prescribing for presbyopia
It's Time to Minimize Monovision
BY DAVID W. HANSEN, OD, FAAO
Monovision has stood the test of time because it is an effective and reasonable prescription for correcting presbyopia. It is easy, it is less expensive and when it works the patients are extremely happy. So why would anyone want to change this concept?
Monovision worked miracles for the early to mid presbyopic patients, but now this generation has advanced with more significant accommodative demand. Increasing quantitative lifestyle demands can cause asthenopic hardships for our patients. For years, lecturers talked about maximizing monovision and giving the full "plus" correction for distance and relaxing the near prescription. It is time to think the opposite and minimize the monovision effect.
Monovision produces an abrupt anisometropia which presents a significant challenge to the presbyopic visual system. Occasionally there is loss of balance during adaptation, hazy vision, poor initial near task performance and the inability to suppress this perception of blur. Anisometropia also produces binocular vision compromise, reduced stereopsis and reduced visual resolution under low-contrast viewing conditions, especially for adds beyond 1.50D. A three-year study by Holden et al suggested that with simultaneous image bifocals, increased design and add power flexibility resulted in better stereo acuity compared to monovision.
Monovision corrections do not reduce size of the binocular field nor peripheral visual acuity. Studies have shown that peripheral blur essentially has no effect on the peripheral functions of spatial localization, perception and movement.
Even though the data shows that monovision has no significant impact on these vital peripheral functions, single binocular vision with aspheric multifocal lenses may enhance visual performance, especially with reduced illumination.
Headlights against a dark background are extremely difficult to suppress, especially for monovision wearers. Schore, Carson, Peterson and associates reported that 33 percent of monovision patients studied experienced glare when driving at night. Other studies report associated night glare with concentric bifocal lenses. Yet another revealed that 80 percent of patients have difficulty with night driving whether they use monovision or aspheric bifocal contact lenses. Nighttime driving produces visual compromise, and patients must be aware of potential risks.
Minimize the Monovision
"Pushing plus" for distance was the traditional standard for fitting monovision. Now we have new bifocal designs that accentuate maximum visual acuity for distance, intermediate and near. Even if a patient is initially fit with the monovision principle, we are relaxing the aniso effect for maximum visual comfort. Today the recommendation for many soft bifocal designs is to prescribe the full distance power, especially on the dominant eye. This ensures good visual acuity and minimizes the dim to dark illumination concerns. Enhancing the distance prescription minimizes the monovision.
I usually find it more difficult to fit presbyopic patients with bifocal lenses once they have adapted to monovision. However, once they experience binocular vision again, they usually embrace it. The bifocal and multifocal designs of today enhance the visual process and produce more satisfactory total vision than monovision's anisometropic effect which requires cerebral adaption.
Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.