Astigmatism is a common condition where the eye’s shape prevents light from focusing evenly on the retina, causing blurred or distorted vision. An eye with astigmatism has a cornea or lens shaped more like a football than a perfectly spherical surface. Laser-Assisted In Situ Keratomileusis (LASIK) corrects this refractive error by reshaping the cornea. The effectiveness of LASIK depends not only on the prescription strength but also on the unique anatomical structure of the individual’s eye.
How LASIK Corrects Astigmatism
LASIK employs a precise laser to smooth the irregular curvature of the eye’s front surface. The procedure involves creating a thin, hinged flap on the cornea’s outermost layer, often using a femtosecond laser, which is lifted to expose the underlying corneal tissue, called the stroma. An excimer laser reshapes the exposed stroma by removing microscopic amounts of tissue in a process called photoablation. For astigmatism, the laser specifically flattens the steeper curvature and steepens the flatter curvature to create a more symmetrical shape. This adjustment ensures light rays focus correctly on a single point on the retina, improving visual clarity. Once the laser process is complete, the corneal flap is repositioned and heals naturally.
Maximum Astigmatism Correction Limits
The degree of astigmatism is measured in diopters (D), which indicates the lens power needed for correction. Modern LASIK technology can safely correct a wide range of astigmatic prescriptions, but practical and regulatory limits exist. The maximum astigmatism correction approved by the FDA for LASIK can be up to 6.00 diopters. However, the typical safe range for most patients treated by surgeons is often considered to be up to 5.00 diopters.
Treating astigmatism higher than 5.00 or 6.00 diopters requires the removal of a greater volume of corneal tissue, which increases the risk of complications. For these more severe cases, alternative procedures like Photorefractive Keratectomy (PRK) or implantable contact lenses (ICL) may be recommended instead of LASIK. The numerical capacity of the laser is one factor, but the true maximum is ultimately determined by the physical health and thickness of the individual cornea.
Anatomical Constraints Affecting Suitability
The anatomical structure of the eye imposes the most significant constraints on LASIK suitability, beyond the diopter limit of the prescription. The most important factor is corneal thickness, specifically the amount of tissue left untouched after the laser ablation. The tissue remaining beneath the flap, known as the residual stromal bed, must be thick enough to maintain the structural integrity of the cornea.
The United States FDA recommends that the residual stromal bed should be at least 250 micrometers thick to reduce the risk of a condition called corneal ectasia, where the cornea weakens and bulges. Many surgeons prefer to leave a thicker margin, sometimes aiming for 270 to 300 micrometers, for added safety and stability. Since a higher degree of astigmatism requires a deeper ablation depth, a patient with a high prescription may not be a candidate if their corneas are too thin.
Corneal stability is another major constraint, particularly concerning pre-existing conditions like keratoconus. This progressive degenerative disease causes the cornea to thin and bulge into a cone-like shape, leading to irregular astigmatism. Because LASIK involves removing tissue and further weakening the cornea, it is contraindicated for individuals diagnosed with keratoconus. The procedure may destabilize the eye and significantly worsen vision.
The size of the pupil in dim light can also affect candidacy, especially with high prescriptions. If the pupil dilates larger than the area of the cornea that received the full laser correction, it can lead to night vision issues like halos and starbursts. Although modern customized wavefront LASIK technology has reduced this risk, a large pupil combined with a high degree of astigmatism still warrants careful evaluation. The overall health of the eyes, including the presence of severe dry eye or certain systemic diseases, is factored into the final determination of whether LASIK is a safe option.