Pearls for LRIs With Presbyopic IOLs

If left uncorrected, even minimal astigmatism can compromise the visual results of these lenses after cataract surgery.

BY JONATHAN STEIN, M.D., AND ERIC DONNENFELD, M.D., F.A.A.O., F.A.C.S.

Until the recent advances in intraocular lens (IOL) technologies, surgical correction of astigmatism was a skill practiced primarily by refractive surgeons. With the increasing popularity of presbyopia correction at the time of cataract surgery, cataract specialists are now finding the need to surgically correct postoperative astigmatic ammetropias. A common misconception is that presbyopic IOL patients will tolerate small refractive errors, but nothing could be further from the truth. These patients are incredibly sensitive to even minor refractive errors. Refractive IOL surgeons must be willing and able to treat small degrees of astigmatism in order to make postoperative patients happy. In fact, the single most important aspect of being able to provide good surgical outcomes for patients with refractive IOLs is the ability to treat residual corneal astigmatism.

There are several different surgical options for treating astigmatism, including excimer laser photoablation, conductive keratoplasty and astigmatic keratotomy. However, not all of these options are available to the cataract surgeon. Furthermore, the ability to reduce corneal astigmatism with limbal relaxing incisions (LRIs) is more cost-effective and convenient than other techniques. Correction of pre-existing corneal astigmatism is not a covered benefit of Medicare or most other insurance plans. Therefore, patients must pay out of pocket for LRIs, or they can be bundled in with presbyopia-correcting IOLs.

Figure 1. Normal cornea (left) is shaped like a basketball, in which both axes are equal. Astigmatic cornea (right) is shaped like a football, in which one axis is steeper than the other.

The Problem of Astigmatism

Astigmatism decreases visual acuity through meridional blur. One axis of the cornea is steeper than the other, causing the cornea to distort images (Figure 1). Astigmatism of a value as small as 0.50 D may result in glare, symptomatic blur, ghosting and halos.¹

There are several types of regular astigmatism, based on the angle between the steep meridian and flat meridian of the cornea. Examples of regular astigmatism include with-the-rule astigmatism (Figure 2A), against-the-rule astigmatism (Figure 2B) and oblique astigmatism (Figure 2C). In general, irregular astigmatism (Figure 2D) should not be treated with LRIs.

LRIs are corneal incisions placed adjacent to the limbus to relax the steep axis of regular corneal astigmatism while steepening the flat axis. The procedure allows the eye to heal into a more spherical shape (Figures 3 and 4). There are several advantages of LRIs over astigmatic keratotomy, which is a similar incisional procedure that is performed more centrally toward the visual axis. These advantages include a decreased likelihood of perforation, less irregular astigmatism, a 1:1 coupling ratio and a reduced tendency to cause axis shift.

Figures 2A-D. The three types of regular astigmatism are defined by which axis is steepest (2A-C). Irregular astigmatism is shown in 2D.

We have found that most patients can tolerate postoperative astigmatism of 0.50 D or less. If their preoperative topography suggests they will have between 0.50 D and 1.50 D of residual cylinder, we plan to perform LRIs. We do not treat such a small degree of astigmatism with excimer photoablation, because we believe this procedure exposes patients to unnecessary expenses and potential postoperative complications. We reserve laser treatment for patients who have more than 1.50 D of postoperative astigmatism.

Planning for LRIs

There are a number of LRI nomograms for correcting small amounts of cylinder and many studies evaluating LRIs have been performed.^1-16 One study on the effectiveness of LRIs shows that there is a 60% average reduction of cylinder, with 79% of patients corrected to less than 1.0 D of cylinder, and 59% corrected to less than 0.50 D cylinder.¹⁷ The 60% reduction in cylinder compares favorably with the results achieved using toric IOLs, which result in a 58.4% mean reduction in cylinder.¹⁸

In general, it is best to practice the techniques of LRIs and develop your own nomogram to achieve consistent results. For surgeons new to LRIs, we suggest beginning by performing the procedure in the OR in conjunction with routine cataract surgery. We recommend adding peribulbar anesthesia to the operative regimen.

LRI Technique

We place LRIs at the beginning of cataract surgery because we prefer to work with a firm, well-hydrated eye. The cornea tends to become thinner as it dehydrates under the operating microscope.

We determine where to place the corneal incisions by referring to a printout of the patient's preoperative corneal topography. We orient the map to the patient'fs position on the operating table by holding it upside down. Next, we grasp the episclera at the limbus with a 0.12-caliber forceps approximately 180° away from the incision'fs intended site.

Figure 3. LRIs relax the steep axis of the astigmatism and allow the eye to heal into a more spherical shape.

