The first U.S. Food and Drug Administration (FDA) approval for laser-assisted in situ keratomileusis (LASIK) was granted in 1998. This means that millions of Americans who previously underwent LASIK are either currently in need of cataract surgery or will be in the near future. Providing the desired outcome for these patients requires extra care and consideration, and an effective approach is needed to achieve the best results.
Those first LASIK patients were early adopters who took steps to reduce their dependency on glasses and contact lenses, so their post-cataract surgery expectations are typically much higher compared to those of an average patient. These early adopters, some now in their 60s and 70s, are typically still active and want to continue a life free from optical devices. They have often done extensive research prior to their cataract consultation and are interested in discussing the latest intraocular lens (IOL) options. These conversations will require more chair time, as clinicians will need to explain their recommendation and set realistic expectations regarding patients’ visual outcomes.
One of the main post-LASIK challenges is measuring the corneal power after refractive surgery. Obtaining accurate measurements is much more difficult in post-LASIK eyes than in previously unoperated eyes; therefore, calculating the power of the IOL is much more complex. For every diopter of correction achieved by previous laser surgery, the measured corneal power changes. When using a standard formula, the correct IOL power after myopic LASIK will be higher than predicted; it will be a lower power than predicted in patients who had prior hyperopic LASIK (Maloney, 2018).
In these situations, it is important to counsel patients that their visual outcome may not be as predictable as it normally would be following cataract surgery. They are at higher risk of having a residual refractive error that may require corrective lenses, additional refractive surgery, or, rarely, an IOL exchange (Epitropoulos, 2018).
Because early excimer lasers did not have the pupil centration or eye-tracking technology available today, early LASIK treatments were often decentered. The ablations were also somewhat irregular and often induced unintended higher-order aberrations (HOAs), because the earliest approved excimer lasers used closing or opening diaphragms to determine laser ablation patterns rather than flying spots (McDonald, 2018). Because of these complications, for many patients who had early LASIK procedures and are now facing cataract surgery, a monofocal IOL may be the best choice. However, patients who have undergone LASIK more recently and have better-centered ablations, larger optical zones, and fewer HOAs may have a wider range of IOL choices (McDonald, 2018).
Optimize the Ocular Surface
In addition to their high expectations, these patients may present with other challenges, such as pre-existing ocular surface disease (OSD) or blepharitis. It is crucial to treat these conditions before taking any measurements to help determine IOL power—including axial length, anterior chamber depth, net corneal power, posterior corneal power, and central corneal thickness (Owens, 2018)—as instruments used for this analysis rely on a normal, stable tear film. Assessing the ocular surface often includes diagnostic testing such as tear breakup time, tear osmolarity, corneal staining, and meibography. Treatment can typically include artificial tears, lid hygiene, or medical treatments such as cyclosporine and/or lifitegrast or meibomian gland treatments (Owen, 2018).
Luchs et al (2010) found that up to 60% of patients preparing to undergo cataract surgery have mild-to-moderate symptoms of blepharitis. Blepharitis can be associated with an unstable tear film, which impacts quality of life and vision and can leave patients less than satisfied with their visual outcome. A healthy ocular surface is the first of many critical steps to successful surgery.
Preoperative and Intraoperative Diagnostics
Several corneal pathologies, such as irregular corneal astigmatism, keratoconus, pellucid marginal degeneration, OSD, and epithelial basement membrane dystrophy, may not be identified with the standard pre-operative workup using biometry, slit lamp examination, and keratometry. These conditions may be better identified with the use of topography or tomography (Epitropoulos, 2018). Addressing these conditions before proceeding with cataract surgery can improve visual outcomes.
Topography and tomography can also help determine whether irregularities in corneal power and shape are contributing to the visual impairment of patients prior to cataract surgery. This information is especially useful in patients who have undergone myopic or hyperopic laser vision correction (Epitropoulos, 2018). Following myopic LASIK, the central cornea is flattened and the midperiphery remains relatively unaltered, although at times it may sustain a degree of postoperative scarring due to the flap incision and epithelial ingrowth. Topography or tomography will also show whether the ablation was decentered (Epitropoulos, 2018).
Calculating IOL Power
Calculating IOL power in previous LASIK patients has improved in recent years with the help of advanced diagnostic equipment. But as discussed earlier, standard IOL power calculations frequently lead to unintended refractive errors. Cataract surgeons are aware of these limitations and have developed several helpful formulas to obtain more accurate IOL calculations.
Previously, many surgeons would request pre-refractive surgery keratometry readings and manifest refraction, believing that these were critical to obtaining an accurate IOL calculation. It now appears that this practice is not necessary and is no longer the standard. Several studies have found that prior history does not result in a more accurate IOL power calculation (Owen, 2018; Wang et al, 2010; Kang et al, 2017).
Current approaches to accurately determine the true power of ablated corneas include the internet-based IOL calculator from the American Society of Cataract and Refractive Surgery (ASCRS; www.ascrs.org ) and Barrett True-K No History, available through the Asia-Pacific Association of Cataract and Refractive Surgeons (APACRS; www.apacrs.org ). Also, Wang et al (2015) reported that the optical coherence tomography-based IOL power calculations using the RTVue (Optovue) were more accurate and predictable compared to the standard IOL calculations using historical data. Overall, these formulas achieved a spherical prediction accuracy of ±0.50D in approximately 70% of cases (Wang et al, 2015).
The calculations to determine the correct IOL power discussed previously take place prior to surgery. However, surgeons now can measure the power of the eye intraoperatively. Intraoperative aberrometry is intended to reduce residual refractive error through aphakic refraction; this allows surgeons to confirm or revise the IOL power choice reached via preoperative biometry, optimize the lens location, and tailor arcuate corneal incisions to eyes’ astigmatic needs (Hill, 2015).
- Numerous patients who previously underwent LASIK are either ready for cataract surgery now or will be in the coming years.
- Calculating the correct IOL power in patients who had previous LASIK requires extra consideration.
- Optimizing the health of the tear film can result in more accurate preoperative measurements, more predictable healing, and overall improved outcomes.
Currently, there are two intraoperative aberrometers available in the United States (Hill, 2015). One is designed to perform aphakic and pseudophakic measurements in real time to help ensure that the correct IOL power was chosen. The system captures sphere, cylinder, and axis information and conveys the data to surgeons. It uses an infrared light and Talbot-Moiré interferometry optimized for the aphakic state to perform a “whole eye” assessment of the optical system. The device is capable of capturing 40 measurements in less than a few minutes, and it displays the scans in sequence. It then combines and analyzes data from the central 4mm to determine the optimal IOL power (Cionni, 2015).
The second is the most recent aberrometer to receive approval. It is based on the principal rapidly rotating micro electro-mechanical system (MEMS) mirror that projects a wavefront from the eye through an aperture. The scanned wavefront is focused onto a quad detector that translates the magnitude of the wavefront displacement of a scanned spot into real-time refractive data. The instrument is able to take 90 measurements per second and, similar to the other system, it attaches to the surgical microscope (Hill, 2015).
These methods have improved the chance of a successful refractive outcome for patients who have a history of laser vision correction. Nonetheless, a careful and informed discussion with patients undergoing cataract surgery after LASIK is necessary to manage their expectations and improve patient satisfaction. CLS
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