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Postsurgical challenge: Correcting the vision of the pseudophakic patient
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 A piggy-back IOL may be an option for patients who need correction for an existing IOL. |
 The Keystone Stereogram can be used to measure aniseikonia. |
Today, a patient returns after cataract surgery, and we are amazed at the results. We usually find good vision with a perfectly placed IOL, sometimes requiring reading glasses and sometimes IOL-monovision. But what about those rare cases when everything does not work out so well?
One of my patients, M.M., is an example of one of these rare cases. A simple extracapsular cataract extraction (ECCE) was intended, but M.M. had no thick vitreous in the operative eye as a result of a vitrectomy a few years before. After the surgeon dropped the IOL he was about to place in her capsule, another emergency vitrectomy had to be performed. An IOL was eventually placed, but the placement was off, and the final power was incorrect. The complicated surgery produced an inflammatory storm that lasted a year. At some point in the surgery - I suspect during the prolonged vitrectomy - the iris sustained focal damage, which gave the patient polyopia.
For the next 6 months, she came in to see me regularly. I dreaded each visit because she was frustrated and I could do little for her. Gradually, the inflammation cooled and with it her temper. We both longed to get her monovision contact lenses back on her eyes, although I was dubious about the quality of the vision in the operated eye because of the polyopia. After a pinhole contact lens yielded no improvement, I should not have been surprised when monovision produced an ecstatically happy patient
The major problem was not the small blurred second image from the erosion of the iris but the aniseikonia from the misplaced IOL and the myopia in her other eye. Monovision completely disrupted any attempt to be binocular, and because she had worn monovision contact lenses before the surgery, she felt as though she had "her old eyes back."
Aniseikonia rare but difficult
Aniseikonia occurs as a complication of ECCE in about 2% to 3% of pseudophakic patients, according to Leonard Achiron, OD, FAAO, Emory Eye Center in Atlanta. Even if no anisometropia results, a 2.5% image difference can be found.
For instance, if one IOL is a haptic sitting in the sulcus and the other is placed in the posterior capsule, the nodal points are profoundly affected. Constants that surgeons use depend on location, so if the location does not work out quite right, the power will not be as intended.
"When I have patients like that, " said Dr. Achiron, "they're a nightmare. Even with a size lens, it's still difficult to correct. Pseudophakias with displaced IOLs give me my poorest success rate for correction of aniseikonia."
Dr. Achiron and his colleague Ned Witkin, OD, also from Emory Eye Center, published an article in the January/February 1997 issue of Survey of Ophthalmology. They used research previously done by Dartmouth Eye Institute and made their own additions to serve today's aniseikonic patients without meridional lenses, thick cosmetically unacceptable lenses or the use of the space eikonometer. Dr. Achiron said he and his group use various methods to approximate the aniseikonic correction, such as the Keystone Stereogram or Awaya's New Aniseikonia Test, or empirical methods.
Prescribing for aniseikonia
Dr. Achiron said it does not matter how the aniseikonia is measured. You can calculate base curves and center thicknesses and devise aniseikonic lenses, but they are "horrendous cosmetically and increase lens aberrations because they are changing best form designs." Meridional lenses are no longer available. His approach is to use available stock lenses of different materials and designs.
For instance, "a CR 39 +6 D lens has about an 8 D base curve. Instead you can use an aspheric with a 5 D to 6 D front curve," he said. "With a 4 D difference in base curve, you can get about a 2% reduction and you'll probably be OK, but if you want to go further, you can decrease center thickness by using a high-index aspheric, which fits flatter and closer to the eye and further reduces magnification."
Dr. Achiron and colleagues did a study with 30 patients who were anisometropic to see: "if you get it close, is it good enough?" They found the answer was "yes."
"Twenty-eight of the 30 subjects preferred their modified pair of glasses to their standard prescriptions," he said.
Dr. Achiron said another study has shown that, on average, 3 D is about the extent of difference between the two eyes before the patient will have symptoms. "If you have a myopic patient with -8 D in one eye and -10 D in the other eye you're going to operate, your final goal after surgery in the -10 D eye should be -5 D, 3 D difference from the phakic eye," Dr. Achiron said.
"If you can measure or estimate the amount of aniseikonia you can design an eikonic lens and get a laboratory to grind it," said Troy Fannin, OD, FAAO, of Houston. "It's a matter of manipulating the front surface curvature, the center thickness and the vertex distance to obtain the desired magnification. An eikonic lens is simply a thick afocal unit combined with an infinitely thin power unit containing the spectacle correction."
