Ординатура / Офтальмология / Английские материалы / Phakic Intraocular Lenses_Hardten, Lindstrom, Davis_2004

INTRODUCTION

High refractive errors are difficult to correct optically and surgically. Traditional options for correcting refractive error—spectacles and contact lenses—have less than optimal performance as refractive error increases. Both high myopes and hyperopes experience distorted peripheral vision with optical correction due to increased radial astigmatism and curvature of field. The diminished quality of vision and poor cosmesis of glasses cause many of these patients to rely heavily on contact lenses. Unfortunately, there are fewer contact lens options for the extreme myope and hyperope than there are for the mild to moderate ametrope. Patients intolerant of rigid gas permeable lenses may be forced to wear conventional, yearly replacement lenses instead of utilizing a more frequent replacement schedule. A frequent replacement schedule, such as every 2 weeks or even every day, is healthier for the eye; however, lenses for extreme ametropes are simply not manufactured in those parameters. Contact lens wear causes corneal hypoxia, especially when thicker lens designs needed for correcting high refractive errors are combined with longer wearing time. Hypoxia, in turn, increases the risk for complications like keratitis and corneal neovascularization. It also accelerates endothelial cell loss.

With the advent of corneal refractive surgery, alternative treatments for correcting refractive errors became available. Radial keratotomy, which is one of the first refractive surgeries developed, is limited to corrections of no more than -5.00 diopters (D). Long-term instability of the cornea with ectasia created by the incisions and the subsequent and progressive hyperopia have all but elimi-

nated its use as a standard refractive procedure. The increased risk of subepithelial haze for moderate to high myopes treated with photorefractive keratectomy (PRK) makes this option less than ideal. Newer PRK techniques, like laser-assisted subepithelial keratectomy (LASEK) and intraoperative use of mitomycin C, may help to decrease the occurrence of haze formation.1,2 Regression rates of both PRK and LASIK increase, however, as the amount of correction increases. Excimer laser procedures for high myopia create an oblate cornea and may decrease contrast sensitivity and increase higher order aberrations. Additionally, there are concerns of iatrogenic keratectasia with high ablation depths.

Other refractive procedures for myopia have been developed recently to maintain prolate corneas and avoid potential keratectasia. INTACS intracorneal ring segments (ICRS) (Addition Technology, Des Plaines, Ill) are inserted into the midperipheral cornea. The arc shortening mechanism of ICRS maintains the positive asphericity of the cornea (ie, the curvature of the central cornea remains steep compared to the peripheral cornea). INTACS, however, demonstrated limited clinical success in cases of low to moderate spherical myopia.3 For the hyperope, a similar though opposite effect is created with conductive keratoplasty (CK). Midperipheral contraction created by the thermal effects of the CK treatment results in central steepening and retains the prolate nature of the natural cornea. The nomogram for CK has demonstrated modest success for refractive errors less than +3.00 D but has not been shown to reliably correct astigmatism.4 Neither ICRS nor CK subtract tissue from the cornea, thus reducing concerns of keratectasia.

The emergence of the phakic intraocular lens (IOL) in the past two decades has given new hope to the extreme ametrope. Strampelli and Barraquer used the first phakic IOLs—anterior chamber angle-fixed lenses—in the 1950s. Early surgical techniques and limited knowledge of corneal physiology led to corneal edema and chronic uveitis, and the lenses were soon abandoned. There are currently four phakic IOL designs under US Food and Drug Administration (FDA) consideration: the Artisan (Ophtec BV, Groningen, Netherlands), the STAAR Implantable Contact Lens (ICL) (STAAR Surgical AG, Nidau, Switzerland), CIBA Phakic Refractive Lens (PRL) (CIBA Vision, Duluth, Ga), and the OII Phakic 6 lens (Ophthalmic Innovations International Inc, Ontario, Canada) (Table 3- 1). The predecessor to the Artisan lens, the Worst-Fechner iris-claw lens, was first used in 1986, while, at the same time, Fyodorov was implanting posterior chamber lenses that were to be early generations of the current STAAR ICL. A more detailed history of the evolution of lens design can be found in other chapters of this text.

PATIENT SELECTION

The refractive outcomes of LASIK when compared to the phakic IOL have been shown to be similar in cases of myopia between -8 and -12 D.5 Surgeon discretion will determine appropriate treatment in these borderline cases when pachymetry is adequate and topography is normal. Some of these patients may have irregular corneal topographies that classify them as keratoconus suspects. Despite good spectacle corrected vision and a refractive error that has been stable for many years, a cautious refractive surgeon may advise against corneal refractive surgery in keratoconus suspects (Figure 3-1).

