PRESBYOPIA

Presbyopia is the physiological decrease in accommodation resulting from the natural loss of elasticity of the lens and the tone of the ciliary muscle. It can also be defined as the progressive distancing of the near focus point in near vision.

Classification of presbyopia

Presbyopia can be classified into four types: incipient, functional, absolute, premature.

Etiology

Presbyopia is the product of the physiological aging of the visual system. Classically, two different mechanisms have been proposed for the appearance of presbyopia. On the one hand, the loss of contraction power of the ciliary muscle, and on the other hand the decrease in the elasticity of the lens¹.

Currently, it is accepted that the main cause of presbyopia is the loss of the elasticity of the lens while the ciliary muscle plays a secondary role. The lens grows throughout life through the migration and proliferation of epithelial cells towards its nucleus.

These cells differentiate to form the elongated fibers that form the lens, so that layers of new cells are placed on older fibers that make up the nucleus, which with time becomes compressed and hardens by the constant addiction of new cells (Figure 1).

Presbyopia can be corrected by refractive surgery techniques, existing different surgical procedures, both at the corneal level, with the use of different lasers (excimer, femtosecond, holmium, among others), and at intraocular level³.

CORNEAL SURGERY FOR PRESBYOPIA

There are multiple published studies that support the results of corneal surgery for presbyopia in patients without lens involvement.

In our experience, it is necessary to know the optical foundations and the changes that will occur after treatment, in order to achieve good results.

We understand that, although the patient requires accurate visual acuity for near, we have to perform a surgery that does not penalize neither the distant vision nor the intermediate one in quality and quantity.

Given the multiple elements that individually are necessary to know and control in this surgery, it is true that we need to internalize this knowledge to master the art of corneal surgery of presbyopia.

BASIC PRINCIPLES FOR UNDERSTANDING THE CORNEAL SURGERY OF PRESBYOPIA

1. SPHERICAL ABERRATION, DEFOCUS, CENTER OF FOCUS AND DEPTH OF FOCUS

The simulation with adaptive optics has allowed us to interrelate all these parameters⁴.

When studying one by one the effect of optical aberrations in pupils of 6 mm under cycloplegia, we see the greatest impact on the depth of field of the spherical aberration with respect to trefoil.

For changes of 0.3 microns of comma or trefoil, the changes were not very striking, while when changing the spherical aberration 0.6 microns, towards positive or negative values, there is a significant increase in the depth of field (2 diopters) and center of focus. But, nevertheless, for changes higher than 0.9 microns the depth of focus, instead of continuing to increase, stabilizes and even decreases smoothly (especially when the aberration is negative).

The increase in depth of field takes a "toll", and this is the decrease in the quality and quantity of visual acuity from a certain amount of aberrometric change.

2. DEPTH OF FIELD (DOF), VISUAL QUALITY AND SPHERICAL ABERRATIONS

2.1 PREVIOUS CONCEPTS:

The PSF (extended point function) is the distribution of luminance in the image of a point source of light. The comparisons are much easier if we apply to the PSF a simple number that specifies the quality of vision in a simple scale. For this we use the Strehl Ratio.

The Strehl Ratio (SR) is a measure of the effect of aberrations in reducing the value of the PSF.

Its average value is 0.212 ± 0.043 in the 6 mm pupil and is a good parameter to evaluate the optical quality of imaging formation. Its value is between 0 and 1 (the greater the number of aberrations, the lower the value of SR and the poorer optical quality of the image formed in the retina).

2.2 BEST VISUAL IMAGE QUALITY THRESHOLD

Fan Yi⁵ chooses the Strehl Ratio based on the optical transfer function (VSOTF) to estimate the quality of the retinal image and its correlation with the visual acuity measured in the logMAR, indicating the importance of maintaining an acceptable level of visual quality after any increase in depth of field (DOF).

The ABSOLUTE DOF value is defined as the range of blur in which the subject's visual acuity is 0.2 log MAR (0.66 VA) according to Collins, Franklin and Davis⁶ (2002), although authors such as Plakitsi and Charman⁷ (1995) consider it in 0.3 log MAR (0.5 VA).

The direct focus algorithm, the VA level of 0.2 log MAR, corresponds to a Strehl Ratio based on VSOTF of approximately 0.12.

