Corneal refractive surgery (LASIK, PRK, and SMILE) is one of the most popular elective surgeries in the world, and with good reason. It is one of the safest and most efficacious surgical procedures,1 with remarkable satisfaction rates of 96%.2 Despite this overwhelming success, complications occasionally arise. One of the most feared and most vision-threatening issues that may occur is postoperative corneal ectasia.
AT A GLANCE
- Postoperative ectasia is a rare but potentially devastating complication of corneal refractive surgery.
- Prevention is the best treatment. Thorough screening is imperative. Consider lens-based refractive procedures for those with risk factors associated with postoperative ectasia.
- Identify ectasia early and halt the progression with corneal collagen crosslinking.
- Visual rehabilitation includes glasses, soft contact lenses, rigid gas permeable contact lenses, and scleral lenses, with corneal transplant as a last resort.
Ectasia is a rare complication of corneal refractive surgery, occurring in just 0.04% to 0.6% of procedures.3 One large study found that 96% of cases of ectasia occurred as a result of LASIK and 4% as a result of PRK.4 Postoperative corneal ectasia results from a loss of biomechanical integrity of the cornea with subsequent thinning and steepening of the tissue. In LASIK, the stromal flap is functionally decoupled from the cornea and no longer provides tensile strength.5 This is of no consequence for most patients, but a certain subset of individuals may have predisposing factors that make them more likely to develop ectasia.
SMILE is thought to reduce the incidence of ectasia in comparison with LASIK, through its greater maintenance of anterior lamellar fibers. This theory is supported by mathematical models that predict a stronger postoperative tensile strength with SMILE compared with LASIK and PRK.6 Despite this, cases of ectasia after SMILE have been reported.7
There may never be a way to completely eliminate this complication. Some individuals who undergo refractive surgery may have developed an ectatic condition such as keratoconus even in the absence of a corneal procedure. Nevertheless, with improved screening protocols, enhanced understanding of corneal biomechanics, and judicious application of patient selection criteria, we can strive to reduce the incidence of postoperative ectasia as much as possible.
Patients susceptible to development of ectasia must be identified preoperatively. These individuals are intending to proceed with an elective surgery that they expect will improve their vision and their lives. We must be diligent in identifying those at higher risk for complications. There is no infallible or universally agreed upon screening protocol to entirely eliminate postoperative ectasia development, but use of multiple imaging modalities and sensible patient selection criteria can reduce the risk for this potentially sight-threatening event.
Patient age and refractive error should be considered before any screening tests are performed. Younger age has been associated with increased risk for ectasia. It is possible that the inverse relationship between age and risk for ectasia is due to a natural age-related increase in stromal crosslinking and subsequent corneal stiffening.8
Refractive error must be considered in conjunction with corneal thickness to determine if the patient will have an adequate residual stromal bed thickness. Approximately 15 µm of stromal tissue is ablated for every diopter of myopia correction, and a low residual stromal bed thickness (250–300 µm) increases the risk for postoperative ectasia.9
Recently, percentage of tissue altered (PTA) has been proposed as a metric for calculating ectasia risk in individuals undergoing corneal refractive surgery. PTA is calculated as the combined flap thickness and ablation depth divided by central corneal thickness. Any value greater than 40% represents an increased ectasia risk.10
Corneal imaging is a crucial component of preoperative screening. Sensitive technologies to assess corneal structure should be used to identify findings that may increase risk for ectasia. Corneal tomography imaging, with devices such as the Pentacam (Oculus Optikgeräte), Orbscan (Bausch + Lomb), or Gallei (Ziemer Ophthalmic Systems), produces a 3D image that characterizes anterior and posterior corneal curvature as well as thickness distribution. These devices also provide indices that incorporate data such as central corneal thickness, thinnest corneal thickness, changes in corneal thickness over the entire cornea, anterior radius of curvature, and posterior radius of curvature. These indices assist in the identification of individuals who may be at increased risk for ectasia after refractive surgery.11
Epithelial thickness mapping via anterior segment spectal-domain OCT may also be useful in identifying patients with early changes associated with keratoconus and those at risk of developing postoperative ectasia. Thicker than average epithelium, highly irregular epithelial thickness distribution, and thin epithelium over a keratoconic protrusion are indicative of early ectatic changes that may not be evident with other imaging technologies.12
Clinicians must never rely solely on one measurement or data point in determining a patient’s suitability for corneal refractive surgery or ectasia risk. Rather, we must make use of all relevant data and demographic information in the decision-making progress. It is important to take a cautious approach and consider alternatives to corneal refractive surgery, such as a lens-based procedure, for patients who are at risk of developing postoperative ectasia.
