Considerations in Cataract Surgery Following Postrefractive Error Correction

6 Considerations in Cataract Surgery Following Postrefractive Error Correction

William B. Trattler

Abstract

Considerations in Cataract Surgery Following Postrefractive Error Correction is a chapter that provides surgeons with a comprehensive guide to evaluating and planning cataract surgery in patients who have a history of corneal refractive surgery. Surprisingly, patients with a history of corneal refractive surgery such as LASIK, PRK, or SMILE can have significant differences in their visual results as well as their quality of vision following cataract surgery. This can be related to findings in their preoperative topography, which is detailed in the chapter. Also, a small percentage of post-refractive patients may have ectasia, which is a progressive condition that may require crosslinking before or after cataract surgery. This condition can be identified preoperatively with corneal tomography or topography. With careful preoperative planning, the visual outcomes can be excellent in patients with a history of corneal refractive surgery. However, patients with irregular astigmatism or microstriae should be informed preoperatively that their quality of vision following cataract surgery may be impacted by their pre-existing condition.

Keywords: LASIK, PRK, post-LASIK ectasia, Barrett formula, irregular astigmatism, preoperative topography, preoperative tomography, microstriae, cataract surgery

6.1 Introduction

This chapter deals with all the considerations and precautions that a surgeon should consider in patients who have previously undergone refractive error correction.

There have been significant advances in the preoperative evaluation of the eye prior to cataract surgery, and a number of these advances can help deliver improved outcomes for patients with a history of previous laser corneal refractive surgery, also described as laser vision correction (LVC). This chapter will focus on the evaluation and management of patients with a history of LVC, either laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), as both have a similar impact on the corneal shape. Improvements in intraocular lens (IOL) calculation formulas for patients with a history of LVC have resulted in improvements in visual outcomes following cataract surgery. While most patients can benefit from these technologies and advanced IOL formulas, preoperative imaging technologies can also identify patients who have undergone LVC and have developed irregularities in their corneal shape that can lead to reduced quality of vision following cataract surgery. In comparison to what was available in the past, surgeons have the ability to provide improved visual outcomes to patients with a history of LVC, and also provide education to patients with corneal irregularities as to how their condition may impact their visual results following cataract surgery.

One of the main impacts of LVC on IOL outcomes is related to how LASIK changes the anterior corneal shape of the eye but does not impact the posterior corneal shape. IOL formulas extrapolate the posterior corneal shape based on the power of the anterior cornea. After myopic LVC, the anterior cornea is flatter, and after hyperopic LVC, the anterior cornea is steeper. Using the actual anterior corneal keratometry after LVC in standard IOL formulas results in an incorrect keratometry value, and therefore leads to an incorrect IOL power recommendation. One early method to compensate for this issue was developed by Jack Holladay, MD, and is called the historical method.1 The historical method requires the pre-LVC keratometry power along with the change in refractive error (as measured at the corneal plane) from before to after LVC. The dioptric power of the refractive change is subtracted from the patient’s original keratometry value, which results in an adjusted keratometry value that can be used in standard IOL formulas. One challenge is that over many years and decades, the corneal curvature may differ from the shape of the cornea immediately after LVC, making the historical method less accurate. Also, in many cases, records that provide the original corneal power and change in refraction following LVC are no longer available. Fortunately, more advanced IOL formulas have become available that do not require historical information.

Many studies have looked at the visual outcomes in post-LVC eyes using the many available IOL formulas. Currently, the Haigis L and the Barrett true K formula appear to provide the best outcomes.2 However, other formulas can also be used. While the newer formulas provide better outcomes than in the past, there can still be significant variability in postoperative visual results. Therefore, it is important to educate patients with a history of LVC that they are at a higher risk of ending up off target as compared to patients who have not undergone corneal refractive surgery.

6.2 Importance of Preoperative Topography

One of the most important first steps when evaluating a patient with a history of LVC is to perform a topography and/or tomography. Evaluation of these tests is a critical step in determining whether a patient has undergone previous myopic or hyperopic LVC, as well as whether the corneal shape is regular, irregular, or ectactic. Fig. 6.1 demonstrates the topography of an eye that has previously undergone myopic LASIK. The topography demonstrates a round central flat optical zone surrounded 360 degrees by a red band of elevation. This is a classic appearance of an eye that has undergone previous myopic LVC (either LASIK or PRK). Note that the central Ks are flat. Fig. 6.2 demonstrates the topography of an eye that has undergone previous hyperopic LVC. The central cornea is steep. Patients who have undergone hyperopic LVC for very high levels of hyperopia can have an appearance similar to keratoconus ( Fig. 6.3). A number of IOL formulas for eyes that have previously undergone LVC, including the Haigis L and Barrett true K, have a specific formula for either hyperopic or myopic LVC, so it is critical to correctly identify which LVC procedure (myopic or hyperopic LVC) a patient has undergone so that the correct IOL formula can be used.

Preoperative topography is also critical, as some patients with a history of previous LVC can develop irregular astigmatism that can impact the quality of vision postoperatively. Identifying this issue preoperatively and discussing with the patient prior to cataract surgery can help set expectations. Fig. 6.4 shows a topography of an eye that underwent LASIK more than 15 years ago, and now has developed irregular astigmatism that impacts the visual axis. Postoperatively, the patient was very disappointed with her vision and felt that her vision was not improved with cataract surgery. Fig. 6.5 shows the topography of a patient who previously underwent myopic LASIK, but now has significant irregularity within the visual axis. This patient ended up with reduced UCVA (uncorrected visual acuity) and BCVA (best-corrected visual acuity). It is also important to identify patients who have developed ectasia following LVC. Fig. 6.6 demonstrates an eye that had previously undergone LASIK and now has developed post-LASIK ectasia. When ectasia is identified, the cataract surgeon can discuss the treatment options with the patient, which can include cataract surgery first with the future possibility of crosslinking, or crosslinking first followed by cataract surgery either many months or many years later (depending on the cataract severity). One challenge in eyes with ectasia after LASIK is that the traditional post-LASIK IOL calculation formulas are not accurate, and IOL planning reverts toward methods that are used with keratoconus.