Correcting Astigmatic Error in Cataract Surgery
As cataract surgical techniques and IOL materials and design have advanced, both patients’ and surgeons’ postoperative expectations have increased dramatically. Cataract surgeons now routinely achieve the goal of emmetropic 20/20 postoperative visual acuity by using new technologies for accurate axial-length measurement and a wide power range of IOLs that yield predictable and consistent results. However, the issue of how to correct underlying preoperative astigmatism is still controversial, and the treatment modality may be critical to obtaining an emmetropic result. I have asked Michael E. Rosenberg, MD; Tal Raviv, MD; and Thomas S.K. Chi, MD, to share their approaches and to answer several specific questions about the following case study.
CASE PRESENTATION
A 66-year-old businessman complains of decreased vision in both eyes and is unable to perform his daily activities adequately. His exam was significant only for nuclear sclerotic cataracts, which account for this visual compromise, and the patient desires bilateral cataract surgery.
His preoperative refraction is +2.50 – 3.75 X 125 OD and +2.50 – 4.00 X 35 OS. The keratometry readings are 44.50/46.50 @ 55 OD and 44.25/46.75 @ 145 OS. The axial length is 22.57 mm OD and 22.49 mm OS.
The patient’s desired refraction is plano for distance. He has never tried monovision but is interested in the idea. Additionally, he has worn progressive spectacles successfully.
QUESTIONS
1. How would you address the case?
2. How should the astigmatic error be corrected (eg, corneal astigmatic cuts, clear corneal wound placement, LASIK, PRK, toric IOLs)?
3. Are multifocal IOLs an option?
4. Do you think this patient could be happy with only one trip (per eye) to the OR suite?
5. If the patient were a high hyperope (+12.0 D) with similar astigmatic issues, would piggyback IOLs be an option?
ABLATION IS THE ANSWER
Michael E. Rosenberg, MD, of Fort Lee, New Jersey, says he would need this patient’s corneal topography in order to ascertain whether refractive surgery could treat the astigmatic error. Additionally, he would have to measure the patient’s corneal thickness to determine if enough corneal tissue were present to allow safe photorefractive ablation after cataract surgery.
“As both eyes have almost 2.0 D of astigmatism, which is not corneal, according to keratometry readings, we can assume that it is partially lenticular astigmatism,” he says. “Unfortunately, we cannot predict how much of this lenticular astigmatism will be removed with cataract surgery.”
Dr. Rosenberg recommends sequential cataract surgery with an incision just anterior to the limbus in order to leave a large area of untreated cornea for future refractive surgery. He would aim for a postoperative spherical equivalent of -0.25 to -0.50 D. In addition, he would not use a toric or multifocal IOL in this case.
“I would not advise using a toric IOL, because we are unsure preoperatively of how much lenticular astigmatism will be corrected by the cataract procedure,” Dr. Rosenberg comments. “Although a multifocal IOL is an option, I would discourage its use, as any decrease in contrast sensitivity will only be compounded by the further decrease … [resulting from] the planned refractive procedure to eliminate astigmatism.”
After completing sequential cataract surgery in this patient, Dr. Rosenberg would perform a trial of soft contact lenses for monovision. Assuming patient satisfaction, he would proceed with either bilateral PRK or LASIK, in accordance with preoperative corneal thickness. Although Dr. Rosenberg notes that astigmatic keratectomy is an option if not attempting monovision at this point, he prefers to perform excimer photoablation because of the ease and predictability of enhancement procedures. If the patient were dissatisfied with the preoperative monovision evaluation, however, Dr. Rosenberg would instead perform trials with soft contact lenses in an effort to determine the amount of myopia with which to leave the patient after excimer surgery.
Dr. Rosenberg would not consider piggyback IOLs in a similar patient with high hyperopia, because he believes that the keratometry and axial lengths would still be within IOLs’ normal parameters. He sees no need for piggyback IOLs and would prefer to avoid their associated complications.
CHOOSE A TORIC IOL
To Tal Raviv, MD, of New York City, this highly astigmatic cataract patient is a good candidate for the STAAR Toric IOL (STAAR Surgical) (Figure 1).
“The IOL comes in two cylindrical powers of 2.0 D and 3.5 D, which translates to 1.4 D and 2.3 D [respectively] of cylindrical correction at the corneal plane,” he says. “I would perform phacoemulsification via an astigmatically neutral temporal corneal incision and implant the 3.5-D lens aligned to the corneal steep axis. I would rely on the topographically measured cylinder. Typically following cataract removal, the ‘intraocular’ or ‘lenticular’ portion of the astigmatism (ie, the difference between preoperative keratometric and manifest refractive cylinder) becomes negligible, although there can be exceptions. As a result, I would counsel the patient on the possibility of needing a second procedure, such as LASIK or PRK, postoperatively to ensure a more certain refractive outcome.”
