Laser-Assisted In Situ Keratomileusis (LASIK) is a common procedure used to correct refractive errors such as nearsightedness and farsightedness. Traditional LASIK corrects both eyes for a single focal point, typically distance vision. Monovision LASIK is a specialized technique designed to address presbyopia, the age-related loss of the eye’s ability to focus on close objects. This condition is caused by the natural stiffening of the eye’s crystalline lens, usually affecting people in their early to mid-40s. Monovision LASIK offers a solution by strategically adjusting the focus of each eye for a broader range of vision without the constant need for reading glasses.
How Monovision Redefines Eye Focus
Monovision LASIK functions by intentionally creating a slight refractive difference between the two eyes, a state known as anisometropia. This difference is carefully calibrated to assign each eye a distinct primary function. The dominant eye is typically corrected for clear distance vision, optimizing clarity for activities like driving or viewing a movie screen.
The non-dominant eye is purposefully under-corrected to focus at a near or intermediate distance, enabling tasks such as reading or working on a computer screen. This surgical approach utilizes the excimer laser to reshape the cornea of each eye to its specific target prescription. The result is a system where one eye provides focus far away, and the other provides focus up close.
The success of this technique relies on the brain’s capacity for neuroadaptation—the learned ability to integrate these two different images. The brain automatically selects the clearer image from the appropriate eye depending on the viewing distance, filtering out the slightly blurred image from the other eye. Over time, the brain learns to seamlessly alternate between the distance eye and the near eye. This allows the individual to maintain functional vision across various distances, reducing dependence on corrective lenses.
The difference in focus is usually a mild degree of nearsightedness in the non-dominant eye, typically ranging from -1.00 to -2.00 diopters. This small power difference is sufficient to bring near objects into focus for the reading eye. The precise power difference is determined based on the patient’s lifestyle and visual needs.
The Essential Pre-Surgical Adaptation Test
Monovision requires a mandatory testing phase before any permanent surgical alteration is considered. This preliminary step, known as the Monovision trial, simulates the visual outcome using contact lenses. The trial involves placing a distance-corrected lens in the dominant eye and a near-corrected lens in the non-dominant eye.
This simulation is crucial because not all patients can successfully adapt to the induced difference in focus; approximately 20% of individuals find the sensation intolerable. The patient wears the trial lenses for a specific period, often several days to a week, to experience Monovision in their everyday environment and gauge their comfort level and functional success.
During the trial, the doctor assesses whether the patient can comfortably perform their daily routine, including reading, driving, and computer work, without experiencing eye strain or persistent disorientation. If the patient reports comfort and functional clarity, they are considered a good candidate for the Monovision LASIK procedure. This test manages expectations and ensures a higher likelihood of long-term satisfaction.
Navigating Visual Compromises
While Monovision offers a significant reduction in the need for reading glasses, it introduces specific trade-offs. The most notable compromise is a measurable reduction in depth perception, which is the ability to accurately judge the relative distance of objects. Depth perception (stereopsis) relies on both eyes focusing on the same point to create a single, three-dimensional image.
Because the eyes are intentionally focused at different distances, their ability to work together binocularly is lessened, leading to a decrease in stereo vision. Although the brain compensates over time, this reduction can be noticeable, particularly in tasks requiring precise depth judgment, such as parking a car. Patients are cautioned about this effect if their hobbies or profession demand high levels of stereoacuity.
Another compromise involves visual performance in low-light conditions, particularly during night driving. Some Monovision patients report experiencing increased glare, halos, or starbursts around bright light sources at night. This is often due to the differing focal points and how light interacts with the newly reshaped corneas.
Furthermore, Monovision does not guarantee complete freedom from all corrective lenses. Very fine print or prolonged reading may still require the use of low-power reading glasses, as the near eye is typically set for a comfortable intermediate distance. As presbyopia naturally progresses, the near vision correction may also drift, potentially requiring an enhancement surgery or increased reliance on supplemental glasses.