Laser Blended Vision

Laser Blended Vision is a form of corneal refractive surgery for the correction of presbyopia. Presbyopia occurs as eyes age and is characterized by a decreased ability to view objects that are close by. This problem has traditionally been treated using glasses, contact lenses or LASIK surgery. Laser blended vision is an alternative, surgical treatment that corrects each of a patents eyes differently. Typically, the dominant eye will be corrected, using LASIK surgery, to have good distance vision, while the non-dominant eye will be corrected to be short sighted, i.e., to see objects that are close by. At the same time, the depth of field (i.e. the range of distances at which the image is in focus) of each eye is increased. As a result, the brain merges the two images, creating a blend zone, i.e. a zone which is in focus for both eyes. This allows the patient to see near, intermediate and far without glasses. The surgery typically only takes 10-15 minutes, and physical recovery occurs within a few hours. The merging of the images, however, may take a short time to learn, so that after surgery many patients experience a few days to a few months of dizziness and nausea, until their brain adapts to their new vision.

Patients with myopia, hyperopia and astigmatismcan also be treated using the concepts of laser blended vision.

History

There has recently been a tremendous increase in interest of surgical presbyopic correction. The Carl Zeiss Meditec team started working in 2003 with an ophthalmic surgeon to develop a module for pure presbyopia correction, and over the next few years modified and adapted things to a point where it is finally a commercially available reality.

Before Laser Blended Vision was available, surgical presbyopic correction was achieved by using traditional Laser Monovision correction, where one eye was focused for distance vision and one eye was focused for near vision. In traditional Laser Monovision, the depth of field is not increased, which means there is no blend zone. This results in creating a gap of blurred vision at intermediate distances where neither the distance eye nor the near eye is in focus. The gap in the range of clear vision makes it more difficult for the brain to fuse the two images from the two eyes and, as a consequence, the tolerance of Laser Monovision is much lower than that of Laser Blended Vision.

Technology

To better understand the way Laser Blended Vision works, instead of viewing presbyopia as the inability to accommodate – change the focus of the eye from distance to near, presbyopia can be considered as a decrease in depth of field – the inability to see distance and near objects. The laser ablation profile in Laser Blended Vision is optimized to increase the depth of field of the eye without compromising visual quality, contrast sensitivity or night vision.

It is known that one way of increasing the depth of field is to increase the amount of spherical aberration.

Based on that knowledge, a non-linear aspheric ablation profile was developed to safely increase the depth of field of the cornea to 1.50 Diopters for any refractive error. Given a 1.50 D depth of field, it would not be possible to get full distance and full near vision with one eye; therefore Laser Blended Vision borrowed from the time-tested concept of monovision and set up the non-dominant eye (near eye) to be slightly myopic, so that the depth of field of the predominantly near (non-dominant) eye was able to see at near and up to the intermediate range while the predominantly distance (dominant) eye was able to see at distance down to intermediate.

In the intermediate region both eyes have similar visual acuity, which helps the process of binocular fusion. The cortical processes of the neural system are able to direct the conscious attention to the part of the visual field with the best image quality.

The low nominal myopic refraction of the near eye combined with the increased depth of field result in an improvement of the distance vision of the near eye compared to Laser Monovision correction. For example, with a refraction of -1.50 D, the near eye is able to see at distance as if it was a -0.75 D refraction. A further component of Laser Blended Vision is the increase in depth of field afforded by pupil constriction during accommodation: a component that persists even in eyes that have lost the ability to accommodate. The combination of controlled induced corneal spherical aberration and pupil constriction gives a significant increase in depth of field on the retinal image, albeit not a perfect image. However, intra-retinal and cortical processing and edge detection is the final component working in Laser Blended Vision: the pure retinal image, which is modified by spherical aberration, is further enhanced by central processing to yield the perception of clear and well-defined edges.

In summary, Laser Blended Vision draws on 5 mechanisms for its success as a procedure; the increased depth of field increase is achieved by:

1. A specific controlled increase in corneal spherical aberration
2. Pupil constriction during accommodation
3. Retinal and cortical processing for increasing contrast of the retinal image of each eye
4. Blend Zone to enable continuous distance to intermediate to near vision between the two eyes
5. Central cortical processing for binocular fusion

Preoperative examination

Patient selection

Inclusion criteria are the same as for standard LASIK. Ideally the corrected distance visual acuity (CDVA) should be no worse than 20/25 in either eye. The range of manifest refractive error is the same as for standard LASIK; the range of spherical equivalent refraction in publications using Laser Blended Vision is from -8.50 D to +5.75

General assessment

The preoperative assessment should include all tests used to evaluate patients for standard LASIK. All patients should have a full ophthalmologic examination including uncorrected distance visual acuity (UDVA) (light-controlled ETDRS chart), uncorrected near visual acuity (UNVA) (verified near vision reading card), subjective refractive error, corrected distance visual acuity (CDVA), distance corrected near vision (DCNV), cycloplegic refraction, slit-lamp microscopic evaluation of the anterior segment, dilated fundus examination, and Goldmann intraocular pressure measurement, placido front surface corneal topography, anterior and posterior best-fit sphere tomography, wavefront aberrations assessment, dark adapted pupil size measurements, pachymetry, and mesopic contrast sensitivity. A line of acuity should be deemed read if at least three of the five letters of that line are recognized correctly.

