From the Cornea and Refractive Surgery Services, Asociación Para Evitar la Ceguera en México, Hospital “Luis Sánchez Bulnes,” Universidad Nacional Autónoma de México, México City, México.
Supported by Asociación Para Evitar la Ceguera en México I.A.P., Mexico City, México.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Manuel Ramírez, MD, Asociación Para Evitar la Ceguera en México Hospital “Luis Sánchez Bulnes,” Vicente García Torres # 46, San Lucas Coyoacán 04030, México City, México. E-mail: firstname.lastname@example.org
The epi-LASIK technique is a refractive surgery procedure that produces an epithelial flap using a mechanical microkeratome and involves repositioning of the flap over the laser-ablated corneal surface. It was developed to minimize postoperative pain and haze formation secondary to other surface refractive surgeries.1 This technique also eliminates corneal flap complications of LASIK surgery, such as button holes2 and partial flaps.3
The purpose of this study was to evaluate the confocal microscopy findings at the corneal epithelial cells and the depth at which we can find activated keratocytes at the anterior stroma after epi-LASIK surgery.
Patients and Methods
Twelve eyes of 12 patients underwent epi-LASIK including epithelial flap creation using the Amadeus II epi-LASIK microkeratome (Ziemer Ophthalmics Systems AG, Port, Switzerland). A Visx Star 4 system (Visx, Santa Ana, CA) was used to perform the laser ablation in all eyes. After excimer laser ablation, the epithelial sheet was placed in all cases. A bandage soft contact lens (Pure Vision; Bausch & Lomb, Rochester, NY) was placed after surgery and retained for 5 days. Topical tobramycin 0.3% (Tobrex; Alcon Laboratories, Fort Worth, TX) was installed three times daily for 2 weeks. After corneal epithelial closure, fluorometholone acetate 0.1% (Flarex; Alcon Laboratories) was instilled four times daily for 1 week, three times per day the second week, and twice daily the third week after surgery.
The mean preoperative spherical equivalent refraction was −3.39 ± 1.81 diopters (D) (range: −1.37 to −6.25 D) and best spectacle-corrected visual acuity was 20/20 in all cases. There were no complications observed after surgery.
In Vivo Confocal Microscopy
A Confoscan 4 confocal microscope (Nidek Technologies, Artigianato, Italy) was used to obtain a central corneal scan of each eye. The front lens of the microscope was disinfected with 70% isopropyl alcohol wipes before and after each examination. A drop of gel (Viscotears; Novartis Pharma AG, Basel, Switzerland) was placed on the tip of the front lens to provide an immersion liquid within which the front lens could move forward and backward, and a Z ring adapter (Confoscan; Fortune Technologies, Vigonza, Italy) was placed to maintain contact with the eye to obtain thickness measurements for a full-thickness central corneal scan without anterior-posterior eye movement. Each scan consisted of two full-thickness corneal sequence of images, with 350 images in total, which were digitized using NAVIS software V.3.1.2 (Nidek; Multi-Instrument Diagnostic, Gamagori, Japan). Each image represented a coronal section of approximately 340 × 255 μm. Each scan was stored in the computer’s memory.
Confocal scans were taken before and 2 weeks and 1 month after surgery. Corneal epithelial thickness was defined as the distance between images of the surface epithelium and sub-basal nerve plexus (or the sub-basal first image when the plexus could not be visualized) (Fig. 1). The anterior stroma morphology was also analyzed. Both measurements were analyzed using NAVIS software V.3.5.0 (Multi-Instrument Diagnostic System; Nidek).
Figure 1. Confocal image of (A) the superficial epithelium (340 × 255 μm) and the (B) subepithelial nerve plexus (340 × 255 μm).
The uncorrected visual acuity was 20/30 or better in all eyes after epi-LASIK, and the best spectacle-corrected visual acuity was 20/20 in all cases (measured at 1 month after surgery). The mean spherical equivalent postoperatively was −0.28 ± 0.40 D.
The superficial epithelium was morphologically normal in all observed corneas, and the subepithelial nerve plexus was present in all central corneas before surgery. After epi-LASIK, the subepithelial nerve plexus was absent in all of the central corneas at 2 weeks and 1 month postoperatively. The mean epithelial thickness was 46.5 ± 5.5 μm before surgery, but decreased by 23.4% (35.6 ± 5.6 μm) at 2 weeks postoperatively and remained decreased at 22.1% at 1 month postoperatively compared with the preoperative measurement (36.2 ± 5 μm). The change in epithelial thickness was statistically significant when preoperative values were compared with the two follow-up visits (2 weeks and 1 month) (Fig. 2, Freedman P < .05).
