Article

PRESCRIBING FOR PRESBYOPIA

THE BLUR TOLERANCE TEST

A 47-year-old secretary presented with complaints of blur at both distance and near with her newly fit daily disposable soft multifocal lenses. She had successfully worn monthly soft multifocals but was refit after developing giant papillary conjunctivitis. Investigation revealed that both the monthly and daily disposable lenses were fabricated by the same manufacturer employing the same optical design. Why was this previously successful patient returning with complaints?

Role Reversal An analysis of lens powers indicated that the daily disposable lenses had been prescribed with a low add power in the right eye and a high add power in the left eye. This was the opposite of what the patient had worn successfully in the monthly modality. The daily disposable add powers were reversed to match the monthly lens add powers and the patient achieved clear comfortable vision at both distance and near.

Multifocals with a Mild Monovision Twist

A common multifocal contact lens fitting strategy to boost near vision is to provide more plus power in one eye. The additional plus may be in the form of a higher add or added plus in the distance power above what would be prescribed based on the spectacle distance prescription (i.e., for a +2.00D distance spectacle power, a +2.50D distance contact lens power is prescribed in addition to an add power).

Distance Distress

While the previous approach improves near vision, it often degrades distance vision, which is why this strategy is employed in one eye only; the opposite eye is prescribed the expected power based on the spectacle power and the fitting guide recommendation, leaving it biased toward better distance vision. When both eyes are open, the patient is generally able to see at both distance and near. The patient discussed above demonstrates that, at least in some cases, it matters which eye is chosen to be biased for near vision.

Does It Matter for Everyone?

To explore this question, eye-care practitioners in our office performed the following procedure:

  1. The patients were instructed to view the distance acuity chart binocularly through their best distance spectacle prescription.
  2. Plus power in +0.25D increments was added to the prescription in front of one eye only.
  3. The patients were then asked to report when they just noticed blur while viewing the acuity chart.
  4. Once blur was reported, the finding was noted and then the sphere power in that eye was returned to the best distance spectacle prescription.
  5. The above steps were repeated for the patients’ other eye.

Figure 1 shows the amount of difference in accepted plus between eyes for each subject. Patients who had a greater difference took more plus in one eye before reporting distance blur. This suggests that this eye tolerates distance blur better and should be the eye fit with a lens biased for near viewing.

Figure 1. Difference in accepted plus between eyes.

The majority of patients demonstrated no, or very little, difference between eyes. This suggests that these individuals would accept distance blur in each eye equally. In these cases, it may not matter which eye is biased for distance or near vision. In fact, this has been our clinical experience.

More study must be undertaken under controlled scientific protocols to fully explore the value of the Blur Tolerance Test, but practitioners in our office have found it to be helpful in selecting multifocal lens power. CLS

Special thanks to Shane Foster, OD, Rachel LeFebvre, OD, and Heather van Law, OD, for their assistance in collecting the data for the case series.