Between surgeries
What do you do about the patient waiting for surgery on the second eye? Dr. Fannin said he would try to make the patient monocular.
Theodore Grosvenor, OD, PhD, FAAO, from Indiana University College of Optometry, agrees. He said that
as long as the unoperated eye is seeing 20/40 or worse, the aniseikonia resulting from an IOL in one eye and a cataract in the other is usually not a problem. He judges that the patient is usually "essentially dominant in the eye that has the IOL." Problems with binocularity
Bruce Wick, OD, PhD, FAAO, associate professor at the University of Houston College of Optometry and also in private practice, is referred patients who have not done well with IOLs and have problems with binocularity. He said that in patients who have an IOL in one eye alone, problems occur only if the cataractous eye is capable of 20/25 to 20/30 at worst.
Dr. Wick is referred about five or six of these cases per year. He generally recommends contact lenses, but because these older patients usually prefer not to wear them, he often prescribes spectacles first, "explaining to patients that it may not work," he said.
Some surgeons today plan monovision with IOL placement. "Some patients like it," said Dr. Wick. He and Elizabeth Westin, OD, presented a paper at the American Academy of Optometry meeting in December 1997 documenting prescription changes induced by monovision. He believes this may happen with IOL monovision also because he believes the "mechanism is blur, regardless of age." Although usually not possible because of severely reduced acuity, Dr. Wick would prefer to test monovision with contact lenses in patients before recommending monovision IOLs.
IOL-induced diplopia
Dr. Wick also sees about two or three patients a year with diplopia induced by IOLs. For these patients, the "challenge is deciding between Fresnel prisms, vision therapy, aniseikonic lenses and sometimes occlusion," he said.
He has also seen a few cases in which the optics of the IOL seemed to be blurred, "just as you sometimes see with spectacle lenses." In these cases, the blur is usually "attributed to something wrong with the patient's eye" by the surgeon, but Dr. Wick is not sure that is always true.
For the patient corrected with monovision who has problems reading, spectacles may be prescribed. I have prescribed single-vision reading glasses, bifocals that give both binocular distance and near vision and, for the patient with occupational concerns, sometimes a bifocal with the intermediate correction in the top and near correction in the segment.
Dr. Achiron is experimenting with prescribing different types of multifocals for patients who would otherwise require slab-off. He makes use of the differently sized corridors, prescribing the lens with the shorter corridor in the more powerful eye.
Replacing the IOL
When do you ask the surgeon to replace the IOL? The optometrists interviewed for this article said "never." But this is changing, too. Jack T. Holladay, MD, of Houston Eye Associates, describes three surgical alternatives: refractive corneal surgery, IOL exchange and secondary piggy-back IOLs. If the patient is young and the cornea is healthy, refractive corneal surgery may be the best option.
This may also be the best option if there were retinal problems following the first surgery, making the prudent surgeon reluctant to reopen the eye. However, most psuedophakes are not young, and their eyes take a longer time to recover from corneal surgery than from intraocular surgery.
IOL exchange - replacing the old IOL with a more appropriately powered new IOL - is rarely performed anymore because removal may disrupt tissue. New piggy-back lenses can be placed in front of the old IOL, either in the sulcus or in the anterior chamber, to correct the existing IOL.
For example, if the patient began as an 8-D hyperope in both eyes, Dr. Holladay concurs that most surgeons try to stay within 3 D of the refractive error of the unoperated eye unless that second eye will be undergoing surgery soon. If the surgeon places an IOL to make the operated eye +5 D until the second eye needs surgery, then when the time comes to operate on the second eye, he or she may make the second eye plano and use a piggy-back lens in the +5 D eye to make that plano, as well. This keeps the patient binocular without aniseikonia from first surgery until finished no matter what the time lapse may be. The piggy-back is also useful when the measurements taken before surgery were flawed or the lens placement was complicated and the final outcome is a problem.
How often is this done? Dr. Holladay said he receives phone calls every day requesting advice on ordering piggy-back lenses and performs two or three of these surgeries himself every month. These numbers will undoubtedly increase as news of the piggy-back alternative spreads.
For Your Information:
- Leonard Achiron, OD, FAAO, assistant professor of ophthalmology, may be contacted at Emory Eye Center, 1365-B Clifton Road, NE, Atlanta, GA 30322; (404) 778-3340; fax: (404) 778-2244; e-mail: LAchiro@EMORY.edu. Dr. Achiron has no direct financial interest in any products mentioned in this article, nor is he a consultant for any companies mentioned.