Refractive eye surgery was revolutionized with the introduction of excimer laser-based procedures. Overall, laser-based procedures dramatically improved the outcome, cost effectiveness and popularity of refractive surgeries thereby making it a viable choice for correcting these common vision problems. A number of surgical procedures are currently in use for correcting myopia (nearsightedness), hyperopia (farsightedness) and astigmatism. However the surgical correction of presbyopia is still not well developed.
Laser-assisted in situ keratomileusis (LASIK)
Laser-assisted in situ keratomileusis or LASIK is currently the most frequently performed surgery for correction of refraction errors. It offers a predictable, safe and effective treatment option for refractive errors. Recovery following LASIK is rapid and pain is minimal. The LASIK procedure is widely accepted by surgeons and enjoys high patient satisfaction compared to other laser-based procedures like PRK. It is performed under local anesthesia and is generally an out-patient procedure. LASIK is useful in mild to high degree myopia (up to -12 diopters) which may or may not be associated with astigmatism, in hyperopia (up to 4 diopters) with or without astigmatism and also in astigmatism (up to 5 diopters).
- A flap of the cornea is made with a microkeratome. This is a major difference compared to PRK in which the layer is removed. The corneal flap of about 100 to 180 micrometer thickness is raised exposing the stroma beneath it. A modified LASIK procedure called IntraLasik uses femtosecond laser instead of microkeratome for flap creation.
- A precisely calculated amount of the corneal stroma is then ablated using an excimer laser and removed.
- The corneal flap is then replaced back to the original position after clearing the ablated tissue remnants by irrigation.
- An endothelial pump is frequently used to produce a marginal corneal dehydration that can stabilize the corneal flap without the requirement of any sutures. The adherence of the flap and its stability is carefully confirmed after the surgery.
- Topical steroid, NSAID and an antibiotic are then applied and the patient is usually discharged almost immediately. Use of medications including steroids is seldom required beyond the first post-operative week. The use of bandage contact lens (as done in PRK) is not required in LASIK procedure.
Unlike PRK, performing the LASIK procedure on same day in both eyes is safe. Correction of any residual errors is relatively easy with LASIK. The flap made in the initial surgery can be lifted within a period of 6 to 12 months for repeat procedures.
Intra-operative flap related complications like formation of button holes, irregular or incomplete flaps, thin small flaps, amputation of flap, and rarely corneal perforation may be associated with LASIK procedure. Post-operatively the flap may wrinkle, can get distorted or dislocated. Some patients can develop keratitis with infiltrates on the flap which may sometimes necessitate topical steroid therapy to resolve. Epithelial ingrowth can occasionally develop under the flap. Flap related complications are more common in patients with a thin cornea. Some complications similar to PRK like over/undercorrection, dry-eyes and gradual regression of correction are occasionally seen with LASIK. Rare complications of LASIK include optic neuropathy, corneal ulceration, infiltrates in the corneal periphery and infectious keratitis. LASIK is not associated with pain or corneal haze like PRK and recovery is rapid unlike PRK. Patients with high degree refractory errors having thinner cornea are not suitable for LASIK. Other contraindications of LASIK are similar to that of PRK below.
Laser Assisted Sub-Epithelium keratomileusis (LASEK)
Laser assisted sub-epithelium keratomileusis (LASEK) is a surface based procedure using the excimer laser. The procedure keeps the corneal epithelium intact and aims at changing the corneal shape by ablation of the corneal stroma. LASEK is ideal in patients with thin cornea who are not suitable for PRK.
- An ultra-thin layer of epithelium is lifted with a trephine blade after loosening the layer with alcohol.
- The stromal ablation is then performed with an excimer laser.
- The intact corneal epithelium acts like a bandage in this procedure.
- The healing of epithelium is slightly slower than LASIK but comparable to PRK.
EPI-LASIK is an alternative to LASEK procedure which avoids exposure of the cornea to alcohol. In EPI-LASIK an epi-keratome is used to remove the top layer of corneal epithelium instead of a trephine blade.
