prescribing for presbyopia
Translating Bifocals into
BY THOMAS G. QUINN, OD, MS, FAAO
Ima Challenge is an energetic 50-year-old English professor whose passion for Shakespeare is superseded only by her love for her grandchildren, whom she frequently visits after a several-hour
hour drive. She came to you to fulfill her long-awaited desire to get rid of the glasses she has been "imprisoned" by for years.
Based on her high visual demands at distance and near, and your astute observation that her lower lid lies tangent to her lower limbus, you recommended she pursue translating bifocal gas permeable contact lenses. However, now she is not happy with her near vision. You suspect the bifocal lens may not be translating enough to provide good near acuity. How can you verify your suspicions?
All You Have To Do Is Ask
Oftentimes subjective response will answer your question. Ask the patient to maintain her head in a straight-ahead position as you hold a near target about 40 centimeters away in front of her line of vision. Slowly lower the target, asking the patient to follow it and report when the near vision "kicks in." This should occur at a comfortable angle (around 45 degrees). However, if the patient's response is variable, or you suspect the lens power may be off, objective information can save the day.
One objective method is to observe lens movement during downgaze with the slit lamp or Burton lamp. If the lens shifts up relative to the cornea, good translation takes place. The drawback to this method is you are not able to determine how much of the near zone is actually encroaching into the pupil.
An Arm's Length View
My favorite way of assessing lens translation is via use of a monocular direct ophthalmoscope. Dim the room lights and stand arm's length away from the patient (Figure 1). Add plus to the ophthalmoscope until the edge of the pupil is focused in the red reflex. Before assessing translation, ask the patient to maintain fixation in straight-ahead gaze and blink. You should see the seg line rise into the pupil zone immediately following the blink and then drop quickly. If the seg remains in the pupil for a prolonged period, the patient is likely to complain of distance blur. If this is the case, first try thinning the upper lens edge to reduce the lifting action of the upper lid. This is always the first thing I try because it can be accomplished at the time of the visit. If this is not successful, order a new lens with a flatter base curve or more prism.
|Figure 1. Stand arm's length away from the patient to assess
Figure 2. Move into position where the patient views reading
Once you assess lens translation performance in straight-ahead gaze, move yourself into the position where reading material will be viewed (Figure 2). As the patient looks down, you should see the seg translate into the pupil. If you are not seeing adequate translation, try flattening the edge at the prism base to help the lower lid "catch" the lens. If this is not successful, flatten the base curve by at least 0.50D. If at least half of the pupil is covered by the near segment while the patient is viewing down, you can anticipate good near vision, turning Ima Challenge into Ima Success!
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.
Contact Lens Spectrum, Issue: March 2001