Myopia, or nearsightedness, is a refractive error where distant objects appear blurred. It occurs because the eye is either too long or the cornea is too steeply curved, causing light to focus in front of the retina instead of directly on it. Laser eye surgery is a widely used method designed to correct this focusing error. By permanently altering the shape of the cornea, these procedures shift the focal point back onto the retina, providing clear distance vision.
Reshaping the Cornea to Correct Vision
The cornea, the clear, dome-shaped front surface of the eye, is responsible for the majority of the eye’s focusing power. In a myopic eye, the cornea’s curvature causes light rays to converge too quickly. The core principle of laser eye surgery for myopia is to flatten the central curvature of the cornea, which reduces its refractive power.
This precise flattening is achieved using an excimer laser, a device that emits a cool beam of ultraviolet light. The laser vaporizes microscopic amounts of tissue from the middle layer of the cornea, known as the stroma, in a highly controlled manner. By removing more tissue from the center than the periphery, the laser reshapes the cornea into a flatter profile. This change in shape ensures that incoming light rays are refracted less sharply, directing them to focus precisely on the retina.
The amount of tissue removed is customized based on the individual’s prescription and the specific shape of their eye. This process is known as ablation and is the mechanism by which the refractive error of myopia is neutralized. The precision of modern laser technology allows for highly accurate correction.
Different Types of Laser Eye Surgery
Three main procedural methods are used to deliver the corneal reshaping for myopia correction, each varying in how the excimer laser accesses the corneal tissue. These techniques are Laser-Assisted in Situ Keratomileusis (LASIK), Photorefractive Keratectomy (PRK), and Small Incision Lenticule Extraction (SMILE). While the goal is the same—to reshape the cornea—the procedural steps determine the recovery time and suitability for various patients.
LASIK
LASIK is the most common technique, involving the creation of a thin, hinged flap on the cornea’s surface, usually made with a femtosecond laser. This flap is lifted to expose the underlying stroma. After the excimer laser performs the tissue ablation to correct the myopia, the flap is repositioned, adhering naturally without sutures. The quick adherence of the flap contributes to LASIK’s characteristic rapid visual recovery.
PRK
PRK, or variations like Trans-PRK, is a surface ablation technique that does not use a permanent flap. Instead, the surgeon gently removes the cornea’s outermost layer, the epithelium. The excimer laser then reshapes the underlying corneal tissue directly. A bandage contact lens is placed over the eye while the epithelium regenerates, resulting in a longer initial healing period and recovery time compared to LASIK.
SMILE
SMILE is a newer, minimally invasive procedure using a femtosecond laser to create a lenticule, a small, lens-shaped piece of tissue, entirely within the intact cornea. The laser creates a small, “keyhole” incision, typically less than four millimeters. The surgeon extracts the lenticule through this small incision, changing the corneal shape without a large flap or extensive surface removal. This small incision is thought to offer biomechanical stability advantages and potentially reduce the incidence of post-operative dry eye compared to LASIK.
Determining Candidacy and Realistic Outcomes
Candidacy for laser eye surgery depends heavily on a thorough pre-operative assessment of eye health and physical characteristics.
Degree of Myopia
The primary consideration is the degree of myopia, measured in diopters (D). While many clinics safely treat myopia up to approximately -8.0 D, some advanced centers can correct prescriptions up to -12.0 D. Individuals with extremely high prescriptions, exceeding -10.0 D, may be better suited for alternative refractive procedures like an implantable collamer lens (ICL).
Corneal Thickness and Stability
Corneal thickness is a limiting factor because the laser must remove tissue for correction. If the cornea is too thin, removing the necessary tissue could compromise the eye’s structural integrity and increase the risk of ectasia, a condition where the cornea bulges outward. A minimum residual stromal bed thickness must be maintained after the procedure to ensure safety and long-term stability. Additionally, the patient’s refractive error must be stable, typically meaning the prescription has not changed significantly for one year. This stability ensures the correction is based on a fixed visual error. The patient’s overall eye health is also assessed, as conditions like severe dry eye or certain autoimmune disorders can disqualify a person from surgery.
Expected Outcomes
Realistic expectations regarding post-surgical vision are part of the candidacy discussion. The goal of the surgery is to significantly reduce or eliminate dependence on corrective lenses, with many patients achieving 20/20 vision or better. For those with high myopia, the outcome may be slightly less predictable, but the majority still achieve functional vision of 20/40 or better. In a small percentage of cases, a minor undercorrection may occur, which can often be refined with a secondary procedure, known as an enhancement, after the eye has fully healed.
Managing Post-Surgical Complications
As with any surgical procedure, laser eye surgery carries the potential for side effects and complications, although serious issues are rare. The most common side effect is temporary dry eye, which occurs because the procedure can temporarily decrease tear production or affect the corneal nerves responsible for tear signaling. This dryness usually resolves within the first few weeks or months, though it may require medicated eye drops and diligent management during the healing phase.
Visual disturbances are also frequently reported in the early post-operative period, including light sensitivity, glare, and halos around lights, particularly at night. These symptoms are usually temporary as the cornea heals and the brain adapts to the new visual input. In LASIK, there is a small risk of issues related to the corneal flap, such as displacement or epithelial ingrowth, where surface cells grow beneath the flap.
A less common complication is regression, where the vision slowly shifts back toward the original myopic prescription over time. Regression is more likely in patients who began with a very high degree of myopia. In the rare event of an undercorrection or overcorrection, a follow-up enhancement procedure can often be performed to fine-tune the result.