When considering custom soft multifocal contact lens options available today, such as aspheric, concentric ring design, and wavefront-guided optics, pupil size in an important fitting factor.
A lab will typically incorporate all patient fitting data, including pupil size, into computerized software that will dictate multifocal/progressive contact lens optics typically at the geometric center of a lens to be fabricated. In theory, this setup can yield great visual quality when lenses are worn; however, if multifocal optics are not lined up over the pupil center, symptoms of glare, halos, and shadowing around letters can be common patient complaints.
In Figure 1, a progressive design multifocal contact lens has settled onto a 48-year-old female’s left eye. Upon slit lamp inspection of the lens, an eyecare practitioner would likely remark that this lens is “centered” on the eye, based on the near equal overlap of the lens beyond the iris borders. However, from the tangential power difference topography map in Figure 2, there is a temporal displacement of the central optics of the lens (red arrow) away from the center of the pupil (black plus sign) and the center of the topography map (white plus sign). In patients who have normal ocular alignment, the center of the topography map is clinically referred to as the line of sight or the visual axis. Recall that each are slightly different optical paths that connect the point of fixation to the often temporally decentered anatomical fovea (Mandell et al, 1995).
What Is Happening Here?
Scenario 1: The lens is truly decentered. In Figure 1, the lens appears to have settled slightly temporally on eye. This likely does not fully explain the temporal location of the center lens optics in relation to the central reference point of the topographical map in Figure 2.
Scenario 2: There is a high angle lambda. Angle lambda is the angle that is formed between the pupillary axis, defined as the line from the pupil center that is perpendicular to the cornea, and the line of sight (Mandell et al, 1995). From Lampa et al (2013), it was reported that the presence of a non-zero value for angle lambda can account for the decentration of optics observed.
Scenario 3: The pupil is displaced. While this patient has normal ocular anatomy, we also know that the human eye is an asymmetric optical system with various optical and neural axes. Natural pupil location may also play a role.
As we advance in multifocal contact lens options and explore methods to better align final lens optics over a patient’s pupil, a more in depth analysis of pupils in relation to the various topographical optical reference points will likely be needed. CLS
For references, please visit www.clspectrum.com/references and click on document #261.