That is, the ideal in these cases is to achieve the greatest possible depth of focus while maintaining acceptable visual acuity at a distance, determined by 0.2 log MAR or its equivalent 0.66 of VA.

3. PREOPERATIVE FACTORS ASSOCIATED WITH HIGH ORDER ABERRATIONS. GENERAL ASPECTS

We must understand that high order aberrations (HOAs) are intrinsic to the optical system, that they appear in all people, and that they change influenced by different factors⁸.

Age: changes of statistical significance appear in people older than 40 years with respect to HOAs: commas, spherical aberration. In young patients, corneal aberration predominates over total aberration and the crystalline attempts to compensate for this difference. This does not occur in older patients where there is a loss of balance between corneal and lens aberrations, as we will explain later. In addition, the shape of the cornea changes with age, the astigmatism tending to go from the rule to against the rule. The transparency of ocular media also changes (for example, the opacity of the lens, the vitreous degeneration...), contributing to changes in the optical aberrations.

Sex: statistical significance of defocus in women.

K-Steepest: We know that the most pronounced mean value of K shows the highest correlation with total HOAs increase of all the factors studied (P <0.001).

Pupillary diameter: There are also significant differences (P <0.001) in different aberrations (total RMS, total HOAs, defocus, total coma) when comparing different pupil sizes. Oshika et al⁹ (1999) shows an increase in total HOAs and comma-type aberrations in pupillary dilatation of 3 to 7 mm. Campbell & Gubisc¹⁰ (1999) showed that the image quality is relatively good in pupils of medium size and deteriorates with the increase in diameter, as we will explain later.

Defocus: Simonet et al¹¹ (1999) and Marcos et al¹² (2000) report an increase in aberrations with an increase in myopia.

Astigmatism: Wu¹³ concludes that residual or induced astigmatism limits visual acuity and causes nocturnal halos. Irregular astigmatism causes loss of BCVA, monocular diplopia and phantom imaging.

Trefoil: The borderline ratios between "invalidating" and "tolerable" blurring is higher in the trefoil (ratio 3.5) than in the defocus (ratio 2.5) and astigmatism (ratio 2.2)^14,15.

Lacrimal film: Tutt et al¹⁶ (2000) and Koch et al¹⁷ (2002) demonstrated that irregularities of the anterior surface of the cornea due to impaired lachrymal stability lead to significant optical aberrations.

Other factors: White-White, ACD, IOP, all of lesser statistical significance.

4. MONOVISION AND MODIFIED MONOVISION EFFECTS OF DIFFERENT COMBINATIONS OF Z04 AND Z06 IN DOF AND VISUAL ACUITY

80% of patients tolerate traditional monovision after surgery, but with alterations in stereopsis and intermediate vision impairment when there are high additions in near. With the modified monovision, "binocular summation" attempts to improve the quality of vision and the depth of field, using, together with a small anisometropia, various combinations of Z04 and Z06, either in the two eyes or only in the non-dominant one¹⁸.

In a study published by Fan Yi⁵ in real eyes we observed that the best combination of aberrations is that of Z04 and Z06, but of opposite sign, being its effect in terms of increase in depth of field greater than that measured separately from Z04 and Z06 or when they are added with the same sign.

Regarding the effect on visual acuity decrease, the introduction of Z04 up to 0.6 microns, either positive or negative, reduced the average visual acuity to 0.30 log MAR per micron.

The effect of Z06, positive or negative, of up to 0.25 microns reduced average visual acuity 0.83 log MAR per micrometer, but the combination of Z04 and Z06 with opposite signs induced an average decrease in visual acuity of 0.4 log MAR per micrometer.

It has also been observed that Z04 and Z06 separately are able to increase the mean depth of field by 0.27 D and 0.24 D respectively for a loss of visual acuity of 0.1 log MAR, while their combination with opposite signs does so in 0.4 D for the same visual acuity loss of 0.1 log MAR.

As for the combinations of spherical aberrations and the center of focus, the introduction of Z04 induces an average change of 2.9 D of center of focus per micron. The one of Z06 induces an average displacement of 3.5 D per micron and the combination with opposite signs of Z04 and Z06 achieves 3.9 D per micron.

