Laser eye surgery, such as LASIK and PRK, reshapes the cornea to correct refractive errors like nearsightedness, farsightedness, and astigmatism. This process allows light to focus precisely onto the retina, improving vision and reducing the need for glasses or contact lenses. While highly effective, not everyone seeking correction is eligible. Strict, individualized standards must be met to ensure the safety of the eye and the long-term success of the outcome.
Vision Stability and Refractive Error Limits
A fundamental requirement for laser eye correction is refractive stability, meaning the patient’s prescription must not have changed significantly over a defined period. Surgeons generally require documentation showing a stable prescription for at least 12 months, and often 24 months. This stability ensures the procedure addresses a permanent optical condition rather than a temporary or still-progressing one.
The patient must also be at least 18 years old, as the eyes of younger individuals are frequently still developing. Many clinics prefer candidates to be 21 years or older to confirm the cessation of major refractive changes. Performing surgery on an unstable eye increases the likelihood of needing a retreatment later.
The range of the refractive error itself is another limiting factor, measured in diopters (D). Standard laser procedures can typically correct nearsightedness (myopia) up to approximately -12.00 D and farsightedness (hyperopia) up to about +6.00 D. Astigmatism, an uneven curvature of the cornea, is generally treatable up to plus or minus 6.00 D. These limits exist because correcting higher prescriptions requires removing more corneal tissue. Excessive tissue removal can compromise the structural integrity of the eye, so alternative procedures, such as Implantable Collamer Lenses (ICL), may be recommended for those with very high prescriptions.
Corneal Health and Structural Eligibility
The physical condition and thickness of the cornea represent a frequent reason for disqualification. Since laser correction works by reshaping the cornea, enough tissue must remain after the ablation to maintain the eye’s structural integrity. A typical, healthy cornea measures around 540 to 550 microns in thickness.
During a procedure like LASIK, a corneal flap of about 90 to 120 microns is created. The remaining underlying tissue, known as the residual stromal bed, must measure a minimum of 250 to 300 microns to prevent the cornea from weakening and bulging outward, a serious complication known as ectasia. For patients with thinner corneas, a surface procedure like PRK or LASEK, which removes only the epithelial layer, is often the preferred and safer option.
The cornea must also have an ideal shape. Structural conditions like keratoconus, where the cornea progressively thins and takes on a cone shape, are an absolute contraindication for laser vision correction. The eye must also be free from active eye diseases or infections. Existing conditions such as advanced glaucoma or severe cataracts also generally preclude laser surgery.
Pre-existing severe dry eye syndrome is another common ocular hurdle. The creation of a corneal flap during LASIK can temporarily worsen tear production. Patients with insufficient tear quantity or quality must often undergo treatment to manage their dry eye before the surgery can be performed safely.
Systemic Health and Lifestyle Contraindications
A patient’s overall health status is a significant factor in determining eligibility, as systemic conditions can interfere with the eye’s ability to heal post-procedure. Active or uncontrolled autoimmune diseases, such as lupus or rheumatoid arthritis, pose a risk because they can lead to unpredictable healing responses and increased inflammation. These conditions can delay recovery and potentially compromise the final visual outcome, so they must be well-managed prior to surgery.
Uncontrolled diabetes also presents a challenge, as it can impair the body’s wound-healing mechanisms and increase the eye’s susceptibility to infection. Fluctuating blood sugar levels can also cause temporary changes in the refractive error, making the stable prescription requirement difficult to meet.
Temporary biological states and certain medications can also be grounds for postponement or exclusion. Pregnancy and breastfeeding are temporary contraindications because hormonal fluctuations can lead to corneal changes and unstable vision measurements. Certain medications, such as the acne treatment isotretinoin, are known to induce severe dry eye and may be an absolute contraindication until a patient has been off the drug for a specified period.
The Comprehensive Pre-Surgical Evaluation
The final determination of candidacy is made through a thorough pre-surgical evaluation, which integrates optical and health criteria into a detailed patient profile. This consultation begins with a comprehensive medical history review, ensuring all systemic and ocular risk factors are accounted for. The assessment then moves to a series of instrument-based measurements that map the eye’s anatomy.
Corneal topography is a central component of this testing, creating a detailed 3D map of the cornea’s curvature to detect irregularities or early signs of conditions like keratoconus. Pachymetry is used to precisely measure the corneal thickness in microns, providing the surgeon with the data needed to calculate how much tissue can be safely removed. This measurement is individualized based on the patient’s prescription strength.
Measuring the pupil size, particularly in dim lighting conditions, is another important step, as a large pupil may increase the risk of night vision disturbances like glare or halos after surgery. The internal pressure of the eye is also checked to rule out undiagnosed glaucoma. These objective measurements, combined with a manual refraction test, generate the data set used to program the laser.
The surgeon reviews all this information to determine which procedure—LASIK, PRK, or SMILE—offers the greatest chance for a successful outcome based on the patient’s unique anatomical profile. This final discussion also involves setting realistic expectations regarding post-operative vision and the possibility of still needing reading glasses later in life due to age-related changes.