LASIK is one of the most widely performed elective procedures used to correct refractive errors like nearsightedness, farsightedness, and astigmatism. This procedure uses a laser to precisely reshape the cornea, the clear, dome-shaped front surface of the eye, allowing light to focus correctly onto the retina. While highly effective, LASIK is not a universal solution, and a patient’s vision can be too severe for the procedure to be performed safely. Eligibility for LASIK is determined by two main factors: the magnitude of the refractive error needing correction and the physical health and structure of the eye.
Maximum Refractive Error Limits
The capacity of LASIK to correct vision is directly limited by the amount of corneal tissue the laser must remove. Correction is measured in diopters (D). Attempting to correct prescriptions that are too high requires ablating tissue beyond the point of safety. Removing too much tissue compromises the cornea’s structural integrity, leading to a risk of the cornea bulging forward, a condition known as ectasia.
For nearsightedness (myopia), correction limits range from -10.00 D to -12.00 D of spherical error. This represents the maximum corneal flattening performed while maintaining a safe residual structure. When treating farsightedness (hyperopia), the limits are much lower, maxing out around +4.00 D to +6.00 D. Correcting hyperopia involves reshaping the cornea to make it steeper, which is less predictable at higher powers.
Astigmatism can be corrected up to 6.00 D when combined with either myopia or hyperopia. These numerical thresholds are not absolute, as the final decision depends on the patient’s unique corneal thickness and shape. Prescriptions falling outside these ranges are universally rejected for LASIK due to the unacceptable risk profile.
Structural and Medical Disqualifications
Even if a patient’s prescription falls within the acceptable diopter range, physical and medical factors can render them ineligible for LASIK. The most common structural barrier is insufficient corneal thickness, as the cornea must be thick enough to withstand the procedure. During LASIK, a thin flap is created, and the underlying tissue is reshaped.
After the laser ablation, a minimum of 250 microns of untouched corneal tissue, known as the residual stromal bed (RSB), must remain to ensure the eye’s long-term stability. If the cornea starts too thin, or if the prescription is high enough to consume too much tissue, that minimum RSB cannot be guaranteed. Therefore, a thorough measurement of the cornea’s total thickness is a necessary pre-operative step.
Certain ocular conditions also serve as disqualifiers, such as chronic severe dry eye syndrome. The creation of the corneal flap severs some corneal nerves, which temporarily reduces sensation and tear production. For a patient who already suffers from significant dry eye, LASIK can exacerbate the condition and prolong the recovery process.
The size of the pupil is another physical factor. Patients with naturally large pupils may experience night vision disturbances like halos and glare after the procedure. In dim light, the pupil dilates beyond the area the laser treated, causing light to scatter at the untreated edges. While modern wavefront-guided technology has reduced this risk, it remains a consideration during candidacy screening.
Systemic medical conditions that affect wound healing and immunity are disqualifiers. Uncontrolled autoimmune disorders like rheumatoid arthritis and lupus can impair the eye’s ability to heal after surgery. These conditions raise the risk of severe complications, including corneal melting and irregular healing, which can be detrimental to vision. Patients must demonstrate a stable vision prescription for at least one to two years prior to surgery, as a changing prescription indicates that the eye is still developing or unstable.
Alternative Vision Correction Procedures
For patients disqualified from LASIK due to high prescriptions or structural limitations, several effective alternatives exist. Photorefractive Keratectomy (PRK) is a surface ablation technique recommended for those with thinner corneas or professions involving a high risk of eye trauma. Because PRK does not involve creating a permanent corneal flap, it maintains more of the cornea’s structural integrity, leaving a stronger eye.
Implantable Collamer Lenses (ICL) are the primary alternative for individuals with very high myopia, sometimes correcting up to -20.00 D. This procedure involves surgically placing a permanent, thin lens inside the eye, behind the iris, without removing any corneal tissue. Since the cornea is left untouched, ICL is ideal for patients with thin corneas or severe dry eye who are disqualified from laser-based procedures.
Refractive Lens Exchange (RLE) is an option considered for patients over the age of 40 who have high refractive errors or are experiencing early age-related vision changes. The procedure is similar to cataract surgery: the eye’s natural lens is removed and replaced with an artificial intraocular lens designed to correct the refractive error. RLE is useful for correcting high hyperopia and eliminating the need for reading glasses, providing a long-term solution unconstrained by corneal thickness.