Presbyopia is the reduction in near focusing ability resulting from the changes to the eye's crystalline lens. It is a natural consequence of ageing, and most people begin to notice symptoms-- difficulty with small print, need for more light when reading, holding reading materials further away—around the age of 40.
For generations, patients relied on reading glasses or bifocals to provide clear near vision. But nowadays, many find these options limited and cosmetically unappealing. Monovision is a good option to provide good near acuity without compromising appearance and comfort. Multifocal lenses will be another option but not available in the NHS. This is discussed in length in another section.
Monovision involves one eye, usually the dominant eye, being corrected for distance viewing, and the other eye being corrected for near viewing. Monovision is a good option as it gives the option of GLASSES-LIGHT lifestyle but for some patients it may not be Glasses-Free.
Monovision is actually a misnomer since both eyes work still together when viewing distance and near. When driving, for instance, the out of focus eye is slightly suppressed by the brain, but it still contributes important visual information regarding the periphery. This degree of teamwork between the two eyes will vary from patient to patient; therefore, a trial with monovision spectacles or monovision contact lenses is strongly advised before choosing monovision via refractive surgery.
Monovision does involve some degree of mutual concessions between glasses dependency and reduced binocular visual acuity and reduced depth perception, especially early on. The most common issues can be addressed as follows:
1. Visual fatigue or strain due to prolonged near work (such as needlepoint). Part-time glasses and better lighting condition are recommended for prolonged reading.
2. Patients with monovision correction may notice halos and glare with night driving. Again, part-time compensation driving glasses may be prescribed.
Monovision can be an excellent alternative to multifocal Lenses or Standard Lenses. The compromises associated with monovision seem to be more acceptable than a high degree of dependency on reading glasses. Understanding how the monovision optical system works is the key to success with monovision, and it requires your time and patience.
You and your doctor can decide what is best for you by communicating your exact visual tasks, both occupational and recreational, and by participating in a monovision trial with contact lens prior to cataract surgery.
The success rates with contact lens monovision range from 50% to 70% which indicate patient selection is very important before choosing monovision. Although monovision should be part of every eye surgeon's armamentarium, knowing its limitations and carefully selecting patients are paramount.
Interocular blur suppression and neuroadaptation is the adaptive mechanism that allows monovision to succeed. If this mechanism is not adapted, the patient may experience reduced binocular visual acuity and reduced depth perception, especially early on. As with most adaptive mechanisms, the brain needs time to adapt to this new visual function before it embraces the visual compromise that is monovision.
In order for a 60-year-old patient to achieve this quality of vision, the reading eye may require -1.5 D of myopia. Increased amounts of correction decrease the synergy for binocular visual acuity. There is a fine balance in providing the patient with excellent reading vision while minimizing anisometropia and maximizing binocular visual acuity. I try to never induce more than 1.5 D in the difference between the two eyes. As we age, the prescription for near and distance vision increases and can be corrected by changing glasses.
During the monovision education process, it may be wise to suggest that higher intensity light may be necessary to enjoy monovision to the fullest. You may need distance glasses for night driving and other visually demanding events such as theatrical performances or spectator sports. Make sure to state this information clearly to the patient before the surgery.
Despite perfect technique and surgical outcomes, patients may not adapt to monovision and therefore want reversal in the form of converting the reading eye to distance. I recommend waiting at least 3 months to allow the patient an adequate trial with his new monovision, and to allow the eye a chance to heal adequately after surgery. If after this trial he is still unhappy, I proceed to reverse the monovision in order to achieve our ultimate goal: patient satisfaction. This will involve either use of contact lenses, exchanging or reinserting new intraocular lens or laser refractive surgery.
Possible additional cost:
Because this option requires additional clinic time and small chance of possible resurgery, please check with your insurance provider. Chances of needing additional procedure is very small.
Test contact lenses first
Proceed cautiously with monovision in patients who have not tried it in the past with contact lenses. Encourage a trial to ensure the patient's ultimate success and happiness with the result. For many of our patients, comfort while wearing contact lenses is difficult to achieve due to contact lens intolerance but encourage them to ignore the discomfort and emphasize whether “visual comfort” is either achieved or possible.
Reduction in stereo acuity/depth perception
While this effect usually improves following adaptation, I will rarely perform monovision on patients who require excellent stereo acuity. I proceed cautiously with (1) patients who are marginally ambulatory or disabled and at high risk of falling, (2) patients whose hobbies include golf, tennis, and baseball, and (3) pilots, truck drivers, or law enforcement officers.
Monovision entails concessions from both the surgeon and the patient. The key to a successful outcome is preoperative counselling and straightforwardness prior to surgery. To each of my monovision patients, I state that monovision is a compromise: “You will give up something (binocular balance/acuity) to get something else (ability to read without glasses).” Only the patient knows whether the advantages of monovision will outweigh the disadvantages.
Perhaps in no other situation does the patient's occupation, personality type, goals, age, and visual demands play as large a role as when we decide our patients to help them make informed choices regarding monovision.