Cataract Surgery After Prior Refractive Surgery

How to obtain positive outcomes for these challenging patients.

BY UDAY DEVGAN, M.D., F.A.C.S.

Cataract surgery is to correct cataracts, but with our modern techniques we can also use it as an opportunity to provide a specific refractive outcome. While the refractive predictability of cataract surgery is good, it's not as precise as excimer laser-based corneal refractive surgery. And it becomes even less accurate when we are performing cataract surgery in eyes that have undergone prior corneal refractive surgery.

Patients who have had prior LASIK, PRK or LASEK are exacting and challenging individuals who would like to achieve a specific refractive result in order to minimize their dependence on glasses after surgery. They have already elected to undergo prior refractive surgery and will be expecting the same level of success with their cataract surgery.

ALL IMAGES FOR THIS ARTICLE ARE COURTESY OF UDAY DEVGAN, M.D., F.A.C.S.

Figure 1. This patient had undergone 8-cut RK for myopia, then LASIK surgery 10 years later for hyperopia, and now presents for cataract surgery. An eye like this poses many challenges to the surgeon.

There are already more than 10 million Americans who have undergone corneal refractive surgery. They are all getting older, and with time, they will all develop cataracts. In my private practice, which is one of the major refractive surgery centers in Los Angeles, a large portion of the patients on whom I perform cataract surgery have had prior refractive surgery. By treating a few hundred of these patients every year, I have learned how to deal with these challenging situations.

The Spectrum of Prior Refractive Surgery

The most common form of corneal refractive surgery is excimer laser-based, such as LASIK or PRK. These procedures achieve their refractive effect by changing the anterior curvature and power of the cornea, with minimal effect on the posterior corneal curvature. The results tend to be stable and after a healing period, there is minimal change or drift. The resultant corneal curvature tends to be regular, with any corneal astigmatism symmetric in nature.

Prior to the FDA approval of the excimer laser, most corneal refractive surgery was incisional with radial keratotomy (RK) for myopia, astigmatic keratotomy (AK) for astigmatism and hexagonal keratotomy for hyperopia (though I don't recall seeing too many cases where it has been successful). With incisional corneal refractive surgery, the anterior and posterior corneal curvatures are affected, and these patients are more likely to have irregular topography and asymmetric astigmatism. In addition, these patients often have a significant degree of instability in their refraction, with daily fluctuations being quite common, and RK patients tending to drift towards hyperopia indefinitely.

For patients with prior phakic IOL surgery, there is very little change to the corneal curvature. Cataract surgery in these eyes is similar to that of virgin eyes, other than the removal of the phakic IOL at the time of surgery. If using ultrasound to measure the axial lengths, be aware that the phakic IOL can affect the measurement and that an optical method may be more accurate.¹

Making IOL Calculations

The commonly used theoretical formulae for IOL calculations (Hoffer Q, Holladay 1 and SRK-T) require just two measurements to calculate an IOL power for a specific postop refraction: the corneal power and the axial length (Figure 2). These formulae are able to estimate the effective lens position (ELP) of the IOL to determine the final calculation of lens power.

Figure 2. The common 2-variable theoretical IOL calculation formulae (Hoffer Q, Holladay 1, and SRK-T) use the K value to determine the effective lens position of the IOL.

In eyes with prior corneal refractive surgery, there are two questions: (1) what is the ELP of the IOL? and (2) what is the true power of the cornea?

The first question can be addressed by using the Aramberri double K method.² Because the IOL calculation formulae use the corneal power to determine the effective lens position, they can be fooled in eyes with prior corneal refractive surgery. A virgin eye with a flat cornea, perhaps a power of 40 D, is likely to have a shallow anterior chamber and an ELP that is more anterior. Similarly, a virgin eye with a steep cornea, perhaps a power of 47 D, is likely to have a deeper anterior chamber and an ELP that is more posterior.

But what about an eye that used to be myopic with a corneal power of 46 D, but had LASIK for a correction of –7 D and it now has a power of about 39 D? The IOL calculation formulae may erroneously assume that this low corneal power implies a shallow anterior chamber and a more anterior ELP and thus determines an incorrectly low IOL power. Thus, the patient ends up with a hyperopic result after surgery.

The Aramberri double K method uses the measured corneal power (K) for the IOL power calculation but uses a fixed K power of about 44 D for the ELP determination. The Holladay 2 formula requires many more input variables, a total of seven, including K, axial length, anterior chamber depth and refraction, among others, but it does not use the K value as the sole determinant of the ELP.³ The Haigis formula is not as widely used and is more complex than the 2-variable formulae but may prove to be more accurate in these post-refractive surgery eyes.⁴

To determine the true corneal power, many methods have been described, which Hoffer and Savini have organized into a few categories based upon whether the preoperative K values and preoperative refraction data are available.⁵ With more than 20 methods described to determine the true corneal power, it's clear that none are perfect.

To facilitate the IOL power determination, there are multiple Web sites to perform the calculations for you:

► http://iol.ascrs.org is a service of the American Society of Cataract and Refractive Surgery and was developed by Drs. Doug Koch, Li Wang, and Warren Hill.