Photorefractive Keratectomy (PRK)
PRK is one of the excimer laser-based photo ablative procedures that are currently performed. Energy from the beam of the excimer laser is used to ablate corneal tissue which is controlled by a computer. The laser beam ablates and removes microscopic quantities of the corneal tissue as required to suitably reshape the cornea. The ablation done with PRK exerts minimal damage to the surrounding tissues. PRK can be used to treat patients with mild to moderate myopia of up to -6D (diopters), astigmatism of up to 3D and low degrees of hyperopia. The treatment of patients with myopia involves flattening the central anterior part of cornea by ablation. In patients with hyperopia, the ablation is done in the periphery and this makes the cornea more vertical.
PRK vs LASIK
The outcome of PRK is stable and predictable in individuals with mild to moderate myopia. The overall results are comparable with that of LASIK which is now the preferred method. The main disadvantage of PRK compared to the LASIK procedure is the long recovery time required for PRK. However, in a person with a thin cornea where LASIK is not ideal, PRK may be the preferred option.
- PRK is usually done as an outpatient procedure under local anaesthesia.
- Laser ablation of the cornea involves removal of the corneal epithelium which is completed in 30 to 60 seconds time.
- A topical antibiotic, NSAID and steroid is applied after the laser ablation is completed.
- The cornea is then covered with a bandage contact lens. The bandage lens is retained in the eye until the healing of corneal epithelium. This may take 3 to 4 days after the procedure.
- In the absence of any signs of infection the antibiotic therapy is discontinued 2 to 3 days after the removal of bandage. The topical steroid may be continued for a period of 2 to 3 months following the procedure.
PRK is generally considered to have excellent safety profile. The complications in the post-operative period include photophobia (light sensitivity), blurred vision, watery eyes, corneal discomfort, pain and delayed corneal healing. These complications usually ease significantly within one month and completely resolves within 3 to 6 months. Use of sunglasses with suitable UV protection while in bright light is helpful in patients with photophobia during the post-operative period. The pain in the postoperative period is generally mild due to the application of topical analgesic and bandage lens. Some patients may experience severe pain requiring systemic analgesics.
A corneal haze can develop postoperatively leading to glare at night. This may be present for 3 to 6 months. The vision normally improves with healing of the epithelium but it may slightly fluctuate in the first three months following the procedure. The cornea normally heals within 2-3 days but sometimes it may be delayed. Occasionally PRK is associated with corneal scarring and corneal defects causing halos and hazy vision, worsening of the refractive error, overcorrection or undercorrection, astigmatism, reduced corneal sensation, infection, acute corneal necrosis, dry eyes, slow regression of the corrected refractive error and decreased night vision.
PRK is contraindicated in patients with viral (herpetic) keratitis, unstable and progressive myopia, glaucoma, corneal diseases, and cataracts. PRK is also not recommended in patients with autoimmune diseases, pregnant and nursing women, and immune-suppressed individuals.
Laser Thermal Keratoplasty (LTK)
Thermal keratoplasty (TK) is a procedure to change the shape of the cornea by performing a ring of small burns on the periphery of the cornea. Following the burns the cornea constricts producing the desired change. The procedure can be done with help of electric probes of high-frequency or by laser. The latter is performed with a Holmium-YAG laser and is known as laser thermal keratoplasty (LTK). It differs from procedures using the excimer laser in that LTK uses infrared (thermal) energy whereas the excimer laser uses a cool beam. The laser in thermal keratoplasty ablates the anterior most part of the corneal stroma. Thermal keratoplasty procedures are safe and effective in treatment of low hyperopia and in some patients with astigmatism. This procedure is also of importance in improving presbyopia. TK and LTK procedures are generally performed as outpatient procedures under local anesthesia. Procedure-induced astigmatism and regression of hyperopia requiring repeat treatments are the most common complications associated with thermal keratoplasty procedures.