3.2.3.1. Strategies with intraocular lenses in the correction of presbyopia
3.2.3.1. Strategies with intraocular lenses in the correction of presbyopia
Instituto de Oftalmología Cançado Trindade, en Belo Horizonte, Minas Gerais, Brasil. Universidad Federal de Minas Gerais, Brasil
Presbyopia correction can be achieved with the implantation of several types of intraocular lenses (IOLs): accommodative, pseudo-accommodative, spherical/aspherical and toric monofocals. With the exceptional development of IOLs, as well as the improvement of surgical technique and equipment, there has been a marked improvement in the safety and efficacy of dysfunctional lens surgery, thus achieving the status of an eminently refractive procedure1,2, where the independence from glasses has become a primary goal. Besides this independence, what everyone wants with the surgical correction of presbyopia is the ease/speed of reading and, above all, the quality of vision.
Whatever the surgical strategy adopted, there will always be some kind of loss. There is no ideal method, just as there is no perfect IOL. Each type of IOL has its advantages and disadvantages. Correct indication of the type of IOL for correction of presbyopia should take into account the lifestyle, the patient's personality, postoperative expectations, associated ocular morbidity, pupil size, and important physical characteristics such as, for example, the size of the arm/forearm. In this way, each case must be carefully analyzed and clarified before surgery, to avoid frustrations and problems in the postoperative period.
Accommodative IOLs simulate the natural mechanism of accommodation and do not promote the division of light, as with multifocal IOLs. The amplitude of accommodation is proportional to its dioptric power. Its mechanism of action can be by the anteriorization of the single optical part, by the increase of the curvature of the optical part or by the withdrawal of the optics, when double.
Of course, for this to happen, it is indispensable for intra-saccular accommodative IOLs to maintain a certain capsular elasticity, a situation not yet observed in practice. The currently available models have a narrow accommodative range at best. In addition, they do not yet exhibit postoperative refractive predictability comparable to other IOLs3. In fact, the results obtained with accommodative IOLs have been disappointing, mainly due to the progressive contraction of the capsular bag.
Some were wrong in predicting that at the present time multifocal IOLs would be replaced by accommodating IOLs4, which did not occur at all. Although very desirable, because they are the most physiological, accommodative IOLs are still used very little, because they present a restricted accommodative amplitude, questioned by many.
Multifocal IOLs promote pseudo-accommodation by dividing light into two or more foci, which leads to a loss of the amount of light reaching the retina in each of these foci. The basic pillar of multifocal IOLs lies, then, in the concept of simultaneous long-distance and mid-distance/close-up vision, as opposed to the alternating vision seen with multifocal glasses, for example.
This multiplicity of simultaneous (far, intermediate, and near) focal points, characteristic of multifocal IOLs, reduces contrast sensitivity and gives rise to dysphotopsies that many patients report after surgery, which requires a neuro-adaptation, not always obtained mainly in those hypercritical patients.
The first multifocal IOLs were launched in the 1980s. With the notable technological advances observed in recent years, the acceptance of multifocal IOLs has increased, especially after the introduction of trifocal IOLs and IOLs with minor addition, with lower collateral effect and consequently greater tolerance.
Multifocal IOLs are able to return a continuous vision for near, intermediate, and far distance, especially when implantation is bilateral. One feature that can be used to increase continuous vision at various distances is the implantation of IOLs with different additions.
Regarding multifocal IOLs, an important trend observed is the use of models with reduced addition. Such models prioritize the intermediate vision at the expense of close vision, with a lower incidence of adverse optic phenomena, such as halos and glare5.
Trifocal IOLs are differentiated by the presence of an intermediate focal point, promoting improved vision for mid-range activities. The creation of the intermediate focal point is inevitably accompanied by a reduction in the percentage of light energy directed to each focal point. However, studies show that visual performance with trifocal IOLs is similar to that found in bifocal IOLs, with the advantage of presenting a better defocus curve in the intermediate distance6.
It is important to emphasize the importance of the binomial judicious selection of patients plus impeccable surgery to achieve the desired success, which is life without glasses after surgery.
In addition to the need for neuroadaptation, which is directly proportional to the patient's visual requirement, the multifocal IOLs depend on emmetropia and require an ocular clarity for their perfect performance.
Therefore, multifocal IOLs work poorly in the presence of (increasingly rare) postoperative refractive residues, which will require surgical correction, generally with excimer-laser, those inconvenient "retouching", unpleasant and problematic, mainly for older patients. It is precisely in this group of patients that we find most commonly associated ocular morbidities, such as glaucoma and macular degeneration, which are contraindications for multifocal IOL implantation.
Despite the remarkable advances in multifocal IOL technology in recent years, in addition to the industry's efforts to introduce ever-better IOLs, as well as the evident financial interest of those who market them, multifocal IOLs have not yet reached 10% of the IOL market.
Monovision is a strategy widely used in the surgical correction of presbyopia7. In fact, in the past it was common to aim for a discreet myopia in one or both eyes, after cataract surgery with IOL implantation, to help near vision without greatly diminishing vision for distance. Boerner and Thrasher8 were the first to describe in 1984 the pseudophakic monovision method, which is based on the induction of a discreet myopia in one eye. Thus, there seems to be an adequate balance between the mild loss of stereopsis and near vision gain. This modality of presbyopia correction can be performed with spherical, aspheric and toric monofocal IOLs. Generally, the dominant eye is intended for emmetropia and for the non-dominant one, a low myopia around -1.50 diopters9. This is conventional or traditional monovision. There is nothing to prevent the opposite: dominant eye for near and non-dominant eye for far, a situation called cross-monovision, the results of which are as satisfactory as those obtained with the conventional method10.
