Monovision is a specialized vision correction technique designed to address presbyopia, the age-related difficulty in focusing on close objects. This condition typically begins around the mid-40s when the eye’s natural lens loses flexibility, making near tasks like reading increasingly difficult. Monovision offers an alternative to bifocals or reading glasses by intentionally creating a difference in focus between the two eyes. This approach allows individuals to regain visual flexibility for various tasks.
How Monovision Works
Monovision operates on the principle of having one eye corrected for distance vision and the other eye corrected for near vision. Typically, the dominant eye is chosen to see clearly at a distance, while the non-dominant eye is set to focus on close-up objects. This setup creates a visual environment where each eye provides a clear image at a specific range, with the other image being slightly blurred.
The brain plays a significant role in making this system function effectively, a process known as inter-ocular blur suppression. When a person looks far away, the brain instinctively prioritizes the clear image from the distance-corrected eye and suppresses the blurred input from the near-corrected eye. Conversely, when performing a near task like reading, the brain favors the input from the near-corrected eye. This neural processing allows the person to achieve functional vision across multiple distances without the need for additional eyewear.
Methods of Correction
Monovision can be achieved through both non-surgical and surgical means, depending on the patient’s existing vision and long-term goals. Contact lenses are the most common and least invasive method, often used as a trial before a permanent commitment. A practitioner prescribes a different lens power for each eye—one for distance and one for near—to simulate the effect.
For permanent correction, monovision can be created through refractive surgery, such as LASIK or PRK. During these laser procedures, the surgeon intentionally reshapes the cornea of one eye for distance and the other for near vision. Another surgical option involves implanting monofocal intraocular lenses (IOLs) with different powers into each eye during cataract surgery or refractive lens exchange.
Evaluating the Trade-offs
Monovision offers the significant advantage of reducing or eliminating the need for reading glasses for most daily activities, providing a high degree of visual freedom. However, this convenience comes with several well-documented visual compromises that must be considered. The primary trade-off is a reduction in stereopsis, which is the ability to perceive depth accurately.
Depth perception relies on the brain receiving two slightly different images from the two eyes. The difference in focus created by monovision degrades this binocular function, reducing stereoacuity. This reduction is noticeable at greater viewing distances, potentially affecting tasks like judging the distance of a moving car or navigating stairs.
Another common compromise relates to visual quality in low-light conditions, such as night driving. The difference in focus between the two eyes can increase issues with glare or halos around lights. Furthermore, the brain must work continuously to suppress the blurred image from one eye, which can lead to visual fatigue or strain in some individuals. The overall success depends on the brain’s ability to adapt to this differential blur, a process that can take time and may not be fully successful for every person.
Trialling and Determining Suitability
Suitability for monovision is almost always determined through a trial period using contact lenses before committing to surgery. This trial allows the patient to experience the visual effect and assess their ability to adapt to the different focal points. The process includes a comprehensive eye examination to determine the visually dominant eye, which is usually designated for distance correction.
During the trial, the eye care professional assesses the patient’s comfort and visual performance in various real-world situations, such as reading, using a computer, and driving. A successful outcome is defined by the ability to perform daily tasks without persistent discomfort, headaches, or significant difficulty with depth perception. The practitioner also tests how well the brain suppresses the blur in each eye.
Candidates with professions requiring high-level depth perception, such as certain trades or sports, may be advised against monovision. Individuals with a very strong visual dominance in one eye may also struggle to accept the near correction in their non-dominant eye. The amount of power difference between the eyes, known as anisometropia, is carefully managed, often aiming for no more than 1.50 to 2.00 diopters of difference.