Corneal Ectasia Fast Facts
Corneal ectasia is a group of uncommon, noninflammatory, eye disorders characterized by bilateral thinning of the central, paracentral, or peripheral cornea. It is both a contraindication and a potential consequence of refractive surgery.
Signs to look for can include, but are not limited to1:
- Inferior steepening
- Superior flattening
- Skewing of radial axes on power topographic maps
- Abnormal corneal thinning or rate of change of corneal thickening from the center to the periphery
1. Cornea ectasia PPP – 2018. American Academy of Ophthalmology. www.aao.org/preferred-practice-pattern/corneal-ectasia-ppp-2018. Accessed February 8, 2020.
POSTOPERATIVE FINDINGS AND MANAGEMENT
My post–refractive surgery patients are often the easiest exams I have all day. They’re generally young, healthy, and seeing exceptionally well. On the uncommon occasion when the patient isn’t doing as well as expected, we must investigate to identify the cause.
Decreased UCVA with increasing myopia and/or astigmatism may easily be mistaken for simple treatment regression. Epithelial thickness mapping may be a useful tool in identifying early ectasia. Myopic regression will demonstrate thickening in the central zone corresponding to the flattened region of the cornea following myopic laser refractive surgery. Conversely, corneal ectasia will reveal thinning of the epithelium over the steepest portion of the cornea.13 It is of utmost importance to differentiate regression from ectasia, as retreatment of an ectatic cornea will hasten progression.3
Findings suggestive of ectasia include decreased BCVA and topographic irregularities such as irregular astigmatism and focal corneal thinning or steepening. In advanced cases, topographic findings become indistinguishable from keratoconus or pellucid marginal degeneration.3
When ectasia is identified, the first goal in management is to halt its progression. Corneal collagen crosslinking (CXL) increases stromal rigidity and has demonstrated efficacy in stopping the progression of post–refractive surgery ectasia.14
After progression of ectasia has been halted, vision must be addressed. We have many options to correct an individual’s vision, depending on the degree of corneal irregularity.
Glasses and soft contact lenses may be considered in mild ectasia. However, this is usually inadequate for most cases, given the high incidence of irregular astigmatism. Rigid gas permeable (RGP) lenses are often the first treatment option for irregular astigmatism. One study found that 80% of ectatic eyes were successfully managed with RGP lenses.15 Hybrid lenses have also been shown to be beneficial for irregular astigmatism, and they may improve comfort in those intolerant of traditional RGP lenses.16 Scleral lenses are invaluable in the management of irregular astigmatism, and they are often of great value in individuals who cannot be adequately fit with or cannot tolerate RGP or hybrid lenses.17
Controversy surrounds the use of intrastromal corneal ring segments (ICRS) for post–refractive surgery ectasia. Pinero et al demonstrated improved BCVA, decreased astigmatism, corneal flattening, and reduced higher order aberrations at 6 months after ICRS implantation, but these benefits did not persist at the 1 year time point.18 Additionally, the presence of an ICRS may make the fitting of scleral lenses more challenging.19
THE LAST RESORT
Corneal transplant is the treatment of last resort for postoperative ectasia. Deep anterior lamellar keratoplasty has largely replaced penetrating keratoplasty as the procedure of choice for these patients, as the lamellar procedure is associated with a lower risk of graft rejection.20Approximately 8% of patients who develop ectasia postoperatively will require a transplant.3 It is to be hoped that this number will decrease with earlier detection and the use of CXL to stabilize the irregular corneas.
