Astigmatism is a type of refractive error — a condition related to the shape and size of the eye that causes blurred vision. If we break down the meaning of the word, the “a-” at the beginning means “without” and “-stigma” means “point.” Astigmatism, therefore, is a refractive error that keeps light from coming into focus at a single point on the retina (the retina is a very important structure inside the eye that acts like a projector screen). So, to follow the analogy, having astigmatism means the “projector” is out of focus.
There are two causes of astigmatism: corneal and lenticular. The cornea is the clear dome-shaped structure at the front of the eye. Before reaching the back of the eye, light must travel through the cornea where it is bent or focused. When light enters the eye, the cornea is the very first surface in our eyes to focus or bend the light. Corneal astigmatism occurs when the cornea is not a perfectly spherical shape, and therefore, the light is bent unevenly. After the light passes through the cornea, it is also focused by a second structure — the crystalline lens (natural lens inside the eye, which we are born with). Lenticular astigmatism is a result of an irregular shape of the crystalline lens, which can also cause the light to be focused unevenly onto the retina.
Astigmatism can be combined with other refractive errors — either myopia (nearsightedness) or hyperopia (farsightedness). When an individual has both nearsightedness and astigmatism, it is called myopic astigmatism. In myopic astigmatism, the curvature in both orientations causes the light to focus in front of the retina. Likewise, if someone has farsightedness and astigmatism it is called hyperopic astigmatism and the focal points are behind the retina. If the light is focused both in front and behind the retina it is called mixed astigmatism.
It is still unclear why certain individuals develop a high amount of astigmatism, while others do not. Medical professionals do know that astigmatism appears to have some hereditary factors, so if you have a close relative who has developed a high amount of astigmatism, you should mention it to your eye doctor. Even if you do not have astigmatism now, you could be at increased risk for this common condition as you get older.
Regular vs. Irregular Astigmatism
Regular astigmatism is common and occurs when the steep corneal axis and flat corneal axis are oriented exactly 90 degrees away from each other. This type of astigmatism can simply be corrected with glasses or contact lenses. Irregular astigmatism is less common and more difficult to treat. It usually is the result of a dry ocular surface, trauma, scarring, corneal transplants, or eye diseases such as keratoconus or pellucid marginal degeneration. With irregular astigmatism, the shape of the cornea is highly irregular and the problem is not easily fixable with glasses, standard disposable soft contact lenses, or refractive surgery.
Astigmatism is a very common eye problem that is present in at least small amounts in all people. In fact, astigmatism is often present from birth. While some cases of astigmatism are very mild, some cases can be severe and lead to amblyopia (lazy eye) if untreated.
Sometimes symptoms can take a long time to become apparent and they may differ in each person. Symptoms of astigmatism may include:
- Blurry, distorted, or fuzzy vision at all distances
- Eye strain
- Eye irritation
- Difficulty seeing at night
- Squinting to see clearly
You may not be able to tell you have astigmatism — especially during the early stages. This is why it is important to schedule an annual eye exam to catch any vision problems that may develop. Our eye doctors can test your eyes and make a definitive diagnosis. The methods for determining astigmatism classification, type, and power are the same methods we use to determine nearsightedness and farsightedness.
In many cases, we use automated instruments to estimate the amount of astigmatism a patient has and then refine it with a subjective test we call a refraction. Refraction is a process in which an instrument called a phoropter is used to dial a variety of lens choices in front of a patient’s eye, and the patient chooses the options that look clearest. With each choice the patient makes, the lenses in the phoropter get closer to the final measurement of the refractive error.
Astigmatism can usually be treated with glasses, contact lenses, or refractive surgery.
Prescription eyeglasses for astigmatism include a special cylindrical lens to compensate for how light passes through the cornea (seen as CYL on your eyeglass prescription). Generally, a single-vision lens is prescribed, but in some patients over 40 years old, your eye doctor might recommend a bifocal.
Soft Contact Lenses for Astigmatism
Soft contact lenses are a good treatment option for regular astigmatism. These lenses are often called toric lenses. Typically the toric lenses are designed a little larger in diameter and fit a little tighter than basic spherical soft contact lenses. The idea is to have as little movement as possible in toric lenses to keep the correct lens powers in the proper position for optimal vision. If these lenses rotate, the corrective powers will be misaligned and therefore will not properly focus the light on the retina.
Gas Permeable Contact Lenses for Astigmatism
Gas permeable contact lenses (hard contacts) can be used to correct regular or irregular astigmatism. They can also correct higher amounts of corneal astigmatism than soft contact lenses. These lenses are very rigid and give the cornea an artificial spherical surface. In some cases, the lenses are used for irregular astigmatism from trauma, scarring, post corneal transplants, or diseases such as keratoconus or pellucid marginal degeneration.
LASIK for Astigmatism
LASIK is a safe and effective laser eye surgery that corrects regular astigmatism and other common refractive disorders. In LASIK, a laser is used to permanently alter the shape of the cornea, causing light that passes into the eye to be focused clearly on the retina. LASIK is an appropriate astigmatism treatment for young patients with no other significant eye problems who wish to eliminate their need for glasses or contacts.
PRK for Astigmatism
Photorefractive keratectomy (PRK), is a proven LASIK alternative that corrects astigmatism. Like LASIK, PRK reshapes the cornea to dramatically improve vision. The two procedures are similar with the main difference being that PRK does not utilize the creation of a flap in the top layer of the cornea that is routine with LASIK.
Higher amounts of astigmatism can be corrected by Toric ICL (implantable collamer lenses), which are similar to contact lenses but are surgically implanted. When the surgeon places the ICL into the lens capsule, the lens has to be rotated to the appropriate axis to properly correct the astigmatism.