Figure 4. Limbal relaxing incisions can easily be performed in the operating room for pre-existing astigmatism.

To place the incision, we apply a diamond blade (usually preset to 0.6 mm) to the eye and hold it in place for 1 second to ascertain that we have achieved the full depth before extending the cut to the required length. The length of the incision is determined by the amount of astigmatism we want to correct. We prefer to draw the diamond knife toward the surgeon to increase control. For 0.75 D of cylinder or less, I do not mark the cornea. For larger cylindrical errors, an astigmatism marker can be placed on the cornea and the cornea can be marked (Figure 5). One of the most common mistakes that novice LRI surgeons make is not pressing the LRI blade firmly against the cornea, which results in a shallow, ineffective incision.

When placing LRIs, it is important to consider how the surgically induced cylinder will affect the patient'fs existing astigmatism. For a patient who has against-therule cylinder of 0.5 D, it would be appropriate to perform a limbal relaxing incision at 180° preoperatively. For a patient who has 0.50 D preexisting cylinder with-therule, astigmatism will be corrected by using a superior incision for the cataract extraction. For surgeons who prefer an oblique cataract incision approach or are treating a patient with oblique astigmatism, a vector analysis of the pre-existing cylinder and the cylinder induced by the cataract surgical incision will determine the location and length of the LRI.

Figure 5. An astigmatism marker can be used and the cornea can be marked for correction of larger cylindrical errors.

Using an LRI Nomogram

Many LRI nomograms are adjusted for age and cylinder axis, making them detailed and complex, and giving the impression that the procedure is extremely precise and unforgiving. However, in my opinion this simply is not the case. LRIs are as much an art as a science. For this reason, we have developed a very simple nomogram that works extremely well (Table) and is ideal for the novice LRI surgeon. The Donnenfeld nomogram (DONO) is available on the internet at www.lricalculator.com (Figure 6). The online LRI calculator uses vector analysis to calculate where to make LRI incisions based on preoperative patient keratometry and the surgically induced astigmatism. The LRI calculator employs the Donnenfeld nomogram and provides a visual map of the axis and length of incisions that should be placed. A printout of the LRI calculator can be brought to the operating room and used as a guide when marking the cornea and performing LRIs. Another inaccurate assumption about why surgeons hesitate to perform LRIs is that they are not comfortable with this technique. Instead, I think that surgeons do not perform preoperative LRIs as often as they could because they do not have access to operating microscopes in their offices. A simple solution in these situations is to perform LRIs at the slit lamp (Figure 7). We prepare patients for in-office LRIs by anesthetizing their eye with lidocaine gel. When we are satisfied that the patient'fs head is positioned correctly on the chin rest, we look at the eye through the phoropter and use a diamond blade to create a small LRI, just as we would under an operating microscope. One small incision will correct 0.50 to 0.75 D of cylinder. The whole procedure takes approximately 30 seconds, and the patient'fs vision improves immediately. To prevent postoperative inflammation and infection, we prescribe prednisolone acetate 1% (Pred Forte, Allergan) and gatifloxacin 0.3% (Zymar, Allergan) four times daily for 5 days.

Figure 6. The Donnenfeld Nomogram is available on the internet at the website: www.lricalculator.com.

Avoiding, Treating LRI Complications

As with any surgical procedure, there are potential complications associated with LRIs, but most are either temporary or correctable. The procedure is generally not associated with glare or starburst as may be seen with radial keratotomy or astigmatic keratotomy.

The possible problems with LRIs include overcorrection, undercorrection, infection, perforation of the cornea, decreased corneal sensation, induced irregular astigmatism and discomfort. For patients with significant remaining astigmatism, it may be necessary to retreat by redeepening or extending the LRI. For overcorrections, we recommend waiting and then later cleaning out the wound with a Sinskey hook. then suturing it with 10-0 nylon if necessary. For smaller overcorrections, an excimer laser photoablation may be employed. We never recommend placing LRIs perpendicular to the original LRIs for consecutive cylinder, as this may induce irregular astigmatism. If the cornea is perforated, it may be self-sealing or a suture may be needed. To reduce postoperative discomfort, a topical NSAID can be prescribed.

We believe that LRIs are underutilized for correcting residual astigmatism after the implantation of presbyopia-correcting IOLs. One of the most important concepts is to treat small residual refractive errors. Ophthalmologists who successfully transition from cataract to refractive IOL surgery do so because they pay attention to the details that improve their patients' visual outcomes. For many of them, this means learning to perform LRIs. OM

Figure 7. Performing a limbal relaxing incision at the slit-lamp in the office.

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18. Package Insert. Acrysof Toric TM SA60T4IOL, Alcon Laboratories, Inc., Fort Worth, TX.