LASIK, or Laser-Assisted in Situ Keratomileusis, is a popular refractive procedure designed to reshape the cornea and correct common vision problems like nearsightedness, farsightedness, and astigmatism. The surgery can be performed on only one eye, but this is a deliberate decision made for specific, medically sound reasons to optimize a patient’s overall visual function. The two primary rationales for this approach are a technique called monovision and the need to correct a significant imbalance between the two eyes.
Monovision: The Intentional Correction of One Eye
The most common reason for intentionally treating only one eye is to achieve monovision, often referred to as “blended vision.” This technique is designed to counteract presbyopia, the age-related loss of near focusing ability that typically affects people around age 40 and older. Instead of correcting both eyes for optimal distance vision, the surgeon corrects the patient’s dominant eye for clear distance vision.
The non-dominant eye is then intentionally left slightly nearsighted, or myopic, to allow it to focus clearly on close-up objects. This planned refractive difference allows the patient to use the distance-corrected eye for tasks like driving and the near-corrected eye for reading or using a phone. The goal is to reduce or eliminate the need for reading glasses while maintaining functional distance vision.
The degree of nearsightedness left in the non-dominant eye is carefully calculated, usually targeting a refractive error of around -1.00 to -1.50 diopters. This difference allows the brain to merge the two images effectively, providing a continuous range of vision from near to far.
Monovision is a visual strategy that can also be simulated with contact lenses. For patients over age 40 who are frustrated by the need for reading glasses, this procedure offers a long-term surgical solution to maintain spectacle independence for most daily activities.
Addressing Asymmetrical Vision Needs
A separate reason for treating only one eye is to correct anisometropia, a significant difference in refractive power between the two eyes. This difference is typically two or more diopters, and when substantial, it can lead to issues with binocular vision, depth perception, and amblyopia, or “lazy eye,” especially if developed in childhood.
In these situations, the surgeon may elect to treat only the eye with the much higher prescription to bring its refractive power closer to the other eye. The primary goal is to achieve better binocular balance, allowing the eyes to work together more comfortably and effectively. This differs from monovision because the aim is not to set one eye for near vision, but rather to equalize the visual input.
Correcting only the more highly myopic or hyperopic eye can eliminate the visual distortion that occurs when a patient attempts to wear spectacles with a large power difference between lenses. By reducing the disparity, the procedure improves the overall quality of vision without the compromise inherent in monovision.
The Pre-Operative Assessment and Trial Period
The decision to proceed with monocular correction, particularly for monovision, requires a thorough pre-operative assessment. This screening includes an extensive eye exam, measuring ocular dominance to determine which eye is best suited for distance correction. Diagnostic tools are used to map the corneal surface, measuring thickness, curvature, and elevation.
The contact lens trial period simulates the intended visual outcome before surgery. The patient wears a contact lens that corrects the dominant eye for distance and the non-dominant eye for near vision. This trial allows the patient to experience blended vision and allows the eye care professional to assess the patient’s ability to adapt.
The trial period is designed to evaluate the patient’s brain’s ability to manage the visual disparity. If the patient struggles with the simulated monovision, experiencing symptoms like persistent blur or headaches, they are often not considered a good candidate for the surgical version.
Adapting to Monocular Correction
After undergoing monocular LASIK, the patient’s brain must begin adaptation to process the new, intentionally unequal visual input. The brain must learn to automatically select the clearest image from the appropriate eye. For many, this process happens quickly, allowing them to feel comfortable within a few days.
Full adaptation can take 6 to 8 weeks as the brain becomes more efficient at merging the distance and near images. During this initial period, some individuals may notice a temporary reduction in depth perception, especially when performing tasks that require precise visual judgment. Visual disturbances such as glare or halos around lights at night may also be more noticeable.
Patients who have undergone monovision correction may find they still require glasses for certain activities, such as driving at night or reading very small print in low light conditions.