Figure 2. Epithelial thickness individual values before epi-LASIK (preop), and at 2 weeks and 1 month postoperatively. Statistically significant values were found between preoperative and the following postoperative measurements (*Freedman P < .05).
The anterior stroma showed activated keratocytes immediately posterior to the epithelium in a layer that extended posteriorly 31.0 ± 7.9 μm (range: 18 to 45 μm) and 21.7 ± 5.2 μm (range: 15 to 32 μm) in stromal depth at 2 weeks and 1 month, respectively, and activated keratocytes appeared to be brighter at 1 month than at 2 weeks postoperatively (Fig. 3).
Figure 3. Confocal image of the anterior stroma activated keratocytes (A) 2 weeks (340 × 255 μm) and (B) 1 month (340 × 255 μm) after surgery.
Epi-LASIK surgery has been reported to be safe and effective for the correction of low myopia, with good visual results4 and stable refraction up to 1 year after surgery.5 This surgical technique offers visual and refractive results similar to other surface ablation techniques, and lower levels of postoperative pain during the first 2 hours. However, epi-LASIK has demonstrated more pain than photorefractive keratectomy on days 3 and 6, along with a high rate of flap failure and conversion to photorefractive keratectomy.6,7
The absence of the subepithelial nerve plexus after refractive surgery is a consistent finding across different techniques, including return of this plexus at the center of the cornea 6 months postoperatively.8,9 This nerve absence is thought to provide a temporary decrease in corneal sensitivity and tear film function.10
In the current study, we found that the superficial epithelium was morphologically normal in all observed corneas, whereas a previous study showed that epithelial mechanical separation (epi-LASIK) did not affect the normal cell morphology, with the lamina densa and lamina lucida preserved and the hemidesmosomes retaining normal morphology along almost the entire length of the basement membrane.11
Epithelial corneal thickness measurement has been performed using various methods, including high-frequency ultrasound,12,13 and confocal microscopy has been used to measure both corneal epithelium and corneal stroma thickness14,15 with similar results to those of high-frequency ultrasound.16 An increase of epithelial thickness has been demonstrated after myopic photorefractive keratectomy.17,18 Erie et al. reported an increase of epithelial thickness after LASIK surgery of 22% at 1 month that persisted up to 1 year after surgery.16 In contrast, we found that epi-LASIK decreased corneal epithelial thickness by 23.4% at 2 weeks after surgery, which persisted (22.1% decrease) at 1 month after surgery. Of course, it is possible that the thickness could continue to increase in our study with time.
Keratocyte activation is a consistent finding in studies of photorefractive keratectomy and LASIK surgery.9,19–21 Mitooka et al.9 reported activated keratocytes within 22.7 ± 13.0 μm of stromal depth behind the corneal flap interface at 1 week after LASIK surgery. In the current study, we found activated keratocytes behind the corneal epithelium within 31.0 ± 7.9 μm at 2 weeks, which decreased to 21.7 ± 5.2 μm in stromal depth at 1 month after epi-LASIK surgery. It is possible that epi-LASIK surgery produces more keratocyte activation than LASIK.
The current study showed a decrease in epithelial thickness after epi-LASIK up to 1 month postoperatively and similar stromal findings than previous studies found following photorefractive keratectomy and LASIK surgery.
- Pallikaris IG, Katsanevaki VJ, Kalyvianaki MI, Naoumidi II. Advances in subepithelial excimer refractive surgery techniques: epi-LASIK. Curr Opin Ophthalmol. 2003;14:207–212. doi:10.1097/00055735-200308000-00007 [CrossRef]
- Leung AT, Rao SK, Cheng AC, Yu EW, Fan DS, Lam DS. Pathogenesis and management of laser in situ keratomileusis flap buttonhole. J Cataract Refract Surg. 2000;26:358–362. doi:10.1016/S0886-3350(99)00414-9 [CrossRef]
- Rao SK, Padmanabhan P, Sitalakshmi G, Rajagopal R. Partial flap during laser in-situ keratomileusis: pathogenesis and timing of retreatment. Indian J Ophthalmol. 2000;48:209–212.