Even though he believes that a toric IOL would likely address most of this patient’s refractive needs, Dr. Raviv notes that he would offer the patient no guarantees and would prepare him for the possibility of two procedures. He states that, despite technological advances, “the IOL calculation is still just an indirect approximate measurement,” and surgeons must appropriately manage patients’ expectations.
Dr. Raviv acknowledges incisional keratotomy, whether corneal or limbal, as a good option for patients with low astigmatism. Nevertheless, he adds that, despite being easy and economical to perform, astigmatic keratotomy lacks the accuracy, reproducibility, and long-term stability of toric IOLs and excimer lasers, especially for the treatment of more than 2.0 D of cylinder.
Like Dr. Rosenberg, Dr. Raviv expresses concern about diminishing the patient’s contrast sensitivity. He believes that current multifocal IOLs would not address the astigmatic error and would therefore need to be combined with LASIK or PRK. Dr. Raviv states that, because both procedures are independently associated with a decline in contrast sensitivity, the combination might too greatly affect the patient’s quality of vision.
He would discuss monovision in detail with this patient and perform a contact lens trial preoperatively, if possible. If the patient were unsure, Dr. Raviv would aim for a plano result and use the excimer laser for fine-tuning postoperatively.
For a highly hyperopic patient with a similar astigmatic error, Dr. Raviv would elect a bioptic approach, such as IOL implantation combined with LASIK, over a piggyback IOL. He comments that surgeons’ understanding of complications associated with piggyback IOLs (eg, interlenticular opacification) is limited, and he remarks that “newer, very high plus lenses are becoming available and should diminish the need for piggybacking.”
INCISIONS WILL HELP
“In the current environment of refractive cataract extraction, patients’ expectations are much higher than in the previous era of cataract surgery,” states Thomas S. K. Chi, MD, of Medina, Ohio.
His preference in this patient would be to use two arcuate transverse incisions at the 7-mm optical zone, rather than limbal relaxing incisions or a toric IOL, to correct the pre-existing corneal astigmatism (Figure 2). He has made astigmatic arcuate incisions for years, even before limbal incisions became popular, and has found them to be fairly predictable without significant postoperative irregular astigmatism or glare problems. Oftentimes, he will combine one arcuate incision with a limbal one that also functions as the site of the cataract wound when he wants to achieve 1.0 to 2.0 D of astigmatic corrections in a cornea exhibiting with-the-rule astigmatism.
“I will incorporate a 3-mm cataract incision within a 6-mm limbal incision at the steep meridian and place an additional arcuate incision at the usual 7-mm optical zone opposite the incision site,” he comments. “My limbal incision blade depth is set at around 600 µm.”
Dr. Chi estimates that this patient has approximately 2.0 D and 2.5 D of corneal astigmatism in his right and left eye, respectively. He would obtain corneal topography in this case in order to confirm the corneal astigmatism and detect any irregular astigmatism. Generally, if the astigmatism is mostly corneal, he will attempt to undercorrect the patient slightly. When a significant amount of lenticular astigmatism is present, Dr. Chi is more aggressive, because some of this astigmatism remains after cataract surgery.
He uses a personal nomogram based on the patient’s age, axis, and amount of corneal astigmatism to determine the length of the (usually) 30º, 45º, or 60º incisions. Frequently, Dr. Chi will take pachymetry readings at the astigmatic incision sites during preoperative office evaluations. He will set his astigmatic blade to 90% depth, or to approximately 550 µm if there are no pachymetry readings. In this patient, Dr. Chi would place two 45º arcuate incisions at the steepest axis in each eye at the 7-mm optical zone—the 55º meridian OD and the 145º meridian OS. Prior to making the arcuate incisions, Dr. Chi would inject viscoelastic into the anterior chamber in order to elevate IOP and decrease the astigmatic blade’s indentation of the cornea. If considering a laser refractive procedure after cataract surgery, he will place the astigmatic incisions at the limbus versus the 7-mm optical zone and use the limbal incision site as the cataract site.
