LASIK, or Laser-Assisted In Situ Keratomileusis, is a widely performed surgical procedure designed to correct common refractive errors that cause blurry vision. The technique uses a laser to permanently reshape the cornea, the clear front surface of the eye, improving how light is focused onto the retina. Determining candidacy involves a comprehensive evaluation of a person’s visual stability, physical ocular characteristics, and overall systemic health. These criteria ensure the procedure is both safe and effective.
Essential Vision and Prescription Requirements
A fundamental requirement for LASIK is that the patient must be at least 18 years old, though many surgeons prefer candidates to be 21 when vision has stabilized. The eye’s prescription must have remained stable for a minimum of one to two years, meaning no significant change greater than 0.5 diopters during that period. This stability is crucial because the procedure is a permanent correction, and operating on a changing eye would lead to a poor long-term outcome.
LASIK corrects myopia (nearsightedness), hyperopia (farsightedness), and astigmatism within specific boundaries. Generally, the procedure can safely treat myopia up to approximately -12.00 diopters, hyperopia up to +6.00 diopters, and astigmatism up to 6.00 diopters. Prescriptions exceeding these limits may require too much corneal tissue removal, increasing the risk of structural compromise.
The thickness of the cornea is the most important physical measurement, as the procedure removes microscopic layers of tissue to reshape the eye. A healthy cornea is typically around 550 microns thick. Surgeons must ensure that the residual stromal bed—the tissue remaining after the flap is created and ablation occurs—is at least 250 to 270 microns thick for structural integrity. If the cornea is too thin, an alternative procedure may be necessary to prevent a rare but serious complication known as corneal ectasia. Pupil size is also assessed, as very large pupils may cause visual disturbances like halos or glare in low light conditions post-surgery.
Medical Conditions That Disqualify Candidates
Certain systemic health conditions can negatively affect the post-operative healing process, making LASIK an unsafe option. Autoimmune diseases such as lupus, rheumatoid arthritis, or Sjögren’s syndrome compromise the body’s ability to repair itself following the surgical creation of the corneal flap. These conditions interfere with the predictable healing necessary for a successful outcome. Immunodeficiency conditions, including uncontrolled HIV, can similarly delay or impair wound healing and increase the risk of infection.
Uncontrolled diabetes presents a risk due to its potential to cause fluctuating vision prescriptions and poor peripheral circulation, which hinders corneal tissue healing. Patients with diabetes must demonstrate stable blood sugar control to be considered for surgery. Temporary hormonal changes also act as contraindications, which is why pregnant and nursing women are typically advised to wait. Hormonal shifts can cause temporary changes in vision and increase the severity of dry eye, necessitating postponement until stability returns.
Several pre-existing ocular conditions also disqualify a patient from LASIK due to the risk of exacerbating the problem or causing structural failure. Keratoconus, a progressive condition where the cornea thins and bulges into a cone shape, is an absolute contraindication because LASIK further weakens the cornea. Severe dry eye syndrome is also a concern, as the procedure can temporarily worsen symptoms, and a pre-existing severe case increases the risk of chronic discomfort and poor visual results. Patients with advanced cataracts or uncontrolled glaucoma, characterized by high intraocular pressure, are generally unsuitable for LASIK. Certain medications, like those used to treat severe acne, can also impact corneal healing and must be discontinued well in advance of the procedure.
The Comprehensive Pre-Surgical Screening Process
Determining candidacy involves a detailed consultation that begins with the patient temporarily discontinuing contact lens wear, typically for one to four weeks. This allows the cornea to return to its natural, uncompressed shape, ensuring accurate diagnostic measurements. Failure to stop contact lens use can result in an overestimation of the refractive error and a poor surgical plan.
The eye care professional uses specialized equipment to gather the necessary biometric data. Corneal topography and tomography map the precise curvature, elevation, and shape of both the front and back of the cornea to detect subtle irregularities like keratoconus. Pachymetry measures the exact corneal thickness, ensuring enough tissue remains for safe ablation.
A wavefront analysis measures how light travels through the eye, identifying specific, higher-order visual errors unique to the patient that a standard glasses prescription cannot account for. The size of the pupil in low light conditions is also measured to plan the treatment zone and minimize night vision side effects. After all data is collected, the surgeon discusses the patient’s visual goals and outlines the expected outcome. If the patient is disqualified from LASIK due to thin corneas or high prescriptions, the consultation provides an opportunity to explore alternatives, such as Photorefractive Keratectomy (PRK) or Small Incision Lenticule Extraction (SMILE).