PATIENT HISTORY

A thorough patient history is an important part of every exam, especially for any patient considering an elective surgery. To the astute observer, the case history offers more than a chance to gather data; it provides an opportu-

nity to assess the motivations and expectations of the patient. Establishing realistic expectations is the first step toward a positive outcome. Making a note of how the patient expresses his or her expectations of the surgery is useful. For example, does he or she use words like “20/20” or “life without any glasses”? The many successes of LASIK have shifted focus from functional uncorrected vision and lifestyle changes to expectations like “20/20 the next day.” Learning about a patient’s occupation and avocations is important in determining goals and visual needs.

The patient should be questioned carefully about any history of amblyopia, stability of refraction, and satisfaction with wearing glasses or contact lenses. Best-corrected visual acuity (BCVA) of 20/25 or 20/30 may be normal in high myopes due to the minification caused by spectacle lenses. These patients may prefer contact lenses for this reason and report that they see better with their contacts. BCVA less than 20/30, especially in the hyperope, should raise questions about amblyopia and warrants further investigation into amblyogenic factors. The surgeon should inquire about the patient’s age of first spectacle correction and any history of strabismus. Refractive stability is also important. Frequent changes in glasses prescriptions and/or constantly increasing amounts of sphere or cylinder may indicate a condition such as keratoconus or pellucid marginal degeneration. On the other hand, the patient who has corneal topography with asymmetric astigmatism and/or inferior steepening but with a stable refractive error and no clinical signs of keratoconus may be a candidate for a phakic IOL. A thorough history will also uncover possible contraindications to implant surgery (Table 3-2).

AGE

Age is an important factor when considering a candidate for implant refractive surgery. In the high ametrope, who is not a candidate for LASIK, age may be the deciding factor between remaining phakic or becoming pseudophakic. The concept of presbyopia should be clearly explained when counseling any refractive surgery candidate. The surgeon should discuss several factors with the

patient: phakic IOL implantation, presbyopia, and senile cataract formation. The longer the time period is between factors one and two, the more benefit a patient receives from implanting the phakic IOL. The risk-to-benefit ratio may be considered lowest if the time between factors one and three is maximized. Unfortunately, the time at which a patient enters presbyopia is more predictable than the age at which a senile cataract will develop.

The surgeon should also take into account how agerelated changes to the lens affect the anterior chamber depth. The anterior chamber has been shown to shallow with age, most likely as a result of continued growth of the crystalline lens.6 Therefore, the younger the patient, the more important it may be that he or she has sufficient anterior chamber depth to allow for this occurrence. This may lead surgeons to adopt minimum chamber depths based on age.

PUPIL SIZE

Pupil size is also extremely important in patient selection for phakic implant refractive surgery. The optical zone of the implant limits the maximum scotopic pupil size allowed. Glare and halos from pupils larger than the optical zone can be a nuisance at best, debilitating at worst, and cause for explantation of the lens. Pupil size, however, is not an absolute contraindication in the motivated patient. A patient with a large scotopic pupil may still wish to proceed with surgery after careful explanation and education on the risks of glare and halos. Many of these patients may feel they have glare and halos with their current forms of vision correction and may be willing to trade these potential side effects for the decreased dependence on glasses or contacts. It is advised that standard, metered, and reproducible scotopic lighting conditions be used for testing pupils. A scotopic illuminance level of 0.5 to 0.6 lux has been recommended.7 An assessment of pupil size should be taken at each preoperative visit and an accurate medical history elicited to uncover any medication that may affect pupil size, such as antidepressants and antihistamines.

CORNEAL EVALUATION

A careful and thorough slit lamp examination is essential in ruling out any problems that may lead to an unsatisfactory surgical and refractive outcome. Corneal anomalies that are not considered visually significant and nonprogressive could be considered relative contraindications, whereas progressive and visually deleterious conditions should be classified as absolute contraindications (Figure 3-2). Other instruments that can help with this evaluation are the corneal topographer and the noncontact specular microscope. Corneal topographers are indispensable instruments to the refractive surgeon and should be used to screen all patients interested in corneal refractive surgery in order to detect patients with irregular corneal surfaces. Endothelial cell density, as determined by specular microscopy, is another important index of corneal health and stability and should be considered important to the surgeon performing phakic or pseudophakic intraocular surgery.

