Presbyopia, the age-related inability of the eye to focus sharply on close objects, is caused by the natural stiffening of the eye’s crystalline lens. This condition often leads people to rely on reading glasses. Contact lenses designed to correct vision at multiple distances, including both near and far, are widely available as an alternative.
Contact Lenses for Presbyopia
While the term “bifocal” is frequently used due to its familiarity in eyeglasses, the contact lenses that correct presbyopia are more accurately described as “multifocal” lenses. A true bifocal lens has only two distinct optical powers, separated by a sharp line or boundary. Multifocal contact lenses, in contrast, incorporate a gradual progression of power or multiple zones for near, intermediate, and distance vision within a single lens. The objective of these lenses is to restore the ability to see clearly at all viewing ranges.
Understanding Lens Design Principles
Multifocal contact lenses achieve simultaneous distance and near correction through two primary optical mechanisms. The most common design is called simultaneous vision, which presents both the near and distance prescription to the retina at the same time. The brain then selectively focuses on the clear image and ignores the blurred one.
This simultaneous design includes two main subtypes: concentric and aspheric. Concentric lenses feature alternating rings of near and distance power around the pupil. Aspheric designs use a gradual change in power from the center to the edge of the lens, similar to progressive eyeglass lenses.
The other mechanism is the alternating or translating vision design, which is more commonly found in rigid gas permeable (RGP) lenses. These lenses are designed to shift position on the eye when the wearer looks down, causing the pupil to align with the lower, near-vision segment. The lens is typically shaped with a flattened bottom edge, or truncation, which interacts with the lower eyelid to facilitate this translation.
Selecting the Right Lens Material and Schedule
Multifocal designs are available in various materials to suit different needs for comfort and eye health. Soft lenses, particularly those made from highly breathable silicone hydrogel, are the most common choice, offering high oxygen permeability and moisture retention. Rigid Gas Permeable (RGP) lenses are also available in multifocal designs, often offering sharper vision, though they require a greater initial adaptation period due to their firmer nature.
Replacement schedules are another consideration, with options including:
- Daily disposables.
- Two-week lenses.
- Monthly lenses.
The fitting process for multifocal lenses is more complex than for standard single-vision lenses. The eye care professional must consider the wearer’s pupil size and visual demands, often involving multiple trial lenses and follow-up appointments to achieve the optimal balance between distance, intermediate, and near vision.
Adjusting to Multifocal Vision
Adapting to multifocal contact lenses requires a period of “neuro-adaptation,” as the brain must learn to manage the multiple focal points presented by the lens. This adjustment can take anywhere from a few days to several weeks, with most people reaching a functional level of satisfaction within one to two weeks of consistent wear.
During this time, wearers may notice a slight reduction in contrast sensitivity and a potential for glare or halos around lights, particularly when driving at night. These effects occur due to the simultaneous presentation of multiple images. The goal of multifocal contacts is functional vision across all ranges, and the brain’s ability to filter out blur improves significantly with consistent use.