eye disorders
Refractive Surgery
Types of refractive surgery
The twomost common refractive surgery proceduresare Laser in situ keratomileusis (LASIK) Photorefractive keratectomy (PRK) Other refractive surgeries include Small incision lenticule extraction (SMILE) Phakic intraocular lenses (IOL) Corneal inlays Clear lensectomy Intracorneal ring segments (INTACS) Radial keratotomy Astigmatic keratotomy 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. 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 Overview of Refractive Error , 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. 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 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 formation, Corneal inlays are implants placed into the corneal stroma via a lamellar pocket or flap to treat 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 withgreatcaution in young patients with myopia because they have a higher risk of postoperative retinal detachment than older patients with high myopia and cataract. 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 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 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 keratotomyhas 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 keratotomyis 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. 