The combination +Z04-Z06 / -Z04+Z06 and the combination -Z04+Z06 / +Z04-Z06 are considered the most effective and contribute to changes in optical aberrations.¹⁹

According to the quantity of the same the results on the quality of vision and depth of field are different (Figure 2).

When we decide to leave a considerable anisometropia in the non-dominant eye or there are large differences of spherical aberrations between the two eyes, we can put in conflict the effect of "binocular summation" and change it to "inhibition or suppression".

In "summation" the result of binocular vision is superior to the best in monocular. In "inhibition", the best monocular vision is superior to binocular vision.

In these cases of large differences between both eyes, stereopsis can also be affected. Previous tests with contact lenses help predict the tolerance limit and the final result.

5. LIMITS OF BLUR

Changes of spherical aberrations can produce vision limits with "invalidating" blur that does not allow daily activities derived from the change in visual quality caused by their effect on the increase in depth of field.

The amount differs according to various authors: Fan Yi 2.59 D+-0.52 D (2011)⁵, Atchison 1.77 D (2005)²⁰, Benard et al 1.67 D (2010)²¹. Regarding the "tolerable" blur or lack of clarity in studies by Yi et al, the amount decreases to 0.79 D +/- 0.15 D (2010)²². These values ​​are not fixed and are conditioned by the favorable positive effect of the neurosensory adaptation over time.

6. OPTICAL ABERRATIONS AND CRYSTALLINE LENS

The lens changes as the age progresses, entail changes in the relationship between the aberrations of the anterior face of the cornea and the total optical aberrations.

The horizontal commas of the anterior face and the totals are similar in young patients and this relationship is practically maintained in middle ages. The magnitude of the spherical aberration is greater in the anterior cornea than in the total of the eye in young patients. However, with age the total spherical aberration of the eye increases significantly, due to changes in the posterior surface of the cornea and especially at expense of the lens, even surpassing that of the anterior cornea (Figure 3)²³.

The changes of predominance of horizontal coma to vertical coma with the ages is one of the causes that explains the transformation of astigmatisms with the rule to against the rule. K Rocha finds that in the initial phases of nuclear lens alterations there is a statistically significant increase in spherical aberration towards positive values, while when there are cortical alterations, coma is predominantly affected²⁴.

This is of interest in time, after the treatment of corneal presbyopia, because the lens, when it begins to opacify in its nucleus, will compensate for the negative spherical aberrations produced in hyperopic treatments, or increase them if the patient was myope, with the consequent effect on depth of field and its visual consequences.

In conclusion, with age, spherical aberrations tend to be more positive because of changes in the lens. Laser surgery in high myopia causes an increase in spherical aberration towards the positive, which can be increased if the lens nucleus begins to be affected. The hyperopia surgery has an increase in spherical aberration towards the negative, which over time can be partially compensated by the nuclear affectation of the lens.

However, in low myopia, the laser treatment produces a more discrete increase of spherical aberration towards the positive that in cases of the onset of nuclear sclerosis produces a summing effect of negative spherical aberration that causes an improvement in the near vision by increasing the depth of field and a longer duration of treatment effect, provided that the nuclear affectation is mild and does not advance rapidly.

The ideal is to achieve optimum spherical aberrations with the highest binocular visual quality and to foresee the aberrometric changes of the near future by combining the effects of spherical aberration change, both positive and negative.

7. OPTICAL ABERRATIONS, Q FACTOR, PUPIL AND ACCOMMODATION

The accommodation and the pupil dynamics entail a change of spherical aberration and therefore a refractive effect. Therefore, they are KEY elements in corneal surgery of presbyopia.

The total defocus changes produced in the accommodation are mainly due to the increase of the anterior curvature of the lens (responsible for 82%), and also of the posterior face (33%).

While changes in the anterior face of Z04, which produce -19% (in the opposite direction), and of Z06, which produce 4%, also have influence, the changes of Z04 and Z06 produced by the posterior face of the lens are not significant (Figure 4)²⁵.

As a final effect, during accommodation an increase to the negative of the spherical aberration takes place while the rest of the aberrations barely change (H. Cheng)²⁶.

The tilt changes of the lens during accommodation determine, in a more individualized way and without fixed pattern, comma and astigmatism changes.

Also, some authors have observed subtle changes in the shape of the cornea with very small changes towards the positive of the corneal aberrations (He)²⁷.

