Blended Vision, is a sophisticated laser or catract eye treatment which is used to treat presbyopia (ageing eyes, progressive loss of the ability to focus on nearby objects) or other age-related eye conditions. It can be used to help people that simply need reading glasses, but also those who have started to need bifocal or varifocal spectacle correction due to ageing changes in the eye. It can be used for people who are also short-sighted (myopia) or long-sighted hyperopia and who also may have astigmatism.
Primarily the treatment is for condition called presbyopia Blended vision can be achieved through laser eye surgery such as LASIK, PRK or LASEK, by increasing the depth of field of each eye through subtle changes in the optics of the corneal spherical aberration. The increase in depth of field allows for the eyes to be corrected in such a way that the dominant eye is set for distance and intermediate vision while the non-dominant eye sees best in the intermediate to near range. Because of the similarity in the visual performance of each eye in the intermediate range the brain is able to fuse the images between the eyes rendering a binocular visual environment. This is in contradistiction to traditional monovision where the image disparity between the eyes is too high for image fusion by the brain and instead the brain needs to apply suppression of the blurred eye in order to perceive a clear visual field. In Laser Blended Vision, the eyes are effectively working together to allow good vision at near, intermediate and far, without the use of glasses. The effects of Laser Blended Vision tend to laset between 5 to 10 years but can be further adjusted by enhancement procedures.
Laser Blended Vision provides a range of benefits, particularly in comparison to tradition monivision solutions, such as bifocal glasses or contact lenses. The key advantage of laser blended vision is the freedom from reading glasses. Over 98% of laser blended vision patients can read normal newsprint and over 90% can read the small print on medicine bottle inserts. There is a creation of intermediate and far-intermediate distance zone of fusion, allowing the merging of images from each eye in the brain so no dissociation between the eyes. And due to increased depth of field in both eyes, the vision is significantly better than would otherwise be anticipated. The technology of the excimer laser provides maximum safety particularly as the eyetracker’s feedback frequency is higher. The laser treatment is a comfortable and gentle procedure and provides an extremely rapid rate of recovery.
Patients treated using laser for blended vision, have an increased depth of field compared to traditional monovision. There is a diminishing effect of the depth of field with use of contact lenses. According to a study conducted by Dr. Dan Reinstein, more than 95% of patients he treated tolerated Laser Blended Vision, compared with only 59% to 67% of patients who tolerate contact lens monovision. After a year of the laser treatment, 94.5% of patients achieved 20/20 vision.
The aim of this surgery is to produce one eye that sees objects close up, whilst the other sees objects that are far away. This can produce problems initially, as you may feel dizzy or nauseous until your brain adjusts to this new method. This is usually quickly, as the brain begins to ‘blend’ the images from both eyes together, to produce corrected vision. As with all laser eye procedures, there are risks and side effects of the treatment. You should understand these before you decide to go ahead. The Excimer laser enables safe treatment of higher levels of myopia, and provide accurate calibration. After the laser surgery, one may require artificial tears and antibiotic eye drops which can caused blurred vision temporarily. Dryness of eyes can be a little uncomfortable for a few weeks after the surgery. Presbyopia is progressive and therefore a boost maybe required after some years. Typically the effects of laser blended vision surgery can last between 5–10 years.
By Graham D. Barrett, MD, FRACO
The ability to accommodate for near and intermediate visual tasks greatly diminishes with age due to the onset of presbyopia. Patients in their fifth decade considering contact lenses, laser refractive surgery, or cataract surgery will require glasses for near and intermediate vision if they achieve emmetropia and excellent distance UCVA in both eyes.
Monovision has been widely practiced, historically with contact lens correction and more recently with LASIK to assist with near UCVA after refractive surgery. However, the largest group of patients who are suitable for monovision are those undergoing cataract surgery.
Although multifocal implants are an alternative to monovision, the patient compromises visual quality and may experience dysphotopsia, such as halos, which some patients find unacceptable. An accommodating IOL would be ideal, but the efficacy and refractive outcomes of today’s models are somewhat unpredictable. Therefore, my personal preference for the past 5 years has been to utilize monovision as an effective solution for patients who desire a greater level of spectacle independence, both for distance and near, following cataract surgery.
I target emmetropia for distance in the first—preferably dominant—eye and a refractive outcome of -1.25 D sphere in the second eye following cataract surgery. All patients who achieve 6/9 or better UCVA in the first eye are offered the choice of a target refraction of emmetropia or modest myopia (-1.25 D) in the second eye.
The counseling required for this modified monovision technique is straightforward and time efficient. After surgery in the first eye is complete, I demonstrate the type of vision that will be achieved with the addition of 1.25 D sphere. This is done with a trial frame in the recently operated distance eye. The patient can immediately perceive the reduction in distance vision and the improvement in near vision that he will experience if the target refraction is achieved in the second eye. Approximately 50% of patients elect to be emmetropic in both eyes for distance and rely on reading glasses; the remainder select monovision.
My preference is to perform surgery in the dominant eye first; however, I will still elect to perform surgery initially in the eye with the greater level of cataract and poorer acuity aiming for emmetropia, regardless of dominance. In these circumstances, the dominant eye may remain myopic—a situation referred to as cross dominance. I do not perform an initial contact lens trial because results are difficult to interpret in the presence of significant cataract. I always counsel patients that they will still need reading glasses for fine print but will be spectacle independent for intermediate and most near visual activities. The majority of patients, however, find that their need for reading spectacles is minimal, with many achieving spectacle independence following surgery.
