Laser-Assisted In Situ Keratomileusis, commonly known as LASIK, is a procedure designed to correct refractive errors by precisely reshaping the cornea. This outpatient surgery uses an excimer laser to modify the eye’s front surface, allowing light to focus correctly onto the retina. LASIK is not universally applicable and requires a thorough screening process to ensure patient safety and predictable outcomes. The strength of an individual’s prescription, measured in diopters, is the primary factor determining eligibility and establishing treatment safety boundaries.
Specific Numerical Limits for LASIK Eligibility
The question of what prescription is too high for LASIK is answered by established safety ranges based on the type of refractive error. For myopia, or nearsightedness, the maximum prescription generally treatable is between -10.00 and -12.00 diopters (D) of spherical equivalent. Treatment beyond this range significantly increases the risk of complications and reduces the predictability of the final visual outcome.
Farsightedness, or hyperopia, has a considerably narrower safe treatment window because the reshaping process is mechanically less efficient than for myopia. The upper limit for hyperopic correction is typically set between +4.00 D and +6.00 D. As the prescription approaches this maximum, the results may become less predictable compared to lower prescriptions.
Astigmatism, an imperfection in the curvature of the cornea, can be treated concurrently with myopia or hyperopia. The typical limit for astigmatic correction, measured in cylinder, is up to 5.00 D or 6.00 D. These numerical cut-offs are guidelines, and the actual maximum treatable prescription is uniquely determined for each eye during a comprehensive pre-operative examination.
The Role of Corneal Thickness in Safety Limits
The numerical limits for LASIK exist because the procedure physically alters the cornea by removing tissue, a process known as ablation. The higher the prescription, especially for nearsightedness, the greater the volume of tissue that must be vaporized by the laser. This tissue removal directly impacts the structural integrity of the eye, which is why a prescription can be deemed too high.
During LASIK, a thin flap is created on the corneal surface, and the excimer laser then reshapes the underlying tissue, called the stroma. The critical safety concern is the amount of tissue remaining beneath the flap after the ablation is complete, which is known as the Residual Stromal Bed (RSB). If the RSB is too thin, the cornea can weaken, leading to a progressive bulging and thinning condition called ectasia.
To prevent structural instability, a minimum safety thickness for the RSB must be maintained, commonly set at 250 microns. Many surgeons adopt a more conservative threshold, aiming to leave 300 microns or more for an extra margin of safety. An individual with a high prescription but a naturally thick cornea may still be eligible, while someone with a moderate prescription but a thin cornea may be disqualified.
Options When the Prescription is Too High
When a patient’s prescription or corneal anatomy exceeds the safe limits for LASIK, several alternative vision correction procedures are available. Photorefractive Keratectomy (PRK) is a surface ablation technique suitable for prescriptions just outside the LASIK range or for patients with thin corneas. Because PRK does not involve creating a corneal flap, it preserves more stromal tissue, making it a safer option for borderline thin corneas.
For individuals with extremely high myopia, such as prescriptions beyond -10.00 D or -12.00 D, or those with very thin corneas, Implantable Collamer Lenses (ICLs) are frequently the preferred method. This procedure involves surgically placing a corrective lens inside the eye, in front of the natural lens, without removing any corneal tissue. ICLs can safely correct myopia up to -20.00 D, offering a predictable and reversible solution for the highest refractive errors.
Refractive Lens Exchange (RLE) is another option, typically reserved for older patients with high hyperopia or those beginning to experience age-related focusing issues. RLE replaces the eye’s natural lens with an artificial intraocular lens, similar to cataract surgery. These alternative procedures ensure that vision correction is still possible even when LASIK is not recommended due to prescription strength or corneal thickness.