- Pallikaris IG, Kalyvianaki MI, Katsanevaki VJ, Ginis HS. Epi-LASIK: preliminary clinical results of an alternative surface ablation procedure. J Cataract Refract Surg. 2005;31:879–885. doi:10.1016/j.jcrs.2004.09.052 [CrossRef]
- Katsanevaki VJ, Kalyvianaki MI, Kavroulaki DS, Pallikaris IG. One-year clinical results after epi-LASIK for myopia. Ophthalmology. 2007;114:1111–1117. doi:10.1016/j.ophtha.2006.08.052 [CrossRef]
- O’Doherty M, Kirwan C, O’Keeffe M, O’Doherty J. Postoperative pain following epi-LASIK, LASEK, and PRK for myopia. J Refract Surg. 2007;23:133–138.
- Torres LF, Sancho C, Tan B, Padilla K, Schanzlin DJ, Chayet AS. Early postoperative pain following epi-LASIK and photorefractive keratectomy: a prospective, comparative, bilateral study. J Refract Surg. 2007;23:126–132.
- Kauffmann T, Bodanowitz S, Hesse L, Kroll P. Corneal reinnervation after photorefractive keratectomy and laser in situ keratomileusis: an in vivo study with a confocal videomicroscope. Ger J Ophthalmol. 1996;5:508–512.
- Mitooka K, Ramirez M, Maguire LJ, et al. Keratocyte density of central human cornea after laser in situ keratomileusis. Am J Ophthalmol. 2002;133:307–314. doi:10.1016/S0002-9394(01)01421-0 [CrossRef]
- Kalyvianaki MI, Katsanevaki VJ, Kavroulaki DS, Kounis GA, Detorakis ET, Pallikaris IG. Comparison of corneal sensitivity and tear function following epi-LASIK or laser in situ keratomileusis for myopia. Am J Ophthalmol. 2006;142:669–671. doi:10.1016/j.ajo.2006.04.054 [CrossRef]
- Pallikaris IG, Naoumidi II, Kalyvianaki MI, Katsanevaki VJ. Epi-LASIK: comparative histological evaluation of mechanical and alcohol-assisted epithelial separation. J Cataract Refract Surg. 2003;29:1496–501. doi:10.1016/S0886-3350(03)00348-1 [CrossRef]
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- Spadea L, Fasciani R, Necozione S, Balestrazzi E. Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg. 2000;16:133–139.
- Li HF, Petroll WM, Moller-Pedersen T, Maurer JK, Cavanagh HD, Jester JV. Epithelial and corneal thickness measurements by in vivo confocal microscopy through focusing (CMTF). Curr Eye Res. 1997;16:214–221. doi:10.1076/ceyr.126.96.36.19912 [CrossRef]
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- Erie JC, Patel SV, McLaren JW, Ramirez M, Hodge DO, Maguire LJ, Bourne WM. Effect of myopic laser in situ keratomileusis on epithelial and stromal thickness: a confocal microscopy study. Ophthalmology. 2002;109:1447–1452. doi:10.1016/S0161-6420(02)01106-5 [CrossRef]
- Gauthier CA, Epstein D, Holden BA, et al. Epithelial alterations following photorefractive keratectomy for myopia. J Refract Surg. 1995;11:113–118.
- Gauthier CA, Holden BA, Epstein D, Tengroth B, Fagerholm P, Hamberg-Nystrom H. Factors affecting epithelial hyperplasia after photorefractive keratectomy. J Cataract Refract Surg. 1997;23:1042–1050.
- Frueh BE, Cadez R, Böhnke M. In vivo confocal microscopy after photorefractive keratectomy in humans: a prospective, long-term study. Arch Ophthalmol. 1998;116:1425–1431.
- Moller Pedersen T, Li HF, Petroll WM, Cavanagh HD, Jester JV. Confocal microscopic characterization of wound repair after photorefractive keratectomy. Invest Ophthalmol Vis Sci. 1998;39:487–501.
- Erie JC, Patel SV, McLaren JW, Maguire LJ, Ramirez M, Bourne W. Keratocyte density in vivo after photorefractive keratectomy in humans. Am J Ophthalmol. 2000;129:703. doi:10.1016/S0002-9394(00)00449-9 [CrossRef]