Therefore, the ability of a patient to accommodate after presbyopia surgery, will determine its prognosis depending on the effect derived from the aberrometric change and defocus that may occur. Maintaining a better accommodation with specific exercises is of interest to achieve good results over time. Amigó²⁸ and López-Gil²⁵ studied refractive changes and their relationship with spherical aberration (SA) depending on the pupil. Amigó²⁸ finds that by decreasing the size from 6 to 2.5 mm, a mean change of 0.6 D/micron of SA occurs, regardless of the possible changes in depth of field (Figure 5).

In the presbytes, when the eyes converge in the near gaze, the provoked myosis produces a refractive change towards myopia when spherical aberration is negative, and towards hyperopia when it is positive.

Presbyopic patients operated on for myopia have a reduction in their accommodation and high positive spherical aberrations that secondarily produce a hyperopic effect, so they need to make a greater accommodative effort to compensate for the refractive changes caused by miosis. On the contrary, hyperopic presbytes, with negative spherical aberrations, benefit from the myopic effect caused by miosis.

These changes are often called "pseudo-accommodation", but Amigó²⁹ recommends the term "disaccommodation" since it only occurs when there is a significant change in the pupillary diameter.

In the opposite direction, when the pupil dilates in situations of low luminosity, in the myopic patient the increase towards positive values ​​of the spherical aberrations would produce a myopic effect and frequent occurrence of nocturnal myopia.

Therefore, the defocus produced by the number of spherical aberrations induced by the laser in presbyopic treatments must be controlled to reduce the symptoms of dysphotopsia and lack of nocturnal visual quality, especially in myopic patients²⁸.

If we look at the spherical aberration prior to surgery, a patient with positive SA – if hyperopic after presbyopic surgery the SA tends to become negative – will benefit from the convergence in myosis and the symptoms of presbyopia will decrease.

If the patient is myopic, with negative previous SA, this can be positivized after treatment with less effect of the accommodation induced in miosis and worse results.

When we perform corneal presbyopia surgery based on the Q factor, there is no reference limit to be influenced by the final spherical aberration that will determine a secondary refractive effect, and this will depend very significantly on the balance of the pupillary dynamics. Therefore, it is an individualized value in each surgery (Figure 6)²⁹.

We must be careful, because if we add the TOTAL refractive effect of negative spherical aberration (or of the Q factor) caused by (1) the treatment in a hyperopic presbyte, (2) the refractive effect of the "disaccommodation" of the convergence in miosis, (3) the change towards the negative of the SA due to the residual accommodation of the lens, and (4) the degree of myopic defocus in the non-dominant eye, we can have a disastrous result of visual quality, due to excess of aberrations and also with its corresponding refractive effect.

In these cases, it is recommended to make a total balance of all the "actors involved" and sometimes obviate the myopic defocus, which is not necessary.

8. OPTICAL ABERRATIONS INDUCED BY THE TREATMENT ITSELF

When treating myopia, the spherical aberration increases towards the positive, and this has a slight myopic effect that the laser nomogram must compensate. In hyperopia, the effect is just the opposite, causing an increase towards negative values ​​of the spherical aberration. Amigó²⁸ describes a change of 1.6 D/micron, Bernard³⁰ of 2.09 D/micron, and Rocha³¹ of 2.6 D/micron.

9. THE IMPORTANCE OF THE CORRECT CENTERING OF THE REFRACTIVE TREATMENT

The focus of treatment on presbyopia is vital, since these processes are guided by aberrometer, there is very little tolerance for decentralization, which would cause less efficiency and dysphotopic phenomena resulting from increased optical aberrations, mainly comma. Okamoto³² proposes to focus on coaxially sighted corneal light reflex instead of line of sight centration (pupillary center), indicating a better safety index, especially when the difference between both points is greater than 0.25 mm, and also better efficiency index, especially if the distance between both is greater than 0.15 mm.

On the other hand, J Chang³³ proposed to center the treatment at 80% of the distance between the coaxially sighted corneal light reflex and the line of sight centration (pupillary center), encountering less astigmatism and hypo-corrections.

This is of great interest especially with large kappa angles, as many hyperopes present.

