Non-linear aspheric corneal ablation can be used to treat presbyopia

Presbyopia combined with any refractive error has been a significant treatment challenge for refractive surgeons. Traditionally, the principles used for monovision contact lenses have been applied to corneal refractive surgery.¹ However, this retains many of the limitations found with such contact lenses, including loss of fusion and stereoacuity.²

Multifocal corneal ablation profiles have also been suggested; however, although an overall improvement in visual acuity has been recorded for both near and distance vision, the efficacy has remained relatively low³, and safety and quality of vision can be compromised.⁴ A better solution that offers improved visual results and greater tolerance is still required.

Laser blended vision

It is helpful to consider presbyopia as the inability to accommodate rather than a decrease in depth of field of the eye. This decrease can be overcome, at least in part, by using an optimised ablation profile that controls postoperative spherical aberration, thus increasing the depth of field of each eye without significantly compromising visual quality, contrast sensitivity or night vision. The optimisation is based on the patient’s age, refraction, preoperative spherical aberration, tolerance for anisometropia and treatment centred on the corneal vertex.

We learned in the 1990s that spherical aberration increased in myopic ablations, leading to a decrease in visual quality and contrast sensitivity.⁵ My early work in wavefront-guided repair of night vision disturbances using what was at the time the highest resolution aberrometer (210 µm) coupled with Gaussian small-spot (0.7 mm) high repetition rate excimer laser ablation taught me that even a modest (27%) decrease in harmful levels of spherical aberration restored contrast sensitivity and night vision quality to normal.

This led me to consider up to approximately 0.6 µm of spherical aberration (Optical Society of America, 6 mm) as tolerable – this level can be filtered by the brain. This led to the concept of using spherical aberration to increase the depth of field of the eye.⁶

Within a few years, several researchers were able to experimentally duplicate this concept using adaptive optics systems and to demonstrate that extended depth of field increased linearly with the increase in spherical aberration, but only up to a certain point.⁵ Most important to note here is that adaptive optics studies proved that the depth of field increased with both positive and negative spherical aberration, showing that the effect was due to the spherical aberration itself rather than a zonal change in refractive sphere power (e.g., in positive spherical aberration, the larger the pupil, the more myopic the sphere of the refraction).

These laboratory experiments confirmed our surgical clinical research findings that a ‘therapeutic’ range for spherical aberration producing extended depth of field existed, beyond which there were ‘toxic’ effects of halos and reduced contrast sensitivity. During my early work developing an algorithm for presbyopic correction, the initial aim was to be able to adjust depth of field enough to provide clear vision from distance through intermediate to near, creating an eye that could see 20/20 at distance and also see a computer screen and read J1.

We discovered that with photopic pupil diameters, the depth of field could be safely increased to 1.50 D for any starting refractive error. Given a 1.50 D depth of field, it would not be possible to get full distance and full near vision monocularly; therefore, based on the time-tested concept of introducing a degree of anisometropia between the eyes, the non-dominant eye was set up to be slightly myopic, so that the predominantly distance (dominant) eye was able to see at distance to intermediate while the predominantly near (non-dominant) eye was able to see in the near range and up to intermediate.

Both eyes had similar acuity in the intermediate region, an optimal situation for stereopsis. Micro-anisometropia in this case draws on the inherent cortical processes of neuronal gating and blur suppression by ‘interocular rivalry’ (the ability for conscious attention to be directed to the specific area with the best image quality within the entire visual field of both eyes). This contrasts with other attempts to treat presbyopia by inducing a cornea with two distinct focal points within the same eye: ‘intraocular rivalry’.

A further component contributing to the increase in depth of field, which persists even in eyes that have lost the ability to change crystalline lens power during the accommodative effort, is the increase in depth of field afforded by pupil constriction during accommodation. The combination of controlled induced corneal aberrations and pupil constriction significantly increases the depth of field on the retinal image.

