The Classical option, monovision, is still required in the treatment of presbyopia
Taking advantage of the rich musical history of Vienna, Prof. Barrett described the importance of monovision in presbyopia treatment using the basic principles of composing as an analogy. "Now the essence of any musical composition, and I think monovision is a classic, can be described in terms of its rhythm, melody, key and harmony," he added. "There are several options available to correct presbyopia and each, as is with a musical composition, has its own rhythm."
He went on to reveal that the majority of lens implantations being performed are still monofocal with many patients having monovision. "So, this is probably the most widely practiced presbyopic solution today," Prof. Barrett said. Looking at the through focus or defocus curve of the monofocal lens it is possible to see how the focus varies when compared with diffractive multifocal lenses he explained. Although multifocal lenses offer good vision for distance and near there is a deficit for intermediate range visual acuity. "Monovision provides an additional focus in the second eye for near and the combined binocular defocus curve, is not dissimilar to the normal accommodative response," he said.
Traditionally, for monovision, there is a target of -2 or -2.5 D for the near eye, which provides good unaided near vision. However, with this traditional approach, stereoacuity can be effected. To avoid this, Prof. Barrett pointed out that the myopic defocus needs to be limited to perhaps no more than -1.5 D. "I routinely assess stereoacuity in normal patients and with modest monovision I find that the impact is not nearly as severe," he added.
"So, what we are faced with is a balance between the impact of myopic defocus and distance acuity versus the amount of near vision that can be achieved and also the impact on stereoacuity," Prof. Barrett asserted.
Melody and tone
"Often a person's preference for musical compositions can be determined by the melody and tone of the piece and in the instance of ophthalmic patients we are talking about patient selection and counselling," said Prof. Barrett. "We are all familiar with the caution that multifocals may not be well suited to discriminating individuals but with modest monovision, architects, engineers, artists and truck drivers are all possible candidates."
Additionally, he noted that minor levels of defocus encountered with astigmatism, PCO and even macula dysfunction are not affected to the same extent as they are with multifocal lenses. "So, really all we need is a potential visual acuity of at least six lines in both eyes and adequate comprehension of the refractive strategy," he added.
To ensure patient selection is appropriate and their expectations are managed Prof. Barrett explained his processes and methods in three steps. "Firstly, I address the alternative, explaining that a monofocal IOL does provide the best quality of vision but does require reading glasses, multifocal IOLs have issues with contrast, while accommodating lenses are relatively unpredictable," he said. "Only then do I broach the possibility of monovision explaining that this provides optimum quality of vision that can be corrected. Patients will have excellent unaided intermediate vision. But they will require reading correction for some activities."
After these steps, Prof. Barrett chooses the eye with the denser cataract, aiming for distance vision. "With a clear lens extraction I'm more concerned with dominance and then I do prefer to have the dominant eye for distance. But with a modest level of defocus, it's not such a critical factor," he continued. "And if I do achieve the first eye on target for the first time I will demonstrate the amount of myopic defocus that the patient can expect with 1.25 D lens in a trial frame. So the patient knows beforehand what the impact on distance vision will be, how much medium vision they will obtain, and the good thing about this is that the patient can see what they will achieve."