Astigmatism
Astigmatism is a refractive error where the cornea (this is the clear window at the front of your eye) is not completely round but instead shaped more like an American football. This creates distortion, double images or shadowing in the vision at all distances. Correcting astigmatism with LASIK is the treatment of choice and can make a dramatic improvement to the quality of vision.
Occasionally astigmatism can be found by itself, but much more commonly together with nearsightedness (myopia) or farsightedness (hyperopia). When it is combined with these other refractive errors, it will increase the blur and distortion that the patient sees. When someone suffers from significant astigmatism (usually anything more than 0.5 of a diopter) they will require corrective lenses, usually glasses. Contact lenses are available for this condition, known as toric lenses, but are not well tolerated by some patients due to the lens rotating on the cornea, blurring vision.
LASIK is the treatment of choice for astigmatism and highly effective. The laser part of the procedure takes 5-10 seconds to evaporate a very thin layer of tissue within the cornea, reshaping its curvature to produce a more spherical cornea. This eliminates the double focus that is characteristic of astigmatism, allowing all rays of light to come to a single focus. The whole procedure is complete in approximately 10 minutes per eye.
In a normal eye, light enters through the clear cornea, passes through the pupil (the central opening in the iris) and continues through the lens located just behind the iris. If there is no prescription (refractive error), the cornea and lens focus parallel rays of light to converge together to make a clear image on the retina. This light-sensitive layer converts the light into information that passes along the optic nerve to the brain. The brain then processes this information and makes a picture for you to see.
Astigmatism is most commonly caused by an asymmetric curvature across the cornea. Imagine the shape of an American football instead of a basketball. Commonly, horizontal rays of light are focused at one point, and vertical rays at a separate point, rather than all rays arriving at a single focal point. Indeed, the word ‘astigmatism’ is derived from ‘a-stigma’, where ‘a’ means without and ‘stigma’ means point – hence there is no single point of focus of light. Astigmatism is a common refractive error.
Risk Factors
Factors that may increase your chance of astigmatism include:
- Heredity – a family history of astigmatism, eye disease, or disorders such as keratoconus
- Eye surgery – certain types of eye surgery, such as cataract extraction and lens replacement
- A history of corneal scarring or thinning
- A history of higher degrees of nearsightedness or farsightedness (myopic astigmatism and hyperopic astigmatism)
Laser surgery for astigmatism
Laser vision surgery (LASIK) for astigmatism is the treatment of choice in most cases in younger patients while Lens Replacement Surgery with Toric IOLs is often the best treatment for more mature patients. In LASIK, a femtosecond laser is used to create the LASIK flap, following which an excimer laser removes exactly the correct amount of corneal to tissue necessary to create a remarkably accurate focus for precision treatment of astigmatism.
Treatment Options
- Corneal Laser Treatment of Astigmatism
- The LASIK procedure for astigmatism
- The PRK procedure for astigmatism
The choice of procedure is mainly made on anatomical factors, plus the degree of your refractive error (the size of your prescription).
- Glasses and Toric Contact Lenses
Glasses have been used to correct vision since the 13th century, and possibly earlier and are the most common method of correcting astigmatism. Glasses usually incorporate a correction to help near or farsightedness which often occur together with astigmatism. Contact lenses that are specially designed to not rotate on the cornea, known as toric lenses, are also available. Some patients do not tolerate toric lenses well and often present for laser vision correction.
The main risk from contact lenses is bacterial infection, a potentially blinding condition. A recent study from Australia showed the 20 year long-term risk of bacterial abscess from contact lenses is 100 times greater than from LASIK.
Refractive Lens Exchange (RLE)
RLE treatment is possible using intracular implant lenses (IOLs) that also correct astigmatism, known as toric lenses, ordered on a case by case basis. You can learn more about refractive lens exchange surgery here.
Myopia typically begins in the teenage years, and gradually progresses with worsening distance vision over the next 10-15 years. With higher levels of nearsightedness, the condition may begin before the age of 10. Some children do not realize at first that their sight is not as good as it could be. They may be able to read books and do close work without difficulty. However, seeing distant objects such as the board at school may become hard. They may think this is normal and not tell anyone. Schoolwork may suffer for a period before the condition is identified and treatment provided.
Myopia typically stabilizes in the early 20s and you no longer need regular increases in the strength of their glasses to see. That is correct time to consider laser vision correction. Most patients are ready to have laser eye correction between 25 and 30, although some may be ready earlier, depending on, e.g. when they first needed glasses.
Like other forms of refractive surgery, LASIK and PRK reshape the cornea to enable light entering the eye to be correctly focused on the retina for clear vision. To correct astigmatism, the cornea needs to be reshaped so that the curvature is more equal in all directions. It needs to be changed from a football shape to more like a basketball.
This is achieved using an excimer laser, a cool-beam ultraviolet device that can evaporate corneal tissue without heat or burning. Each pulse of the excimer beam will remove a depth of 0.2 microns (thousandths of a millimeter). Pulses are placed across the cornea in a pre-determined pattern to remove enough tissue to correct the specific astigmatic prescription. Using the extremely fast WaveLight laser, most prescriptions can be fully corrected in only 10 seconds. Your two options for astigmatic laser correction are:
- LASIK for astigmatism
- PRK/LASEK procedure for astigmatism
The choice of procedure will depend on the severity of prescription and anatomical factors inlacing the shape and thickness of your corneas.
LASIK, or “laser-assisted in situ keratomileusis,” is the most commonly performed surgery to correct astigmatism. It can also be used for farsightedness (hyperopia) and nearsightedness (myopia).
First, our surgeon uses a Ziemer femtosecond laser to create a very thin, precise circular “flap” in the cornea. N.B. An older version of LASIK used a mechanical surgical tool called a microkeratome – this is outdated and should be avoided.
The surgeon then gently folds back the hinged flap to gain access to the underlying cornea tissue (known as the stroma) before using the excimer laser to correct your astigmatic prescription. After the laser has corrected the astigmatism and any other associated prescription, the flap is then laid back in place covering the area where the corneal tissue was removed. The cornea is allowed to heal naturally. The skin surface will heal across the edge of the flap within 24 hours. The surgery requires only topical anaesthetic drops, and no bandages or stitches are necessary
In most cases LASIK is pain-free and completed within 15-20 minutes for both eyes. The results are usually obvious instantly – most patients can see fairly well as soon as they sit up at the end of the procedure.
PRK (an acronym for photo-refractive keratectomy) is a form of laser surgery to correct astigmatism. It can also be used for mild degrees of farsightedness (hyperopia) and nearsightedness (myopia).
PRK was the initial type of laser eye surgery for vision correction and was first performed in 1988. It is the fore-runner to the most commonly performed procedure: LASIK
PRK recovery takes a longer than recovery from LASIK eye surgery, between 3 and 5 days for most patients. The first 2-3 days are often uncomfortable and vision is not very clear.
PRK is still commonly performed and is used mainly when LASIK is not possible, for example a thin cornea or when the patient expresses a preference. Like LASIK excimer surgery, PRK works by reshaping the cornea allowing light entering the eye to be correctly focused on the retina for clear sight. With the WaveLight system, we do not tell the laser which procedure is being performed – the excimer laser ablation is exactly the same for PRK and for LASIK.
For both PRK and LASIK, the excimer laser sculpts the stromal layer of the cornea to correct your astigmatism and other associated correction. The main difference between PRK and LASIK is that with LASIK a thin, hinged flap is created on the cornea to access the inner layers; in PRK no flap is created – just the surface skin layer is removed and the excimer laser energy is applied to the top layer of corneal collagen (the stroma).