Additional assessment

In addition to standard LASIK testing, ocular dominance should be assessed to determine which eye is dominant.

Surgical procedure

The surgical procedure is exactly the same as for standard LASIK treatments with the MEL80.

Postoperative examination

The procedure for postoperative evaluation is exactly the same as for standard LASIK patients.

Results

The results reported below for the myopic and hyperopic populations were recently published in the Journal of Refractive Surgery. The results for the emmetropic population were presented at the ESCRS in 2008. The results are reported after all treatments (i.e. including retreatments).

Safety

The safety of Laser Blended Vision is the same as for standard LASIK with the MEL80. No eye lost two lines of Corrected Distance Visual Acuity (CDVA) and there was a loss of one line of CDVA in 8% of eyes in the myopic population and 17% of eyes in the hyperopic population (NB these populations were treated using the Hansatome microkeratome).

Contrast sensitivity

The safety in terms of contrast sensitivity is also equivalent to the results for standard LASIK with the MEL80. The publications reported that the mean postoperative contrast sensitivity was either the same as preoperative or slightly better than preoperative for all spatial frequencies measured (3, 6, 12, and 18 cpd) for both myopic and hyperopic populations.

Efficacy

Distance vision of distance eyes was excellent, with 98% of myopic, 86% of hyperopic, and 92% of emmetropic eyes achieving 20/20 or better. The binocular distance vision was 20/20 or better in 98% of myopic patients, 95% of hyperopic patients and 96% of emmetropic patients. The distance vision of the near eye was better than expected given the -1.50 D refraction, and approximately 80% of the eyes were 20/63 or better. (An untreated eye with myopia of -1.50 D would only be expected to achieve a Uncorrected Distance Visual Acuity of 20/80).

To demonstrate the effect of neural summation (see section on binocular summation in Binocular vision), it was interesting to note that for all three groups combined (+5.75 to emmetropes to -8.50 D), monocular distance vision was 20/63 in 80% of near (non-dominant) eyes and 20/20 in 92% of distance (dominant) eyes. When the relatively blurred non-dominant (near) eye was added for binocular distance vision, the distance vision increased from 92% monocularly to 96% binocularly. This indicated that the addition of a blurred non-dominant eye to the distance eye resulted in even better distance vision.

Near vision was at least J3 (better than newsprint) for 99% of myopic patients, 94% of hyperopic patients and 99% of emmetropic patients, while 96% of myopic patients, 81% of hyperopic patients and 95% of emmetropic patients were able to read J2 (very small print). It is interesting to note that this level of near vision was achieved with a -1.50 D anisometropia. (A person 55 years of age would be expected to need a near spectacle addition in the range of -1.50 to -2.25 D.)

For combined distance and near vision, 94% of myopic and 92% of emmetropic patients reached 20/25 and J2; despite the slightly lower accuracy of hyperopic corneal LASIK, 80% of these patients including hyperopia up to +5.75 D achieved 20/25 and J2 (and 99% are 20/25 and J5).

Stereopsis

While postoperative uncorrected stereopsis was lower than preoperative near-corrected stereopsis, a functional level of stereopsis was maintained postoperatively; 68% of patients had stereopsis of 100 secs or better and 93% had stereopsis of 200 secs or better. The study also found that near-correction restored preoperative near-corrected stereopsis in the majority of patients; 5% of patients with 40–50 seconds of stereopsis preoperatively showed a 1 patch decrease in best corrected stereopsis, while 100% of patients with initially 60 seconds or less showed no loss at all.

Retreatments

The retreatment rate for Laser Blended Vision is expected to be higher than that for standard LASIK. The main reason for this is that a high degree of accuracy is required in both the distance and near eyes to enable the patient to achieve excellent unaided vision at both distance and near. This is in contrast to standard distance only correction, where if one eye is slightly off target, the patient does not tend to notice as the distance vision binocularly is still good. Another reason for a slightly higher retreatment rate than generally reported is that it is only older patients who would have Laser Blended Vision. The retreatment rate is higher for older patients as there is greater inaccuracy in the refractive outcome most likely due to a less repeatable healing response between individuals.

