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Astigmatic effect from phaco incision placement
Careful planning can reduce pre-existing astigmatism and improve final visual outcomes in cataract surgery patients.
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Many patients desire a refractive correction at the same time as cataract surgery. This makes sense because we can correct a large degree of spherical refractive error as well as astigmatism with proper planning. The phaco incision that we use during cataract surgery can have a significant effect on the astigmatism of the eye, and it needs to be accounted for in our calculations.
Many ophthalmologists split astigmatism patients into two primary groups: eyes that have with-the-rule (WTR) astigmatism and eyes that have against-the-rule (ATR) astigmatism (Figure 1). While a smaller number of patients will have oblique astigmatism, which typically has steep and flat meridians at about the 45° and 135° positions, the vast majority are either WTR or ATR. Our usual temporal phaco incision will affect these types of eyes differently.
Source: Uday Devgan, MD
With-the-rule astigmatism
According to convention, patients with a steep corneal axis at about 90° are the rule, thus this type of astigmatism is called WTR. The steep axis of the corneal astigmatism can be on either side of 90°, within about a clock hour. This means that the typical range for WTR astigmatism is a steep axis of between 60° and 120°. This type of WTR astigmatism is more commonly found in younger patients and myopic eyes and less so among the seniors who compose our typical cataract population. With our alphanumeric characters, a small degree of WTR astigmatism may help to increase depth of focus for enhanced reading vision in eyes with little or no accommodation.
Against-the-rule astigmatism
The cornea tends to slowly change over time and with age, and patients typically develop a shift from either no astigmatism or WTR astigmatism to a degree of ATR astigmatism. Among our cataract patients, who tend to be senior citizens, the most commonly seen astigmatism is ATR. Against-the-rule astigmatism has a steep corneal axis at the 180° meridian, with a span of a clock hour on either side, giving a range of 150° to 30°. This position means that our typical cataract surgery incisions, which are placed temporally, tend to be near this steep axis.
Temporal incisions
Using a temporal corneal incision for phacoemulsification gained in popularity about 20 years ago, and it is now the most common entry site for cataract surgery. There are advantages to a temporal incision: It provides easier access to the anterior chamber even in patients with prominent brows or narrow palpebral fissures, and it is farthest from the visual axis and therefore less prone to induce astigmatism. We have moved toward using smaller incisions, going from the wide 3- to 3.5-mm incisions to the smaller 2.2- to 2.8-mm incisions that are more commonly used now. While it is possible to make an even smaller incision for cataract surgery, the narrow lumens of the smaller instrumentation can impede the efficiency of the procedure. In addition, IOL choices are limited when it comes to designs that will fit through a sub-2-mm incision.
Corneal phaco incisions induce flattening and a decrease in the astigmatism where they are placed. Due to the corneal coupling effect, if one meridian of the cornea is flattened, then the corresponding meridian 90° away will be steepened by approximately the same amount. This means that the net or average corneal power, as used in IOL power calculations, is minimally affected by the main phaco incisions, accessory incisions or even limbal relaxing incisions.
Temporal phaco incision reduces ATR astigmatism
The total corneal astigmatism is the difference between the steep and flat axis powers. For example, if an eye has a keratometric power of 44 D at 180° and 43 D at 90°, the total astigmatism is the difference of 44 and 43, which equals 1 D of astigmatism (Figure 2). With the phaco incision placed at the 180° position, and with the assumption that it will induce 0.5 D of flattening, the new keratometric values will be 43.75 D at 180° and 43.25 D at 90°, giving a total astigmatism of 0.5 D (calculated by 43.75 minus 43.25). Note that the average corneal power before the incision (43.50 D) is the same as the average corneal power after the incision (43.50 D), thus illustrating the corneal coupling effect. We can see in this example that patients who have ATR astigmatism will benefit from a reduction in corneal astigmatism due to placement of the phaco incision at this axis.
Temporal phaco incision will worsen WTR astigmatism
For the smaller number of patients who have WTR astigmatism during the preoperative consultation for cataract surgery, we must carefully consider the placement of the incisions. Because WTR astigmatism means the 90° meridian is steepest, placing the phaco incision at this superior position could be helpful but it would require the surgeon to change positions. In many patients, particularly those with deep-set eyes, overarching brows or tight palpebral fissures, sitting superiorly and making the phaco incision at the 90° position may not be possible. We can still make our phaco incision temporally at the 180° position, but we must realize that this will worsen the corneal astigmatism (Figure 3).
In this example, the eye has a total of 1 D of WTR astigmatism that is steep at 90° with a corneal power of 44 D at 90° and 43 D at 180°. Then the phaco incision is made temporally at the 180° position. This will induce 0.5 D of flattening at this position so that the new corneal powers become 42.75 D at 180° and 44.25 D at 90°, thus giving a total of 1.5 D of corneal astigmatism. Note that the average corneal power both before and after the phaco incision is 43.50 D, thus having no effect on the IOL power calculation. In this example, the resultant 1.5 D could be addressed by either limbal relaxing incisions or use of a toric IOL.
We can even plan for the future shift from WTR to ATR astigmatism by choosing to leave our patients with just a little WTR astigmatism so that they have more years of less than 1 D of astigmatism. A patient who has 0 D of astigmatism after surgery can be expected to have 0.5 D ATR after 5 years and then 1 D ATR after 10 years. A better result may be to leave a patient with 0.5 D WTR after cataract surgery so that at 5 years the eye will have 0 D of astigmatism and then at 10 years, 0.5 D ATR. This gives more years with 0.5 D or less of astigmatism.
For patients who desire a refractive outcome at the time of cataract surgery, careful analysis and planning can reduce the pre-existing astigmatism and improve the final visual outcome.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: devgan@gmail.com; website: www.DevganEye.com.
Disclosure: Devgan reports no relevant financial disclosures.