► http://iol.ocularmd.org, developed by Dr. Dennis Goldsbery, also provides multiple methods of calculations.

► http://www.eyelab.com allows a free download of a computer spreadsheet created by Drs. Kenneth J. Hoffer and Giacomo Savini which will perform the appropriate calculations based upon the data that you have available.

No matter which formulae you use, make sure that you track your results so that you can determine what works best in your hands. It's also helpful to err on the side of postoperative myopia since it can be useful for near vision in pseudophakes and it is usually easily treated with an excimer laser ablation. This is particularly true for eyes with prior RK because these eyes often end up hyperopic postoperatively.⁶ Some over-simplifications of lens calculations in RK eyes, such as the concept of adding +3 D to the IOL power after a routine calculation, further emphasize that these eyes tend to have hyperopic surprises.

Intraoperatiive Considerations

The incision for the cataract surgery must be carefully placed in order to avoid the prior corneal refractive surgery sites. In LASIK eyes, the corneal incisions must avoid the LASIK flap in order preserve the ability to relift the flap to fine tune the refractive state at a later date (Figure 3).

Figure 3. The clear corneal incision is carefully placed to avoid intersecting the LASIK flap.

In RK eyes, the corneal incisions should be placed between the RK incisions since any intersection of existing incisions can cause them to rip open and create excessive fluid leakage intra-operatively, causing anterior chamber instability, and a higher risk of posterior capsule rupture (Figure 4).

Figure 4. The clear corneal incision for cataract surgery is carefully placed between the existing RK incisions.

RK incisions can swell during routine cataract surgery as the irrigation fluid moves around the anterior chamber. This is why there is often some postoperative transient hyperopia: the peripheral RK incisions swell, causing central corneal flattening and temporary decrease in corneal power resulting in hyperopia. This swelling will resolve in a few weeks; the central corneal power will be stronger and the refraction will be closer to emmetropia.

High pressure during surgery, particularly during IOL insertion, can even cause the RK incisions to rupture and leak. For this reason, all incisions should be carefully checked by using a fluorescein dye leakage test at the end of the surgery (Figure 5).

Figure 5. Fluorescein dye is used to paint all of the incisions to check for microscopic amounts of fluid leakage.

For patients with phakic IOLs who are now undergoing cataract surgery, the primary difference is removing the phakic IOL, which can be done at the beginning of the case while the eye is maintained with viscoelastic.

Most companies now offer phaco platforms with microincisional capability, so that the entire cataract surgery, including IOL insertion, can be done through a smaller incision, approximately 2 mm vs. the 3 mm in the past.

IOL Selection

Monofocal IOLs are available with positive spherical aberration, zero spherical aberration or negative spherical aberration. Prior hyperopic LASIK tends to induce negative spherical aberration in the cornea, so a traditional monofocal IOL with positive spherical aberration is a good choice to return the eye to an overall status of no spherical aberration. Most non-aspheric IOLs are of this variety.

Prior myopic LASIK tends to induce positive corneal spherical aberration, so an aspheric monofocal IOL with negative spherical aberration is a good choice to return the eye to an overall status of no spherical aberration. The IOL with the highest ability to balance out a cornea after myopic LASIK is the Tecnis (Advanced Medical Optics [AMO], Santa, Ana, Calif.), which comes in both a 1-piece acrylic design (ZCB00) as well as a three-piece acrylic design (ZA9003). The Acrysof SN60WF (Alcon, Fort Worth, Texas) is a one-piece acrylic design that has less ability to balance out the spherical aberration of a post-LASIK cornea, but offers a yellow-tint should you desire it.

Eyes with complex aberrations, such as post-RK eyes, may benefit from IOLs which induce zero spherical aberrations. These IOLs don't help to offset any corneal spherical aberration, but they also don't induce any aberrations. When there is a significant degree of corneal irregularity, a zero spherical aberration IOL like the SofPort AO silicone three-piece IOL, Akreos AO 1-piece acrylic IOL (both Bausch & Lomb, Rochester, N.Y.), or the Collamer and Silicone Aspheric IOLs (STAAR Surgical, Monrovia, Calif.) are good choices.

In eyes with significant degrees of corneal astigmatism, toric monofocal IOLs can be a good choice. The Alcon Acyrsof platform has toric models to correct up to 2 D of corneal astigmatism and the STAAR toric platform is able to correct up to 3.5 D. Extra care should be taken when using these IOLs in eyes with prior RK because determining the exact axis of placement can be difficult, particularly if there is any degree of irregularity.

Multifocal IOLs May Be an Option

Because these patients have had prior refractive surgery, they are of the mindset of achieving as much freedom from glasses as possible. This means that they are often interested in presbyopia-correcting IOLs. Multifocal IOLs such as AMO's ReZoom and Alcon's ReSTOR can be appropriate if there are no unusual aberrations, the corneal topography is normal and the patients have appropriate expectations.

