prescribing for presbyopia
Measurement Tips to Increase Presbyopic RGP Fitting Success
BY CRAIG W. NORMAN, FCLSA
During the contact lens fitting process for the presbyopic patient, an analysis of the patient's refractive error and corneal topography is necessary to mathematically design a presbyopic lens.
But it's the additional measurements that may ultimately determine the level of success. Described here are some key measurements that I've found will help in choosing the presbyopic patient's initial lens.
Figure 1. 1. Upper and lower lid
position; 2. Distance from lower lid to pupil edge; 3. Lid tonicity.
While the upper lid may aid in rotation control of prism ballasted bifocal designs, the lower lid position is of greater importance (Figure 1.1). In rigid gas permeable bifocal designs, the ideal position for the lower lid is at the lower limbus. This lid position stabilizes translating bifocals in primary gaze, while holding the lens when the eye alternates from distance to reading gaze. The lower lid positioned above the inferior limbus may cause the reading segment of translating RGP bifocals to ride high in front of the pupil. This is a relative contraindication because lowering the segment position or decreasing the overall diameter will properly position the segment height.
If the lower lid comes to rest below the inferior limbus, it is a warning sign that ballasted RGP bifocals may position excessively downward in primary gaze and an alternative lens design would be better suited.
Lower Lid To Pupil Edge Distance
The near segment position of an alternating vision RGP lens must be positioned so that it does not interfere with distance vision while translating minimally in the vertical meridian to provide reading vision. In a segmented design, this normally means that the segment line is positioned slightly below the lower pupil edge in normal room lighting.
Performing this measurement is simple. Adjust the room lighting to approximate daylight. While the patient is looking straight ahead, place a millimeter ruler vertically in front of the eye and measure the distance from the top of the lower lid to the lower edge of the pupil (Figure 1.2). Keep in mind that if the light is too bright or too dim the smaller/larger pupil size will result in inaccurate measurements.
In primary position, the alternating vision RGP should rest inferiorly on the lower lid (Figure 1.3). The prism ballast incorporated in these designs automatically places the lens in this position. The role of the lower lid is to prevent the lens from slipping underneath it. Thus, the tone or tautness of the lower lid is another measurement helpful to RGP presbyopic fitting. This is best evaluated using a diagnostic lens.
Usually, presbyopic translating RGP designs can be ordered in any diameter. Measurement of the horizontal visible iris diameter (HVID) provides an approximation of corneal diameter. More important in translating lens designs is the vertical measurement because it is along this meridian that the RGP lens will actually translate. As a starting point, choose an overall lens diameter 2.5mm smaller than this vertical measurement.
Craig W. 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 an advisor to the RGP Lens Institute.