The problems of correcting aphakia with high-plus spectacle lenses include:
magnification of approximately 20%–35%
altered depth perception resulting from the magnification
pincushion distortion; for example, doors appear to bow inward
difficulty with hand–eye coordination
ring scotoma generated by prismatic effects at the edge of the lens (causing the “jack-in-the-box” phenomenon)
extreme sensitivity of the lenses to minor misadjustment in vertex distance, pantoscopic tilt, and height
in monocular aphakia, loss of useful binocular vision because of differential magnification
In addition, aphakic spectacles create cosmetic problems. The patient’s eyes appear magnified and, if viewed obliquely, may seem displaced because of prismatic effects. The high-power lenticular lens is itself unattractive, given its “fried-egg” appearance (Fig 4-36).
Figure 4-36 Aphakic lens with magnification and pincushion distortion.
(Courtesy of Tommy Korn, MD.)
For these reasons, intraocular lenses and aphakic contact lenses now account for nearly all aphakic corrections. Nevertheless, spectacle correction of aphakia is sometimes appropriate, as in bilateral infantile pediatric aphakia.
Because of the sensitivity of aphakic glasses to vertex distance and pantoscopic tilt, it is difficult to refract an aphakic eye reliably by using a phoropter. The vertex distance and the pantoscopic tilt are not well controlled, nor are they necessarily close to the values for the final spectacles. Rather than a phoropter, trial frames or lens clips are used.
The trial frame allows the refractionist to control vertex distance and pantoscopic tilt. It should be adjusted for minimal vertex distance and for the same pantoscopic tilt planned for the actual spectacles (approximately 5°–7°, not the larger values that are appropriate for conventional glasses). Pantoscopic tilt is desirable in spectacle lenses to maintain the vertex distance in downgaze. Excess tilt will induce oblique (marginal) astigmatism in the axis of rotation.
Refracting with clip-on trial lens holders placed over the patient’s existing aphakic glasses (overrefraction) keeps vertex distance and lens tilt constant. Take care that the center of the clip coincides with the optical center of the existing lens. Even if the present lens contains a cylinder at an axis different from what is needed, it is possible to calculate the resultant spherocylindrical correction with an electronic calculator, by hand, or with measurement of the combination in a lensmeter.
Guyton DL. Retinoscopy: Minus Cylinder Technique, 1986; Retinoscopy: Plus Cylinder Technique, 1986; Subjective Refraction: Cross-Cylinder Technique, 1987. Reviewed for currency, 2007. Clinical Skills DVD Series [DVD]. San Francisco: American Academy of Ophthalmology.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.