Multifocal intraocular lenses (IOLs) are artificial lenses implanted during cataract surgery or refractive lens exchange, designed to provide vision at multiple distances. Unlike standard monofocal lenses that correct vision for only one distance, multifocal IOLs aim to reduce or eliminate the need for glasses for both far and near tasks. While offering substantial visual advantages, these lenses involve inherent trade-offs that need careful consideration before implantation.
Managing Nighttime Visual Effects
The most commonly reported downside of multifocal IOLs relates to unwanted visual phenomena, especially in low-light conditions. These lenses work by splitting incoming light into multiple focal points simultaneously, a mechanism achieved through concentric rings or diffractive steps etched onto the lens surface. This light-splitting action causes some light to be scattered in unintended ways.
This light scattering manifests as visual artifacts like halos, glare, and starbursts. Halos appear as bright rings around point sources of light, such as headlights or streetlights. Glare is the excessive brightness that spreads beyond the source, and starbursts are streaks radiating outward from lights.
These effects are often collectively referred to as dysphotopsias and are most noticeable when the pupil dilates in dark environments, such as during night driving. A larger pupil allows more light to pass through the different optical zones, making the scattered light more prominent. Studies show that up to 70% of patients experience halos initially, although the severity tends to decrease over time for most individuals.
Compromised Contrast and Intermediate Vision
Apart from the light artifacts, multifocal IOLs inherently affect the overall quality of vision, particularly contrast sensitivity. Contrast sensitivity is the ability to distinguish an object from its background, especially in low-light or low-contrast situations.
Multifocal lenses reduce contrast sensitivity because the incoming light energy is permanently divided among two or more focal points. This division means that no single focal point receives the full amount of light, which naturally “softens” the image quality compared to a monofocal lens. This effect is particularly pronounced in dim lighting.
Another functional compromise is the quality of vision at the intermediate distance, which refers to arm’s length tasks like using a computer or viewing a car dashboard. While multifocal lenses typically provide excellent near and distance vision, older designs may have a “gap” in the intermediate range. Objects at this middle distance may not be as sharp as desired, sometimes requiring low-power glasses for comfortable computer work. Newer trifocal and extended depth of focus (EDOF) IOLs have been developed to specifically improve this intermediate zone, but a slight compromise in clarity can still occur.
The Need for Brain Adaptation and Realistic Expectations
A significant factor in patient satisfaction is the adjustment period the brain must undergo after the lens is implanted, a process known as neuroadaptation. The visual cortex is accustomed to receiving a single, clear image, but a multifocal lens presents two or more images simultaneously—one in focus and the others out of focus. The brain must learn to selectively ignore the blurred images and favor the clear one for the distance being viewed.
This neuroplasticity takes time, typically ranging from a few weeks to several months, and is related to how quickly a patient’s brain can process this new visual input. The successful suppression of the out-of-focus images allows the patient to perceive a visually functional image. Age and cognitive processing speed may influence the pace of this adaptation.
Managing expectations is important, as multifocal IOLs do not guarantee absolute freedom from glasses for every task. Patients must understand that while their dependence on glasses will likely be greatly reduced, they may still require low-power spectacles for fine print or prolonged reading in low-light conditions. Dissatisfaction can arise even from a technically successful surgery if the preoperative expectation was 100% glasses independence.