Presbyopia and accommodation
What is it?
It’s possible that the vision problem which ends up affecting a greater number of people is presbyopia, more commonly known a "eyestrain". It is currently estimated that there are 1,800 million people affected by presbyopia, and by 2020 there are expected to be 2,300 million. It is not therefore surprising that the possibility of solving presbyopia through surgery has become one of the new frontiers in ophthalmology.
Presbyopia is the result of the loss of accommodation, the physiological capacity that allows us to keep a sharp image of an object as this approaches us. Accommodation involves an increase of the optical power of the eye, by bringing the focal point from a far point to a closer one, like when reading. To focus 33 centimeters (cm) from the eye, starting from the further focus, we must accommodate 3 diopters (D).
Accommodation decreases with age, and by the age of 40 only about 3 or 4 D remain: the patient begins to notice difficulties when read comfortably without the aid of glasses. After 40 years of age, presbyopia makes itself increasingly evident until the accommodation reaches its minimum at 65 years.
Some people in the age of presbyopia can apparently read unaided. Although this may be due to many factors, it happens most often when presenting a slight defect such as myopia or myopic astigmatism. They require less amplitude of accommodation to see closely; however, at the age of presbyopia, even myopic people need a different correction to see clearly from near and far.
Currently, in addition to reading glasses, bifocals and progressive lenses, there are multifocal and bifocal contact lenses. Another effective option is called monovision, which consists in correcting one eye (dominant) for far vision and the other one for near.
None of surgical methods proposed so far have been able to restore the correct adaptation, meaning the dynamic and continuously variable ability to focus at all distances; there is only a certain form of remedy at a greater or lesser extent, which is called pseudo-accommodation.
The most used treatments include include bifocal, multifocal and variable focal intraocular lenses (IOL). In general, this surgery is similar to cataract surgery, exchanging the lens with an IOL. The optics for near and far in the same IOL are superimposed, and simultaneously produce near and far focal points on the retina. From this disposition, some limitations in visual quality and defects in night vision are derived, but it is the brain that ultimately chooses the image you are interested in at all times, and adaptability can compensate for many of the system imperfections.
Corneal techniques for presbyopia include monovision, by conventional laser procedures (LASIK or similar type) correcting one eye for each distance, as well as the creation of bi- or multifocal corneas (PresbyLASIK). The main disadvantage is the practical irreversibility of corneal ablative laser procedures.
Intracranial implants can be inserted in the thickness of the thin corneal lenticules, by either lifting a flap as LASIK or creating an interlaminar pocket. These implants can be of refractive type (intracorneal lens), even for a bifocal cornea, or in diaphragm shape without optical power (AcuFocus KAMRA, Bausch & Lomb). Sufficient sharpness is achieved for a wide range of distances, from near to far.
Although various exercises and nutritional supplements (such as lutein) have been proposed to delay the onset of presbyopia, there is no scientific evidence for their effectiveness.
New accommodative IOL designs appear, some of which may meet a patient’s expectations. One way to really recover the physiological accommodation would be the substitution of the content of the lens with a transparent and elastic gel that would fill the capsular bag (Phaco-ersatz). This idea is not new, but for its practical application it must still overcome some technical problems. Finally, with the advent of ultrafast lasers (femtosecond), the possibility of surgery in the lens without affecting its transparency begins to raise, eventually regaining its elasticity and accommodative function.