Parts 1 through 4 of this series described the importance of determining the visual axis measurement to maximize patient visual comfort and satisfaction with PALs. This final installment introduces methods and devices to obtain that measurement.
Part 3 discussed the effect of angle Kappa on PAL fitting. Part 4 addresses the effect of the dominant eye and alignment error on PAL success. A patient’s adaptive response is for the dominant eye to center on the clearest portion of the segment in the lens in front of the dominant eye. If the lenses are incorrectly aligned, the non-dominant eye’s visual axis will now fall outside the umbilic. That results in a head or eye turn to make near vision clear.
Ask just about any eye care professional when knowing the fitting vertex distance of a pair of glasses is important, and they'll no doubt respond: "For powers over 6 diopters, you'll have to compensate the power for the difference between the exam vertex distance and the wearing vertex distance."
For most of the last century, eyeglasses were fabricated using a binocular measurement for pupillary distance, and simply dividing it in half to center the lenses for each eye. This method delivered apparently great patient satisfaction, as visual complaints were not clearly traceable to the lack of using a "proper" monocular PD....even for segmented multifocals.
Clinical Tip When fitting multifocals, especially in higher powered Rxs, although you normally place the MRPs or optical centers at unequal decentrations to agree with unequal monocular pd's, you may wish to equalize the decentration of the segs for a better cosmetic result.