Renato Meduri

Chair of the Department of Optical Physiopathology
University of Bologna

The eye is a dynamic dioptric system able to adjust its refractometric capabilities according to the variations in distance of the point of focus:

from infinity = parallel rays
to reading = diverging rays







After the image is captured, accommodation allows visual information in the best conditions, through focus of the optical image on the retina, according to spatial dimensions (depth).

The first valid description and explanation of this phenomenon was formulated by German doctor Herman von Helmholtz, in the middle of the XIX century, in one of his publication on optical physiology, Handbuchs der physiologischen Optik. With changes in the curb of the crystal lens caused by the state of contraction of the ciliary muscles, he concluded on the mechanism of the phenomenon. It was a brilliant observation given that it was done with means of research that were decidedly modest, and that it is still fully valid today.

The relaxation of the ciliary muscles induce an increase of the sphincter's diameter, bringing tension to the ligaments of the zonules, which exert a traction on the middle of the lens, and a flatness of anterior and posterior areas. The lens' dioptric capabilities are at their lowest. This situation is appropriate for an emmetropic eye when distance rays hit the retina that has focused (for man this means over six meters).

And vice-versa, the contraction of ciliary muscles corresponds to the state of accommodation to see at close range: this means the crystalline lens absorbs maximum reflective powers.

This accommodation capability does not remain intact throughout life. The emmetropic eye (that is not myopic, hypermetropic or astigmatic) in youth reaches a power over 10, then it tends to progressively reduce: at 40 years of age, it is at 3 1/2; at 45, 2 1/2; at 50, 1 1/2, and at 55, 1/2.


That is why at 25 years of age, the next level is above 25 cm, increasing its distance until the age of 60, age at which is lost all residue of accommodating function in the emmetropic eye. Since to go from infinite vision (parallel rays) to close vision (diverging rays), like in reading, it is necessary to have an accommodation level of 3 1/2. It is clear that at 45 years of age, there are potential difficulties for seeing at close range, and these difficulties will increase over the years.

The eclectic school founded in the first century AC by Agatino Da Sparta, a school which tended to choose the most significant notions from various theories, found in Galeno a distinguished representative. Galeno described presbyopia and believed it was caused by the shrinking of cornea in old age, becoming rarefied according to the state of mind. Presbyopia later was discussed by Merculiar, doctor in the years 1500, in a voluminous and confused chapter, which included all pathologies relating to visual impairment.

Increase in the crystalline lens

In fact, this phenomenon called presbyopia is not a disease, but simply the consequence of the continuous growth of the crystalline lens. In the newborn, the shape of the crystalline lens is almost spheric. There are no differences between the curves of the anterior and posterior areas. The diameter is of 6-8 mm in the newborn, and 9-11 in the adult. The thickness is 3,7 mm in the newborn, and 4,5-5 in the adult.
The weight and volume are subjected to obvious increase from the time of birth to old age. From a weight of 65 mg and 64 mm cubic volume, it reaches 230 mg and 213 mm cubic in old age.
This increase in diameter and volume of the crystalline lens brings it closer and closer to the ciliary muscles, reducing sensitivity to its contraction and relaxation.

Aside from this, the proteins inside the crystalline lens become increasingly more rigid (like proteins in vessels, in articulations, etc.), creating less crystalline plasticity, which means a slower and more difficult process when changing shape, or focusing.

How presbyopia can be corrected

Following the discovery and making of negative and positive lens thanks to the work of Antonio Manzini (1660), positive lens began to be used to compensate difficulties in close vision. Even today, positive lens represent the basic solution to correct presbyopia.

Things have changed. It is now possible to use lenses that are particularly comfortable, allowing a clear close up vision as well as middle range distances, ideal for working in the office. For people who need to see up close, as well as in middle and far range distances, progressive lenses may be used. These lenses modify the refraction rate from up to down, allowing, with small adjustments in the positions of the eyes, a good vision in all distances. For those who do not wish to have glasses, it is possible to use contact lenses available today even in progressive version. For those who do not wish to have contact lenses, the problem can be resolved with refractive surgery which allows one eye to be made myopic at 2 diopters and 1/2, adapting to close vision, and the other eye emmetropic, adapted for distance vision. This is called monovision and, contrary to what we may think, it is considered to be comfortable, and prosthesis are no longer necessary.

Good news for those who need to have cataract surgery: there are multifocal lenses and accommodative lenses, which are artificial crystalline lenses allowing the person who has had cataract surgery to have a good vision up close and in distances. However, they are crystalline lenses which require careful selection by patients, and their cost is clearly superior to ordinary artificial crystalline lenses.

In the future, but not yet clinically applicable: after a graft of the internal portion of the crystal lens with its envelope, putting in a substance of gelatinous consistency would recreate a crystalline lens a lot similar to the crystalline of youth.

Lastly, and this is to correct a wildly spread belief: use of sight for prolonged periods postpones the manifestation of presbyopia and does not hurt the eyes.

- Back to the beginning -