Astigmatism: Causes, Types, Symptoms, Treatment- Lenses, Refractive Surgery
What is Astigmatism?
Astigmatism is an eye condition characterized by imperfection in the eye's curvature. Astigmatism is a common and mild condition and it usually can be treated easily. Astigmatism causes blurriness of the vision.
When the anterior surface of the eye, i.e. the cornea, has a mild difference in the curvature in one place from the other, then it results in astigmatism. The corneal surface is supposed to be even and smooth in all directions. If the corneal surface becomes flat or steep in some places, then it results in astigmatism which causes blurred vision in all directions. Astigmatism is usually congenital in nature and can be present in a combination with farsightedness or nearsightedness. In majority of the cases, it is not serious enough to warrant treatment; however, when this condition progresses then treatment comprises of corrective lenses and surgery.
Causes and Types of Astigmatism
There are two regions in the eye, cornea and lens, which help in focusing on images. In a normal, healthy eye, both these focusing features have a completely smooth curvature. However, if the cornea or lens isn't curved evenly and smoothly, then the light rays do not get refracted properly resulting in a refractive error. Astigmatism is a type of refractive error where the cornea or lens is curved more steeply in one place than another. If the cornea is distorted in shape, then it is known as corneal astigmatism. If the lens is distorted, then it is known as lenticular astigmatism. Both these types of astigmatism cause blurred vision which may be in many directions, such as vertical, horizontal or diagonal.
Other refractive errors which can occur in combination with astigmatism are:
- Nearsightedness (Myopia): This condition results when there is an exaggerated corneal curvature or if the eye is longer than normal. In this condition, the light is focused in front of the retina instead of being focused on the retina itself, resulting in blurry vision for distant objects.
- Farsightedness (Hyperopia): This condition occurs when the curvature of the cornea is less or if the eye is shorter than normal. Hyperopia is the opposite of myopia where the light is focused behind the posterior region of the retina resulting in blurry vision for nearby objects.
Astigmatism is often a congenital condition, i.e., it may be present from birth. Astigmatism can also develop after an injury or trauma to the eye or any disease or surgery to the eye. Astigmatism isn't caused or worsened if:
- You are reading in insufficient light
- You are squinting
- You are sitting very close to the television.
Symptoms of Astigmatism
- Distorted/blurred vision.
- Strain on the eye.
- If the vision worsens to the point that the patient is not able to perform daily activities of living, then a visit to the doctor is necessary.
Investigations for Astigmatism
- Measuring the light which is reflected from the corneal surface using a device known as a keratometer helps in quantifying the severity and the direction of corneal astigmatism.
- Measuring the curvature of the cornea is done with the help of a device known as keratoscope where a light is used which projects rings on the cornea. Keratoscope helps in measuring the severity of the corneal curvature and helps in diagnosing astigmatism.
- Corneal topography is a procedure where the changes in the corneal surface curvature can be measured. Corneal topography comprises of a videokeratoscope where the keratoscope is fitted with a video camera.
Treatment for Astigmatism
The main aim of treating astigmatism is to correct the uneven curvature of the cornea which is causing the blurred vision. Treatment options include corrective lenses and refractive surgery.
Corrective lenses for Astigmatism:
Corrective lenses can be worn to treat astigmatism. They correct the astigmatism by counteracting the uneven curvature of the cornea. The different types of corrective lenses are:
- Eyeglasses: Eyeglasses with special lenses can be made to counterbalance the uneven shape of the eye. Along with correcting astigmatism, eyeglasses can also help in correcting other refractive errors like farsightedness or nearsightedness.
- Contact lenses: As with eyeglasses, contact lenses can also correct astigmatism. There are many types of contact lenses available such as soft, hard, disposable, extended wear, bifocal and rigid gas permeable. Patients should discuss with their doctor about the various contact lenses available and decide on the one which is best suited for them.
Refractive Surgery for Astigmatism:
This treatment method for correcting astigmatism does so by reshaping the eye's surface. Refractive surgery options include:
- LASIK Surgery: Laser-Assisted In-Situ Keratomileusis, i.e. LASIK surgery is a procedure where the ophthalmologist, with the help an instrument keratome, makes a fine, circular hinged cut into the cornea. A special cutting laser can also be used to make this cut. The ophthalmologist lifts the flap and with the help of an excimer laser sculpts the shape of the cornea. The difference between an excimer laser and other lasers is that it doesn't generate heat.
- Photorefractive Keratectomy (PRK): In this procedure, the outer protective layer of the cornea is removed, after which an excimer laser is used to change the cornea's curvature.
- Laser-Assisted Subepithelial Keratomileusis (LASEK): In this method, the layer of cornea which is folded back is extremely thin and this makes the eye less prone to damage or injury. For patients having thin cornea, LASEK is a better option for them. Those patients who are involved in sports or are at a higher risk for an eye injury should also opt for this procedure.
- Radial Keratotomy: This procedure was used in the past for treating astigmatism, but is now obsolete and is not commonly performed these days.