ANTERIOR CHAMBER EVALUATION

Determination of the anterior chamber depth is also of importance in the evaluation of the phakic IOL candidate. Adequate distance between the implant and the posterior surface of the cornea is essential in minimizing damage to the endothelium and ensuring the long-term health of the cornea. Past lens designs had high vaults that resulted in a

higher incidence of corneal touch and decompensation. The high myope frequently has a deep anterior chamber; therefore, most of these patients will not have a problem meeting minimum chamber depth requirements. Unfortunately, as hyperopia increases, axial length and anterior chamber depth decrease. A shallow anterior chamber depth is often what precludes the high hyperope from phakic IOL implantation. As lens designs improve, it is hoped that the minimum chamber depth will decrease, making phakic IOLs a viable option for the high hyperope with a shallow chamber.

The anterior angle configuration is a second important aspect of the anterior chamber to assess. This is equally important for anterior chamber and posterior chamber phakic IOLs. It has been observed that the sulcus placement of the posterior chamber lens haptics can cause changes to the angle structure over time. Ultrasound biomicroscopy has also shown that current posterior chamber-style lenses do come into contact at times with the pigmented epithelium on the posterior surface of the iris.8 Therefore, it is important to document angle configuration and pigmentation prior to surgery, should any changes occur.

PATIENT EDUCATION/

PATIENT COUNSELING

Whether he or she has done extensive internet research, saw a special feature on the nightly news, or knows that one of his or her family members or coworkers has had the pro-

cedure, chances are the patient who comes in for an evaluation for refractive surgery knows something about LASIK. Two of the appealing aspects of LASIK are the quick visual recovery from the surgery and the short duration of the procedure. It is important to explain the differences between LASIK and refractive implant surgery. LASIK is an extraocular procedure, whereas phakic IOL surgery is intraocular. While LASIK has near immediate visual recovery in most cases, phakic IOL surgery may take 1 to 3 months to stabilize. While LASIK is often times done as a bilateral procedure, patients must understand that phakic IOLs are typically implanted one eye at a time. More importantly, they need to be prepared for a period of anisometropia in the interim between the two surgeries. Contact lens placement in the nonoperated eye achieves the best correction and minimizes aniseikonia during this time.

For obvious ethical and legal reasons, the risks, benefits, and alternatives for any treatment should be clearly explained to the patient in language that he or she can easily understand (Table 3-3). Clearly, refractive surgery is an elective procedure and this should be emphasized. The patient always has the option of nonsurgical correction.

The most serious risks related to phakic IOLs are primarily a result of the surgical procedure of implantation. The incidence of complications like endophthalmitis, intraocular bleeding, and inflammation, fortunately, are very low but can be vision threatening. Surgical complications specific to phakic IOL placement include decentered or displaced lenses. Glare and halos due to mismatched optic vs pupil size can be avoided with careful testing and proper preoperative screening. Patients should be educated about the possibility of under and over correction with phakic IOLs. They should be informed that, postopera-

tively, IOL exchange, LASIK or PRK (bioptics), or conventional treatments, like spectacle or contact lenses, might be needed to fully correct vision.

Long-term complications, such as corneal endothelial decompensation and cataract formation, should also be discussed and presented as a reason for good follow-up care in the years after the surgery. Retinal complications, such as detachments and macular holes, should also be discussed. The high myope is at a higher risk for developing retinal complications that are unrelated to the surgery. As lens designs continue to improve and surgeon experience increases, both the operative and postoperative complication rates should decrease.

Given the current state of technology of the phakic IOL and the inability at this time to tailor the implant to the patient’s refractive error, it is important to tell the patient that a realistic goal is decreased dependence from other optical devices. Hence, he or she should not expect to be completely free from some additional form of vision correction. Myopes who primary rely on spectacles may have an added benefit of retinal image magnification relative to their glasses. This can often times result in improved BCVA.9 Additional benefits include maintaining the integrity of the central cornea and preserving its natural prolate shape. This may preserve contrast sensitivity when compared to other types of refractive surgery and make postoperative contact lens fitting easier.

A final alternative for highly ametropic patients is clear lens extraction. This is not a perfect solution, as it carries a similiar surgical risk as phakic IOL implantation, an increased risk for retinal detachment, and results in a loss of natural accommodation.10

SUMMARY

The high myope and hyperope are often poor candidates for corneal laser vision correcting surgery. On the other hand, with proper patient selection and counseling, these same patients may do extremely well with phakic IOLs. With the US FDA approval of these lenses, we are likely to witness an increasing number of patients choosing this option for refractive surgery.

