January 2013

 

REFRACTIVE SURGERY

 

Presbyond Laser Blended Vision: Another approach to presbyopia


by Dan Z. Reinstein, M.D.

   
Spherical aberration diagram

Spherical aberration diagram Source: Dan Z. Reinstein

The ideal solution for correcting presbyopia would be to restore accommodation, however, no procedure up to now has been proven to reverse presbyopia and restore the natural focusing mechanism of the eye. While there is ongoing research on techniques to achieve this, clinical applications of these techniques will probably not be available for another 10-20 years. Because of our inability to restore accommodation, current treatments for presbyopia rely on splitting the refractive power for distance and near either within the same eye (multifocality) or between eyes (monovision), but all treatments require some compromise from the patient.

The challenge for such treatment options is to achieve good binocular vision at far, intermediate, and near distances while also maintaining optical quality, contrast sensitivity, night vision, stereo acuity, and as a bonus the procedure should be reversible. This was the goal that we set when developing Presbyond Laser Blended Vision with Carl Zeiss Meditec (Jena, Germany), and our approach was to take advantage of the natural mechanisms within our optical system and minimize the need for the patient to adapt. All multifocal approaches require the patient to adjust to the unnatural situation of having to differentiate between two images in the same eye, so it is no surprise that these procedures are associated with loss of CDVA, contrast sensitivity, and night vision disturbances. There have been significant improvements over the years, however, multifocality will always rely on the patient's ability to adapt to this new and unnatural intraocular rivalry. Multifocal treatments are also usually limited to a small range of refractive error (usually low hyperopic patients).

The well-established principles of contact lens monovision have been used in laser refractive surgery, however, many of the limitations of contact lens monovision also affected laser refractive surgery-induced monovision. These limitations include loss of fusion due to the anisometropia between the two eyes, poor intermediate vision, poor distance vision in the near eye, reduced binocular contrast sensitivity, and reduced (or even broken) stereoacuity. However, monovision is based on the natural process of binocular fusion (interocular rivalry as opposed to the unnatural intraocular rivalry experienced in multifocal procedures), and recent studies have demonstrated that many of these limitations could be avoided by limiting the anisometropia to 1.25 D or 1.50 D. But this level of anisometropia does not always give the patient enough near vision.

Therefore, with Presbyond Laser Blended Vision, we incorporated another natural visual processfiltering of spherical aberrationto increase the depth of field in each eye and achieve good binocular vision at all distances. In an eye with no spherical aberration, light is focused to a point, so any forward or backward movement of the object will make it instantly go out of focus. However, if we introduce some spherical aberration into the system, there is dissemination of the focal point, meaning that there is a wider range of distances where the focus is equivalent, although slightly reduced. This of course applies to the retinal image, but the image is still perceived as sharply as if there were no aberrations due to the natural ability of the visual cortex to "process" spherical aberration. This range is the depth of field and can be demonstrated by the better-than-expected distance vision in the near eye (the mean visual acuity is about 20/45 whereas 20/80 would be expected for a 1.50 D refraction).

This concept is simply an extension of the eye's natural state as everyone has some naturally occurring spherical aberration, and the brain is already preprogrammed to do this filtering. If there is too much spherical aberration, however, the visual cortex is no longer able to fully "process" the spherical aberration and will result in loss of contrast sensitivity and other aberration-related quality of vision symptoms, similar to those seen after multifocal ablations. The ideal depth of field in each eye is 3.00 D, but we have found that the maximum depth of field that can be safely induced is 1.50 D. Therefore, this spherical aberration method cannot be used to correct presbyopia by itself but can be combined with monovision to improve the range of vision in each eye.

The increased depth of field in each eye enables good near vision to be achieved with a lower degree of anisometropia than in traditional monovision, which we refer to as micromonovision. With Presbyond Laser Blended Vision, it is possible to displace the foci between the eyes and create continuous vision, from near to intermediate to far. In essence, this strategy creates a blend zone of vision between the two eyes at intermediate distances meaning that much less suppression is required and there is no dissociation between the eyes. In fact, patients even retain a functional level of uncorrected stereoacuityproving that they have binocular function.

In Presbyond Laser Blended Vision, a number of factors are considered including age, accommodative amplitude, pre-op wavefront, tolerance to anisometropia, and the amount of refractive error. The software then combines these factors to generate an ablation profile with the aim of leaving the patient with an appropriate level of spherical aberration in order to maximize the depth of field without compromising contrast sensitivity, stereoacuity, or night vision.

At one year after Presbyond Laser Blended Vision, binocular UDVA was 20/20 or better and UNVA was J2 or better in 95% of 136 myopic patients (≤8.50 D), 77% of 111 hyperopic patients (≤+5.75 D), and 95% of 148 emmetropic patients (within 0.88 D). The safety in terms of contrast sensitivity was the same as for standard LASIK with the MEL 80 excimer laser (Carl Zeiss Meditec) with no eyes losing more than one line CDVA. Mean post-op mesopic contrast sensitivity was either the same or slightly better than pre-op at 3, 6, 12, and 18 cpd for all three populations, using the CSV-1000.

In summary, Presbyond Laser Blended Vision is a solution for presbyopia that meets all the goals of good binocular vision at all distances, no compromise in safety, contrast sensitivity, or night vision, and retention of functional stereo acuity. The procedure is immediately reversible by wearing spectacles, or a simple retreatment can be done using a standard excimer laser ablation with the advantage of keeping the depth of field. All this is achieved while simultaneously correcting a wide range of refractive errors and astigmatism levels. The key to this approach was to base it on the natural mechanisms of spherical aberration processing and binocular fusion, unlike multifocal approaches that require the patient to adjust to the unnatural situation of having to differentiate between two images in the same eye.

Editors' note: Dr. Reinstein practices at the London Vision Clinic, London, and is affiliated with the Department of Ophthalmology, Columbia University Medical College, New York, and the Centre Hospitalier National d'Ophtalmologie, Paris. He has financial interests with Carl Zeiss Meditec and ArcScan Inc. (Morrison, Colo.).

Contact information

Reinstein: +44 020 7224 1005, dzr@londonvisionclinic.com

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