Many patients believe that they are unable to access laser eye surgery due to their prescriptions involving astigmatism. While this may have been true in the past, with advancements in our ability to make precise calculations during laser eye surgery to accurately correct astigmatism, LASIK for astigmatism is now in fact just one of several refractive surgical techniques used to give an astigmatic patient clear vision without depending on glasses or contact lenses.
What is Astigmatism?
Before delving into how LASIK eye surgery, or any laser eye surgery, can be used to correct astigmatism, it helps to first understand what astigmatism is.
Astigmatism is a type of refractive error, an imperfection of the eye’s focusing system which leads to blurry or uncomfortable vision. It may occur alongside either long sightedness (hyperopia) or short sightedness (myopia). Most cases of astigmatism arise due to an uneven curvature of the cornea, which is the transparent dome at the front surface of the eye. This results in light entering at different positions through the cornea being bent, or refracted, by different degrees. The outcome of this is that different rays of light come to focus at two distinct points on the retina at the back of the eye, which we then perceive as blurry or distorted vision. A less common source of astigmatism is the crystalline lens inside the eye, in a case known as lenticular astigmatism.Many eyecare practitioners will use the analogy of an AFL football versus a soccer ball – on an AFL football, following one line along the ball from tip to tip results in a flatter curvature than if you were to trace along the perpendicular meridian, around the girth of the ball. Conversely, a soccer ball is perfectly round and you will find the same degree of curvature no matter which line you follow around the ball. When it comes to the cornea or lens, light is refracted more sharply when passing through the steeper curve, which results in these rays coming to a focus point earlier within the eye than rays that passed through the flatter line of curvature, which will focus to a point further in the eye.
Patients with uncorrected astigmatism may find they experience:
- Glare sensitivity, such as seeing oncoming car headlights or streetlights as a “starburst”
- Ghosting, which refers to the perception a faint double image where there is only one object
- Headaches, often after prolonged reading or other near vision activities
Originally, the only way to correct astigmatism was through the use of spectacle lenses. Since then, contact lens technology has developed to be able to correct higher degrees of astigmatism, and now soft contact lenses can often cover prescriptions of up to almost -6.00 dioptres of astigmatism; custom-made hard contact lenses can often correct astigmatism even higher than this.
Patients with astigmatism can be assured that now LASIK eye surgery, other forms of laser eye surgery such as PRK, and refractive techniques that don’t involve a laser, such as refractive lens exchange, are considered safe and accurate methods of surgically correcting astigmatism.
How Does LASIK for Astigmatism Work?
LASIK for astigmatism works much the same way as general LASIK eye surgery. Laser eye surgery procedures work on the premise of reshaping the cornea through the process of photoablation, the removal of select areas of corneal tissue in order to redirect the passage of light through this surface.
After topical anaesthetic is applied, a flap of superficial corneal tissue is first created using either a manual bladed instrument or a femtosecond laser tool, and then moved aside while still remaining attached to the eye. An excimer laser is then applied to the underlying corneal layers for the reshaping process, and the corneal flap then repositioned when the work is done.
In order to accurately correct refractive error with LASIK eye surgery, whether the script involves astigmatism or not, precise mathematical wavefront measurements are taken of the eye and then fed into a complex computer algorithm, along with other biometric data of your eye. This then guides the excimer laser during surgery to ensure the correct areas of the cornea are subject to photoablation of just the right amount to correct your prescription. As astigmatic scripts can be a complex combination of both corneal and lenticular sources of astigmatism, it is important to be able to achieve precise biometric measurements prior to going into surgery, something that was not always available. With advancements in the ability to make accurate wavefront calculations, LASIK for astigmatism of up to around 5 dioptres has become possible.
As with any laser eye surgery, general eligibility criteria do apply for people interested in LASIK eye surgery. As LASIK involves removal of corneal tissue, it is important to establish a minimum corneal thickness to ensure that a sufficient amount of tissue remains to maintain the mechanical stability and integrity of the eyeball. As higher prescriptions typically require a greater amount of photoablation, a patient with a script within the treatable range may find they are in fact unsuitable for LASIK due to insufficient corneal tissue available to address their script. Such people may be guided towards another refractive procedure, such as PRK, intraocular contact lens, or refractive lens exchange.