Article

Neglect negligible astigmatism no more

How to correct the low-cylinder cataract patient.

Astigmatism correction at the time of cataract surgery has become an important objective for reaching modern patient expectations, which include correcting all aspects of prescription needs at the time of surgery.

Most cataract surgeons have accepted this truth, especially when it comes to patients with higher degrees of astigmatism. And, it is very easy for us to mention, offer and educate patients on potential fixes for those with 3 diopters or more. What has become much more difficult to do, at least for some surgeons, is approach patients on the lower end.

Recent data1 from Warren Hill, MD, shows that roughly 73% of all cataract patients have greater than 0.5 diopters of pre-surgical astigmatism, and half of those patients have between 0.5 and 1.0 diopters. Thus, the majority of patients with preoperative astigmatism that needs to be addressed fall in the lower range, where it is often overlooked.

Fortunately, the modern cataract surgeon has many tools available to treat even these low amounts of astigmatism.

A LOOK AT OUR OPTIONS

We have two general ways to treat astigmatism:

  1. At the corneal plane, with either hand-cut limbal relaxing incisions (LRIs) or femto-cut arcuate incisions.
  2. At the IOL plane with toric IOLs.

Correcting astigmatism at the IOL plane, especially when using modern tools like intraoperative wavefront aberrometry, has been found to be extremely reliable, predictable and stable over time.2 Unfortunately for U.S. surgeons, the toric IOLs to which we have access can only correct down to 1.0 D of cylinder at the corneal plane. If the patient requires less than 1.0 D of cylinder correction, it must be done at the cornea via incision(s) or placement of main incision at steep axis. When creating a surgical plan, it is most important for the surgeon to know how much preoperative astigmatism the patient really has.

Prior to surgery, at a minimum, modern surgeons need to start their surgical astigmatism correction plan with a newer generation optical biometer (such as IOLMaster 700 [Zeiss] and Lenstar [Haag-Streit]). This ubiquitous clinical tool gives the surgeon the basic idea of magnitude and location of the patient’s anterior corneal astigmatism. Surgeons may want to consider additional measurements, including topography, manual keratometry and even the patient’s current spectacle prescription.

In general, the surgeon would use these additional tests to make sure that the astigmatism measured by the biometer makes sense. The measurements, by nature, will not be exactly the same, but they at least need to be in the same ballpark. The additional benefit of topography would be to identify the regularity of the astigmatism or any pre-existing keratoconus or ectasia or other causes of irregular astigmatism, such as epithelial basement membrane dystrophy.

PCA IS CRITICAL

While those tests are great for measuring anterior corneal astigmatism, modern astigmatism correction cannot be accurately addressed without accounting for posterior corneal astigmatism (PCA) as well. In addition to intraoperative aberrometry, Koch’s Rule3 with respect to PCA has been the biggest advance in astigmatism correction at the time of cataract surgery. In simple terms, PCA can subtract from the total corneal astigmatism in patients who have with-the-rule (WTR) astigmatism. Conversely, PCA can add to patient’s total corneal astigmatism in those who have against-the-rule (ATR) astigmatism.

PCA can subtract 0.5 to 0.6 D from total astigmatism in WTR patients and can add 0.2 to 0.3 D in ATR patients. This amount of change could radically alter the surgeon’s plan, especially at the lower range of astigmatism.

Let’s look at an example of a patient with ATR astigmatism, and how applying Koch’s Rule can both drastically alter our preoperative planning and improve our outcome in a low cylinder patient. Fortunately, most modern online toric calculators take PCA into account for us. In Figure 1, I use Alcon’s Legacy Toric calculator vs. Alcon’s modern generation Online Toric IOL calculator to demonstrate the difference:

Figure 1. Results with Alcon’s Legacy Toric calculator, which does not take PCA into account. A non-toric IOL was recommended.

Preop K’s: 42.0 x 090, 42.4 x 180 (0.4 D of ATR astigmatism)

AL: 23.4 mm

ACD: 3.2 mm

SIA: 0.1 D

As you can see, Alcon’s Legacy Calculator, which does not take PCA into account, called for a non-toric lens in this low cylinder ATR patient. However, in Figure 2, Alcon’s Online Toric IOL Calculator, which takes PCA into account, shows this low cylinder patient is a candidate for a T3 Toric IOL.

Figure 2. Results with Alcon’s Online Toric IOL Calculator, which takes PCA into account. A T3 Toric IOL was recommended.

THREE WAYS TO DETERMINE PCA

The above example shows the importance of PCA in modern astigmatism correction, especially at the low cylinder range. Those small adjustments the PCA makes on total corneal astigmatism may make some patients toric IOL candidates and, conversely, may show that a toric IOL could potentially over-correct, meaning that the patient could do better with corneal incisions to control low amounts of astigmatism.