10. IMPORTANCE OF BINOCULARITY AND STEREOPSIS

A correct stereopsis and binocular visual acuity for far and near are key to the success of corneal refractive surgery of presbyopia. A basic element is that the differences in high-order aberrations between both eyes are not so great as to hinder or impede the effect of binocular summation to contrast sensitivity or prevent correct stereopsis measured at maximum disparity (minute of arc) (Figure 8)³⁴.

11. THE EPITHELIAL REMODELING

The effect of the epithelium as a remodeler of the altered surface trying to improve the curvature gradient was revealed by Reinstein³⁵ and Vinciguerra³⁶, together with other authors. This remodeling can have beneficial side effects, such as a "lens" effect that helps near focusing or improves comma aberrations or irregular small-sized astigmatisms due to treatment. Epithelial remodeling is also one of the causes of refractive regressions.

THE PATIENT'S CHOICE AND THE NOMOGRAM, KEY TO SUCCESS

In the non-dominant eye, implementation of spherical aberrations (Z04, Z06) together with residual defocus, which must be variable and dependent on the various factors previously discussed (age, previous and final keratometry, densitometry of the lens, type and amount of ametropia, stereopsis of the patient, pupillary dynamics, initial spherical aberrations Z04 and Z06 and prognosis of change after treatment, expectations and needs of the patient, among others), will allow us to carry out an individualized treatment, thanks to the implementation of new nomograms and software for laser platforms, such as guided and optimized treatments. As an orientation, what was previously indicated in the studies of adaptive optics (Table 1).

In our experience, we consider a wave front-guided-optimized treatment for the dominant eye, with or without implementation of spherical aberrations (Z04, Z06), depending on the individual characteristics of the patient, as previously mentioned. As an orientation, what was previously indicated in the studies of adaptive optics (Table 1).

It is important from our experience to highlight the importance of the ocular surface, prior to surgery, discarding patients with dry eye and especially with symptomatic alterations of Meibomian Glands.

We also highlight the usefulness of exercises to improve the stereopsis of accommodation (Hart letters, Brock cords) after surgery to optimize results.

CORNEAL SURGERY PLATFORMS FOR PRESBYOPIA

SUPRACOR

The correction algorithm was developed by Technolas Perfect Vision GmbH and is available for the Technolas 217P and Technolas Teneo 317 platforms.

The basis of the technique is based on the creation of a varifocal cornea with two well-differentiated areas: a central hyperprolate zone of 3 millimeters in diameter and 12 microns in elevation, responsible for near vision, and a peripheral zone with aspheric profile that extends up to 6 millimeters, responsible for far vision. Between both there is a very smooth transition zone for intermediate vision.

The ablation algorithm of the Supracor technique has been optimized to minimize the induction of aberrations within the pupillary area (Figure 9)³⁶.

Its mechanism of action is dependent on the pupil and is based on the principle of pseudo-accommodation. In near vision, the pupil contracts and the image focuses on the retina thanks to the negative spherical aberration provided by the central area. On the contrary, in distant vision the pupil slightly dilates, allowing 300% more light in the peripheral area, which ensures a vision of quality.

Depending on the addition for near provided by the central zone, we have two treatment options: Mild SUPRACOR (+1 D of addition) and Regular SUPRACOR (+2 D of addition). To achieve better near results, the treatment is usually associated with a small degree of defocus in the non-dominant eye (between 0 and -0.50), obtaining by this micro-monovision additions of up to +2.5. The most relevant results published for the Supracor technique are listed in Table 3.

It is important to note that they are referred to hyperopic patients treated with regular SUPRACOR. The SUPRACOR technique has shown good results in near vision. Most published studies report near visual acuities equal to or greater than 20/25 (J2) in more than 90% of cases^37-40. The results in far vision are modest when both eyes are planned with a final myopic defocus^37,38. The results have experienced a marked improvement with the introduction of asymmetric nomograms with micro-monovision in the non-dominant eye^39,40,42.

In terms of safety, in the different published series, losses of two or more lines of best-corrected distance vision have been reported in 4 to 10% of treatments^37-40.

As a cause of these, several factors have been postulated, including the decrease in contrast sensitivity, the greater incidence of dry eye, difficulty in neuroadaptation to corneal multifocality, excessive pupillary myosis in photopic conditions and increase in optical aberrations after the procedure, due to an imprecise focus of the treatment in the visual axis. The rate of retreatments is very variable in the different series published (between 5.7% and 22%)^37,39,40.