1. Donnenfeld ED. The best for LASIK. Paper presented at: AAO Refractive Surgery Subspecialty Day; November 10-11, 2017; New Orleans.
2. Eydelman M, Hilmantel G, Tarver ME, et al. Symptoms and satisfaction of patients in the Patient-Reported Outcomes With Laser In Situ Keratomileusis (PROWL) studies. JAMA Ophthalmol. 2017;135(1):13-22.
3. Wolle MA, Randleman JB, Woodward MA. Complications of refractive surgery: ectasia after refractive surgery. Int Ophthalmol Clin. 2016;56(2):127-139.
4. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115:37-50.
5. Schmack I, Dawson DG, McCarey BE, Waring GO 3rd, Grossniklaus HE, Edelhauser HF. Cohesive tensile strength of human LASIK wounds with histologic, ultrastructural, and clinical correlations. J Refract Surg. 2005;21(5):433-445.
6. Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg. 2013;29(7):454-460.
7. Moshirfar M, Albarracin JC, Desautels JD, Birdsong OC, Linn SH, Hoopes PC Sr. Ectasia following small-incision lenticule extraction (SMILE): a review of the literature. Clin Ophthalmol. 2017;11:1683-1688.
8. Blackburn B, Jenkins MW, Rollins AM, Dupps WJ. A review of structural and biomechanical changes in the cornea in aging, disease, and photochemical crosslinking. Front Bioeng Biotechnol. 2019;7:66.
9. Belin MW, Ambrosio R Jr. Corneal ectasia risk score: statistical validity and clinical relevance. J Refract Surg. 2010;26(4):238-240.
10. Santhiago MR. Percent tissue altered and corneal ectasia. Curr Opin Ophthalmol. 2016;27(4):311-315.
11. Motlagh MN, Moshirfar M, Murri MS, et al. Pentacam corneal tomography for screening of refractive surgery candidates: a review of the literature, part I. Med Hypothesis Discov Innov Ophthalmol. 2019;8(3):177-203.
12. Kanellopoulos AJ, Asimellis G. Anterior segment optical coherence tomography – assisted topographic corneal epithelial thickness distribution imaging of a keratoconus patients. Case Rep Ophthalmol. 2013;4(1):74-78.
13. Reinstein DZ, Srivannaboon S, Gobbe M, et al. Epithelial thickness profile changes induced by myopic LASIK as measured by artemis very high-frequency digital ultrasound. J Refract Surg. 2009.25(5):444-450.
14. Sharif W, Ali ZR, Sharif K. Long term efficacy and stability of corneal collagen cross linking for post-LASIK ectasia: an average of 80mo follow-up. Int J Ophthalmol. 2019;12(2):333-337.
15. Woodward MA, Randleman JB, Russell B, Lynn MJ, Ward MA, Stulting RD. Visual rehabilitation and outcomes for ectasia after corneal refractive surgery. J Cat Refract Surg. 2008;34(3):383-388.
16. Nau AC. A comparison of Synergeyes versus traditional rigid gas permeable lens designs for patients with irregular corneas. Eye Cont Lens. 2008;34(4):198-200.
17. Stason WB, Razavi M, Jacobs DS, et al. Clinical benefits of the Boston Ocular Surface Prosthesis. Am J Ophthalmol. 2010;149(1):54-61.
18. Pinero DP, Alio JL, Uceda-Montanes A, El Kady B, Pascual I. Intracorneal ring segment implantation in corneas with post-laser in situ keratomileusis keratectasia. Ophthalmology. 2009;116(9):1665-1674.
19. Kramer EG, Boshnick EL. Scleral lenses in the treatment of post-LASIK ectasia and superficial neovascularization of intrastromal corneal ring segments. Cont Lens Anterior Eye. 2015;38(4):298-303.
20. Fogla R, Padmanabhan P. Results of deep lamellar keratoplasty using the big-bubble technique in patients with keratoconus. Am J Ophthalmol. 2006;141(2):254-259.