INTRODUCTION

Around 40 years of age begins a new visual situation, presbyopia. Almost always the patient perceives it as something very negative. It does not matter whether or not he was wearing glasses before. The feeling of losing the possibility of close focus is something that makes daily life very difficult. In addition, the implementation of the problem is rapid and, either, it is the first sign of that “aging” sensation, or it is associated with other symptoms that will remind us that time passes inexorably reducing our aptitudes little by little. The first correction method to solve or alleviate the problem is the glasses, either for near or progressive, but this does nothing but ratify that sense of aging that we all try to avoid, so we will quickly look for any other solution that avoids that functional and aesthetic dependence of the spectacles. Perhaps one of the first methods of correcting presbyopia, seeking this independence of the glasses, becomes the use of contact lenses, but is this really a solution? Let us study it more closely.

MATERIALS

Leonardo Da Vinci, Descartes, are illustrious names that were already directly associated with contact lenses, and almost from the beginning, talking about contact lenses is closely associated with talking about the materials that compose them.

Currently the materials are very diverse, but we can still divide them into "soft" or "hard" lenses. In both cases, the polymers obtained have improved oxygen permeability, wetting and therefore tolerance recently up to unsuspected levels. Rigid gas permeable (RGP) lenses and silicone hydrogel lenses are usually tolerated much better than other options.

When it comes to correct presbyopia using contact lenses, all these materials, or combinations of them as piggy-back, can be proposed, although the proportion of users of soft lenses of daily or monthly use has clearly increased in recent years.

In general, in this type of patients, we must bear in mind that presbyopia is directly related to age and therefore to hormonal changes, hydration, alterations of the ocular surface... Thus, if we talk about soft contact lenses we have to look for high hydration materials, such as silicone hydrogels, aiming for an increase in comfort and discard other materials due to their clear intolerance in these patients.

METHODS

There are several ways to achieve improved vision. We will be developing each of these treatment strategies that we can summarize as:

● Monovision

● Simultaneous vision lenses

MONOVISION

In general, we can say that this strategy seeks to leave one eye for far vision and another for near vision, the non-dominant eye being the one used for close. Although a priori it seems strange, it is one of the most accepted strategies and is considered to have a 70% acceptance. However, the most important key point in this strategy is to choose the patient very well.

When choosing the patient it will be more accepted in low ametropia than in patients with a similarity between the graduation of both eyes, they must be motivated patients and it is better if they were already contact lens wearers since if they were not there are many variables that we are going to introduce in the patient (lens size, different vision with one eye and another...).

It is obvious that by using this technique, while gaining in width of visual field and vision, we are going to have a loss in binocular vision, so we should rule out those patients who have small binocular vision problems.

There will be different approaches, which can be summarized in the following:

● Pure monovision

● Mini-monovision

● Modified monovision

● Mix & match

Pure monovision

We need to create an important dioptric difference between both eyes. We will always leave the dominant eye totally corrected for far, and in the other eye we will leave a myopia of around 2.5 diopters. We must ensure in the clinic that the patient tolerates this well, that it does not generate spatial vision problems that do not allow him to perform his daily tasks. Thus, we could even do the adaptation in two or three stages, so that the user can adapt in a stepped-like fashion to the new situation. The adaptation will be better in patients in whom there was previously a large difference between the graduation of both eyes. In addition, it will be more tolerated in myopic than in hyperopic patients, and it is difficult for those with significant astigmatism to be a good treatment option.

Mini-monovision

This is the same strategy as the previous one, but with less difference between both eyes. In general, a difference of 1.5 diopters is tolerated very well, and if instead of leaving the eye completely emmetrope, we sacrifice some of the far vision, leaving it slightly myopic, we can increase the depth of field, leaving a very good useful vision with a great tolerance.

Modified monovision and Mix & Match

In this case we have two alternatives. In the first one (Modified monovision), we place a monofocal lens for distance in the dominant eye, and a multifocal contact lens in the other eye. This could be done in patients who have not tolerated mini-monovision because they require greater visual acuity for far. It is less tolerated than the previous one, but it can be proposed in hyperopic patients, perhaps with better acceptance. In the second one (Mix & Match), both eyes are going to be corrected with a multifocal lens, but each one with a different design. In the dominant eye we place a contact lens with a central part to correct far vision and a peripheral part for near vision, while in the non-dominant eye we will do the inverse, the central part will be for near and the peripheral for far (see Figure 1). If we decide to use this strategy, it is very important to try it first in the consultation and even warn of a period of adaptation of weeks until getting an adequate vision with a certain naturalness in the day to day life.

SIMULTANEOUS VISION LENSES

Unlike the monovision technique and its variants, in simultaneous vision both eyes carry the same lens design, which can be bifocal, multifocal or aspheric of extended focus. These in turn can have several designs. We may mention:

● Concentric

● Segmented

Concentric contact lenses may have two or more annular zones with alternating far-close powers (see Figure 2).

The segmented ones are bifocals that present a vertical difference, being in general the upper part of it for correction of far and the lower has the addition for close. Like lenses for astigmatism correction, in these lenses it is very important that they maintain their position, so they are constructed with a ballast prism that allows a stable position in the eye against blinking.

Aspherical lenses do not have annular zones, but a gradual transition of power between that corresponding to the far and near compensation (or vice versa) as we move away from the center of the lens (see Figure 2c). These designs are also the ones used in the Mix & Match technique mentioned above. Concentric designs give better results in young presbytes with low additions.

NEW DESIGNS

In recent years, peripheral refraction has received increasing attention in research on myopia, based on the theory (demonstrated in animals²) that a hyperopic defocus on the peripheral retina can promote the onset or the progression of myopia^3,4. It has been clinically demonstrated that specific multifocal contact lens designs are capable of reducing the progression of myopia in children and young people, if they are capable of inducing relative myopia in the peripheral zone of the retina^5-7. One of these designs is based on the concept of fractal lenses⁸, whose design is shown in Figure 3. Numerical simulations made in theoretical eyes show that these lenses induce a growing myopic defocus with the eccentricity towards the peripheral retina in the horizontal meridian (Figure 4). Currently, the first tests in real patients of the effect of these lenses are being carried out.

Therefore, the development of fractal lenses, allow us to work at two levels. On the one hand they will help us design models for the control of the evolution of myopia, and on the other hand we can adapt these new ideas to design contact lenses to be useful when it comes to solving the focus problems that appear in presbyopia.