Aphakic Magnification for Boards

Why Correction Location Changes Image Size Here Too

Aphakia, absence of the crystalline lens, requires very high plus spectacle power if not corrected with a contact lens or intraocular lens. This high-plus correction sits far from the eye and produces substantial spectacle magnification. A unilateral aphakic spectacle correction enlarges the image by about 25 to 35 percent versus the phakic fellow eye. This is usually intolerable for binocular vision because of severe aniseikonia. Because a contact lens rests on the cornea, its minimal vertex distance greatly reduces spectacle magnification despite similar optical power, which is why contact lens and intraocular lens correction are strongly preferred for unilateral aphakia. Aphakic spectacles also cause a ring scotoma, sometimes called the jack-in-the-box phenomenon, and pincushion distortion in the periphery, further limiting their usefulness.

Comparing the Three Correction Methods

• Aphakic spectacles: largest effect, about 25 to 35 percent magnification. They also narrow the visual field and create marked image-size mismatch in unilateral aphakia.
• Aphakic contact lens: intermediate effect, about 6 to 10 percent magnification. Corneal-plane correction greatly reduces aniseikonia and is a practical option when an intraocular lens is absent.
• Posterior chamber intraocular lens: smallest effect. It most closely restores the natural position of the crystalline lens and produces retinal image size closest to normal.
• Bilateral aphakic spectacles: usually better tolerated than unilateral aphakic spectacles because both eyes receive similarly magnified images. There is no phakic fellow eye creating a major interocular image-size difference.