The eye’s natural crystalline lens needs to have two essential qualities to fulfil its function: elasticity, for accommodation and to focus on objects at different distances, and transparency, to see clearly. In a young person without eye problems, both qualities are fulfilled. But the crystalline lens ages relatively early. From the age of 40, it progressively loses its properties: Firstly, it loses its elasticity and therefore its accommodation capacity, which results in a growing difficulty in focusing on close objects (presbyopia). Then, it loses its transparency (cataracts), causing a loss of visual acuity for near and far vision. While cataract surgery has become the most common eye surgery in developed countries, an increasing number of people want to stop their over-dependence on glasses caused by presbyopia or “old eyes”. Dr. Daniel Elies, a cornea and refractive surgery specialist at IMO, explains that these patients should be aware that “although the accommodative power of the crystalline lens cannot be corrected, there are an increasing number of strategies to help minimise its consequences.” They also need to bear in mind that “it is a permanently evolving process and, consequently, most correction techniques need to be modified over time.”
Intracorneal lenses to modify the curvature of the cornea and improve depth of focus
PTo achieve this, experts are increasingly focusing on techniques that enable reversibility, meaning that, once implanted, they can be easily modified without trauma for the patient if the results are not as expected or if the evolution of the presbyopia process so dictates. In this respect, IMO is a pioneer in Spain in the implantation of the latest intracorneal lenses, which do not have optical power as they do not modify the dioptres, but have a direct effect on the curvature of the cornea. They are 2 mm-diameter fully-transparent lenses that are positioned in the centre of the cornea to modify its asphericity. This produces a change in the depth of focus, which combats the difficulty of focusing on near objects, typical of presbyopia. This option can be easily reversed by removing the lens and, if deemed necessary, it can be replaced by another lens or one of the other options currently available to combat the effects of presbyopia.
Among these alternatives, as with other situations in refractive surgery, there exists the possibility of intraocular lens implantation to correct a refractive error without removing the crystalline lens. Bifocal and multifocal models exist. The most common option for correcting presbyopia is the implantation of intraocular lenses to replace the natural crystalline lens (which is what occurs in cataract surgery). If the characteristics of the eye are appropriate and the patient is around the age at which cataracts develop (over 55), this is one of the best options with which to combat presbyopia.
Cataract and presbyopia surgery
After cataract surgery, patients who previously wore glasses for distance vision can usually dispense with them, but sometimes still need glasses for near vision as the lens is not able to accommodate. With relatively high predictability, the eventual refractive error can be programmed by the surgical team and, through monovision, manipulation of the asphericity or a combination of both, greater independence from glasses is frequently achieved for near vision after surgery. “If a patient requires near-vision glasses after cataract surgery and wishes to dispense with them, modification of the curvature of the cornea through implantation of the latest intracorneal lenses can be a good option,” explains Dr José Luis Güell, the coordinator of IMO’s cornea and refractive surgery department and a pioneer in the use of such lenses.
Different techniques exist to improve presbyopia conditions, such as multifocal or bifocal excimer laser ablation of the cornea by means of photorefractive keratectomy (PRK) or LASIK; episcleral implants; or anterior episcleral incisions, although, for the moment, the results of some of these techniques need improving. The strategies that are producing the most effective results are those aimed at modifying corneal asphericity, either by using femtosecond or excimer lasers or by implanting the latest intracorneal lenses (which has the additional advantage of being a reversible procedure).
Whatever the technique chosen, an important concept when addressing presbyopia is monovision, which basically involves using one eye for near vision and the other one for far vision. Although monovision has traditionally been offered to patients with low myopia and presbyopia who want to dispense with the need to wear glasses, it is increasingly becoming an option with greater possibilities, designs and indications. The emmetropic eye (with no refraction defects) is used for distance vision and the myopic eye for near vision. Patients who have a positive disposition and in appropriate conditions generally adapt very well, since optical correction is only required in certain circumstances, such as driving at night and reading small writing. As an alternative to monovision, presbyopia can be corrected with the previously mentioned intraocular lenses, which can be multifocal or accommodative and which, by means of different mechanisms, seek to achieve good vision for all distances. Results vary, with some patients being very satisfied, while others are unable to do without their near vision correction. By studying each individual case, an experienced professional can select the best candidates for this type of surgery. In general, “we can say that multifocal lenses offer a greater range of viewing distances, while accommodative lenses (monofocal) provide a more limited accommodation range,” explains Dr Elies. “In contrast, visual quality in specific conditions (e.g. low light) is slightly higher in the case of accommodative monofocal lenses, so, in some cases, patients who have had this type of lens implanted will require glasses for these situations,” he adds.
Glasses and contact lenses
If the patient does not want to undergo surgery, presbyopia correction can be carried out with glasses. Patients who used glasses before the onset of presbyopia will need to change to bifocal lenses (two optical powers for near and distance vision), trifocal lenses (near, intermediate and distance focusing) or progressive lenses (the upper part of the lens is used for distance vision and the lower for near, while, in the central part, the optical power is progressive, covering all distances). The option that best covers all visual needs is the progressive lens, but adaptation sometimes takes a long time or is even impossible. As presbyopia is progressive, optical power has to be regularly modified. With respect to contact lenses, different types exist: bifocal and multifocal, but as adaptation is difficult, they are not widely used. They can however be useful depending on the activity carried out. It is important to try to limit the amount of time these lenses are used as they are worn by patients in an age group with poor tear film quality and a higher risk of corneal hypo-oxygenation. If the activities that need to be carried out require good near vision, contact lenses are usually not a good option.