The ability to accommodate for near and intermediate visual tasks greatly diminishes with age due to the onset of presbyopia. Patients in their fifth decade considering contact lenses, laser refractive surgery, or cataract surgery will require glasses for near and intermediate vision if they achieve emmetropia and excellent distance UCVA in both eyes.
Monovision has been widely practiced, historically with contact lens correction and more recently with LASIK to assist with near UCVA after refractive surgery. However, the largest group of patients who are suitable for monovision are those undergoing cataract surgery.
Although multifocal implants are an alternative to monovision, the patient compromises visual quality and may experience dysphotopsia, such as halos, which some patients find unacceptable. An accommodating IOL would be ideal, but the efficacy and refractive outcomes of today's models are somewhat unpredictable. Therefore, my personal preference for the past 5 years has been to utilize monovision as an effective solution for patients who desire a greater level of spectacle independence, both for distance and near, following cataract surgery.
I target emmetropia for distance in the first—preferably dominant—eye and a refractive outcome of -1.25 D sphere in the second eye following cataract surgery. All patients who achieve 6/9 or better UCVA in the first eye are offered the choice of a target refraction of emmetropia or modest myopia (-1.25 D) in the second eye.
COUNSELING AND PATIENT SATISFACTION
The counseling required for this modified monovision technique is straightforward and time efficient. After surgery in the first eye is complete, I demonstrate the type of vision that will be achieved with the addition of 1.25 D sphere. This is done with a trial frame in the recently operated distance eye. The patient can immediately perceive the reduction in distance vision and the improvement in near vision that he will experience if the target refraction is achieved in the second eye. Approximately 50% of patients elect to be emmetropic in both eyes for distance and rely on reading glasses; the remainder select monovision.
My preference is to perform surgery in the dominant eye first; however, I will still elect to perform surgery initially in the eye with the greater level of cataract and poorer acuity aiming for emmetropia, regardless of dominance. In these circumstances, the dominant eye may remain myopic—a situation referred to as cross dominance. I do not perform an initial contact lens trial because results are difficult to interpret in the presence of significant cataract. I always counsel patients that they will still need reading glasses for fine print but will be spectacle independent for intermediate and most near visual activities. The majority of patients, however, find that their need for reading spectacles is minimal, with many achieving spectacle independence following surgery.
Patient satisfaction is extremely high with monovision. In contrast to multifocal implants, unhappy patients are an exceptionally rare phenomenon. I believe that the success of this strategy is due to the level of targeted myopia for near vision. A myopic refraction in the near eye of -1.25 D sphere is significantly less than the level utilized with conventional monovision in contact lenses. The range of myopia in this scenario preserves stereoacuity and avoids the reduced contract sensitivity that may occur with higher levels of ammetropia.
Asthenopia due to strong dominance is also unlikely to occur when the difference between refractions in the two eyes is in the range of 1.25 D. This level of emmetropia can be considered physiological, allowing fusion and binocular summation rather than suppression that may be necessary with higher levels of ametropia. This level of monovision does not require prolonged neural adaptation, which sometimes is necessary with multifocal implants due to the processing required to deal with the spatially incongruent images inherent with multifocal implants. A target refraction of -1.25 D sphere for near vision appears to be more effective for pseudophakia versus phakia due to the enhanced depth of focus enjoyed by pseudophakic individuals. This is often described as pseudoaccommodation. Perhaps monovision is not an appropriate term to describe the vision enjoyed by these patients; alternative terms such as mini-monovision, blended vision, and omni-vision may be more appropriate.
I take great care with biometry and utilize toric IOLs as well as limbal relaxing incisions when appropriate to ensure that the target refractions for emmetropia and near vision are achieved. Nevertheless, compared with multifocal implants, the technique is more robust in the presence of minor degrees of spherical defocus and astigmatism. Secondary interventions are extremely uncommon, and most patients are satisfied with refractive outcomes in the range of -1.00 to -1.50 D in the near-vision eye. One of the major advantages of monovision is that any perceived deficit in acuity can be corrected with the occasional use of spectacles, thus restoring full binocular acuity and quality of vision. Patients' acceptance of the use of spectacles is important because many patients will develop against-the-rule astigmatism changes as well as deteriorating macular function with age. It is also feasible to perform LASIK if the patient should experience asthenopia due to monovision, although personally I have not found this to be necessary. Similarly, the need for secondary procedures such as Nd:YAG laser capsulotomy does not increase compared with conventional cataract surgery with monofocal implants.
All currently available techniques to improve near vision following cataract surgery entail some degree of compromise. The compromise in contrast sensitivity and quality of vision with multifocals may be found unacceptable by some patients, thus requiring a lens exchange. Monovision targeting higher levels of myopia for near vision would be expected to offer high levels of spectacle independence for reading; however, the issues of stereoacuity, contrast sensitivity, and dominance are more challenging. In contrast, the technique of modified monovision or blended vision produces a predictable result with a high level of patient satisfaction.
Graham D. Barrett, MD, FRACO, is an Associate Professor of Ophthalmology at The Lions Eye Institute, and is Head of the Department of Ophthalmology at Sir Charles Gairdner Hospital, both located in Nedlands, Western Australia. Dr. Barrett may be reached at tel: +61 8 9381 0872; e-mail: firstname.lastname@example.org.