PresbyMAX

Its presbyopia correction algorithm has been developed by Schwind Eye-Tech-Solutions GmbH and is available for the different Amaris platforms.

Its mechanism of action is based on the creation of a multifocal bi-aspheric corneal surface in which the central area corrects the near vision and the peripheral region the corrects distant vision.

The ablation profile is highly optimized and contemplates the addition of a precalculated amount of high-order aberrations, as well as a defined myopic residual defocus to improve reading ability.

There are three profiles available for the PresbyMAX procedure (Figure 10):

• Symmetric PresbyMAX. It treats both eyes in the same way by adjusting a -0.4 D defocus of myopia with 1.5 D of corneal multifocality for both eyes. Its depth of focus ranges from 2.5 m to 52 cm.
• PresbyMAX µ-monovision. This treatment applies a different ablation to each eye, focusing the dominant eye towards distant vision (defocus -0.12 D) and the non-dominant eye towards near vision (defocus -0.88 D). In both eyes the degree of multifocality is the same: 1.5 D.
• Hybrid PresbyMAX. This profile, introduced in 2013, combines micro-monovision in the non-dominant eye (defocus of -0.88 D) with a different multifocality for each eye. In the dominant eye, the multifocality will be reduced (0.8 D) and in the non-dominant eye it will be complete (1.5 D). In this way the eye for distant vision covers from more than 6 m to 1.1 m, and the eye for near vision does it from 1.3 m to 44 cm.

The most relevant results published for the PresbyMAX technique in myopia and hyperopic presbytes are listed in Table 4.

In all of them, bilateral PresbyMAX is proposed, with the only exception of the series by Chan⁴², in which only the non-dominant eye is treated with PresbyMAX.

Most of the published studies report very good visual acuities close to or equal to 20/25 (J2) in more than 90% of the cases^43-46.

There is a marked difference in the results of distant vision between studies before and after the introduction of hybrid PresbyMAX. The reduction in the degree of multifocality and myopic defocus in the dominant eye has allowed series such as that of Luger in 2005 to reach UDBVA greater than or equal to 20/20 in 93% of cases⁴⁶.

In terms of safety, there have been reports in the different published series of loss of two or more lines of corrected distance vision between 3 and 10% of the treatments^43-45,47.

The series published by Chan stands out for not presenting any case of loss of two or more lines in distant vision. These results would be explained by the absence of multifocality in the dominant eye responsible for distant vision⁴². Retreatment rates range between 14% and 19% in the different series^42,45,46.

There is a specific ablation algorithm (Reversal PresbyMAX) to completely reverse the multifocality of a cornea treated with PresbyMAX⁴⁷.

PRESBYOND

The correction algorithm, which is based on an aspheric nonlinear ablation profile, has been developed by Carl Zeiss Meditec and is available for the Mel 80 and Mel 90 platforms. PRESBYOND is based on the creation of a slight monovision (micro-monovision) associated with an increased depth of focus by modifying spherical aberration within limits that are tolerable to maintain adequate night vision and good contrast sensitivity.

Another important component that differentiates PRESBYOND is related to the epithelial thickness profile, which takes advantage of the fact that the epithelium is remodeled to compensate for changes in the curvature of the stromal surface.

The initial goal of treatment is to convert positive corneal spherical aberration (flatter in the center) into negative (more pronounced in the center).

The nomogram is calculated so that epithelial remodeling is able to completely mask this central stromal island, so that the anterior surface topography appears normal. The result is a profile of epithelial thickness superimposed on the stroma that looks and acts similarly to a set of multifocal lenses due to the difference between the refractive index between the epithelium and the stroma (1.401 versus 1.377).

There are also two very important brain phenomena: the increase in contrast of the slightly defocused retinal image (due to the existence of spherical aberration) and binocular neuroadaptation with a suppression of the blur coming from the non-dominant eye in the distant vision.

The dominant eye is planned for emmetropia and the non-dominant eye for a myopic defocus of approximately -1.5 D. The different depth of focus ranges for each eye make it possible to create a binocular combined intermediate vision zone (blended vision zone) (Figure 11).

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