Ectasia After SMILE Lacking Perceivable Risk Factors

Abstract

PURPOSE:

To describe 3 eyes from 2 patients who had corneal ectasia after small incision lenticule extraction (SMILE) without tangible risk factors.

METHODS:

A 27-year-old woman and a 24-year-old man with compound myopic astigmatism and normal corneal topographies underwent uneventful SMILE in both eyes.

RESULTS:

The first patient complained of decreased visual acuity 6 months after SMILE. She presented with uncorrected distance visual acuity (UDVA) of 20/60 and 20/200 in the right and left eyes, respectively. Topography revealed decreased thickness and inferior steepening in both eyes. The second patient presented at 5 years after SMILE with UDVA of 20/400 and 20/100 in his right and left eyes, respectively. Tomography revealed an inferior steepening in the right eye. These findings were compatible with post-refractive surgery ectasia.

CONCLUSIONS:

Even though SMILE is considered a safe procedure, it is not exempt from developing corneal ectasia even without evident risk factors, which implies the need for a strict preoperative evaluation.

[Journal of Refractive Surgery Case Reports. 2022;2(3):e63–e67.]

Small incision lenticule extraction (SMILE) is a safe, effective, and predictive refractive procedure. It has been postulated to have some advantages over laser in situ keratomileusis (LASIK). Because a flap is unneeded, the strength of the anterior stroma is better preserved, so corneal biomechanical stability may be greater.¹

Corneal ectasia is one of the most feared complications after refractive surgery. Ectasia occurs when the biomechanical integrity of the cornea is compromised beyond a safe threshold required to maintain its shape and curvature and there is consequent, irreversible corneal thinning and steepening.¹ The purpose of this report is to describe 3 eyes of 2 patients with corneal ectasia after SMILE who did not have tangible risk factors.

Case Reports

Case 1

A 27-year-old female weightlifter had an uncorrected distance visual acuity (UDVA) of 20/400 and a corrected distance visual acuity (CDVA) of 20/25 in both eyes. The preoperative refractive error was −6.00 −4.00 × 30° and −5.75 −4.00 × 150°, respectively. At pre-operative evaluation, there was no history of keratoconus, stable refraction, no eye rubbing, no atopy, and a normal slit-lamp examination. Without any relevant clinical findings, Scheimpflug topography (SIRIUS; CSO) was performed (Figure 1) and was programmed for SMILE. For the procedure, in the left eye the cap thickness was 140 µm and the lenticule thickness was 137 µm, leaving a residual stromal bed of 260 µm.

Figure 1. - Standard 4-map refractive views of corneal topography of the (A) right and (B) left eye of case 1 with unremarkable findings.

One month postoperatively, UDVA was 20/40 in the right eye and 20/50 in the left eye with a refractive error of −0.75 −1.00 × 35° and −0.75 −1.00 × 174°, respectively. Biomicroscopy presented no relevant findings in the right eye, whereas several small opacities were localized at the interface of the left eye (Figure 2A). Because the opacities were judged to have no consequences on visual acuity, conservative management was chosen.

Figure 2. - Clinical and topographical images of case 1. (A) Clinical image of the right eye where a clear interphase can be observed. (B) Clinical image of the left eye where residual opacities can be observed along the inferior interphase. (C) Preoperative and 8-month postoperative tachy-metric map of the right eye with central thinning. (D) Preoperative and 8-month postoperative pachymetric map of the left eye with an important central thinning. (E) Preoperative and 8-month postoperative keratometric map with a slight inferior curvature. (F) Preoperative and 8-month postoperative keratometric map of the left eye with a marked increase in superior-inferior index and an increased inferior curvature.

Six months after surgery, the patient complained of decreased visual acuity with UDVA of 20/60 in both eyes, with a refractive error of −1.50 −1.50 × 20° in the right eye and −1.00 −3.00 × 110° in the left eye. Topography of the left eye showed significant central thinning and steepening of anterior curvature. Four weeks later, UDVA in the left eye was 20/400 and a significant change in refraction (−6.00 −4.50 × 109°) was noted. The interphase was more opaque due to inflammatory haze and therefore a pocket lavage using unpreserved topical dexamethasone (Etacortilen; Sifi Laboratories) was performed. Two weeks later, UDVA in the left eye improved to 20/60 with a refractive error of −1.25 −5.25 × 114° without improvement regarding the interphase opacities (Figure 2B).

At her last visit 8 months after SMILE, UDVA was 20/50 in the right eye and 20/200 in the left eye, with refractive errors of −1.00 −1.00 × 30° and −0.50 −7.25 × 110° and a CDVA of 20/20 and 20/50, respectively. In the right eye, there were no significant changes in the pachymetric maps (Figure 2C), but a slight inferior steepening presented in the keratometric maps (Figure 2E). In the left eye, there was significant central corneal thinning that decreased another 20 µm in the last topography (Figure 2D) and progression in the inferior steepening (Figure 2F). Due to these findings, she was scheduled for corneal cross-linking (CXL) in both eyes.

