Astigmatism Basics | Jan/Feb '17

What Is Astigmatism, Anyway?

Cataract surgery has entered the arena of refractive surgery, and surgeons are now aiming for LASIK-like outcomes after their cataract procedures. To attain that goal, it is important to understand, diagnose, and treat astigmatism. This first article in this series will delve into the basics of astigmatism.

BREAKING IT DOWN

Astigmatism is present in more than 90% of the global population, and nearly 50% of individuals worldwide have more than 0.50 D of astigmatism.1 Although the cause of astigmatism is not fully understood, many theories concerning genetics and mechanical interactions between the cornea, eyelids, and extraocular muscles might have an effect on its development.

In an emmetropic eye, light that emanates from a point on a distant object is focused to a point on the retina, creating clearly focused images. Refractive errors, such as myopia and hyperopia, occur when the focusing power of the eye is too great or too little for its axial length, respectively, which results in blurred images. In eyes with astigmatism, refractive power is not uniform across all meridians, ie, light entering along the vertical meridian may be focused more strongly than light along the horizontal meridian. This differential refractive power will cause images to be smeared along the optical axis, resulting in blurred vision.

When the principle meridians of astigmatism (highest and lowest refractive powers) are orthogonal to each other and refractive power is uniform across each meridian, the astigmatism is considered regular. Typically, regular astigmatism can be corrected with glasses or contact lenses. When the principle meridians are not orthogonal or the refractive power varies along a given meridian, the astigmatism is considered irregular. Irregular astigmatism is much more difficult to treat.

TYPES OF ASTIGMATISM

There are five types of regular astigmatism. In compound myopic astigmatism, the real images from each principle meridian are focused anterior to the retina. In compound hyperopic astigmatism, the virtual images from the principle meridians would be focused posterior to the retina. In simple myopic astigmatism, the image from the stronger principle meridian is focused anterior to the retina, while the image from the other principle meridian lies on the retina. Similarly, simple hyperopic astigmatism results in the virtual image of the weakest principle meridian focused posterior to the retina, with the image from the other principle meridian focused on the retina. Finally, in mixed astigmatism, the real image from the stronger principle meridian is focused anterior to the retina, and the virtual image from the weaker principle meridian lies posterior to the retina.2

Ocular astigmatism may occur at any of the major refractive interfaces of the eye: the anterior cornea, posterior cornea, anterior lens, or posterior lens. Refractive power depends primarily on the curvature of the refractive interface and the difference in the refractive indices of the materials on each side of the interface. Therefore, astigmatism occurs when the principle meridians have different curvatures. Because the anterior cornea is the refractive interface with greatest power, the analogy that an eye with astigmatism is shaped more like a football than a basketball is often used to explain this to patients. Corneal astigmatism is typically classified according to the location of its steepest meridian.

In with-the-rule (WTR) astigmatism, there is greater refractive power (and increased curvature) along the vertical meridian as compared with the horizontal. Conversely, in against-the-rule astigmatism (ATR), the highest refractive power is along the horizontal meridian, and, in oblique astigmatism, the steepest corneal meridian is oriented at an oblique angle.3 Young adults typically display WTR astigmatism, and, with increasing age, there is a shift of astigmatism toward ATR. Internal astigmatism—the astigmatic contributions of the posterior cornea, anterior lens, and posterior lens—stays relatively stable over time.3

MEASURING ASTIGMATISM

Advances in technology and instrumentation have significantly improved our ability to measure and analyze the eye’s shape and optical properties, including astigmatism. Manifest refraction (retinoscopy) and wavefront analysis take the entire optical system into account and measure total astigmatism.4 Various devices can measure corneal astigmatism, such as automated and manual keratometry, Placido-based topography, Scheimpflug elevation mapping, and slit-scanning tomography. Corneal topographers (ie, Placido-based devices) measure the anterior corneal curvature and estimate the contribution of the posterior corneal surface. Tomographic devices use elevation-based analysis of cross-sectional images of the cornea to measure curvature and thus can provide information on both anterior and posterior corneal astigmatism.

Current devices can accurately measure anterior corneal astigmatism and are rapidly improving in their measurement of posterior corneal astigmatism. At this time, we can only measure internal or lenticular astigmatism indirectly by subtracting corneal astigmatism from total refractive astigmatism. The accuracy of all these measurements rests on the ocular surface status and tear film composition.

IMPORTANCE OF ASTIGMATISM

From a clinical standpoint, astigmatism management is important in both corneal laser vision correction (LASIK and PRK) and cataract surgery. Uncorrected astigmatism results in blurred vision, diplopia, glare, ghosting, and asthenopia. In refractive surgery, the total refractive astigmatism is treated by changing the shape of the anterior corneal surface, which has implications for cataract surgery in the future.

Cataract surgery has become a refractive procedure with increasing patient expectations. Treatment of astigmatism is vital in achieving spectacle independence and excellent visual outcomes. Traditionally, astigmatism from the posterior corneal surface was ignored because it was presumed to be minimal, but recent studies by Koch et al showed that posterior corneal astigmatism is important in the determination of total corneal astigmatism. If total corneal astigmatism measurements are based only on the anterior surface, this may lead to overestimation of astigmatism magnitude in WTR eyes and underestimation in ATR eyes.5,6 It is advisable to use more than one method (ie, automated keratometry and corneal topography/tomography) to measure corneal astigmatism and reconcile any disagreements between devices.

CONCLUSION

The reduction of astigmatism during surgery is critically important because it can result in significant improvement in visual quality and uncorrected vision. Surgical methods such as peripheral corneal relaxing incisions (both manual and femtosecond laser) and toric IOLs can be used to correct astigmatism at the time of cataract surgery.7

Educating patients about astigmatism is very important, especially if a patient will pay a premium price, whether for a toric IOL or a limbal relaxing incision. The concept of astigmatism is a tough topic to explain to patients, which is why educational materials such as brochures or websites with simulated vision images that represent blurriness secondary to astigmatism is helpful. It is also helpful to show color topography images when talking to patients.

1. Lee BS, Lindstrom RL, Reeves SW, Hardten DR. Modern management of astigmatism. Int Ophthalmol Clin. 2013;53(1):65-78.

2. Riordan-Eva P. Optics & refraction. In: Riordan-Eva P, Whitcher JP, eds. Vaughan & Asbury’s General Ophthalmology. 17th ed. McGraw-Hill Medical; 2007.

3. Read SA, Collins MJ, Carney LG. A review of astigmatism and its possible genesis. Clin Exp Optom. 2007;90(1):5-19.

4. Mozayan E, Lee JK. Update on astigmatism management. Curr Opin Ophthalmol. 2014;25(4):286-290.

5. Koch DD, Ali SF, Weikert MP, et al. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38:2080-2087.

6. Koch DD, Jenkins R, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: the effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013;39(12):1803-1809.

7. Lee H, Kim T, Kim EK. Corneal astigmatism analysis for toric intraocular lens implantation: precise measurements for perfect correction. Curr Opin Ophthalmol. 2015;26(1):34-38.

author
Zaina Al-Mohtaseb, MD
  • Assistant Professor of Ophthalmology and Associate Residency Program Director, Baylor College of Medicine, Houston
  • zaina@bcm.edu; Twitter @zaina1225

NEXT IN THIS ISSUE