Corneal refractive surgery alters the curvature of the cornea to focus light more precisely on the retina. (See Overview of Refractive Error Overview of Refractive Error In the emmetropic (normally refracted) eye, entering light rays are focused on the retina by the cornea and the lens, creating a sharp image that is transmitted to the brain. The lens is elastic... read more .) The goal of refractive surgery is to decrease dependence on eyeglasses or contact lenses. Most people who undergo refractive surgery achieve this goal; over 95% do not need corrective lenses for distance vision.
Ideal candidates for refractive surgery are healthy people aged 18 and older with healthy eyes who are not satisfied wearing eyeglasses or contact lenses.
Contraindications to refractive surgery include
Active ocular diseases, including severe dry eye
Autoimmune or connective tissue diseases, which can impair wound healing
Use of isotretinoin or amiodarone
Refraction should be stable for at least 1 year prior to surgery. Latent herpes simplex virus may be reactivated after surgery; patients should be advised accordingly.
Adverse effects of refractive surgery include temporary symptoms of
Foreign body sensation
Occasionally, these symptoms persist.
Potential complications include
In excimer laser procedures done on the superficial corneal stroma, haze formation is possible. If infection, irregular astigmatism, or haze formation causes permanent changes in the central cornea, best-corrected acuity could be decreased. The overall complication rate is low; chance of vision loss is < 1% if the patient is considered a good candidate for refractive surgery preoperatively.
Types of refractive surgery
The two most common refractive surgery procedures are
Laser in situ keratomileusis (LASIK)
Photorefractive keratectomy (PRK)
Other refractive surgeries include
Small incision lenticule extraction (SMILE)
Phakic intraocular lenses (IOL)
Intracorneal ring segments (INTACS)
Laser In Situ Keratomileusis (LASIK)
In LASIK, a flap of corneal tissue is created with a femtosecond laser or mechanical microkeratome. The flap is turned back and the underlying stromal bed is sculpted (photoablated) with the excimer laser. The flap is then replaced without suturing. Because surface epithelium is not disrupted centrally, vision returns rapidly. Most people notice a significant improvement the next day. LASIK can be used to treat myopia, hyperopia, and astigmatism.
Advantages of LASIK over photorefractive keratectomy (PRK) include the desirable lack of central stromal healing response (the central corneal epithelium is not removed, thereby decreasing the risk of central haze formation that occurs during PRK healing), the shorter visual rehabilitation period, and minimal postoperative pain.
Disadvantages include possible intraoperative and postoperative flap-related complications, such as irregular flap formation, flap dislocation, and long-term corneal ectasia. Ectasia occurs when the cornea has become so thin that intraocular pressure causes instability and bulging of the thinned and weakened corneal stroma. Blurring, increasing myopia, and irregular astigmatism can result.
Photorefractive Keratectomy (PRK)
In PRK, unlike laser in situ keratomileusis (LASIK), no corneal flap is created. In PRK, the corneal epithelium is removed and then the excimer laser is used to sculpt the anterior curvature of the corneal stromal bed. PRK is used to treat myopia Overview of Refractive Error , hyperopia Overview of Refractive Error , and astigmatism Overview of Refractive Error . The epithelium typically takes 3 to 4 days to regenerate; during this time a bandage contact lens is worn.
PRK may be more suitable for patients with thin corneas or epithelial basement membrane dystrophy.
Advantages of PRK include an overall thicker residual stromal bed, which reduces but does not eliminate the risk of ectasia, and lack of flap-related complications.
Disadvantages include the potential for corneal haze formation (if a large amount of corneal tissue is ablated) and the need for postoperative corticosteroid drops for several months. The intraocular pressure of postoperative patients who are using topical corticosteroids should be monitored carefully because corticosteroid-induced glaucoma has been reported after PRK.
Small Incision Lenticule Extraction (SMILE)
In SMILE, a femtosecond laser is used to create a thin, intrastromal lenticule of tissue, which is then removed through a small (2- to 4-mm) peripheral corneal laser incision. SMILE is available to treat myopia and myopic astigmatism.
The efficacy, predictability, and safety of SMILE are similar to those of laser in situ keratomileusis (LASIK), with the additional benefit that it eliminates flap creation and the attendant risks. Another benefit of SMILE is the reduced degree of postoperative corneal denervation and an accelerated rate of corneal nerve regeneration relative to LASIK.
Disadvantages include increased incidence of suction loss (which may necessitate that the procedure be aborted) and difficulty with enhancements (additional surgeries to correct residual refractive error).