Case 2

A 24-year old man without a history of keratoconus and with stable refraction, no eye rubbing, no atopy, and a normal slit-lamp examination presented for refractive surgery evaluation. Preoperative refraction was −4.75 −1.00 × 5° in the right eye and −4.50 −1.00 × 7° in the left eye, with a CDVA of 20/30 and 20/25, respectively. Scheimpflug imaging (Pentacam; Oculus Optikgeräte GmbH) showed a normal preoperative tomography (Figure 3).

Figure 3. - Standard 4-map refractive views of corneal topography of the (A) right and (B) left eye of case 2 with unremarkable findings.

Uneventful bilateral SMILE (VisuMax; Carl Zeiss Meditec) was performed. In the right eye, the cap thickness was 160 µm and the lenticule thickness was 102 µm, leaving a residual stromal bed of 306 µm. The postoperative course was otherwise normal and UDVA was 20/25 in both eyes at the 1-week postoperative examination. At the 1-month postoperative examination, the UDVA was 20/25 in the right eye and 20/20 in the left eye, with refractive errors of +0.12 −0.25 × 67° and +0.62 −0.37 × 62°, respectively.

At the 5-year examination after SMILE, the patient referred to vision fluctuations in both eyes. The UDVA was 20/400 in the right eye with a manifest refraction of −5.00 −1.25 × 65°, and 20/100 in the left eye with a manifest refraction of −2.00 −0.75 × 75°. Corneal tomography showed an inferior steepening in the right eye, inferior-superior asymmetry of 5.30 diopters, and elevated points in the posterior and anterior maps (Figure 4). The left eye showed no tomographic signs of ectasia (Figure 5). With these findings, corneal CXL was suggested for the right eye, whereas observation was indicated for the left eye.

Figure 4. - Tomographic images of the right eye, case 2. Upper row showing preoperative pachymetric, keratometric, and posterior elevation maps of the right eye. Bottom row showing the same tomographic maps 5 years after refractive surgery with an important increase in inferior curvature, thinning of the central cornea, and elevated points.

Figure 5. - Tomographic images of the left eye, case 2. Upper row showing preoperative pachymetric, keratometric, and posterior elevation maps of the left eye. Bottom row showing the same tomographic maps 5 years after refractive surgery, without frank ectasia patterns.

Discussion

We describe 3 eyes of 2 patients with ectasia following SMILE without sufficient alterations in topographies/tomographies to fulfill ectasia risk criteria. Currently, a total of 1,988 SMILE procedures have been performed at our center since 2011, and after an average of 1 year of follow-up, we have an estimated prevalence of ectasia of 0.1% after SMILE.

Refractive corneal surgery reduces the biomechanical strength of the cornea.¹ SMILE preserves approximately 60% of the collagen network of the anterior stroma.³ Previous studies have suggested that a cap thickness of 160 µm might cause less biomechanical weakening after high myopic correction (−8.00 diopters).⁴ Interestingly, these cases developed a corneal ectasia with cap thicknesses of 160 µm. Further studies are needed to establish the role of cap thickness in corneal ectasia.^3,4

Case reports of ectasia following SMILE are scarce, with only one report of unilateral ectasia following SMILE in a 26-year-old man with a normal preoperative topography.⁵ Abnormal preoperative topographies have been documented in other case reports.^2,6,7 with a high ectasia risk factor score⁸ for LASIK. Santhiago⁹ demonstrated that for LASIK the percentage of tissue altered (PTA) is significantly associated with development of ectasia. Nevertheless, it should also be considered that PTA is less relevant in patients with suspicious topographies because it is imperative to alter less tissue in these predisposed corneas.¹⁰ As for SMILE, a modified PTA (mPTA) has been proposed that ponders the theoretical advantages of cap versus flap. Applying this concept, the mPTA for both eyes was 15% for case 1 and 14% for case 2. Although there is no safety limit for mPTA, the authors suggest that a value of 20% may be considered high risk. This novel value needs to be further evaluated in future studies.³

Physical activities such as yoga, weightlifting, and eye rubbing have been reported to possibly contribute to ectasia development.¹¹ The first case was a weight-lifter who did not take a break from exercise in the postoperative period, which could reinforce these ectasia risk factors. On the other hand, the second patient had an unremarkable medical history and had clear tomographic signs of unilateral ectasia in the right eye despite not having risk factors.

Conclusion

Although some authors have proposed combining SMILE with CXL for borderline cases,^12,13 preoperative candidacy evaluation for patients to undergo SMILE should be as strict as it is for LASIK, and SMILE should be avoided in borderline cases.¹⁴ Further studies are required to develop safety indicators and thresholds for SMILE such as the PTA for LASIK. We need to be aware that ectasia after SMILE can be developed without having evident risk factors.

References

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