequent acquired macular problems.
If significant retinal disease is truly present, it is unfortunately generally incompatible with perfect visual acuity and it is important to candidly discuss visual prognosis with these patients before their surgical or medical management.
It should be underscored to the patient that damage to retinal structures can cause significant, uncorrectable, and irreversible visual decrease independent of their IOL function. Part of a good informed consent discussion is to explain ahead of time the possible complications of retinal surgery upon the premium lens, up to and including the possibility of lens explantation. Similarly, informed consent for a patient with previous corneal refractive surgery should detail that even with successful retinal surgery, there is a possibility of mild refractive shift even with vitrectomy or of more significant myopic shift if scleral buckling is to be performed.
If treating medical disease, the natural history of the disorder must be clearly described and separated from IOL-related issues. A wise doctor manages expectation appropriately, and a safe approach is to underpromise and overdeliver.
As these presbyopia-correcting IOLs gain popularity among cataract patients, retinal surgeons need to be aware of intraoperative pitfalls, valid subjective visual decreases that may not appear on simple Snellen acuity, and challenges with patients' heightened expectations. However, with careful preoperative planning, proactive familiarization with these premium IOLs, and appropriate communication with patients, retinal surgeons need not fear these sophisticated lenses.
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