August 18, 2021
5 min read
August 18, 2021
5 min read
Welcome to another edition of CEDARS/ASPENS Debates. CEDARS/ASPENS is a society of cornea, cataract and refractive surgery specialists, here to discuss some of the latest hot topics in ophthalmology.
William B. Trattler, MD, joins us this month to discuss some scenarios in which PRK may be a better option for patients seeking refractive surgery. We hope you enjoy the discussion.
Kenneth A. Beckman, MD, FACS
OSN CEDARS/ASPENS Debates Editor
Laser corneal refractive surgery, which includes LASIK, PRK and SMILE, has the potential to eliminate the need for contact lenses and glasses for many patients. However, not all patients are optimal candidates for these procedures.
For those who are potentially a good candidate for corneal refractive surgery, the preoperative exam is important to determine the safest and most effective options for a particular patient. Important findings during the preoperative exam include the patient’s medical and ocular surgery history, uncorrected visual acuity, refraction, best corrected visual acuity, corneal thickness, corneal shape, and health of the cornea, lens and retina. The preoperative exam, along with an understanding of the patient’s goals and expectations, will help determine which corneal refractive technique to recommend.
A patient’s history can play a role in determining candidacy. For example, a patient with a history of LASIK 12 years earlier who has developed some mild myopia may do best with a PRK enhancement with mitomycin C over the flap. A patient with a history of a contact lens-related corneal ulcer that has left a small anterior stromal scar in the visual axis may do well with PRK. On the other hand, a patient with a stable refraction and no significant medical history may be an excellent candidate for LASIK, SMILE or PRK.
When comparing techniques, many patients prefer the rapid visual recovery with LASIK and SMILE. While PRK provides a similar visual outcome to LASIK and SMILE, the recovery time is longer with PRK. On the positive side for PRK, new methods and surgical techniques have resulted in less discomfort and faster visual recovery as compared with the past.
One of the main differences between the three methods of laser corneal refractive surgery is their impact on the strength of the cornea. While the vast majority of patients who undergo laser vision correction will maintain a stable corneal shape long term, a small percentage of patients can experience the development of keratoconus, which can present many years after the procedure. One of the main risk factors for the development of keratoconus after corneal refractive surgery is an abnormal preoperative topography. Additional risk factors include the residual stromal bed depth, age at the time of the procedure and/or eye rubbing months or years after the procedure. Studies have identified other potential risk factors with LASIK including the percentage of tissue altered. While the preoperative corneal thickness by itself is not an independent risk factor, thin corneas can be a sign of early keratoconus, so careful evaluation of the preoperative topography is important in patients with thin corneas. When comparing the three methods of corneal refractive surgery, PRK appears to have the lowest risk for leading to keratoconus and is therefore often the procedure selected when patients have some suspicious but inconclusive findings on their preoperative exam.
The preoperative topography/tomography is one of the most important factors in determining whether or not a patient is an appropriate candidate for corneal refractive surgery. The goal of the preoperative corneal map is to determine whether or not a patient has a condition such as keratoconus or pellucid marginal degeneration (Figure 1). The corneal map may also identify patients who have other forms of irregular astigmatism, which may affect their visual outcome. Corneal mapping may identify patients who have subtle corneal shape irregularities that are suspicious but not definitive for keratoconus. For example, in Figure 2, this patient has some mild inferior steepening present. In cases in which the corneal shape as well as the corneal thickness profile is suspicious for keratoconus, some surgeons may recommend that laser vision correction should be delayed or potentially avoided. Other surgeons may still feel comfortable offering laser vision correction, especially when the degree of refractive error is on the lower side. In cases in which laser vision correction will be considered, surgeons may recommend PRK over LASIK or SMILE as the impact to the strength of the cornea is less with PRK. Studies have reported that SMILE may affect corneal strength less than LASIK, and more work is being conducted to further evaluate the differences.
In patients who have borderline corneal maps, such as the patient in Figure 2, there are a number of options and considerations.
1. The patient can consider PRK. Following PRK, the patient can return for annual visits that include corneal mapping and refraction at each visit. If signs of keratoconus are detected at a future exam on corneal mapping, the patient could undergo corneal collagen cross-linking (CXL) at that time to strengthen and stabilize the cornea. Figure 3 provides an example of how annual exams following corneal refractive surgery can identify early signs of keratoconus. This patient developed changes in topography over 5 years and eventually underwent CXL to stop further progression.
2. The patient can consider CXL (off label) to strengthen the cornea as a first-line procedure and months later undergo PRK. Currently, the Glaukos CXL procedure is FDA approved for progressive keratoconus. Using the Glaukos CXL system to strengthen the cornea in patients who have early nonprogressive keratoconus is considered off label. Once the patient’s cornea heals and the refraction becomes stable, PRK can be considered.
3. The patient can consider combined (simultaneous) CXL and PRK. However, the long time of the FDA-approved CXL procedure in the U.S. results in a high risk for haze when combining these two procedures. Outside of the U.S., where there are CXL devices that allow for a shorter treatment time, the risk for haze when combining PRK and CXL appears less.
4. The patient can consider other forms of refractive surgery, including the Visian ICL (STAAR Surgical) or a refractive lens exchange.
5. The patient can choose to not undergo refractive surgery and continue with their contact lenses and/or glasses.
With these options available, the refractive surgery team and patient can develop an effective treatment plan. Other new technologies that can also help in the evaluation of patients include epithelial thickness maps (Zeiss, Heidelberg and Optovue corneal OCT as well as ArcScan high-resolution ultrasonography), genetic testing (Avellino) and corneal biomechanical testing (Oculus Corvis and Reichert Ocular Response Analyzer). All of these tests can further help evaluate patients before surgery and determine which procedures would be most appropriate for a particular patient.
As the number of patients interested in corneal refractive surgery rises in the U.S., refractive surgeons will encounter more patients who are borderline candidates for corneal refractive surgery. The good news is that there is a variety of options available for patients. The preop exam can help determine the best options for a particular patient as well as help determine the importance of postoperative testing to monitor patients long term.