prescribing for presbyopia
Assessing Presbyopic GP Lens Translation
BY CRAIG W. NORMAN, FCLSA
Gas permeable lenses for presbyopia fall into two design categories — rotational and non-rotational. While different, one similarity is that each of these design concepts needs to move during the blink cycle and during various visual tasks.
Most presbyopic GP designs fit today, such as aspheric multifocals and center-distance lenses, are in the rotational category and move in a circular fashion throughout the day as the wearer blinks. These lenses must position centrally to superior centrally during distance gaze, then move vertically about 1mm to 2mm in an upward translation as wearers shift their gaze.
True bifocal or trifocal designs are non-rotational, meaning they have a top distance portion and a bottom reading component that need to be properly positioned in different fields of gaze. The inferior-positioned weighting naturally positions the lens lower, thus translation is important to move the lens upward from its distance resting position when the patient has near tasks.
Figure 1. An illustration of where to place a mirror to assess translation.
The location and size of the reading segment is critical. It's ideally 1mm to 1.5mm below the center of the pupil in distance gaze, translating in downgaze so the segment line rises slightly above the pupil center.
To evaluate translation and segment position, one method is to simply pull the patient's upper lid upward while he looks down-ward and note how much the lens moves along the vertical meridian. If the lens has adequate room for translation, it should cross the upper limbus. Using a penlight to illuminate the lens, see if the lower lid has pushed up (translated) the lower edge of the lens so that the patient is viewing through the near zone. Unfortunately, it's often difficult to see the segment line with this technique.
Craig Woods, PhD, MCOptom, DipCLP, FAAO, and Gina Sorbara, OD, MSc, FAAO, from the Centre for Contact Lens Research at the University of Waterloo described an alternative technique in Correction of Presbyopia with GP Contact Lenses. They suggest placing a small (2.5mm × 1.5mm) rectangular mirror between your patient's cheek and lower eyelid (Figure 1). While the patient is looking down, tilt the mirror until it reflects the eye and lens, then observe through the slit lamp. (Figure 2).
Figure 2a (top). Good reading position in downgaze, segment over pupil. Figure 2b. Poor reading position in downgaze, segment not over pupil. The lens has slipped under the lower lid margin.
This simple technique provides a much better picture of the influence of the upper and lower lids on lens translation and is especially helpful in viewing the bifocal segment position. CLS
Craig Norman is director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute. He is also a consultant to B&L.