prescribing for astigmatism
TIMOTHY B. EDRINGTON, OD, MS
My comfort level with prescribing toric soft
contact lenses has recently increased due to
improved lens reproducibility, more frequent
replacement options and generous lens exchanges. However, I still have an occasional need to problem-solve a toric soft contact lens case. Some tidbits I utilize to problem-solve follow.
Blurred vision throughout the day. The most likely cause of blurred vision is misalignment of the correcting cylinder axis. Re-evaluate the location of the prism base, and adjust the prescribed axis accordingly. To fine-tune your assessment of lens rotation, use a protractor on the housing of the slit-lamp light source.
A sphero-cylinder overrefraction often solves the mystery. Prism-ballasted lenses often require additional plus for the sphere (when compared to the manifest refraction sphere) due to flexure and/or tear lens effects. If the correcting cylinder of a lens is positioned 30 degrees away from the patient's astigmatic axis, the full amount of the lens cylinder will be found in the overrefraction; 15 degrees misalignment will result in an overrefraction with one-half of the amount of the correcting cylinder and 10 degrees misalignment will result in one-third of the correcting cylinder in the overrefraction.
If the prism base is not rotated too far from the six o'clock location, you can prescribe by entering the lens sphere, cylinder and axis and the sphero-cylinder overrefraction values into a resultant cylinder calculator or into a program found on several manufacturers' websites.
Blurred vision upon lens application. If the patient's vision is blurred at both distance and near only for minutes after lens application, instruct the patient to insert the lens with the prism base down.
Variable vision. The most likely cause of variable vision is lack of rotational stability. In other words, the orientation of the correcting cylinder and prism base position are unstable from one moment to the next. Refitting the patient into a lens design that incorporates more prism or one that has a larger overall diameter might alleviate the patient's symptom of variable vision. Increasing overall diameter allows for a larger carrier portion of the lens. An increased carrier portion enables the manufacturer to accentuate thickness differences to minimize lens rotation during and after a blink.
Observe lens rotational stability through the slit lamp microscope. A lens that "swings" back and forth from blink to blink may be too flat. Consider prescribing a steeper base curve or larger overall diameter. If the lens rotation appears to "creep" in one direction from blink to blink, the lens may be fitted too steep. A flatter base curve should be considered.
"Hey Doc, I thought you said monovision works." Patients who successfully wear toric soft contact lenses prior to presbyopia might not adjust to monovision as well as their spherical lens counterparts. For pre-presbyopes, both eyes contribute to the patient's satisfaction with distance and near vision. If an individual lens is not quite perfect, either occasionally or consistently, the patient generally is satisfied with his vision. When this patient wears monovision, his attitude towards his vision might change if one or both of his astigmatic corrections is misaligned.
On the other hand, patients with low amounts of astigmatism who wear spherical soft contact lenses might benefit from a toric soft contact lens when they mature into a monovision correction.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry.
Contact Lens Spectrum, Issue: June 2000