Astigmatism: What is it? How can it be corrected?
Most of us have probably been told we have some degree of astigmatism. Symptoms can include blurred vision and monocular double vision, but what exactly is astigmatism?
The term astigmatism is used to describe a cornea that is not round. It is shaped more like a football instead of a basketball. Like a football, the astigmatic cornea has two curves: a steeper one and a flatter one 90 degrees away. These two curves bend light entering the eye, causing two images to form in the back of the eye on the retina.
As a comparison:
- A person with standard myopia or hyperopia (farsightedness) may see a dot as a blurred circle;
- A person with astigmatism may see the same dot as a blurred oval or a frankfurter-shaped blur. The variation depends on the how much astigmatism is present.
What are the treatments for astigmatism for patients with cataracts?
Our doctors will determine if a visually significant cataract is present. Astigmatism will be corrected with the cataract surgery, if the astigmatism is in the lens.
However, if the astigmatism is in the cornea, the astigmatism remains when the cloudy opaque lens is removed. If good, uncorrected vision is desired, the astigmatism needs to be addressed in addition to the standard lens implant surgery.
How can Astigmatism be corrected?
Astigmatism can be corrected with eyeglasses, toric (astigmatic) soft contact lenses and gas permeable (rigid) contact lenses. Surgically, astigmatism can be corrected with laser vision correction (99% accurate), or for small amounts (less than 1.5 diopters) of astigmatism, Limbal Relaxing Incisions LRI (80% accurate), and Toric Intraocular Lenses in conjunction with cataract surgery (95% accurate).
Several intraocular lenses are approved by the FDA for the surgical correction of astigmatism during cataract surgery. The most commonly recommended are:
- the Tecnis Symfony Toric Lens
- the Alcon IQ toric IOL
Typically, a patient’s eye is marked while sitting upright as the eye may rotate (cyclotorsion) when they lay supine for surgery. Surgery is painless and takes about 15 to 20 minutes. If the lens rotates more than 5 degrees it may be necessary to return to the operating room weeks later to rotate the lens to the desired axis.
If no cataract is present, LASER vision correction is an excellent modality to treat astigmatism.
The usual type of astigmatism is symmetric regular astigmatism. In case of asymmetric or irregular astigmatism other treatments are necessary.
Keratoconus is a progressive non-inflammatory (usually) bilateral thinning of the cornea associated with asymmetric or irregular astigmatism.
It typically is a contraindication for LASIK as well as any of the advanced technology lenses used in cataract surgery such as Crystalalens AO, Tecnis Multifocal or ReSTOR lenses which treat presbyopia (inability to read up close after age 40).
Sometimes keratoconus can be treated with a toric intraocular lens (Off-label) if the astigmatism is stable and the patient is not expected to ever wear gas permeable contact lenses. Collagen cross linking is a new treatment worldwide for keratoconus, but it is not yet approved by the FDA. It uses ultraviolet light to cure or harden the cornea after it has been pre-treated with riboflavin.
Pterygium is an abnormal growth from the conjunctiva (white part of the eye) onto the cornea (clear windshield-like dome in the front of the eye). It can cause asymmetric astigmatism and can be cured surgically. Small pterygia may be safely observed, whereas larger pterygia are best cured by surgical excision, typically with a conjunctival auto-graft secured with tissue glue (Tisseel) instead of stitches.
Tags: astigmatism, LASIK, PRK, cataract surgery, pterygium