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Management of astigmatism during cataract surgery

Correcting corneal astigmatism during cataract surgery helps achieve the freedom from spectacles that patients desire.

Issue: November 15, 2006

ByUday Devgan, MD

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Uday Devgan

Because of increasing interest in refractive cataract surgery, it is important to be able to achieve emmetropia postoperatively. This is done by selecting the appropriate IOL, which typically corrects the spherical portion of the refractive error, and by managing the patient’s corneal astigmatism.

Correcting corneal astigmatism at the time of cataract surgery is instrumental in achieving the freedom from spectacles that patients desire.

In my own practice, I have implemented techniques of astigmatism management, in particular those pioneered by Louis D. “Skip” Nichamin, MD, and Doug Koch, MD. In this article, we will summarize the basic concepts behind astigmatism correction at the time of cataract surgery, including the effects of making corneal incisions.

Coupling

Coupling explains how a change in the steepness of one meridian will affect the meridian 90° away. With peripheral corneal relaxing incisions, the coupling ratio is approximately 1:1, which means that a decrease/flattening in corneal power by 1 D at the incision will result in an increase/steepening of the corneal power in the meridian 90° away, with no change in the spherical equivalent.

Due to coupling, there is no need to change the anticipated spherical IOL power when we perform these astigmatic correcting corneal incisions (Figure 1).

Incisions

Any incisions that are made in the cornea have the potential to change the curvature and therefore the dioptric power of the cornea in that meridian. Typically, corneal incisions cause flattening at the axis where they are made. The basic concepts are:

1. The larger the incisions, the greater the flattening (Figure 2).

   The larger the arc length of the corneal incisions, the more effect it will have in flattening the cornea at that meridian. Due to the coupling effect, arc lengths of more than 90° are ineffective.

Example of coupling. With corneal incisions made at the steep axis of 90°, there is flattening at 90° and a resultant steepening at 180°. The spherical equivalent is the same before and after; therefore, no change in the anticipated spherical IOL power is required.

2. The more central the incisions, the greater the flattening (Figure 3).

   Most surgeons prefer using limbal relaxing incisions (LRIs), which are made at the periphery of the clear cornea. Due to this peripheral location, they tend to be more forgiving and less likely to cause irregular astigmatism, and they tend to heal better, as well. However, due to the distance from the central cornea and the increased thickness of the cornea at the periphery, these incisions have less effect than astigmatic keratotomy incisions, which are more centrally placed.

3. For penetrating incisions, the shorter the tunnel length, the greater the flattening (Figure 4).

   Creating an “astigmatically neutral” clear corneal incision during cataract surgery requires the incision to have a sufficient tunnel length. This reduces the compromise of the corneal structure at the incision site and induces little change in the corneal astigmatism.

   For increasing the astigmatic effect of our corneal incisions, we can make the tunnel length shorter; however, this results in an incision that may be more prone to leaking during the postoperative period. We can also vary the position of the corneal incision so that it is placed on the steep axis and, therefore, any induced flattening will help the patient by reducing astigmatism.

4. For nonpenetrating incisions, the deeper the incision, the greater the flattening (Figure 5).

   Most nomograms for LRIs call for nonpenetrating incisions that are placed perpendicular to corneal tissue. With incisions that are made at 80% or 90% of the corneal thickness, as measured by pachymetry, there is a significant flattening of the corneal astigmatism. As the incisions become more shallow, their effect is less, and incisions at less than half corneal depth have little effect on the corneal curvature and power.

   Incisions that are more than 90% of pachymetry may result in a corneal perforation, and care should be taken to err on the side of safety.

Timing and helpful hints

For cases of mild astigmatism, I find that varying the placement of my incision and varying its construction are sufficient. By making the main corneal phaco incision shorter, it will have a greater flattening effect. Some surgeons have even advocated using paired, full-thickness, penetrating corneal incisions to further decrease astigmatism.

For cases in which there is a more substantial amount of preoperative astigmatism, I prefer making LRIs at the end of the cataract procedure. I have found the nomograms of Drs. Nichamin and Koch to be quite accurate, and they can be implemented easily.

It is imperative that you check and double check your numbers and calculations so that you accurately place your LRIs. If your incisions are off by 90° (the most common error), you will likely double your patient’s astigmatism instead of reducing it. Remember that the plus cylinder axis of the MRx, the steeper K on keratometry and the LRI position should all be at the same axis.

Finally, I usually aim to correct most, but not all, of the astigmatism. Due to the corneal coupling action, an overcorrection of the astigmatism will result in a flipping of the steep axis and an unhappy patient.

By simply reducing the patient’s astigmatism at the time of cataract surgery, we can deliver emmetropia, good uncorrected visual acuities and high patient satisfaction.

For more information:

• Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.

References:

• Nichamin LD. Nomogram for limbal relaxing incision. J Cataract Refract Surg. 2006;32(9):1408.
• Wang L, Misra M, Koch DD. Peripheral corneal relaxing incisions combined with cataract surgery. J Cataract Refract Surg. 2003;29(4):712-722.