6 steps to prepare former LASIK patients for cataract surgery
One of your patients, who previously had LASIK, now needs cataract surgery. The excimer laser was approved for myopic treatments more than 20 years ago, and the early adaptors are now in their 60s and 70s, still active, and still wanting a life free of glasses or contact lenses.
Should these patients have the same expectations regarding the outcomes of their surgery 20 years later when undergoing cataract surgery? The answer may be “yes,” but extra planning is necessary.
1. Set expectations
The most crucial step for the referring OD is to set a realistic expectation. The changes in the cornea from laser vision correction alter the calculations in determining intraocular lens (IOL) power, thus affecting the final refractive outcome.
In our center, we stress with staff who interact with this type of cataract patient to avoid overpromising results. In addition, I do not discuss specific outcomes. I will say things like, “You have a greater chance of not being exactly on the target and a greater chance of needing a touch-up photorefractive keratectomy (PRK) procedure.”
In our clinic, the preference for laser vision correction after cataract surgery is PRK. The eye is not pressurized during the creation of a flap, eliminating that process from disrupting the IOL position.
Unfortunately, the recovery time from PRK is slower.
2. Determine IOL power
Several measurements are critical in determining IOL power. These include axial length, anterior chamber depth, net corneal power, posterior corneal power, and central corneal thickness.
Instruments used to gather these measurements rely on a consistent, normal tear film to provide accurate measurements. The first thing a referring OD should consider prior to referral is the ocular surface. Performing diagnostic tests on tear break-up time, tear osmolarity, corneal staining, and meimography aid in assessing the ocular surface.
When necessary, treating a patient for ocular surface disease (OSD) prior to obtaining IOL calculations is valuable. That treatment ranges from artificial tears, lid hygiene, or medical treatments such as cyclosporine (Restasis, Allergan) and/or lifitegrast (Xiidra, Shire) or meibomian gland treatments.
3. Avoid errors
The diagnostic equipment used in calculating IOL power has continued to improve. Unfortunately, standard IOL power calculations frequently lead to hyperopic errors.1,2
There are two significant reasons for these errors.
First, the index of refraction used in the calculations is not appropriate for these patients. This is due to the relationship between the anterior and posterior cornea changing secondary to laser vision correction.3
Second, the effective lens position becomes inaccurate due the change in the anterior corneal curvature which no longer reflects an accurate anterior chamber depth.4
Cataract surgeons are aware of these limitations, and several formulas have been developed to obtain accurate IOL calculations. Previously, it was thought that having pre-refractive surgery keratometry readings and manifest refraction were critical to obtaining an accurate IOL calculation.
4. Calculate IOL power
Our laser center, like other laser centers, has provided patients with a copy of their pre-operative keratometry measurements for when they may have cataract surgery in the future. It appears that this practice is now not necessary and is no longer the current standard. Several studies have found prior history does not result in a more accurate IOL power calculation.5,6
Specifically, OCT-based IOL power calculations using the RTVue (Optovue) and the Internet-based IOL calculator from the American Society of Cataract and Refractive Surgery (ASCRS; www.ascrs.org) and Barrett True-K No History (www.apacrs.org) were more accurate and predictable that the standard IOL calculations using historical data.7
It is wonderful that IOL calculations have improved and are continuing to improve for these patients. While these calculations take place prior to surgery, surgeons can now measure the power of the eye intraoperatively to help determine the correct IOL power.
Currently, there are two intraoperative aberrometers.
WaveTec (Alcon) aberrometer ocular response analyzer (ORA) uses Talbot moiré interferometry to provide sphere cylinder and axis measurements in both the aphakic and pseudophakic state. This allows the surgeon to select the proper IOL power and then confirm the power once the lens has been implanted.
HOLOS (Clarity Medical) is the most recent aberrometer to receive approval. It uses a rapidly rotating micro electro-mechanical system mirror to quantify the amount of wavefront displacement. HOLOS is able to take 90 measurements per second and, similar to the ORA system, it attaches to the surgical microscope.8
In a study of 215 patients who had previous myopic laser vision correction (LASIK [laser in situ keratomileusis] or PRK) undergoing cataract surgery, researchers found ORA achieved median absolute error of 0.35 D and mean absolute error of 0.42 D, which was significantly better than all the other preoperative calculations tested.9
5. Change lens power
I mentioned predicting the lens power before surgery and measuring the power of the eye during surgery, but what if we could change the lens power after surgery?
Such a lens exists, and more are in the ophthalmic future. Rx Light Adjustable Lens (LAL; RxSight) is a three-piece silicone light adjustable lens that received US. Food and Drug Administration (FDA) approval in November 2017.
The lens is made of a photosensitive material that can be adjusted post-operatively with UV light. In the company’s approval study, 91.8 percent of patients achieved a result within ±0.50 D of the targeted manifest refraction.10
6. Alter changes
The femtosecond laser may have a new role in IOL surgery. A concept of refractive index shaping uses the femtosecond laser to create a negative refractive index change in a hydrophobic acrylic material.11 This would allow for up to ±4.00 D of change to be applied most types of implanted IOLs.12
If the patient is not able to adapt, the lens can be converted back to a monofocal lens. Because the procedure alters only the IOL, one would not expect corneal complications, drying, ocular inflammation, or pain for the patient.
References:
1. Hamilton DR, Hardten DR. Cataract surgery in patients with prior refractive surgery. Curr Opin Ophthalmol. 2003 Feb;14(1):44-53.
2. Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol. 2001Feb;12(1):17-29.
3. Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg. 2006 Dec;32(12):2004-14.
4. Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg. 2003 Nov;29(11):2039-42.
5. Wang L, Hill WE, Koch DD. Evaluation of IOL power prediction methods using the American Society of Cataract and Refractive Surgeons Post-Keratorefractive IOL Power Calculator. J Cataract Refract Surg. 2010 Sep;36(9):1466 73.
6. Kang BS, Han JM, Oh JY, Kim MK, Wee WR. Intraocular Lens Power Calculation after Refractive Surgery: A Comparative Analysis of Accuracy and Predictability. Korean J Ophth. 2017 Dec;31(6):479-498.
7. Wang L, Tang M, Huang D, Weikert MP, Koch DD. Comparison of Newer Intraocular Lens Power Calculation Methods for Eyes after Corneal Refractive Surgery. Ophthmology. 2015 Dec;122(12):2433-2449.
8. Hill W. Intraoperative aberrometer evolves with new standard for accuracy. Ophthalmology Times. Available at: http://www.ophthalmologytimes.com/modern-medicine-feature-articles/intraoperative-aberrometer-evolves-new-standard-accuracy. Accessed 7/27/18.
9. Ianchulev T, Hoffer KJ, Yoo SH, Chang DF, Breen M, Padrick T, Tran DB. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014 Jan;121(1):56-50.
10. U.S. Food and Drug Administration. Summary of Safety and Effectiveness Data (SSED). Available at: https://www.accessdata.fda.gov/cdrh_docs/pdf16/P160055B.pdf. Accessed 7/27/18.
11. Sahler R, Bille JF, Enright S, Chhoeung S, Chan K. Creation of a refractive lens within an existing intraocular lens using a femtosecond laser. J Cataract Refract Surg. 2016 Aug;42(8):1207-1215.
12. Nguyen J, Werner L, Ludlow J, Aliancy J, Ha L, Masino B, Enright S, Alley RK, Sahler R. Intraocular lens power adjustment by a femtosecond laser: In vitro evaluation of power change, modulation transfer function, light transmission, and light scattering in a blue light-filtering lens. J Cataract Refract Surg. 2018 Feb;44(2):226-230.
