Cataract surgery is one of the most common and successful procedures performed globally, involving the replacement of the eye’s cloudy natural lens with an artificial one called an intraocular lens (IOL). While IOLs significantly restore vision, patients often have concerns about potential visual side effects, especially the perception of unwanted light phenomena. The question of whether the standard monofocal lens causes “halos” is a frequent concern for individuals considering cataract surgery.
Understanding Monofocal Intraocular Lenses
A monofocal intraocular lens is the most frequently implanted type of IOL, designed to provide clear vision at a single focal distance. This lens replaces the natural, clouded lens during cataract surgery and functions by focusing light onto the retina in a manner similar to the original lens. Most patients and surgeons choose to set this single focal point for optimal distance vision, which allows for clear viewing of objects far away, such as when driving or watching television.
The fundamental limitation of the monofocal design is its inability to change focus for different ranges, meaning the patient will likely need glasses for near tasks, like reading or using a computer. Monofocal IOLs are typically made from flexible, biocompatible materials like acrylic and are valued for their proven safety record and long-term reliability. Because they do not split light to create multiple focal points, they generally result in a lower risk of visual side effects compared to more complex lens types.
Defining Halos and Other Post-Surgical Light Disturbances
Halos are a specific type of visual disturbance, often described as bright, colored, or white rings that appear to encircle a light source, especially in low-light conditions. These symptoms belong to a broader category of unwanted visual phenomena known as dysphotopsia, which is Latin for “faulty light”. Dysphotopsia is classified into two main types: positive and negative.
Positive dysphotopsia includes light-related artifacts such as halos, glare, starbursts, and streaks. These occur when light interacts with the new synthetic lens in a way that causes it to be scattered or reflected onto the retina. While the occurrence of positive dysphotopsia is reported to be high immediately after surgery, symptoms typically resolve spontaneously in most patients as the brain adapts.
Negative dysphotopsia is a distinct phenomenon, described as a dark, arc-shaped shadow or crescent, usually located in the temporal field of vision. Unlike positive dysphotopsia, which is caused by stray light, negative dysphotopsia is thought to be a shadow cast by the edge of the IOL. Both forms of dysphotopsia are a consequence of the eye adjusting to the new lens implant.
Direct Answer: Monofocals, Halos, and Edge Glare
Monofocal lenses are significantly less likely to cause the complex and pronounced halos associated with multifocal IOLs, which intentionally split light to achieve multiple focal points. However, they are not entirely free from causing visual artifacts, which are typically a result of the lens’s physical design rather than its focusing power. When patients with monofocal IOLs report halos or glare, it often stems from a phenomenon called “edge glare” or is related to spherical aberration.
Edge glare is a form of positive dysphotopsia caused by light reflecting off the sharp, square edge of the IOL. This design feature is intended to reduce the risk of secondary cataract formation, but the internal reflection can be perceived as an arc or a streak of light, especially at night. Modern IOLs often include a modified aspheric surface, which helps minimize this issue by correcting for the eye’s natural optical imperfections.
A different issue is negative dysphotopsia, the dark temporal shadow, which is more commonly linked to monofocal lenses with square edges placed within the capsular bag. While this shadow can be bothersome, studies suggest that approximately 80% of cases resolve spontaneously within weeks or months as the peripheral capsule heals. The occurrence of persistent, clinically significant dysphotopsia requiring intervention is rare.
Options for Managing Persistent Visual Artifacts
If visual artifacts, whether they are halos, glare, or dark shadows, persist beyond the initial healing period of a few months, several management options are available. The first step involves non-surgical approaches, which may include using pupil-constricting drops to reduce the amount of light reaching the problematic lens edge. Managing any residual refractive error with glasses or contact lenses can also improve overall visual quality and reduce the subjective impact of light scatter.
For persistent negative dysphotopsia, a surgical approach may be considered, such as performing a YAG laser capsulotomy or reverse optic capture. In rare and severe cases of positive dysphotopsia, an IOL exchange may be performed to replace the lens with one made of a different material, such as silicone. Consulting with the operating surgeon is crucial to determine the exact cause of the visual artifact and select the most appropriate management strategy.