ONE OF THE CHALLENGES of fitting multifocal contact lenses on presbyopic patients arises when the patient complains of reduced visual acuity (VA) or decreased contrast. He or she may read 20/20, but still say “my vision just isn’t clear.” Following a fitting guide’s troubleshooting suggestions sometimes remedies these issues; in other cases, there seems to be no improvement. What else can be done for these patients? How can eyecare practitioners solve these problems?
A possible solution lies in corneal topography. These instruments measure curvatures on the surface of the eye and display the data through mapping. Many multifocal contact lenses are manufactured with the optics on the front surface of the contact lens. Topography can detect these steep areas (shown in red in Figure 1) where the add is located and contrast them to the flatter (blue) areas where the distance power is located.
Many multifocal contact lenses use center-near designs for simultaneous vision. The centration of this type of contact lens fit can be overlooked. Due to the shape of the eye, contact lenses tend to decenter temporally. This moves the central addition portion out of the visual axis, which can make near vision more difficult. Topography can measure how much decentration there is in relation to the pupil.
Figure 1 shows 1.1mm of inferior-temporal decentration on a patient’s right eye, along axis 222º. This patient was experiencing 20/70 VA in the eye at distance as a result, because the center-distance optic was so decentered that she was looking through the densest part of the near zone. The bifocal zone (red ring) is seen bisecting the pupil (black circle).
Another benefit of topography is being able to view the size of the distance and near zones in relation to the pupil. Most center-near designs use a 2.0mm to 2.5mm zone. This is also a fairly typical size for center-distance optics for early presbyopes, because it allows distance and near prescription inside the patient’s pupil.
Figure 1 also shows a plot of the powers across the surface of the lens. The central portion of the distance zone has a curvature of about 40.00D. The curves gradually change, reaching the maximum add, at a curvature of about 43.00D, slightly more than 1mm away from the center of the distance zone in each direction. This shows the distance zone is about 2mm wide, which can then be compared to the pupil size to ensure that all the multifocal optics, distance and near, are within the pupil.
This type of topography image is generated using a tangential map with a custom scale. The scale is an especially important part of constructing these maps. The scale range should be as narrow as possible, usually focused around the values of the simKs. A good starting point is a 1.00D greater and less than flat K. If the software allows customization, adjust the minimum and maximum values until the central curves are reflected on the color map and curvature graph.
By using topography in this way, practitioners can get more information about lens performance during their next multifocal fit.
