What is visual acuity?
Visual acuity is one of the factors that is commonly measured in ophthalmological examinations, as it determines the ability to clearly perceive the details of the objects we see.
Visual acuity is considered to be clinically normal when it is located in the unit (AV=1.0), i.e. when it reaches 100% of vision. On the contrary, when it is lower than this percentage, it is necessary to establish the patient’s level of vision and study what factors cause the decrease in visual acuity.
Main factors of visual acuity
PHYSICISTS
- The function and characteristics of the eye (pupil size, eyeball size).
- The optical power of the lens.
- Irregularities in the corneal curvature (astigmatism).
- The condition of the optic nerve and the visual cortex.
- The existence of optical aberrations, such as those that affect blurring (refractive errors).
PHYSIOLOGICAL
- At birth, visual acuity is low, but it improves over time until it stabilizes.
- From the age of 40-45 it begins to be lost, especially in near vision, due to tired eyes.
- Other previous eye pathologies can cause vision loss, such as glaucoma or retinal problems.
PSYCHOLOGICAL
- Stress or fatigue can alter vision test results.
- In the case of ophthalmological check-ups for children, it may happen that boredom or loss of concentration make it difficult to draw consistent conclusions about the visual acuity study carried out.
Refractive errors
Also known as ametropia, (myopia, hyperopia and astigmatism) are the main cause of decreased visual acuity. Unlike the normal or emmetropic eye, the image of the object is not projected onto the retina, so vision loses sharpness.
As a result, people who suffer from these refractive problems have to make an effort to accommodate themselves in order to focus correctly at different distances.
The incidence of refractive errors at school age reaches 20% of children. The presence of these undetected ametropias is especially important as they can interfere with the visual maturation process (which ends around the age of 7), generating delays in school development or sequelae that could persist throughout life. For this reason, it is essential to carry out the corresponding ophthalmological check-ups on children according to their age and clinical history, and an annual check-up is advisable.
How to detect it?
To detect the most common refractive disorders and study the patient’s level of vision, visual acuity tests must be performed at different distances (near, medium and far).
Although optical correction can compensate for most refractive errors, there are cases in which the maximum visual acuity is not achieved. It is then advisable to carry out an in-depth ophthalmological examination to detect possible pathologies of the visual system.
The visual acuity test allows us to know with great reliability and precision if the patient suffers from myopia, hyperopia and/or astigmatism and to measure the value of the refractive index, which are the diopters (they express the power of refraction). Once the exact prescription you need has been determined, the most appropriate type of optical correction is sought or the ophthalmologist may recommend surgical intervention after a personalized study of the patient.
The specialist can perform several complementary tests:
- Autorefractometer, a diagnostic device that allows us to know in a few seconds whether or not the patient has a refractive error and the approximate diopters.
- Optotype test, a test that measures the ability to distinguish letters, numbers, symbols and/or figures of different sizes that are shown in different tables (optotypes) and that are cataloged in tenths of vision.
- Tonometry, to measure intraocular tension.
- Corneal topography, to know exhaustively the surface of the cornea.
- Fundus study, especially indicated to check the condition of the retina.
