Zaid Alsafi1, Sara Fatima Memon2, Ammar Mohamed Yusuf3
1The Hillingdon Hospitals NHS Foundation Trust
2The Royal Bournemouth Hospital
3Western Eye Hospital Imperial College Healthcare NHS Trust
Introduction
Refractive error, the second leading cause of blindness across the globe (1) is an umbrella term comprising of conditions where rays of light entering the eye are unable to focus on the retina. Examples of such errors include hyperopia (far-sightedness), myopia (near-sightedness), astigmatism, and presbyopia (age related visual changes). Myopia is particularly common, affecting 23% of the world’s population (2). These values are set to soar with 49.8% of the global population expected to be myopic by 2050, 9.8% of whom will have high degrees of myopia (1).
In the majority cases, refractive errors can be corrected using spectacles or contact lenses. Despite this, laser eye surgery has been shown to improve quality of life metrics that go beyond independence from spectacles (3). A review of 97 articles by Helga et al demonstrated that 99% of patients obtained a visual acuity ≥ 20/40 post operatively, and approximately 99% achieved their refractive target within ± 1·0 dioptre (4). Furthermore, sight threatening complications such as corneal infections and ectasia are rare, with evidence suggesting that there is a lower risk of infection when compared to prolonged contact lens use (5). As a result, it is becoming an increasingly popular option, with over 100,000 patients opting to have refractive surgery in the United Kingdom per year (6).
The transparent cornea is the most anterior structure in the eye, responsible for 2/3rd of the eye’s dioptric power. As such, it is the main target for refractive surgery which utilises lasers to alter corneal curvature, allowing rays of light to better focus on the retina. In this article, we summarise the common refractive surgical techniques.
Laser Assisted in Situ Keratomileusis
Laser Assisted in Situ Keratomileusis (LASIK) is the most commonly performed laser refractive surgery. During the procedure, a femtosecond laser or microkeratome is used to cut a superficial layer of the corneal tissue, leaving a hinge that creates a thin corneal flap. The flap is then lifted, exposing the underlying stroma. An excimer laser is used to sculpt the stromal surface through photoablation before replacing the flap to its original position, thereby changing the surface curvature (6). LASIK is fast, effective and causes minimal discomfort. However, the procedure is not without risk; patients may experience dry eyes due to the disruption of the lacrimal reflex and require artificial tear supplementation. Additionally, 1 in 5 will experience visual aberrations such as halos, glare and haze which may persist for as long as 6 months. Patients may also develop Diffuse Lamellar Keratitis, corneal flap complications such as flap dislodgment, ectasia and infectious keratitis (7).
Surface Ablation Laser Surgery
Like LASIK, surface ablation involves reshaping the corneal stroma using an excimer laser. The stromal radius of curvaturee can be reduced (to treat hyperopia), increased (to treat myopia) or made to be more uniform (to treat astigmatism). However, unlike LASIK, surface ablation does not rely on the creation of a flap. Instead, part of corneal epithelium is completely removed: in Photorefractive Keratectomy (PRK), the corneal epithelium is mechanically removed whilst in Laser Epithelial Keratomileusis (LASEK), an alcohol solution is used to displace the epithelial layer. Both techniques have comparable outcomes to LASIK and can be good options for individuals who are not suitable for LASIK (patients with thinner corneas who are at risk of ectasia) (5). However, due to the removal of the epithelial layer, surface ablation causes more discomfort when compared to LASIK. Additionally, recovery time can be prolonged along with an increased risk of corneal opacification and haze (5).
Small Incision Lenticule Extraction (SMILE)
Small Incision Lenticule Extraction (SMILE) is a newer procedure where an excimer laser sculpts the stroma by creating an intracorneal lenticule. This is effectively a disc-shaped layer of corneal tissue. A femtosecond laser then creates a 2-3mm incision through which the lenticule is dissected and removed, following which the surrounding tissues heal together, resulting in altered corneal curvature. When compared to LASIK, SMILE has been associated with a lower incidence of dry eye, discomfort, corneal inflammation and keratocyte damage (8). Despite this, SMILE can have a slower post-operative recovery in visual acuity compared to LASIK (9). Additionally, SMILE is technically more challenging with a steep learning curve; in dissecting the lenticule one runs the risk of unsuccessful retrieval, fragmentation, and damage to the underlying stroma, resulting in scarring (5).
Conclusion
Refractive eye surgery is growing in popularity, demonstrating high levels of patient satisfaction in addition to good safety and efficacy profiles (10). Effective refractive surgery relies on the identification of suitable candidates and matching them to the appropriate corrective technique. This is set to be amplified by Artificial Intelligence, which has shown potential in using topographic mapping to identify individuals at risk of corneal ectasia (10). As ophthalmologists, counselling patients based on their unique risk-benefit profile is of utmost importance. To do this, further studies assessing the long-term outcomes of all forms of refractive surgery are of paramount importance.
References
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