Ali Adel Ne’ma Abdullah

University Hospital of Wales, UK

Abstract

Uncorrected refractive error (short-sightedness and long-sightedness) is the leading cause of visual impairment worldwide. Traditionally, glasses and contact lenses have been utilised to focus the light onto the retina and hence correct the visual impairment. However, laser refractive eye surgery has emerged as an alternative method to correct refractive error. This article explains the various different types of laser refractive eye surgery by breaking them down into different subtypes of surface and flap-based procedures, followed by evaluating their outcomes, complications, and comparing different procedures against each other.

Introduction

Uncorrected refractive error (short-sightedness and long-sightedness) is the leading cause of visual impairment worldwide (1).  It is primarily the result of the axial length of the eye; in short-sightedness (myopia), the eyeball is too long, causing light to be focussed in front of the retina, and in hyperopia (long-sightedness) the eyeball is too short, causing light to be focussed behind the retina (2). Traditionally, glasses and contact lenses have been utilised to focus the light onto the retina and hence correct the visual impairment.

Laser refractive eye surgery has emerged as a more permanent viable alternative to glasses and contact lenses. It involves the use of lasers to reshape the cornea, and hence focus light onto the retina. This article aims to shed light on the different types of refractive laser eye surgery, as well as their visual outcomes and complications.

Flap procedures

A microkeratome (precise corneal blade) or femtosecond laser is used to create a thin flap of corneal tissue. This flap is then lifted out of the way for the remainder of the procedure to continue, before being replaced at the end of the procedure. Broadly speaking, there are three types of flap procedures.

Laser-Assisted in situ Keratomileusis (LASIK) involves the use of a microkeratome or femtosecond laser to create a flap, followed by the use of a laser to remodel the stroma in a finely controlled manner prior to replacing the flap (3) (Figure 1).

Refractive Lenticule Extraction (ReLEx) is the final flap-based procedure and is subdivided into Femtosecond Lenticule Extraction (FLEX) and Small Incision Lenticule Extraction (SMILE). In FLEX, a flap and a lenticule of corneal stroma are simultaneously cut with a femtosecond laser, followed by manual removal of the lenticule and repositioning of the flap (4). SMILE is a modification to FLEX; a femtosecond laser system is still used to create a lenticule of corneal stroma to be removed, but instead of creating a flap to remove the lenticule, two small incisions are created by the laser system to allow retrieval of the lenticule manually with a spatula and forceps (5) (Figure 2), meaning that technically this procedure is not flap-based.

Surface procedures

These involve the use of an excimer laser to ablate the anterior cornea without creating a flap first. Generally, there are four types.

Photorefractive Keratectomy (PRK) first requires removal of the corneal epithelium mechanically using a spatula (6). Once the stroma is exposed, the laser delivers the final result (6) (Figure 3).

Transepithelial photorefractive Keratectomy (TransPRK) is a modification to PRK where the epithelial layer is removed using a laser rather than by scraping manually (7).

Laser-Assisted Sub-Epithelial Keratomileusis (LASEK) is similar to LASIK except that the flap created is much more superficial and contains no stromal tissue, such that the procedure is deemed to be more of a surface ablation procedure rather than flap-based (8). It was developed as an alternative to PRK to address pain caused by PRK’s epithelial debridement (8).

Epipolis laser in-situ keratomileusis (Epi-LASIK) is similar to LASEK except that there is no alcohol used (9).

Outcomes

Studies generally report high levels of patient satisfaction and good visual outcomes after refractive laser eye surgery. Hansen et al. (2015) followed up 81 PRK and LASEK patients for an average of 5 years and found that 96% of patients were satisfied or very satisfied at final follow-up (10). Reported reasons for dissatisfaction included post-operative short- and medium-term haze and myopic regression. Price et al. (2016) performed a 3-year follow-up of 1375 LASIK patients and demonstrated that 88% of former contact lens wearers and 77% of former glasses wearers were strongly satisfied at year 3, compared to 54% in the contact lens control group (11). Reported reasons for dissatisfaction included dry eyes and difficulty driving at night (haloes, starbursts). Schallhorn et al. (2017) demonstrated a 94% satisfaction rate for patients with moderate to high myopia who underwent LASIK and were followed up at 3 months (12). Reported reasons for dissatisfaction included post-operative haloes, glares and starbursts.

As with any surgical procedure, refractive eye surgery carries risks. Some of these are generally considered by patients to be minor nuisances, although they can be very problematic. Dry eye is one of the most common complaints; a recent meta-analysis demonstrated post-operative reductions in tear break up time after LASIK, SMILE, FLEX and PRK, albeit statistical significance was only demonstrated for LASIK and FLEX patients (13). One study analysing LASIK patients demonstrated that 21% complained of dryness; other complaints included glare (20%), haloes (18%) and increased difficulty driving at night (19%) (14).

Severe complications

Some complications, albeit rare, are serious enough to cause significant permanent visual loss. Examples include post-operative infections and corneal ectasia, a devastating protrusion of the cornea as a result of weakened biomechanical strength secondary to surgical thinning (15-17).

Some complications may only envelop after several years. Flaps created in flap-based procedures never fully heal; indeed, it is possible to peel back an anterior corneal flap years later or displace it traumatically (18-19). Furthermore, keratocyte density in the stroma has been shown to decrease after PRK and LASIK (20). Although this has not been shown to have clinical consequences, it is posited that over decades corneal transparency or curvature could be affected (20).

Comparing different techniques

Wen et al.’s (2017) meta-analysis found no statistically significant differences in either visual outcomes (efficacy, safety) or visual quality (higher-order aberrations, contrast sensitivity) among all major forms of laser refractive surgeries (21). Similarly, Tran and Ryce (2018) reported no difference in patient satisfaction or vision-related quality of life among the different techniques (22). However, few high quality and long-term studies exist; more research is required (23). Regarding complications, surface procedures do not lead to aforementioned flap-based complications.

Conclusions

Laser refractive eye surgery is an appealing treatment option for refractive errors owing to its aesthetic allure and reduced dependency on glasses/contact lenses. Visual outcomes are generally good, most patients are satisfied with their procedure, but the short- and long-term risks must be taken fully into account when embarking on this irreversible path.

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