Laser-Assisted In Situ Keratomileusis, commonly known as LASIK, is a widely performed refractive surgery that corrects vision problems like nearsightedness, farsightedness, and astigmatism. This procedure reshapes the cornea to allow light to focus properly on the retina, reducing or eliminating the need for glasses or contact lenses. Determining candidacy for this procedure involves a thorough review of a patient’s overall health, eye anatomy, and vision history. The following guidelines are generally used by surgeons to assess whether a person can safely and effectively undergo LASIK.
Fundamental Eligibility Requirements
A person must be at least 18 years old, the minimum age approved by the U.S. Food and Drug Administration (FDA) for LASIK. This age threshold is important because the eye needs to have achieved ocular maturity before undergoing a permanent reshaping of the cornea. Many surgeons prefer to wait until a patient is in their early to mid-twenties, as vision can continue to change during young adulthood.
The stability of a person’s vision prescription is another fundamental requirement for a successful long-term outcome. A stable prescription means that the eyeglass or contact lens power has not changed significantly, typically by more than 0.5 diopters, for a period of 12 to 24 months. Operating on an eye with an unstable prescription may lead to a poor result, as the vision could continue to shift after the surgery is complete. Additionally, candidates must be in good general health, which serves as a factor in determining the body’s ability to heal properly after the surgery.
Ocular Health and Measurement Criteria
Detailed anatomical measurements ensure the procedure can be done safely without compromising the cornea’s structural integrity. One of the most important measurements is corneal thickness, as the LASIK procedure involves removing tissue from the corneal stroma. A normal cornea measures around 520 to 555 micrometers, and most surgeons require a minimum preoperative thickness of about 500 micrometers.
The residual stromal bed, the tissue remaining after the flap is created and ablation is performed, must measure at least 250 micrometers. Maintaining this minimum thickness preserves corneal stability and prevents corneal ectasia, a complication where the cornea bulges forward. The degree of refractive error is also measured, and there are typical limits for correction: nearsightedness (myopia) is generally treated up to -12.00 diopters, farsightedness (hyperopia) up to +6.00 diopters, and astigmatism up to 6 diopters.
Pupil size in low light conditions is assessed because a large pupil can affect candidacy. If the pupil is larger than the area of the cornea that is reshaped by the laser, a person may experience visual side effects such as halos or glare, especially when driving at night. Modern laser technology and treatment profiles have helped to mitigate this risk, but a thorough evaluation of pupil diameter is still a standard part of the screening process.
Conditions That Prevent LASIK
Existing eye diseases or systemic health issues can disqualify a person from LASIK. Conditions that cause a progressive weakening or irregular shape of the cornea, such as Keratoconus, are absolute contraindications because LASIK can worsen the condition. Chronic or severe dry eye syndrome is also a concern, as the procedure can temporarily reduce tear production, leading to prolonged discomfort and slower recovery in those already affected.
Other ocular issues that prevent surgery include uncontrolled glaucoma, advanced cataracts, or active eye infections like conjunctivitis. Certain systemic health conditions can interfere with the body’s ability to heal after surgery. Uncontrolled diabetes, for example, can cause fluctuations in vision and delay wound healing, making it a disqualifying factor until blood sugar levels are stabilized.
Autoimmune disorders such as lupus or rheumatoid arthritis, which affect the body’s healing response, may also prevent candidacy. Temporary hormonal changes during pregnancy or breastfeeding can cause temporary shifts in vision. The procedure is postponed until hormones and vision stabilize following the end of nursing.
The Pre-Surgical Evaluation Process
Determining LASIK candidacy begins with a comprehensive pre-surgical evaluation and detailed consultation with the surgeon. This initial meeting collects a complete medical and ocular history to identify potential contraindications before diagnostic testing. The most precise measurements of the eye are then taken using specialized equipment to map the cornea’s unique characteristics.
Diagnostic testing includes corneal topography and pachymetry, which map the cornea’s surface curvature and measure its thickness. A wavefront analysis may also be performed to measure how light travels through the entire eye, identifying subtle imperfections that a standard prescription cannot detect. Other checks include eye pressure measurements and a comprehensive dilation to examine the retina and optic nerve.
Cessation of contact lens wear is mandatory before diagnostic tests, allowing the cornea to return to its natural shape. Since contact lenses rest directly on the eye, they can temporarily warp the corneal surface, leading to inaccurate measurements. The required time off contacts varies depending on the lens type, ranging from several days for soft lenses to three or four weeks for rigid gas permeable lenses. The surgeon uses all collected data to determine final candidacy and will discuss alternative procedures, such as Photorefractive Keratectomy (PRK), if LASIK is not appropriate.