LASIK (Laser-Assisted In Situ Keratomileusis) is a surgical procedure that reshapes the cornea, the clear front surface of the eye. This improves how light focuses onto the retina, aiming to reduce or eliminate the need for glasses or contact lenses and provide clearer natural vision.
Defining Really Bad Vision for LASIK
For LASIK, “really bad vision” refers to high degrees of common refractive errors: myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. Myopia, where distant objects appear blurry, is indicated by a minus (-) diopter value. LASIK can typically correct myopia ranging from -0.5 D up to -12.0 D, though some clinics may treat up to -8.0 D, with others sometimes extending to -13.0 D or even higher depending on individual factors and technology.
Hyperopia, where close objects appear blurry, is measured with a positive (+) diopter value. LASIK is generally effective for correcting hyperopia up to +4.0 D, though some procedures may extend to +6.0 D. Astigmatism, characterized by blurred or distorted vision due to an irregularly shaped cornea, is also correctable. Most patients with astigmatism up to 4.0 D are considered candidates, with some modern lasers capable of correcting up to 6.0 D. These diopter ranges serve as general guidelines, but individual suitability for LASIK depends on various other factors beyond just the prescription numbers.
LASIK’s Capabilities for High Prescriptions
LASIK corrects vision by using an excimer laser to reshape corneal tissue. This laser emits cool, ultraviolet light pulses that remove microscopic amounts of tissue without causing heat damage to surrounding areas. For myopia, the laser flattens the cornea; for hyperopia, it makes the cornea steeper. Astigmatism is corrected by smoothing the cornea into a more symmetrical shape.
The procedure begins with creating a thin corneal flap, usually with a femtosecond laser, which is lifted to expose the underlying corneal stroma. The excimer laser then removes tissue based on the patient’s specific prescription. After reshaping, the flap is repositioned, allowing for quick recovery. Modern excimer lasers often incorporate wavefront-guided technology and eye-tracking systems, which map the eye’s entire optical system with high precision and ensure the laser stays on target even with slight eye movements. This advanced technology allows for customized treatment plans and has expanded the range of correctable prescriptions, even for higher errors.
Factors Limiting LASIK for Extreme Cases
Despite its capabilities, LASIK has limitations, especially for individuals with very high prescriptions. Corneal thickness is a primary concern, as the procedure requires creating a flap and removing tissue. A sufficient residual stromal bed is necessary for corneal stability. Surgeons generally prefer a corneal thickness of at least 500 µm, with a minimum residual stromal bed of 250 µm recommended by the FDA. For every diopter of correction, approximately 12 to 15 microns of corneal tissue are removed, meaning higher prescriptions require more tissue removal. If the cornea is too thin, there is an increased risk of complications like corneal ectasia, where the cornea bulges out, worsening vision.
Pupil size can also be a limiting factor, particularly large pupils, which were historically associated with visual disturbances such as glare and halos at night after surgery. While modern wavefront-guided LASIK has reduced these issues, some patients with very large pupils may still experience these symptoms. An unstable prescription, meaning significant changes in vision over the past year or two, typically disqualifies a candidate, as LASIK requires a stable refractive error for long-lasting results.
Various eye conditions also contraindicate LASIK, including keratoconus, a condition causing corneal thinning and bulging, as it makes the procedure unsafe. Other conditions like severe dry eye, glaucoma, cataracts, and certain retinal diseases can also affect candidacy or worsen post-surgery. Systemic health conditions, particularly autoimmune diseases like rheumatoid arthritis or lupus, or conditions that impair healing such as diabetes, can increase the risk of complications and affect recovery. Additionally, certain medications or hormonal changes, such as those occurring during pregnancy or breastfeeding, can lead to unstable vision or affect corneal biomechanics, making LASIK unsuitable during these periods.
Alternatives for Extreme Vision Correction
For individuals with extreme vision or who are not LASIK candidates due to limiting factors, several alternative procedures can provide significant vision correction.
Implantable Collamer Lenses (ICLs)
ICLs, also known as phakic intraocular lenses, are a viable option. These thin, biocompatible lenses are surgically placed inside the eye, typically between the iris and the natural lens, without removing corneal tissue. ICLs are effective for correcting moderate to high myopia, often up to -20 D, and can be a preferred choice for those with thin corneas or severe dry eyes.
Refractive Lens Exchange (RLE)
RLE, also called clear lens extraction, involves removing the eye’s natural lens and replacing it with an artificial intraocular lens (IOL). This procedure is similar to cataract surgery but is performed on a clear lens to correct significant refractive errors, particularly for those with severe farsightedness or presbyopia who are not suitable for LASIK. RLE can correct a wide range of hyperopia and myopia, and it is often considered for individuals over 40-50 years of age.
Photorefractive Keratectomy (PRK)
PRK is another surface ablation technique suitable when LASIK is not. Unlike LASIK, PRK does not involve creating a corneal flap. Instead, the outer layer of the cornea (epithelium) is gently removed before the excimer laser reshapes the underlying tissue. PRK can be recommended for patients with thinner corneas or very active lifestyles, as it maintains more of the eye’s structural integrity because no flap is created. While the recovery time for PRK is typically longer than LASIK, its outcomes are comparable, and it can sometimes correct higher degrees of refractive errors.