Contact Lens Case Reports
The Multifocal Dilemma
By Patrick J. Caroline, FAAO, & Mark P. André, FAAO
Why can't I see with my multifocal lenses?” patients ask. Every year manufacturers spend millions of dollars attempting to solve this “dilemma.” It's important that manufacturers and clinicians recognize anatomical obstacles that directly impact the optics of these lenses: pupil size and patients' line of sight.
The aging eye undergoes changes that present significant challenges to the successful delivery of presbyopic optics, and one of these changes is that of pupil size.
With aging comes a phenomenon known as senile miosis, an age-related decrease in pupil size and dynamics (Figure 1). Remember that with simultaneous vision, multifocal optics, the only optics available will be those presented over the pupil. Therefore, in the case of a 50-year-old presbyope with a 3.5mm pupil, it is necessary to provide all of the distance, intermediate and near optics within the center 3.5mm of the lens. That is a significant distribution of power in a small area.
Figure 1. The size of the human pupil decreases with age.
Line of Sight
Believe-it-or-not, perhaps the greatest obstacle to the effective delivery of multifocal optics is that soft lenses center on the eye. This may seem counterintuitive in that logic would dictate that you'd want the 3.0mm to 4.0mm distance, intermediate, and near optics of the lens to center over the pupil. Well, we don't. Figure 2a shows a centered multifocal soft lens. The yellow ring represents the perfectly centered optics of the lens. However, humans do not see through the center of our pupil, but instead must view objects 3 to 11 degrees nasally, an optical phenomena known as angle lambda (Figure 2b).
Figure 2. (a) Centered lens, centered optics, (b) Centered lens, centered optics, patient's actual line of sight (pink ring).
In a study at Pacific University, Drs. Matthew Lampa and Kelvin So (2012) performed corneal mapping over the top of a number of commercially available multifocals. All eyes in the study had normal corneal curvatures, normal horizontal visible iris diameters, and less than 1D of corneal astigmatism. Figure 3 shows the geometrically centered soft lens optics (yellow ring) and a patient's actual line of sight (pink ring). All three soft lenses centered perfectly. However, the patient's line of sight was consistently displaced nasally.
Figure 3. Corneal topography (subtractive map) over the top of centered, multifocal soft lenses (yellow ring), patient's actual line of sight (pink ring).
Difficult to Overcome
When you combine the nasal displacement of our visual optics with the age-related decrease in pupil size, it is clear that resulting optical aberrations of multifocals are often too great for the aging optical system to overcome. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #196.
|Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision.|