Monovision is a treatment strategy where one eye is set for distance vision and the other for near vision. It is an alternative to reading glasses in individuals who have trouble reading or using computer screens.
Human beings have a prefered eye known as the dominant eye. In monovision treatment, this eye is made to correctly focus far-away images while the other eye is corrected for close-up vision. Ocular preference/dominance can be tested using the Miles test. The patient is required to stretch both arms and join the hands together using the thumb and the fist knuckles. He/she looks through the small triangular opening formed to view a particular distant object. The eyes should be opened alternatively while looking at the object. The patient is said to be right-eyed if when they close their left eye, they are still able to see the object. They are said to be left-eyed if the object moves out of view. When closing the right eye, the vice versa is true. Majority of people are right-eye dominant.
The brain responds to blended vision by suppressing the image from the eye that is not in focus. It takes up the image from the eye with ‘better' vision. As a result, the person sees clearly both for distant and near objects. Monovision reduces reliance on reading glasses. The downside to the treatment is that it sacrifices depth perception and night vision. Distance vision may also not be clear. It is, therefore, not recommended for truck drivers, sports persons, pilots or people in similar professions.
Types of surgery
- Contact lenses – One lens is placed on the preferred eye for distant vision and another in the other eye for near vision. The difference in the focusing power in the two lenses is usually small to prevent blurriness.
- Monovision correction surgery – Surgeries such as LASIK or PRK can be done to correct the focusing power in the lens. With surgery, one does not need to keep wearing glasses or lenses.
- IOL implants – Artificial intraocular lenses (IOL) can also be implanted. They can replace the natural lenses of the eye or can be used together with them.
Monovision can be prescribed in:
- Presbyopia – The most common use of monovision is in presbyopia where near vision is diminished. The condition occurs when the lens of the eyes can no longer adjust to accommodate close images. Presbyopia is common after the age of 40 when the muscles of the lens lose their flexibility. It also occurs in hyperopic and myopic persons. For the latter, monovision is not usually necessary as the person can simply remove the glasses or contacts to see close images. Presbyopic persons can either choose to wear monovision contacts or undergo corrective surgery.
- Cataract surgery – As part of the operation, the clouded lens can be replaced with lenses that vary in focusing power. The far-powered lens is placed on the preferred eye and the near-powered lens on the weaker eye.
- Astigmatism – This is a condition in which vision is diminished due to the curvature of the cornea or lens. Where lens implant surgery is an option, monovision IOLs can be used.
Preparation & expectation before Surgery
Before undergoing a monovision surgery, patients are first given contact lenses as a trial. Two weeks should be enough to estimate how the surgical monovision will be. In other cases, the lenses can stay for 6-12 months before the operation. The best candidates for the surgery are those who have success with the lenses.
The eye surgeon will also evaluate the health of the eye before the procedure. The tests can include analyzing the thickness of the cornea, checking the size of the pupil and other general eye exams.
The patient should not be wearing makeup or contact lenses on the day of the surgery.
The LASIK surgery is the most often performed operation in monovision. It involves use of an instrument known as a microkeratome to create a circular flap on the cornea. After being folded out of the way, the surgeon obtains access to the inside. The eye is cleaned and numbed using anaesthetic drops.
The lenses are then corrected as per dominance. The dominant eye is modified to be far-sightedness whereas the other is modified for near vision.
The process is pain-free and takes about 15 minutes per eye. No stitches are usually required for LASIK. Antibiotic eye drops can be issued to prevent infections.
After care, recovery & results
For majority of patients, monovision goes really well. Although some take longer, in about 1 to 2 weeks, the brain adapts and vision becomes clear.
It is important for the patient to adhere to instructions given by the eye doctor. The patient should keep both eyes open at the same time. Using only one eye slows down the process of adjustment, resulting in blurry vision.
Risks & complications
Monovision does not work for everybody. It is crucial for the eye doctor to evaluate the effectiveness of the treatment beforehand. The side effects of monovision can include:
- Blurry /foggy distance vision
- Halos or glares at night
- Reduced depth perception
- Decreased night-time vision
In normal circumstances, some of the effects cease as soon as the brain adapts to the changes. When they persist, the patient may need to seek medical care immediately.