Presbyopic Correction – FAQ 2

Troubleshooting Segmented Translating Designs: How Do I Manage Poor Visual Acuity?

Background

Segmented translating bifocal designs are prism-ballasted and are often fit “On K” or slightly flatter. For most designs it is recommended that the seg line position at, or slightly below, the lower pupil margin in normal room illumination. For trifocal designs, the intermediate zone is typically positioned over the inferior pupil area in straight-ahead gaze.

The lens should be positioned inferiorly with the bottom of the lens resting on, or closely adjacent to, the lower lid margin. When the wearer drops his or her eyes to read, the inferiorly positioned segmented zone should push up and be positioned in front of the pupil.

A diagnostic fitting set is often indicated when fitting a specific design.

Poor Vision at Distance

There can be several causes of this problem:

  • High seg position. If the seg line is positioned into the pupil with straight-ahead gaze, it is recommended to decrease the seg height.
  • Lens picked up too high with the blink. These lenses should move only about 1mm with the blink. If the lens is picked up too superiorly, the prism ballast should be increased.
  • Excessive rotation of the seg with the blink. If the lens rotates excessively with the blink such that the seg is in front of the pupil with straight-ahead gaze, the selection of a flatter base curve radius will often allow the lens to release and maintain an inferior position.

Poor Vision at Near

There can be several causes of this problem:

  • Low seg position. If the seg line is positioned 1mm or more below the lower pupil margin, this may not allow the segment zone to cover the pupil sufficiently. A higher seg height is indicated, and as this is often the result of a borderline low-positioned lower lid, a larger overall diameter is often necessary as well.
  • Poor lens translation. If the lens does not translate (i.e., shift upward) with downward gaze, the selection of a flatter base curve radius will increase edge clearance, resulting in a greater likelihood of lens contact with the lower lid with downward gaze. Alternatively, flattening the peripheral curve radius or increasing prism ballast may also help solve this problem. Truncation can be particularly beneficial with plus-power and low minus-power lenses, as this will increase edge thickness.
  • Patient drops head, not eyes, to read. This should also be ruled out and the patient educated to drop his or her eyes to allow translation to occur.

Useful Resources

GP Lens Institute

Publications

Acknowledgements

GP Lens Institute Advisory Committee members: Bruce Anderson OD, Marlane Brown OD, Carmen Castellano OD, Walter Choate OD, S. Barry Eiden OD, John Laurent OD, PhD, Derek Louie, OD, MS, Joe Shovlin OD, Frank Weinstock MD, Bruce Williams OD.