Obvious differences exist between the visual functions of monovision and multifocality, and the ophthalmic surgeon is responsible for providing the best option for each refractive patient. Determining the most appropriate surgical course depends as much on understanding the patient's needs as comprehending the functional properties of monovision and multifocality.
In an eye with monofocality, all of the light from the object of regard is focused on the retina, whereas in multifocality, a portion of the light is focused on the retina from each image in which the patient is interested. For example, with the Array multifocal IOL (Advanced Medical Optics, Inc., Santa Ana, CA) (Figure 1), 50% of the incoming light is focused from distant targets, while 37% is focused from near and 13% from an intermediate range. This refractive division of images allows clear focus at both distance and near simultaneously; the patient chooses the image of regard.
THE MONOVISION OPTION
In my opinion, monovision is an effective treatment for only a small number of patients. My colleagues and I limit the use of monovision to those patients who have successfully worn monovision contact lenses and liked the effect. Also, potential candidates must understand that they may have to return for an enhancement of the undercorrected eye, and that the procedure may leave them somewhat myopic.
In order for a patient to be able to read for a prolonged period of time after undergoing a monovision treatment, he must be slightly undercorrected. Additionally, providing these patients with a full add so that they can read small print will blur their distance vision and compromise their depth perception. This does not mean that multifocal lenses do not also carry limitations; as a matter of fact, many patients who have multifocal lens implants also have to wear glasses for prolonged reading. However, these lenses have the advantageous capability of providing adequate vision at the distance of a computer screen, whereas monofocal lenses do not. Even if a patient's monofocal correction is set at that distance, he is unable to read well up-close. A refractive surgery patient is a somewhat better candidate for monovision correction due to his myopia. The surgeon can undercorrect one eye and then fully correct it soon after if the patient experiences any visual problems.
At the recent meeting of the New Orleans Academy of Ophthalmology, strabismologists explained that certain patients tend toward a tropia and will break down to in-tractable double vision if given monovision correction. In some cases, they may require muscle surgery. Therefore, patients should wear monovision contact lenses for a trial period before undergoing refractive or cataract surgery for monovision correction. Ideally, a prospective monovision patient should test monofocal contact lenses for 1 month, but very few patients are willing to do that. Many instead opt for a multifocal IOL.
MULTIFOCALITY
Ivan Marais, MD, from Johannesburg, South Africa, was one of the earliest ophthalmologists to use the foldable Array multifocal lens bilaterally and extensively in patients. A large group of his patients also had some form of multifocality, either through astigmatism or through monovision (personal communication, Brussels, September 2000). Dr. Marais conducted a quality-of-life survey for the Array implant from which he learned that his bilateral multifocal Array patients were far more satisfied with their vision under all circumstances than were any of his monofocal patients, either via monovision or astigmatism. The survey's most convincing revelation was the patients' responses to the last question, which asked, ?Are you wearing glasses as you fill out this form?? A large percentage of the monovision patients were, compared with a minute proportion of the Array patients.
My colleagues and I usually select multifocality for refractive lens exchange. We carefully screen multifocal lens patients to ensure that they are highly motivated and aware of the potential side effects of the Array IOL, including halos around lights at night. The best candidates for this procedure are hyperopic presbyopes, be-cause they are always happy with their outcome.
POTENTIAL PITFALLS OF EACH TREATMENTPatients Must Adapt to the Array Implant
Currently, the Array IOL is the only available multifocal lens in the US, and it requires a certain amount of adjustment by the patient. First, Array patients see halos around point sources of light at nighttime, although this side effect eventually subsides in most patients. Those who receive this implant, in my opinion, must be highly motivated for the multifocal effect. For example, they may experience some difficulty reading in dim illumination, because the pupil remains small under the near reflex. My colleagues and I have treated this problem in several patients using photomydriasis with a diode laser (Oculight GL; Iris Medical, Mountain View, CA). This technique, described in 1984 with an Argon laser, makes use of multiple broad spots placed circumferentially around the pupil. Shrinkage of iris stromal tissue causes an enlargement of the pupil.1
Issues in Achieving the Correction
Inducing monovision may be somewhat easier if the patient opts for LASIK, because you can undercorrect and then re-treat the eye to achieve the intended refraction. This approach may consume more of the physician's time, but it provides the patient with the most refractive options. If the patient seems highly motivated, I am more willing to take this approach using LASIK than refractive lens exchange. I can change the power of the eye corrected for near by implanting a piggyback lens in the ciliary sulcus (see The Piggyback Enhancement Option).
THE FUTURE OF REFRACTIVE LENS EXCHANGE
Accommodative IOLs will soon become available for refractive lens exchange, thereby widening surgeons' lens choices. My colleagues and I have had a wonderful experience over the last 3 years with the CrystaLens accommodative IOL (C&C Vision, Aliso Viejo, CA) (Figure 2). Of the patients we have implanted with this lens, 100% have at least 20/30 distance, and J3 uncorrected binocular near and intermediate visual acuity. The advantage of an accommodative IOL is that all of the light from the image of regard is focused on the retina. I believe that the availability of accommodative lenses will boost the use of refractive lens exchange significantly.
1. Thomas JV. Pupilloplasty and photomydriasis. In: Belcher CD, Thomas JV, Simmons RJ, eds. Photocoagulation in glaucoma and anterior segment disease. Baltimore: Williams & Wilkins, 1984:150-157.
