Prescribing for Astigmatism
Surgical Correction of Astigmatism, Part 1
BY JARED JAYNES, OD, & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Up to 35 percent of the population has 1.00D or more of refractive astigmatism (Hartstein, 1971). Wolffsohn et al (2011) reported that even 1.00D of uncorrected astigmatism may cause “significantly” decreased vision that could lead to reduced patient independence, affecting their quality of life. One of the available options to correct astigmatism is refractive surgery.
How Much Astigmatism is Acceptable?
In the United States, the Food and Drug Administration establishes guidelines for the amount of cylinder correction approved for individual refractive correcting laser platforms. Some lasers are approved for up to 6.00D of cylinder for myopic corrections and 5.00D of cylinder for hyperopic corrections. Outcomes are now similar for with-the-rule, against-the-rule, and oblique astigmatism. However, results may vary based on a patient’s refractive error and corneal thickness as well as the surgical procedure and laser utilized.
The ablation depth allowed in the most minus power meridian dictates the amount of cylinder that may be corrected. For example, if a patient’s vertexed manifest refraction is −2.00 −6.00 × 180, consider that the ablation depth necessary is essentially the same as for a patient who has −8.00DS vertexed refraction. A residual stromal bed of approximately 250 microns is desired. LASIK is the most utilized procedure, but PRK is a suitable alternative. However, if the cylinder correction needs to be enhanced or if regression occurs, it is generally easier to perform a second LASIK procedure.
A Look at Surgical Outcomes
Ivarsen et al (2013) reported that LASIK correction of highly astigmatic eyes (>2.00D) may lead to undercorrection of the cylinder: 21 percent undercorrected for myopic patients and 28 percent for hyperopic patients. The study authors found that the procedure was safe and that target spherical equivalent refractive error results were precise. However, modifications to the current treatment algorithms could improve the results for astigmatic correction.
Ablative excimer procedures like LASIK tend to be more accurate (fewer over- and under-corrections) and result in more stable outcomes compared to incisional procedures. Up to 3.00D of residual astigmatism may be corrected using limbal relaxing incisions (LRIs) or astigmatic keratotomy after partial-thickness or full penetrating keratoplasty (PK) and after cataract surgery.
LRIs can be made manually with a diamond knife or by using a femtosecond laser. Manual cuts may result in residual astigmatism and can occasionally induce irregular astigmatism. Arcuate incisions using a femtosecond laser for post-PK patients are reportedly safer and more precise for incisional astigmatic correction (Lee et al, 2013). More correction can be achieved with longer incision lengths. Also, limbal incisions tend to correct less astigmatism compared to midperipheral incisions; therefore, a shorter midperipheral incision typically has results similar to those of a longer incision near the limbus. However, limbal incisions usually result in less corneal aberration.
Know What’s Best for Patients
Surgical options continue to become safer and more predictable as technology advances. We must remain informed and up-to-date to provide our patients with the best options for optimally correcting their refractive error. CLS
We would like to acknowledge the helpful insights from Franklin Lusby, MD, and Joseph Stamm, OD.
For references, please visit www.clspectrum.com/references.asp and click on document #211.
Dr. Jaynes, a graduate of Southern California College of Optometry (SCCO), is currently a cornea and contact lens resident at SCCO. Dr. Edrington is a professor at the Southern California College of Optometry. You can reach him at email@example.com.