Phakic Intraocular Lenses (IOLs)
Phakic IOLs are lens implants that are used to treat moderate to high (eg, 4 to 20 diopters) of myopia with or without astigmatism in patients as an alternative to laser vision correction. Phakic IOLs achieve superior visual quality compared to laser vision correction because they do not alter the corneal curvature. Also, there is no risk of corneal ectasia since this is additive technology and no corneal stromal tissue is ablated. Unlike in cataract surgery, the patient’s natural lens is not removed. The phakic IOL is inserted directly anterior or posterior to the iris through an incision in the eye. This procedure is intraocular surgery and must be done in an operating room.
Risks are low overall but include cataract Cataract A cataract is a congenital or degenerative opacity of the lens. The main symptom is gradual, painless vision blurring. Diagnosis is by ophthalmoscopy and slit-lamp examination. Treatment is... read more formation, glaucoma Overview of Glaucoma Glaucomas are a group of eye disorders characterized by progressive optic nerve damage in which an important part is a relative increase in intraocular pressure (IOP) that can lead to irreversible... read more , infection, inflammation, and loss of corneal endothelial cells with subsequent chronic corneal edema that eventually becomes symptomatic. Many complications can be avoided with proper sizing and using a phakic IOL that is designed to be placed in the sulcus (just posterior to the iris).
Corneal inlays are implants placed into the corneal stroma via a lamellar pocket or flap to treat presbyopia Overview of Refractive Error . The only corneal inlay available in the US is made of polyvinylidene fluoride and carbon and is a small aperture inlay, which improves near vision by increasing depth of focus. These inlays are placed only in the nondominant eye of presbyopic patients.
Advantages of corneal inlays are improved near vision with a 1- to 2-line decrease in distance vision in the corrected eye. Also, corneal inlays can be surgically removed if desired.
Disadvantages include risk of corneal haze or inflammation, which requires long-term topical steroid use and can result in glare, halo, and difficulty reading in dim light. Complications can include inlay decentration, dry eye, and epithelial ingrowth.
Clear lensectomy can be considered in patients with high hyperopia who are already presbyopic. This procedure is identical to cataract surgery except the patient’s lens is clear and not cataractous. An extended depth-of-focus, trifocal, multifocal, or accommodating IOL—all of which allow the patient to focus over a wide range of distances without the assistance of eyeglasses—may be inserted.
The main risks of clear lensectomy are infection, retinal swelling, retinal detachment, and rupture of the posterior capsule of the lens, which would necessitate further surgery. Clear lensectomy should be done with great caution in young patients with myopia because they have a higher risk of postoperative retinal detachment than older patients with high myopia and cataract.
Intracorneal Ring Segments (INTACS)
INTACS are thin arc-shaped segments of biocompatible plastic that are inserted in pairs through a small radial corneal incision into the peripheral corneal stroma at two-thirds depth. After INTACS are inserted, the central corneal curvature is flattened, reducing myopia. INTACS are used to treat mild myopia (< 3 diopters) and minimal astigmatism (< 1 diopter). INTACS maintain a central, clear, optical zone because the 2 segments are placed in the corneal periphery. INTACS can be replaced or removed if desired.
Risks include induced astigmatism, undercorrection and overcorrection, infection, glare, halo, and incorrect depth placement. Currently, INTACS are mostly used for treatment of corneal ectatic disorders such as keratoconus Keratoconus Keratoconus is a bulging distortion of the cornea, leading to loss of visual acuity. Keratoconus is a slowly progressive thinning and bulging of the cornea, usually bilateral, beginning between... read more and post-LASIK (laser in situ keratomileusis) ectasia when glasses or contact lenses no longer provide adequate vision or are uncomfortable. Best-corrected vision and contact lens tolerance improve in 70 to 80% of patients.
Radial Keratotomy and Astigmatic Keratotomy
Radial and astigmatic keratotomy procedures change the shape of the cornea by making deep corneal incisions using a diamond or stainless steel blade or femtosecond laser.
Radial keratotomy has been replaced by laser vision correction and is rarely used because it offers no clear advantages over laser vision correction, has a greater need for subsequent retreatment, can lead to visual and refractive results that fluctuate through the day, weakens the cornea, and can cause a hyperopic shift in the long term.
Astigmatic keratotomy is still commonly done at the time of cataract surgery. The incisions are also referred to as limbal relaxing incisions because the optical zone is much larger and closer to the limbus.
Parkhurst GD: A prospective comparison of phakic collamer lenses and wavefront-optimized laser-assisted in situ keratomileusis for correction of myopia. Clin Ophthalmol 10:1209-1215, 2016. doi:10.2147/OPTH.S106120