Intraretinal and cortical processing and edge detection are the final components of 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, well-defined edges. In principle, as described above, the depth of field can be enhanced through the introduction of either positive corneal spherical aberration, in which case corneal power increases with zonal diameter, or negative aberration, where power decreases with distance from the corneal vertex.^{5, 7}

Most patients have some nascent positive spherical aberration before treatment, which is added to by the positive spherical aberration induced by standard myopic ablation. The important thing is to control the induction of spherical aberration to avoid increasing it above the tolerance threshold, which can cause loss of contrast sensitivity and night vision disturbances and can result in a topographic central island. To account for this, the ablation profile includes a pre-compensation factor.

A standard large zone (7 mm) hyperopic ablation induces negative spherical aberration that, in the case of hyperopic correction, is unlikely to increase above the tolerance threshold even with up to +7.00 D correction because most patients start with some positive spherical aberration and the range of hyperopic treatments is smaller than the range of myopic treatments.⁸ In emmetropic patients, you cannot rely on the ablation inducing spherical aberration, so the spherical aberration component of the calculation is increased.

This has an impact on the refractive accuracy. As emmetropic patients have high expectations and low tolerance to refractive inaccuracy, the best option is to increase the depth of field somewhat and make sure that the micro-anisometropia component is as accurate as possible.

The ablation profiles, taking age and preoperative spherical aberration into account, are referred to as non-linear aspheric ablation profiles because the spherical aberration component is governed by a non-linear function.

Results

The outcomes using Presbyond laser blended vision with the MEL 80 excimer laser (Carl Zeiss Meditec) have been published for myopia up to –8.50 D,⁹ hyperopia up to +5.75 D¹⁰ and emmetropia.¹¹ All treatments were performed as bilateral simultaneous LASIK.

For inclusion, patients had to be medically suitable for LASIK, presbyopic with corrected distance visual acuity (CDVA) no worse than 20/25 in either eye and have a tolerance of at least -0.75 D anisometropia.The standard micro-monovision protocol corrected the dominant eye to plano and the non-dominant eye to –1.50 D irrespective of age.

At 1-year follow-up, binocular uncorrected distance visual acuity was 20/20 or better and binocular uncorrected near visual acuity was J2 or better in 95% of myopes, 77% of hyperopes and 95% of emmetropes. Retreatment rate was 19%, 22% and 12%, respectively, although this would have been 5%, 6% and 4% had the criterion for retreatment been 20/32. The safety in terms of CDVA and contrast sensitivity was the same as for standard LASIK, with no eyes losing more than one line.

Mean mesopic contrast sensitivity either remained the same or improved slightly at 3, 6, 12 and 18 cycles per degree for all three populations. Stereoacuity, although slightly reduced, has been shown to be maintained at a functional level of 100–400 seconds. Similar results have been reported by other groups.^12-15

The principle of correcting refractive error while modulating spherical aberration to benefit the depth of field can be equally applied to cataract surgery with IOL placement. A previously pseudophakic patient can be treated by laser blended vision protocols to set a total final spherical aberration of the eye that gives an extraordinary range of vision.

Performing cataract surgery on a patient with prior laser blended vision in the cornea enables the choice of a monofocal IOL of appropriate asphericity to leave the eyes with optimised spherical aberration, without resorting to diffractive optics and all of the quality of vision and adaptation issues that are introduced by intraocular rivalry, reduced contrast and the selective quantisation of the reading distance.

Conclusion

The combination of micro-anisometropia with increased depth of field through appropriate non-linear aspheric ablation profiles substantially improves visual outcomes in comparison with the conventional monovision approach. This can be achieved in the cornea and in conjunction with cataract surgery.

Trials show that laser blended vision is effective in presbyopic patients with refractive errors between +5.75 and –9.00 D, including emmetropic presbyopes. With the safety advantages of modern femtosecond LASIK, the rapid bilateral surgical procedure and a recovery time of a few hours, patient satisfaction is extremely high.

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Dan Z. Reinstein, MA(Cantab), FRCSC, DABO, FRCOphth, FEBO
E: dzr@londonvisionclinic.com
Dr Reinstein founded the London Vision Clinic in 2002, and holds professorships at Columbia, Ulster and Sorbonne Universities. He is a consultant for Carl Zeiss Meditec and has a financial interest in ArcScan Inc.

References

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