Patient satisfaction is extremely high with monovision. In contrast to multifocal implants, unhappy patients are an exceptionally rare phenomenon. I believe that the success of this strategy is due to the level of targeted myopia for near vision. A myopic refraction in the near eye of -1.25 D sphere is significantly less than the level utilized with conventional monovision in contact lenses. The range of myopia in this scenario preserves stereoacuity and avoids the reduced contract sensitivity that may occur with higher levels of ammetropia.
Asthenopia due to strong dominance is also unlikely to occur when the difference between refractions in the two eyes is in the range of 1.25 D. This level of emmetropia can be considered physiological, allowing fusion and binocular summation rather than suppression that may be necessary with higher levels of ametropia. This level of monovision does not require prolonged neural adaptation, which sometimes is necessary with multifocal implants due to the processing required to deal with the spatially incongruent images inherent with multifocal implants. A target refraction of -1.25 D sphere for near vision appears to be more effective for pseudophakia versus phakia due to the enhanced depth of focus enjoyed by pseudophakic individuals. This is often described as pseudoaccommodation. Perhaps monovision is not an appropriate term to describe the vision enjoyed by these patients; alternative terms such as mini-monovision, blended vision, and omni-vision may be more appropriate.
I take great care with biometry and utilize toric IOLs as well as limbal relaxing incisions when appropriate to ensure that the target refractions for emmetropia and near vision are achieved. Nevertheless, compared with multifocal implants, the technique is more robust in the presence of minor degrees of spherical defocus and astigmatism. Secondary interventions are extremely uncommon, and most patients are satisfied with refractive outcomes in the range of -1.00 to -1.50 D in the near-vision eye. One of the major advantages of monovision is that any perceived deficit in acuity can be corrected with the occasional use of spectacles, thus restoring full binocular acuity and quality of vision. Patients’ acceptance of the use of spectacles is important because many patients will develop against-the-rule astigmatism changes as well as deteriorating macular function with age. It is also feasible to perform LASIK if the patient should experience asthenopia due to monovision, although personally I have not found this to be necessary. Similarly, the need for secondary procedures such as Nd:YAG laser capsulotomy does not increase compared with conventional cataract surgery with monofocal implants.
All currently available techniques to improve near vision following cataract surgery entail some degree of compromise. The compromise in contrast sensitivity and quality of vision with multifocals may be found unacceptable by some patients, thus requiring a lens exchange. Monovision targeting higher levels of myopia for near vision would be expected to offer high levels of spectacle independence for reading; however, the issues of stereoacuity, contrast sensitivity, and dominance are more challenging. In contrast, the technique of modified monovision or blended vision produces a predictable result with a high level of patient satisfaction.
Monovision is the optical status of having one eye focus at distance and the other eye focused at near. This optical condition allows the relief of presbyopia symptoms.
To understand how monovision helps a presbyopic person it’s helpful to understand the zoom function of the eye, or accommodation. When accommodation is fully relaxed in an eye, the eye can see an object clearly at distance. In order to see a close object, like a book or sewing needle, the eye increases its accommodative power by changing the shape of its crystalline lens, zooming in onto the near object and allowing it to be seen clearly too.
In childhood, the eye has tremendous power to accommodate, such that objects can be held extremely close to the eye and be seen very clearly. As a person ages the eye steadily looses its ability to accommodate. When a person reaches their mid 40s to early 50s their eyes have usually lost enough of their accommodative zoom power that they can no longer zoom in much closer than an arm’s length. The loss of the zoom function of the eye is called presbyopia. This presents difficulty in seeing small objects that could usually easily be seen by simply holding them closer to the eye when the person was younger. Reading glasses, which artificially set the focus point of the eye closer without requiring accommodative effort by the eye, then become increasingly needed. As a person reaches around 70 years of age their eyes have typically lost all accommodative zoom power and reading glasses are constantly needed to see things up close.
Monovision is an optical trick to get around the problems of presbyopia. In monovision one eye is focused at distance while the other is focused at near. The person’s brain learns to use the distance focused eye for distance viewing and the near focused eye for near viewing. This process of adaptation to monovision usually takes about 1 to 2 weeks, after which the brain begins to seamlessly use the eyes in a monovision manner, and a person becomes mostly unaware which eye is focused at near and which is focused at distance. Reading glasses may occasionally still be required for some near activities, but monovision can help a presbyopic person achieve a good amount of independence from reading glasses.
The most common method of achieving monovision is with contact lenses. Monovision can also be obtained by surgical means. Excimer laser refractive surgery (such as LASIKor PRK) can be used to induce monovision by correcting one eye for distance vision and the other for near vision. And finally monovision may be achieved during cataract surgery by implanting an intraocular lens in one eye that is focused for near, while implanting a distance focused lens in the other.
Most people who try monovision adapt to it well, however about 10 to 15% of people cannot adapt. These individuals may feel “off balanced” when using monovision, or may experience eyestrain or headaches with it.
Also, monovision can cause a mild loss of stereovision. Typically, this loss is not noticeable to a person and is inconsequential to their daily activities. However, if a large amount of near correction is used in the near eye, this loss of stereovision may become more noticeable. As such, most doctors prescribe the monovision eye with enough correction to allow good intermediate distance viewing, such as looking at cans on a grocery store isle, and for reading larger print, such as dinner menus. For very fine print or viewing very small objects monovision patients may still need the help of reading glasses.