Today’s cataract surgeon needs to think about and account for PCA when treating low cylinder patients. Currently, we have three ways to account for PCA:

  1. Using a PCA estimate like the Barrett formula built into the calculators.
  2. Actually measuring the PCA using equipment like a Pentacam (Oculus) and then applying Koch’s Rule.
  3. Using intraoperative aberrometry on the patient in the aphakic stage during surgery.

All of these techniques have pros and cons. The advantage of using the estimate of PCA through latest-generation online formulas is that it does not require the purchase of any new equipment. Using your current biometer, you can enter the patient’s data and the formula will estimate the PCA. Just by performing this step, a surgeon can increase accuracy from about 61% using the Legacy calculator to 75% using a latest-generation formula.4 The downside of this is the estimate is just that: an estimate. There are always outliers in which the estimate under or over reports the effect of PCA on the patient’s total corneal astigmatism.

Using equipment like a Pentacam can have an additional benefit of not relying on an estimate of PCA and its effect on total corneal astigmatism — it will actually measure PCA directly. Using Pentacam PCA measurements with LENSAR femtosecond laser at the time of cataract surgery will allow the surgeon to perform arcuate incisions based on the PCA measurement. The downside of these devices is that they require equipment purchase, which may be out of reach for some practices.

Also, while they directly measure total corneal astigmatism preoperatively, these devices cannot account for any astigmatism changes that occur from the regular corneal incisions created during cataract surgery. While such changes are usually small and they can be tracked and estimated, they are still a variable that must be accounted for.

The final way of accounting for PCA, and thus total corneal astigmatism, is measuring it at the time of surgery in the aphakic state, with intraoperative aberrometry (ORA, Alcon). The advantages: It is a direct measurement done after all corneal alterations are made. The downsides: It requires equipment purchase, and there is a slight learning curve for the surgeon while introducing it to the OR.

Figure 3. The ORA (Alcon) screen in a WTR patient. Notice the measurement (green) is less than the preoeprative astigmatism (blue). This is what we would expect based on Koch’s Rule and the effect the PCA has on total corneal astigmatism.

Figure 4. A hand-cut limbal relaxing incision using a guarded diamond blade knife.

NOW FOR THE PLAN

Once surgeons adequately account for PCA and its effect on total corneal astigmatism, they can plan for which method to employ to correct the astigmatism, either with a corneal incision or with an IOL.

Any patient with between 0.75 and 1.0 D of total corneal astigmatism could be corrected using a toric IOL. The only caution in that scenario is that any over-correction greater than 0.25 D would result in a flipped axis. Over-corrections resulting in a flipped axis under 0.25 D are much better tolerated if they leave the patient with WTR astigmatism rather than ATR astigmatism.

Patients who have a total corneal astigmatism of less than 0.75 D can be dealt with using corneal incisions. Several nomograms are available for surgeons to follow, including LRIcalculator.com for handcut incisions and LaserArcs.com for femto-cut arcuate incisions. Femto-cut arcuate incisions have the advantage of being more predictable than handcut5 due to accuracy of placement and uniformity of depth. Platforms like Lensar or a Verion-coupled LenSx account for cyclorotation for added predictability. Also, femto incisions can be left closed and then opened at a later date to titrate any residual astigmatism as needed.

Figures 5 and 6. Femto-cut arcuate incisions. Note uniform depth and square edge.

NO PATIENT LEFT BEHIND

The low-cylinder patient is often forgotten when it comes to astigmatism correction at the time of cataract surgery. Preoperative measurement and planning is critical to deliver outcomes commiserate with modern patient expectation. Understanding and applying current theories of posterior corneal and total corneal astigmatism is essential to correcting the low-cylinder patient. OM

REFERENCES

  1. Based on data from Dr. Warren Hill. Assumes mid-range distribution of pre-op astigmatism. Excludes irregular and other conditions that impact toric selection.
  2. Kessel L, Andresen J, Tendal B, et al. Toric intraocular lenses in the correction of astigmatism during cataract surgery: A systematic review and meta-analysis. Ophthalmolgy. 2016;123:275-286.
  3. Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013;39:1803-1809.
  4. Abulafia A, Barrett GD, Kleinmann G, et al. Prediction of refractive outcomes with toric intraocular lens implants. J Cataract Refract Surg. 2015;41:936-944.
  5. Roberts HW, Wagh VK, Sullivan DL, et al. Refractive outcomes after limbal relaxing incisions or femtosecond laser arcuate keratotomy to manage corneal astigmatism at the time of cataract surgery. J Cataract Refract Surg. 2018;44:955-963.

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