Laser eye surgery, such as LASIK or photorefractive keratectomy (PRK), can address both nearsightedness and farsightedness in a single procedure, but it requires a specialized approach. The conventional goal is to correct a single refractive error to achieve clear distance vision in both eyes. When patients also need improved near vision, often due to age-related changes, surgeons employ techniques that intentionally create a differential focus between the two eyes. This strategic correction allows the patient to see clearly across a wider range of distances, though the brain must learn to manage the resulting trade-off.
Understanding Near and Farsightedness
Nearsightedness, or myopia, and farsightedness, known as hyperopia, are refractive errors that result from the physical shape of the eye. Myopia typically occurs because the eyeball is slightly too long or the cornea is too steeply curved, which causes incoming light to focus at a point in front of the retina. This results in distant objects appearing blurry while close objects remain clear.
Hyperopia is the opposite, caused by an eyeball that is too short or a cornea that is too flat, leading to light rays focusing behind the retina. Individuals with this condition usually see distant objects clearly but struggle with blurry vision when focusing on items up close. Correcting these two conditions requires opposite adjustments to the corneal shape: flattening the cornea for myopia and steepening it for hyperopia. The most common reason a patient needs simultaneous correction of both near and distance vision is the development of presbyopia.
Presbyopia is caused by the natural stiffening and hardening of the eye’s crystalline lens, which typically begins around age 40. This loss of flexibility limits the lens’s ability to change shape, a process called accommodation, necessary for focusing on near objects. Unlike hyperopia, which is a physical shape error, presbyopia is a loss of the eye’s internal focusing power, requiring a unique surgical strategy.
Surgical Strategies for Dual Correction
The primary method used in laser eye surgery to provide both near and distance vision is called Monovision or Laser Blended Vision. This technique intentionally corrects each eye for a different focal point, allowing the patient to maintain a broad range of vision without glasses. The patient’s dominant eye is typically corrected for clear distance vision, while the non-dominant eye is intentionally left slightly nearsighted to focus on close-up tasks.
The excimer laser performs two distinct ablation patterns on the cornea to achieve differential correction. The dominant eye is reshaped to achieve zero refractive error for distance. For the non-dominant eye, the laser leaves a slight amount of myopia, usually between -1.00 and -2.00 diopters, which translates to a clear focus at reading distance. The brain then learns to prioritize the image from the distance-corrected eye when looking far away and the image from the near-corrected eye for reading.
Laser Blended Vision is a more refined version of traditional monovision, aiming to create a zone of intermediate vision where the focus of both eyes overlaps. This overlapping intermediate range provides a smoother transition between near and distance focus, improving overall visual comfort. The brain is able to “blend” the two images, resulting in a continuous, binocular visual field and reducing the gap in mid-range clarity that can occur with traditional monovision.
Candidate Eligibility and Expected Outcomes
Determining eligibility for dual correction via monovision is a crucial step that relies heavily on a pre-surgical screening process. The most important test is a monovision contact lens trial, where the patient wears contact lenses set to the intended near and distance prescriptions for several days. This trial allows the patient to experience the differential focus and confirm the brain’s ability to adapt to the new visual arrangement before undergoing permanent surgery.
Patients who adapt well to the contact lens trial generally have a high success rate with the surgical procedure, as the brain demonstrates its capacity for neuroadaptation. Not all candidates are suitable, and those with a strong sighting preference or very specific visual demands, such as professional drivers, may be less likely to tolerate the monovision effect. A comprehensive eye exam also assesses the overall health of the eye, prescription stability, and corneal thickness, which are standard requirements for any laser vision correction.
The expected outcome of dual correction is a significant reduction in the dependency on glasses for most daily activities, but it is not a return to the full visual range of a young eye. Common trade-offs associated with monovision include a potential reduction in depth perception, particularly in low-light conditions. Patients may also experience increased glare or halos at night, and they might still need weak reading glasses for prolonged tasks involving very fine print or dim lighting. The brain usually takes several weeks to fully adjust and integrate the two different images into a comfortable, functional view.