REFERENCES

1.Shahinian L. Laser-assisted subepithelial keratectomy for low to high myopia and astigmatism. J Cataract Refract Surg. 2002;28(8):1334-1342.

2.Carones F, Vigo L, Scandola E, Vacchini L. Evaluation of the prophylactic use of mitomycin-C to inhibit haze formation after photorefractive keratectomy. J Cataract Refract Surg. 2002;28(12):2088-2095.

3.Shanzlin DJ, Abbott RL, Asbell PA, et al. Two-year outcomes of intrastromal corneal ring segments for the correction of myopia. Ophthalmology. 2001;108:1688-1694.

4.McDonald MB, Hersh PS, Manche EE, et al. Conductive keratoplasty for the correction of low to moderate hyperopia: U.S. clinical trial 1-year results on 355 eyes. Ophthalmology. 2002;109:1978-1989.

5.Malecaze FJ, Hulin H, Bierer P, et al. A randomized paired eye comparison of two techniques for treating moderately high myopia. Ophthalmology. 2002;109:1622-1630.

6.Hosny A, Alio JL, Claramonte P, Attia WH, Perez-Santonja JJ. Relationship between anterior chamber depth, refractive state, corneal diameter, and axial length. J Refract Surg. 2000;16:336-340.

7.Schnitzler EM, Baumeister M, Kohnen T. Scotopic measurement of normal pupils: Colvard versus video vision analyzer infrared pupillometer. J Cataract Refract Surg. 2000;26(6):859-866.

8.Jimenez-Alfaro I, Benitez del Castillo JM, Garcia-Feijoo J, Gil de Barnabe JG, Serrano de la Iglesia JM. Safety of posterior chamber intraocular lenses for the correction of high myopia. Ophthalmology. 2001;108:90-99.

9.Van der Heijde GL. Some optical aspects of implantation of an IOL in a myopic eye. Eur J Implant Refract Surg. 1989;1:245-248.

10.Colin J, Robinet A, Cochener B. Retinal detachment after clear lens extraction for high myopia. Ophthalmology. 1999;106:2281-2285.

INTRODUCTION

As with any surgical procedure, there are critical factors in the preoperative evaluation and diagnostic evaluation for phakic intraocular lenses (IOLs). To have a good understanding of risk and to improve surgical outcome, this chapter will focus on endothelial cell counts and sulcus measurements. When analyzing endothelial cell counts, the surgeon needs to understand what the normal history of the aging corneal endothelium is and understand the factors that influence a healthy endothelium. The surgeon should also be familiar with what has been published previously regarding endothelial cell counts and phakic IOLs. Finally, this information can be comprised to make a good surgical decision for the patient. Sulcus measurements will also be described, including how they are attained and why they are so critical.

ENDOTHELIAL CELL COUNTS

There are three basic factors that the health of the endothelium is classified by when evaluating the corneal endothelium. The first is the density of the endothelial cells (ie, cell density) (Figures 4-1 to 4-4). The second is the shape of the cells (ie, polymorphism) with hexagonal cells being the most normal shape (see Figures 4-1 to 4-4). Finally, the amount of variation in the size of the cells is also important (ie, polymegathism, or coefficient of variation) (see Figures 4-1 to 4-4). There are many factors that can influence the health of the corneal endothelium. First, history of contact lens use (ie, the type of lens, frequency,

duration in hours per day, and years of use). Other factors that can contribute to the corneal endothelial health are surgical trauma, chemical or physical agents, or pH change.1 Finally, underlying dystrophy or disease can contribute to change.

Effect of a Contact Lens on

Endothelium

The typical patient that undergoes surgery for a phakic IOL implant is a high myope. Because these patients have spectacles that are cosmetically unappealing, they often wear contact lenses for several years and during all waking hours of the day. Therefore, it becomes critical for us to know if the endothelium is compromised to start with before we have qualified the patient for surgery.

Chang et al conducted a study to determine how the corneal endothelium was affected by soft contact lens wear.2 The study had three groups: a control (n = 116), patients who wore soft contact lenses for less than 5 years (n = 34), and patients who wore soft contact lenses for more than 5 years (n = 42). The authors found that the percent of hexagonal cells decreased with increased duration of soft contact lens wear. This occurred as duration was measured in hours per day and length in years of wear. When looking at the polymegathism (ie, coefficient of variation), there was a direct correlation in increased variation in cell size with increased duration in hours per day (note: the duration in years of wear did not matter as much). The authors noted that the cornea also became thinner with increased duration of soft contact lens use.