Welcome back to part 2 of 2 of this blog series titled: Do I Have Astigmatism?
Last month we were able to break down the meaning, causes, classification, and types of astigmatism. This month we will continue the conversation as we focus on treatment options for this easily misunderstood topic.
Testing for Astigmatism
Astigmatism is detected in a routine eye exam, but can often be missed in a basic vision screening. The methods for determining astigmatism classification, type, and power are the same methods we use to determine nearsightedness (myopia) and farsightedness (hyperopia). 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.
An example of a glasses prescription used for nearsightedness and farsightedness would respectively be -2.50 SPH or +2.50 SPH. The SPH stands for sphere indicating there is no astigmatism and a spherical lens can be used to focus or bend the light onto the retina. If a patient has astigmatism, the glasses prescription could read -2.50 + 1.00 x 090, for example. For corneal astigmatism, this would indicate the overall spherical power would be -2.50 (nearsighted) and the amount of curvature difference on the cornea is 1.00 at 90 degrees away from the 3rd number in our prescription called the axis at “090”. So the power difference is 1.00 at 180 degrees (horizontally from 3 o’clock to 9 o’clock). This would mean the overall steepest curve on the cornea is -2.50 at 090 degrees (12 o’clock) and the flattest curve is 1.00 less than -2.50 making it -1.50 at 180 degrees (3 o’clock).
Soft Contact Lenses
Soft contact lenses are also a good treatment option for regular astigmatism and would have a similar prescription to the glasses prescription with a sphere power, cylinder power, and axis number. 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 focus the light on the retina.
Gas Permeable Contact Lenses
Gas Permeable Contact Lenses (Hard Contacts) are also a treatment option for astigmatism. These types of 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. Hybrid contact lenses can achieve the same visual correction as hard contact lenses but are designed with a very rigid center and a soft material as a skirt surrounding the rigid center. This allows for better comfort and centration while allowing the visual benefit of a rigid artificial spherical surface.
Lastly, in some cases astigmatism can be corrected surgically. Elimination of astigmatism, along with associated myopia (nearsightedness) or hyperopia (farsightedness) by surgical methods can allow patients to see clearly without glasses or contacts. There are a few different methods by which astigmatism can be surgically corrected, as we will describe below:
Refractive surgery such as LASIK or PRK can correct regular astigmatism. In both of these procedures, 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 or PRK is an appropriate treatment for young patients with no other significant eye problems who wish to eliminate their need for glasses or contacts.
For patients with cataracts, astigmatism can also be treated at the time of cataract surgery. Correcting astigmatism at the same time as cataract surgery can allow good distance visual acuity without the need for glasses or contacts. Higher amounts of astigmatism can be corrected by implanting a special type of intraocular lens (IOL) called a Toric IOL. When the surgeon places the IOL into the lens capsule the lens has to be rotated to the appropriate axis to properly correct the astigmatism. Another option for correcting milder amounts of astigmatism is through Laser-Assisted Cataract Surgery (LACS). A special laser, in addition to performing some of the initial steps of the cataract surgery, can be programmed to make microscopic arcuate incisions in the cornea, thereby changing the the irregularly curved surface. In so doing, the cornea becomes more spherical and allows the light to focus onto the retina in a single point focus.
With all of the treatment options our goal is to achieve the sharpest and clearest vision possible in the manner that best suits each individual patient. Both Dr. Gregory Hill and Dr. Geoffrey Hill offer cataract surgery with options for astigmatism correction by Laser-Assisted Cataract Surgery or toric lenses. Dr. Geoffrey Hill also offers LASIK or PRK surgery for younger patients with astigmatism who wish to be free of glasses or contacts.
This concludes our two-part discussion on astigmatism. Again, if you have questions or comments, feel free to post them or contact me, Timothy J. Blankenship, OD, directly at firstname.lastname@example.org.