Reported retreatment rates were 19% for the myopic population (up to -8.50 D) and 22% for the hyperopic population (up to +5.75 D). However, these retreatment rates are artificially high since many of the eyes that underwent a retreatment had already achieved either Uncorrected Distance Visual Acuity (UDVA) of 20/25 or better or Uncorrected Near Visual Acuity (UNVA) of J5 or better after the primary treatment. If retreatments had only been performed for patients whose UDVA was 20/25 or worse after the primary treatment, the retreatment rate would have been 9% for the myopic population and 9% for the hyperopic population.

Adaptation period

Patients should be counselled to expect a period of adaptation while the brain adjusts to process the images from the two eyes. This is different for different patients, but for the vast majority the adaptation period is no longer than three months. As with standard LASIK, there is likely to be fluctuation in the vision during the first three months and especially in the first week. For patients after Laser Blended Vision, this might mean that they will see well at near and badly at distance one day and then badly at near and well at distance the next. During this time, the refractive error of either eye might be off from the intended target, meaning that the anisometropia is too large or that the eyes are not focused at the intended distances. This may delay the adaptation period as the brain will not be receiving the right images for processing.

Potential complications

The potential complications are exactly the same as for standard LASIK. Laser Blended Vision does not create any specific complications.

Alternative solutions

Bifocal/varifocal lenses

Bifocal and varifocal lenses can be used to correct presbyopia. However, this mode of correction has some drawbacks. Research studies have indicated that multifocal glasses impair depth perception and edge-contrast sensitivity at critical distances for detecting obstacles in the environment. In varifocal lenses, there is a corridor of continuously changing lens power and optimal vision is only obtained when looking though this corridor and directly facing the object of focus. Outside this corridor, the vision is distorted and peripheral vision is reduced. For these reasons, older people are more likely to fall when wearing multi-focal glasses. Vision through bifocal lenses is the third greatest risk factor for falls in the elderly.

Contact lens monovision

In this technique, the eyes are dissociated by focusing one eye for distance vision and one eye for near vision. However, the large image disparity between the two eyes causes several limitations to the quality of overall vision. One limitation of monovision is the gap in the range of clear vision at intermediate distance (computer, TV).1 Reduced stereopsis is considered to be the major limitation to monovision correction; both distance and near stereopsis have been shown to decrease with monovision correction.. Binocular contrast sensitivity has also been shown to decrease with progressive increase in contact lens power in the near eye. The combination of these limitations means that monovision correction can only be tolerated by between 59-67% of patients.

Laser monovision refractive surgery

The principles used for monovision contact lenses have been applied to refractive surgery. However, many of the same limitations found with monovision contact lenses applied to monovision induced by refractive surgery, including loss of fusion and stereoacuity. Surprisingly, monovision induced by refractive surgery can be tolerated by a higher proportion of patients (92%) than monovision induced by contact lenses.

Other types of refractive surgery

Experimental approaches have been used to create a number of different multi-focal corneal ablation profiles. In such techniques, either a central corneal area is steepened for near vision leaving the mid-peripheral cornea for far vision or vice versa. While an overall improvement in visual acuity has been recorded for both near and distance vision, safety and quality of vision have been compromised. It has been reported that 20% of eyes lost two lines of best-corrected visual acuity at distance and 52% of eyes lost two lines of best-corrected visual acuity at near, while only 48% of eyes achieved 20/20 uncorrected visual acuity. Further, by creating discontinuous optics between the central and the mid-peripheral cornea, contrast sensitivity was decreased and patients have reported night vision disturbances.

Multifocal IOLs

In addition to laser techniques, a popular method of correcting presbyopia is to perform intraocular surgery, removing the patient's crystalline lens and replacing it with a multi-focal or accommodating intraocular lens implant. These lenses aim to correct both distance and near vision through a series of diffractive or refractive circular bands, each band alternating between distance and near vision correction. Clinically, multi-focal lenses do increase the range of vision from distance to near, but there are a few shortcomings. First, there is a limited range to the vision inherent to the type of lens used. As a result, the patient may experience gaps in the vision where poor visual focus is found. Second, multi-focal lenses have discontinuous optics and create more than one image to enable both distance vision and near vision correction. This has been shown to reduce contrast sensitivity and increase night vision disturbances, with approximately, 4-8% of patients experiencing serious night vision disturbances., In addition, these methods ignore the fact that presbyopes under 65 years in age may have some remaining accommodation which is sacrificed when the crystalline leans is replaced by an intra-ocular implant.

External links

Magazine articles on Laser Blended Vision

Presbyopic LASIK procedure corrects distance and near vision simultaneously

• [1]

New LASIK ablation profile increases depth-of-field, presbyopia options

• [2]

Advantages of Laser Blended Vision

• [3]

My personal account of Laser Blended Vision By Pait Teesalu, MD, PhD

• [4]

Videos on Laser Blended Vision

Phillip Schofield from This Morning has laser eye treatment

• [5]

Presbyopic LASIK by Micro-Monovision Using Increased Depth of Field from Non-linear Aspheric Profiles

• [6]