Larry had just gotten new soft lenses, and he was planning to order some spare glasses. He said he knew he needed spare glasses, but he was postponing that expense since the contacts were working well for him. He was having trouble with prolonged viewing of his computer screen because he had to magnify the view. He was emphatic about “hating glasses,” and he was strongly of the opinion that if he were told to wear glasses, he would have no part of it at work or in public. “They are ugly and thick and heavy, and people stared at me when I wore them,” he insisted.
“The headaches worry me. I get them at the end of every day, and I’m pretty sure these headaches are because of all the computer work with this new job,” he told the optician.
“Why don’t you let me take a look at the prescription the doctor gave you for new glasses?” the optician suggested. He handed over a crumpled Rx slip. It said: OD -15.00 -1.75 x 110, OS -14.50 -2.00 X 075.
“They’re gonna be thick and ugly,” Larry sighed, “I’ve had a lifetime of thick, ugly and heavy glasses.”
“Do your contacts correct your astigmatism?” the optician asked.
“Well,” he said. “I tried those but they never worked well. In fact, several have tried but they keep telling me that my eyes are curved funny.”
His optometrist confirmed that no contact would stabilize adequately, so Larry had finally settled for the best spherical equivalent correction in the contacts. “No Glasses” was his mantra.
The optician suggested that he make another appointment with his optometrist and request to be tested for an overcorrection in the form of glasses.
“Oh no,” he exclaimed. “I know what glasses for me are like, and I don’t even want to talk about it. The doc talked to me about glasses combined with contacts, but I just said N-O, NO!
After some discussion and explanation, he agreed to talk to his optometrist about it again. A couple of weeks later he had his overcorrection, the headaches were gone, and he was seeing better than ever.
Patients visit eyecare providers with four goals. They want to see well, look good, be comfortable and achieve these goals at a fair cost. Due to human nature, there is variance in the order of importance of these goals from patient to patient. In Larry’s case, the cost of the overcorrection was of little importance once the optometrist showed him how his acuity improved with cylindrical, spectacle trial lenses over his contacts. His new eyewear is trim and handsome, and he no longer experiences those daily, severe headaches. Larry, in his mid-30s, has had a lifetime of professional eyecare, and several of his doctors, including his current one, had suggested an overcorrection. Unfortunately, no one had taken the time to show him what an overcorrection could do. Corneal surgery was not an acceptable option, and contacts that would give him his full correction were not successful, although many designs had been tried.
Fortunately, his present spectacle lenses have even made his contacts seem to be more comfortable. This could be due to shielding his eyes from the drying effect of air currents and because he no longer must use accommodation to constantly search for the best retinal imagery.
A ROAD LESS TRAVELED
When an eyecare provider (ECP) is counseling patients for problems of refractive blur or discomfort, or functional vision problems such as muscle imbalance the choices are usually either spectacles, contact lenses, or in some cases corneal or eye muscle surgery. The road less traveled is the use of a combination of corrective approaches. An overcorrection is more common with eye muscle surgery when spectacle lenses with prism are used to improve binocularity after the muscles are repositioned. Unfortunately, combining contacts and glasses for a better outcome is sometimes an overlooked alternative in less obvious situations.
Improvements in both gas permeable (GP) and soft lenses, particularly in the area of correcting astigmatism, are highly successful, and very strong refractive errors can now be treated with custom-made soft lenses, as well as GP lenses. That said, there are still patients such as Larry who can benefit by combining spectacle lenses with contacts.
Patients who require prism, particularly lateral prism (i.e., Base In or Base Out) cannot be fitted with contact lenses that include their prism correction. Caution: Patients with relatively mild refractive errors gain little by putting the refractive correction in contacts and the prism correction in an overcorrection. The only gain would be the reduction of lower order peripheral aberrations (primarily marginal astigmatism and/or power error), which are generally reduced well with low-powered corrections.
An overcorrection with 4? of lateral prism OU will cause a blur due to chromatic aberration amounting to a bit over 0.13 prism diopters of image spread from blue to red if the lenses have an Abbe value of 30. If the spectacle lenses alone are used, the least amount of chromatic aberration would be 0.13 prism diopters of image spread at the major reference points (MRPs) of the lenses.
A 3D movie, when viewed without the polarizing lenses, gives a view of two similar images that are “spread” or imperfectly superimposed. This is comparable to the blur caused by the image spread from red to blue resulting from lateral chromatic aberration. Chromatic aberration only affects vision when the patient experiences prism, whether it is prescribed or induced by looking peripherally. Because of this, a patient with BI or BO prism benefit most from a contact lens correction for all or part of their refractive error, with the prism and any residual refractive correction in spectacle lenses.
If an Rx calls for a -7.50 refractive correction, at just 1 cm from the OC (about 20 degrees), the patient will have a 0.25? image spread at just 1 cm from the OC (about 20 degrees) when a lens material with an Abbe of 30 is used (i.e., 7.50/30). Patients are more prone to be bothered by image spreads due to chromatic aberration at longer viewing distances. An image spread of 0.12 to 0.25 prism diopters or more can be problematic.
A clinically practical way to calculate the image spread from red to blue in a lens with no prescribed prism is to divide the power of the lens in its horizontal meridian by the Abbe value of the lens material. If lateral prism is prescribed, the amount of prism in the lens will cause additional issues. Because most of our eye movements are from side-to-side, and because a 20-degree angle from the MRP of a lens is a common viewing angle, using the power in the 180 meridian at 1 cm from the MRP gives a reasonable guideline for anticipating problems related to the Abbe value of the lens material you suggest.
