Mamisoa ANDRIANTAFIKA, MD FEBO
Eye Specialist
Brussels, Belgium

Presbyopia

Left, original picture with Niagara Falls in background. Right, image seen by a presbyopic patient trying to focus in near vision.

Presbyopic vision

Definition of presbyopia

Presbyopia is loss of accommodation power of the eye caused by the decrease lens elasticity with age. Ciliary muscles make lens to change its curvature to focus a closer image on retina. When lens becomes more rigid with age, changes of curvature are smaller and then ranges of accommodation get smaller.

Therefore, at about 45 year-old, an emmetropic person, i.e. who never had glasses before, has a lower accommodation that does not allow him to read correctly at 30cm, mean distance for reading. He will try to increase distance where lower accommodation is required. He will extend his arm to read. Loss of accommodation increases until about 65 year-ols where no accommodation remains.Value of glasses for near vision will therefore changes about every 3-4 years until this age.

Shortsighted persons will complain of presbyopia later, sometime will never complain of it according to the value of myopia, as that even if the image is close, it will still focus on retina as the eye is longer. Venue of cataract induces myopia, older person will be surprised with age to be less dependant of their glasses for reading. A myopia of about -3 dioptries allows to read at 30cm without glasses at the age of about 60 year-old.

Hyperopic persons will complain of presbyopia before 45 year-old if their hyperopia is not completly corrected. Indeed, effort of accommodation allow them to compensate hyperopia and then they are able to see without glasses. Sadly at some age, effort will not be sufficient to compensate supplementary accommodation needed for near vision.

Correction of presbyopia

Correcting presbyopia is not easy as accommodation allows different focuses for different distances: far, intermediate, near. Correction varies along with what we want to see.

At the beginning of presbyopia, it is wise to lighten correctly the lecture plane to lower near vision difficulties.

We can just correct near vision by adding a positive (+) convergent glass to far correction in order to focus the image at adequate distance for reading, usually 30cm. Therefore, an emmetropic person on both eyes can use at the age of 45yo glasses with addition of +1 or +1.25 dioptries to read correctly. If the subject wears a correction for far vision, he will need the use of 2 pairs of glasses or bifocals spectacles which contains the correction for far vision on upper part and addition needed on lower part. According to the same concept we can use segmented contact lenses.

Geometry of a bifocal glass

We can also correct all distances: far, intermediate and near. First we calculate the addition necessary for adequate near vision. We then use a progressive glass which is structured with many focals with increasing addition from center to low part of the glass. In primary vision, the subject will have no addition for far vision. In vision "for computer", the subject will have an addition sufficient for intermediate vision. In vision "for reading", addition will be the one calculated for reading. We can see a corridor of increasing additions, sides of the glass are optimised to the subject to enhance his visual field laterally. According to this principle, althought a bit adapted, we can use different types of multifocal contact lenses.

Geometry of a progressive glass

Surgery for presbyopia is delicate because it needs different additions for different distances. Procedures to model cornea with laser or implantation of intra-ocular lenses use same concepts used with contact lenses. But as surgery is partially or completly definitive, it must be assessed for each case. Sometime use of monovision consist of making non preferential eye slightly shortsighted to maintain one eye with near vision after 45yo, without being able to maintain this state after a certain age.