Visual acuity is traditionally expressed in tenths in France. It corresponds to the ability of the eye to see clearly at a distance. Good distance visual acuity is generally between 8/10 and 10/10. The degree of visual acuity and the degree of visual defect should not be confused. In fact, visual defects are expressed in another unit, the "diopter", which is a unit of power (negative diopters for myopia, positive diopters for hyperopia and presbyopia, negative or positive diopters for astigmatism).
To get good visual acuity, the perceived image of an object must be projected exactly onto the retina, i.e. that the refractive power of the eye must be perfectly adapted. There's no direct proportional relationship between the degree of acuity and the degree of diopter. The visual acuity obtained after refractive surgery will be at least equal to that obtained with the optimal correction before surgery.
Some eyes have an anomaly called amblyopia which limits the maximum possible visual acuity.
Myopia is a very well known visual defect because even the slightest myopia disturbs vision and people are forced to wear a correction lens to see clearly at distance.
Almost a quarter of the population in Europe has this visual defect.
It may run in the family; however lifestyle can also have an influence, including prolonged efforts in near vision such as during studies.
There are different forms of myopia, but they all have the same consequences: imbalance between the excessive power of refraction of the eye relative to the axial length, which leads to forward defocalization of the image perceived by the eye. This image is therefore located at the front of the retina and will be blurred. The goal of treatment is to readjust the settings by changing the refractive power of the eye, so that the image is projected exactly onto the retina.
In the case of Femtolasik surgery, the Excimer laser beam will reshape the cornea to readapt the power by decreasing the radius of curvature.
Hyperopia is often less known by patients than myopia because it can remain latent for a long time when it is moderate. In fact, in young people, it may be compensated by efforts and the vision remains clear. Of course, severe hyperopia is mostly managed early because it can cause associated disorders.
Disorders caused by hyperopia are multiple: far and near blurred vision, headaches, strabismus of the eyes (squint), eye strain...
Hyperopia results from imbalance between the refractive power of the eye and the axial length of the eye. The refractive power is insufficient and the image is formed behind the retina despite the eye's efforts to accommodate.
The goal of treatment is to readjust the power parameters by increasing the total refractive power.
In the case of Femtolasik surgery, the Excimer laser beam will reshape the cornea to increase its power.
Astigmatism is a visual defect that can be isolated or associated to the previous two defects.
It is related to an anatomical peculiarity of the eye. In this case, the cornea is not a normal spherical shape (like a football) but has an oval deformation (like a rugby ball). This deformation may be on all axes, vertical or horizontal.
Visual disturbances are multiple: blurred vision, confusion of letters, superimposed or double vision, eye strain...
Astigmatism causes inhomogeneity in the eye's refractive power which leads to the blurred perception of the image that forms partly on the retina, but another part is defocalized forward or backward.
The goal of treatment is to readjust the power parameters by re-balancing the refractive power.
In the case of Femtolasik surgery, the Excimer laser beam will reshape the cornea to make its cylindrical power uniform.
This is the visual defect that affects everyone from their 40s. It is due to a decrease of the eye power of accommodation.
The accommodative power of the eye is the dynamic action needed to adapt our vision when you move from distance vision to close. It is made possible by a lens action which increases its power when accommodated.
However, this normal and physiologic reflex process becomes insufficient towards the age of 45, as the lens gets stiffer and loses some its power of correction. To compensate this, increasing the distance in near vision for reading is uncomfortable and inadequate.
Presbyopia stabilizes at about the age of 60, when residual accommodative adaptation of the lens becomes very weak.
Presbyopia is a phenomenon which affects a dynamic process. Surgical management of presbyopia therefore aims at compensating for this defect. In all cases, it is a compensatory surgery that offers a compromise to regain near vision because there is no etiological treatment, to date, to repair accommodation. Distance and near simultaneous visions will be made possible again, after individually tailored treatment for each patient. There is no "one" treatment, but there are tailored and customized treatments to suit patients, depending on their age and visual needs.
The goal of treatment is to be free from optical correction for both near and far visions.
So-called "Presbylasik" laser treatments cover a variety of techniques: monovision, micromonovision, simultaneous vision. These techniques are tailored for each patient and they can combine, to variable extents, the complex changes of asphericity and refractive power. Intra-ocular implant treatments are also particularly efficient, but they are offered mainly after the age of 55. Today, there are many types of implants that cover all the specific needs for each case.
The choice of the technique is made based on the preoperative clinical examination. The best solutions are analyzed jointly by the patient and the surgeon and together they choose the technique that best meets their expectations.