It is worth noting that when anisometropia is small, ocular dominance becomes less critical.
Prior use of contact lens monovision may make the choice easier.
The inevitable loss of stereopsis should always be considered, and every precaution should be taken not to indicate monovision for those patients with a previous history of significant forias. Likewise, pseudophakic monovision is contraindicated for those professions that require a high level of stereopsis, as well as for those patients with a very demanding profile.
Hybrid monovision consists of implantation of a monofocal IOL in the dominant eye and a multifocal IOL in the other eye. Good results can be achieved with this mode of monovision11.
And which eye should be the first to be operated?
When the IOL to be implanted is a monofocal type, I prefer to operate the dominant eye first, usually for distance. When the IOL to be implanted is multifocal, I prefer to initially operate the non-dominant eye. The monovision with monofocal or toric IOLs presents some advantages in relation to the use of multifocal IOLs in the correction of presbyopia, such as:
However, the emmetropia in the eye for far is absolutely fundamental to success, that is, independence of the glasses most of the time.
Of course, there are also disadvantages with monovision, such as depth perception in some way compromised, which demands greater care, especially with the elderly patients, when descending stairs, for example. Note the need for glasses for certain activities such as driving at night, and even for a more comfortable near reading, especially with very small characters.
Patients with pseudophakic monovision with exophoria for near of 10.0 prismatic diopters may have the final surgical outcome seriously compromised12. Thus, routine orthoptic exams are urgently needed to diagnose and contraindicate pseudophakic monovision in this group of patients with strabismus problems.
Ito and Shimizu13 compared the ease of reading in a group of patients with refractive multifocal lenses in both eyes with another group of patients with conventional pseudophakic monovision and observed better visual performance in the latter group.
Recently, another type of IOL with diffractive technology has been launched – that of extended focus – aiming to increase depth of focus and thus provide long distance vision and intermediate distance, with superior visual quality, compared to traditional multifocal IOLs. Its mechanism of action is based on the elongation of the focal point. By incorporating a diffraction pattern distinct from traditional multifocal, called echelette (from French échelle, or ladder), this type of IOL is able to correct chromatic aberration, which provides better contrast sensitivity. The result is a more optimized distribution of light energy, with a smoother defocus curve, without great variations in the values of visual acuity, as well as better contrast sensitivity in conditions of low luminosity14.
The purpose of these lenses is to reduce the side effects observed in traditional multifocal – which reduce contrast sensitivity – by correcting chromatic aberration. The extended focus IOL also presents superior performance to the multifocal IOLs, in the presence of ocular morbidity in the postoperative period.
The performance of extended focus IOLs for far vision is superior to that of multifocal IOLs (although some patients report the presence of halos) and lower for near vision.
An alternative to improve vision performance for near with the extended focus IOLs is to induce a mild myopia in the non-dominant eye of approximately -0.75 diopters.
In order to provide an extension of depth of focus, the pinhole effect has recently been incorporated into the mechanism of action of some IOLs: IOL with pinhole mask and supplemental pinhole implant. The manufacturer Acufocus launched the IC-8® hydrophobic acrylic single-piece IOL, with an optical part of 6 mm in diameter, housing a 5 μm thick opaque mask of 3.23 mm of total diameter, with a central aperture of 1.36 mm. This IOL is to be implanted into the capsular bag15.
Following the same principle, the XtraFocus® (Morcher) implant is also proposed to reduce the impact of corneal aberrations in cases of corneal irregularities, such as, for example, keratoconus, post-traumatic leukomas, post- penetrating keratoplasty, post-radial keratotomy with severe irregular corneal astigmatism. This hydrophobic acrylic device with a central orifice of 1.30 mm in diameter does not present refractive power and must be implanted in the ciliary sulcus of pseudophakic patients, primary or secondarily16. The millenary concept of the pinhole orifice to increase the depth of focus, as well as the full acceptance of the implantation of a supplemental IOL in the ciliary sulcus – primary or secondary – authorizes to use of the pinhole implant as an additional alternative in the correction of pseudophakic presbyopia. Of course, implantation of the pinhole device requires perfect cataract surgery, with the primary IOL inside the capsular bag.
It is also important to point out that small pupils (diameter less than 2.5 mm), discrete aberrations, or even a small simple astigmatism against the rule can be very welcome by increasing the depth of focus, thus facilitating near-vision without correction, significantly compromising, however, the vision for far17,18. In fact, we have the satisfaction of observing in ophthalmological practice, with reasonable frequency, pseudophakic patients with senile myosis, operated for more than three decades, with the implantation of spherical monofocal IOLs, who completely neglect the need to wear glasses after surgery.
In conclusion, it can be said that the recent technological advances are very encouraging, and the cataract surgeon has at his disposal new IOLs with several mechanisms of action. The type and model of the IOL that best meets the individual needs of the patient must be carefully chosen in order to provide a quiet and long-lasting independence of the glasses.