If you choose to give your patient’s refractive correction in a contact lens and the prism in spectacle lenses, you should use a high quality AR lens because the low refractive power of the spectacle lens will make ghost images of TV screens, lit candles, oncoming headlights, etc., more annoying. Still this overcorrection approach will give better overall clarity than if the prism and refractive power were combined in a low power spectacle lens.
IMBALANCE AND APPEARANCE
For prescriptions with one lens much stronger than the other (i.e., imbalance), there can be cosmetic and/or vision problems. For example: OD +2.00DS 3? BO, OS +5.00DS 3? BO may cause the left eye to appear significantly larger than the right, particularly if the patient’s vertex distance is long. Even though the difference in power is only 3 diopters, today’s larger frames may dictate a great enough thickness difference between the lenses to cause an unacceptable increase in the apparent size of the left eye. This may be particularly noticeable if the patient has been wearing a smaller, less fashionable frame. Using contacts for the refractive correction will eliminate the appearance problem and will allow the patient to look further above and below the MRP level with comfortable binocular vision.
IMBALANCE AND IMAGE DIFFERENCES
Another problem with imbalance can be an inequality of the size of the retinal images (i.e., aniseikonia). This inequality can interfere with fusing the two images correctly, and spatial distortion, severe headaches and dizziness can result. Aniseikonia problems have been misdiagnosed as chronic stomach problems and inner ear problems. If the spectacle powers are more or less equalized by fitting contact lenses, the aniseikonia may be reduced, and correcting aniseikonia problems with a combination of contacts and eikonic spectacle lenses may be even more advantageous than using contacts alone.
APHAKIA AND OVERCORRECTIONS
The rate at which cataracts develop to the point that surgery is warranted can vary considerably. As a result, patients may present with refractive results such as OD plano DS, OS -3.00 -1.00 x 180 when the cataract has been removed from only the right eye. This presents some interesting lens prescribing challenges. If lenses correcting the refractive error and presbyopia are ordered for such a patient, there will be about a 4? or more imbalance through the add. Also, looking to the right or left (i.e., about a centimeter from the optical center of the left lens) will require fusional efforts that vary from overcoming 3? BI to 3? BO.
If surgery for the right eye is only a matter of a week or two away, simply patching the right eye may serve. For longer periods of imbalance, or for patients who are too troubled by the problem, a temporary contact lens can be fitted that allows the previous glasses to be worn temporarily. For example, a patient who is wearing spectacles with -5.00DS OU prior to surgery, a +4.75 or a +5.00 (depending on the vertex distance) contact OS will restore the myopia, and the existing spectacles may serve. If the spectacle lens in the operated eye has a significant astigmatic error, a toric contact might be needed.
In the case of a looming monocular aphakia complicated by a strong refractive error in the eye that will not be operated, the imbalance due to anisophoria (particularly in the vertical) is predictable, and there are practical alternatives for post-surgical correction. The problem of induced aniseikonia is more complex. Test instrumentation is limited. However, the Aniseikonia Inspector, a software program marketed by the Optical Diagnostics company, may allow an accurate measurement of retinal image size differences both pre- and post-surgery. Such testing could be an avenue that allows predictions of the best ways to handle monocular pseudophakia in binocular patients.
Whether an overcorrection is advisable or not is largely dependent on the doctor’s thorough assessment. Giving each patient the best possible understanding of their own situation and explaining the options and possible outcomes is a responsibility of every ECP. Going to an overcorrection for any reason will probably remain the road less traveled, but it can be an important and beneficial option that should neither be forgotten nor ignored.
Contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.
Abbe Value—A number that signifies how much the power of a lens will vary according to the wavelength of the light passing through it. The lower Abbe values indicate a greater power variance as various wavelengths are considered. Higher Abbe values indicate less variance in lens power among the visible wavelengths. Higher Abbe values generally indicate better optical performance.
Anisophoria—A condition in which there is a tendency for the eyes to be misaligned by varying amounts depending on the direction of gaze.
Aniseikonia—A condition in which the right and left retinal images are not the same size.
Chromatic Aberration—A condition in which lenses have differing powers depending upon the wavelength of light used to measure them. A lens with no chromatic aberration would have the same power, regardless of the wavelength of light used to measure it.
Eikonic Lenses—These are lenses that use special thicknesses and curvatures to provide magnification or minification effects.
They are used to adjust the size of the retinal image, and they are sometimes called iseikonic lenses.
Image Spread—The blur that results from lateral chromatic aberration. It is caused by lens power variations that occur when all visible light wavelengths are not refracted equally. This is not much of a problem when viewing through the optical center of a lens, but Prentice’s Law causes images formed in the lens periphery to be positioned at varying distances from the optical center, giving a rainbow effect rather than a sharp image.
Meridional Aniseikonia—A condition in which the size of each retinal image is stretched or reduced according to the meridian under consideration. If a circle is magnified more in the vertical meridian than in the horizontal, its image will be a vertical oval.
Pseudophakia—The condition in which a lens implant replaces the crystalline lens of the eye.
The Aniseikonia Inspector—A software program marketed by the Optical Diagnostics company that addresses problems of measuring and correcting aniseikonia.