Explain to these patients that they are likely to need an enhancement in the postop period in order to achieve the plano result that is required for the best results from a multifocal IOL. These patients may also experience more glare, halos and issues with lower contrast sensitivity. Next-generation multifocal IOLs such as the Alcon ReSTOR Aspheric and the AMO Tecnis Multifocal Aspheric may help to minimize these issues.

Accommodative IOLs are a particularly attractive option in post-refractive surgery eyes since their accommodative amplitude may help to offset a postop hyperopic refraction. The Crystalens and Crystalens HD (Bausch & Lomb) are the two FDA-approved accommodating IOLs and are less likely to have issues with image quality than multifocal IOLs in these patients. Competing accommodating IOL designs, including the Tetraflex IOL (Lenstec, St. Petersburg, Fla.) and the Synchrony IOL (Visiogen, Irvine, Calif.) are in the pipeline.

Because the Crystalens is normally posterior vaulted in the eye, any anterior shift of the IOL will cause the eye's refraction to become more myopic, though the accommodative amplitude may decline. This can be used to our advantage in the case of a postop hyperopic surprise: the anterior chamber can be deflated via a paracentesis, thereby causing the IOL to shift anteriorly and inducing approximately 1 to 1.5 D of instant myopia. While the power of the IOL is the same, its effective lens position is more anterior, and thus the refraction of the eye is now more myopic.

Another solution is the Calhoun Light-Adjustable IOL (Calhoun Vision, Pasadena, Calif.), which allows the IOL's power to be adjusted after it's already implanted in the eye. This lens is currently undergoing FDA trials.

If the patient has had prior monovision experience with corneal refractive surgery, I prefer to maintain this monovision arrangement. It also gives more flexibility in the postoperative results since we can operate on one eye first, then judge the result and make an adjustment to be more accurate on the second eye (Figure 6).

Figure 6. A simplified decision tree for monovision in post-LASIK patients. It is particularly effective for accommodating IOLs but also works for monofocal IOLs.

Postoperative Management

To evaluate postoperative refractive error, wait until the refraction and the corneal values (K values) stabilize. This is particularly important in post-RK eyes, where many weeks are often needed for stability. Once the postoperative K values return to their pre-operative level, and the IOL has settled into its final position, it is safe to determine the residual refractive error.

Myopic postop results can be advantageous because near vision is provided. If a plano result is desired, a LASIK or PRK enhancement is an option for many eyes. For small hyperopic results, LASIK or PRK is also possible, but for larger degrees of hyperopia, an intraocular procedure such as a piggy-back IOL or even an IOL-exchange would be preferred.

While the postop refraction is an important issue, it's not the only factor in determining the final visual outcome. The tear film is the first refracting surface of the eye and maintaining a healthy ocular surface is critical in achieving good quality vision. The prior corneal refractive surgery coupled with recent cataract surgery can induce a dry eye state and reduce vision. Aggressive management with supplemental tears, Restasis cyclosporine solution (Allergan, Irvine, Calif.) or punctal occlusion should be considered.

Once the light rays have been focused by the tear film, the cornea and the IOL, a focused image is projected onto the retina. Any macular dysfunction will prevent the eye from achieving sharp vision, so care should be taken to take measures to ensure a healthy and dry macula. Use of potent topical NSAIDs, such as Xibrom (ISTA Pharmaceuticals), can help to lessen the risk of cystoid macular edema. NSAIDs also work well to reduce ocular inflammation and irritation in the postop period, so that we can provide patients with a pleasant experience as well as a good refractive result.

Cataract surgery in eyes with prior refractive surgery can be challenging, but with careful planning and patient education, we can achieve great results and patient satisfaction. OM

References

1. Hoffer KJ. Ultrasound axial length measurement in biphakic. J Cataract Refract Surg. 2003 May; 29(5): 961-965.
2. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg. 2003 Nov; 29:2063-68.
3. Probst LE, Holladay JT. Corneal refractive power after myopic LASIK. Ophthalmology, 2003 Sep; 110(9): 1857-58.
4. Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculations according to Haigis. Graefes Arch Clin Exp Ophthalmol. 200; 238: 765-773.
5. http://www.EyeLab.com. accessed on December 14, 2008.
6. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg. 1995 Nov-Dec;11(6): 490-93.

Uday Devgan, M.D., F.A.C.S., is a partner at the Maloney Vision Institute in Los Angeles where he specializes in cataract and refractive surgery. He is also chief of Ophthalmology at Olive View UCLA Medical Center and associate clinical professor at the UCLA School of Medicine. Dr Devgan is a paid consultant to Allergan, AMO, B&L, and ISTA Pharmaceuticals; receives research support from Allergan, AMO, B&L, and Calhoun Vision; and is a stockholder in Alcon, Allergan, AMO, ISTA Pharmaceuticals, and STAAR Surgical. He can be reached at 10921 Wilshire Blvd #900, Los Angeles, CA 90024, tel 310-208-3937, email: devgan@maloneyvision.com, website: http://www.maloneyvision.com