Effect of Spectacle and Contact Lens Correction on Accommodation and Convergence
Both accommodation and convergence requirements differ between contact lenses and spectacle lenses. The effects become more noticeable as the power of the correction increases.
Let us first consider accommodative requirements (see Chapter 5). Recall that because of vertex distance considerations, particularly with high-power corrections, the dioptric power of the distance correction in the spectacle plane is different from that in the contact lens plane: for a near object held at a constant distance, the amount that an eye needs to accommodate depends on the location of the refractive correction relative to the cornea. Patients with myopia must accommodate more for a given near object when wearing contact lenses than when wearing glasses. For example, patients in their early 40s with myopia who switch from single-vision glasses to contact lenses may suddenly experience presbyopic symptoms. The reverse is true with patients with hyperopia; spectacle correction requires more accommodation for a given near object than does contact lens correction. Patients with spectacle-corrected high myopia, when presbyopic, need only weak bifocal add power or none at all. For example, a patient with high myopia who wears −20.00 D glasses needs to accommodate only approximately 1.00 D to see an object at 33 cm.
Now let us consider convergence requirements and refractive correction. Because contact lenses move with the eyes and spectacles do not, different amounts of convergence are required for viewing near objects. Spectacle correction gives a myopic patient a base-in prism effect when converging and thus reduces the patient’s requirement for convergence. (Fortunately, this reduction parallels the lessened requirement for accommodation.) In contrast, a patient with spectacle-corrected hyperopia encounters a base-out prism effect that increases the requirement for convergence. This effect is beneficial in the correction of residual esotropia at near in patients with hyperopia and accommodative esotropia. These effects may be the source of a patient’s symptoms on switching between glasses and contact lenses (see Clinical Example 5-2).
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.