Dr. Chi states that he would adjust the IOL power calculations for this patient because of the astigmatic correction needed. He makes a serious effort to fine-tune IOL power calculations, because the ultimate goal for the patient is a decreased dependency on bifocal spectacles. He usually does not use a 1:1 coupling calculation for the average keratometry reading, because he finds that the astigmatic incisions often result in a 2:1 or sometimes even a 3:1 coupling effect. He states that correcting corneal astigmatism assumes some resultant steepening of the flat meridian upon flattening the steep meridian. “This coupling of steepening and flattening of the cornea in the opposing meridian corresponds to collapsing the Conoid of Sturm to a single plane,” he explains. “For example, a 1:1 coupling would assume a 50% flattening and 50% steepening. A 2:1 coupling would assume a 66% flattening and 33% steepening, and so on and so forth.”
Dr. Chi would also aim for approximately 0.25 to 0.5 D of residual astigmatism when calculating the IOL power in order to minimize overcorrection.
To address the patient’s presbyopia, Dr. Chi would conduct a trial of monovision using contact lenses but would also offer the option of a multifocal IOL, because he has had success with the ARRAY lens (Advanced Medical Optics Inc., Santa Ana, CA). He notes that patients with small pupils may experience diminished multifocality and may need to undergo laser pupilloplasty to enlarge their pupil size. Another possibility would be for the patient to await FDA approval of an accommodating IOL, he says.
If this patient were extremely hyperopic and required a very high-powered IOL, Dr. Chi says that he would consider the use of a piggyback IOL. He has rarely needed this option, however, thanks to the wide range of IOL powers available. If piggybacking IOLs, he would place one implant in the bag and the other in the sulcus in order to maximize the interoptic distance and minimize the formation of interpseudophakic opacification.
COMMENTARY
I agree with Dr. Rosenberg’s approach to the case. The best way to predictably neutralize almost all astigmatic error in this case would be to use a clear corneal, temporal, phacoemulsification procedure (Figure 3) and then perform LASIK several weeks later. However, it may be difficult to convince a patient to undergo two separate procedures per eye with a minimum of three trips to an operating/laser suite; the patient may also have financial concerns, because most insurance carriers will not cover the LASIK astigmatic correction.
Realistically, I would probably perform temporal clear corneal phacoemulsification and make arcuate incisions along the steep axis, as indicated by topography, with a 7- to 8-mm optical zone, as Dr. Chi suggests. I have not, however, had success with the ARRAY IOL; despite extensive presurgical counseling, several of my patients have complained of glare symptoms and lack of clear vision at both near and distance. I agree with Dr. Chi that there are very limited indications for the use of piggyback IOLs, owing to the wide range of high-powered single and multifocal lenses currently available.
I have not used toric IOLs extensively because of the possibility of their rotation and need for readjustment. I tend to have a larger capsulorhexis with my PhacoTilt supracapsular technique, and the capsulorhexis’ greater size decreases IOL/capsule contact and might increase the off-axis decentration of the toric IOL. Moreover, most patients would construe a trip back to the OR to correct the IOL’s position as a complication, despite the best preoperative counseling, and I have concerns about again subjecting the patient to intraocular surgery’s inherent risks. As toric IOL technology improves, however, Dr. Raviv’s approach will clearly become the optimal choice.
In summary, there are multiple choices for correcting astigmatic error in cataract surgery. Generally, arcuate incisions in a large optical zone can correct up to 1.5 D of corneal astigmatism without causing adverse visual symptoms. Correcting more than 1.5 D of astigmatism is more problematic and depends on the surgeon and patient’s preference. It also entails extensive patient counseling about the possible need for additional surgery for which the patient may be required to bear financial responsibility.
Michael E. Rosenberg, MD, is in private practice in Fort Lee, New Jersey, and specializes in corneal and refractive surgery. Dr. Rosenberg may be reached at (201) 947-5929.
Tal Raviv, MD, is in private practice in New York City and is an attending cornea and refractive surgeon at the New York Eye and Ear Infirmary. Dr. Raviv does not hold a financial interest in the STAAR Toric IOL. He may be reached at (212) 717-4609; tal.raviv@nylasereye.com.
Thomas S. K. Chi, MD, is Assistant Clinical Professor at the Case Western Reserve University and is in private practice at Eye Care of Medina in Medina, Ohio. Dr. Chi does not hold a financial interest in the ARRAY lens. He may be reached at (330) 725-3937; tskchi@aol.com.
Douglas K. Grayson, MD, is Assistant Clinical Professor of Ophthalmology at the New York Eye and Ear Infirmary and Medical Director of Omni Eye Surgery of New York and Omni Eye Services of New Jersey. Dr. Grayson does not hold a financial interest in the Grayson Nucleus Manipulator. He may be reached at (212) 353-0030; douglas.grayson@verizon.net.
