The visual acuity test is a non-invasive examination that is an essential part of the comprehensive eye examination and aims to determine the clarity and sharpness of vision. These parameters are tested by assessing the ability to distinguish different optotypes (graphic representations of letters or symbols) from a standard distance. This process, which may seem simple and automatic, actually requires multiple functions on the part of the eye: the retina must be healthy and be able to be reached by light with adequate refraction, and the visual stimuli received must be able to be interpreted correctly by the brain.
Visual acuity testing has been around for a long time and as early as 1800, the desire to standardise visual tests led to the creation of several charts with different octotypes. The first, and still the most widely used today, is the Snellen chart. More recently, the LogMAR chart (also known as the ETDRS chart) has also become established. For adults, octotypes with letters are usually used, while for children, symbols adapted to their level of understanding are used.
Currently, there are also many smartphone applications available to carry out the test, which seem to obtain reproducible results.
Visual acuity test results are reported using a scale in tenths or twentieths. The value of 10/10 and 20/20 is what a person with the highest visual acuity gets, but it only represents one aspect of vision and does not include other elements, such as depth perception, peripheral vision and colour blindness.
One of the most common causes of visual impairment are refractive errors, such as myopia and hypermetropia. Other causes of visual impairment include astigmatism, amblyopia (or 'lazy eye'), retinal detachment, macular degeneration, ischaemia, cataracts, glaucoma, corneal abrasion or other traumatic injuries. Visual acuity testing is essential in ophthalmological examinations because many of these conditions can be identified early, allowing early therapeutic intervention.
When the visual acuity test is performed
A visual acuity test is performed as part of a comprehensive eye examination, particularly in these circumstances:
- - screening for visual impairment
- - support in the diagnosis of relapsing eye diseases
- - assessment of acute visual changes, traumatic or non-traumatic
- - assessment of vision after corrective surgery
- - vision monitoring for driving licence renewal
It plays a role in both the acute and screening contexts. In the former case, it is important to document an initial visual acuity score, both for mechanical injury to the eye and for sudden vision loss (e.g. in the case of glaucoma, retinal detachment or ischaemia). Visual acuity measured under these circumstances can be compared with the previous test, if available, and will serve as the basis for re-evaluation after surgery in the acute and sub-acute setting.
The visual acuity test from children to adults
Visual screening tests, including visual acuity tests, have numerous advantages in the field of paediatric. In particular, visual acuity testing is the best way to detect refractive defects, which are the main cause of visual impairment and reduced vision in children. During the early stages of development, it is important to subject children to more frequent visual examinations.
In asymptomatic adults without significant risk factors, a comprehensive eye examination can be performed every 5-10 years under 40 years, every 2-4 years between 40 and 54 years, every 1-3 years between 55 and 64 years and every 1-2 years over 65 years. The change in the frequency of check-ups serves to better assess age-related eye diseases.
The visual acuity test in case of pathologies
Individuals at higher risk for glaucoma should have comprehensive eye examinations more frequently, even if they are asymptomatic. Individuals diagnosed with type 1 diabetes should have an eye examination, including a visual acuity test, five years after diagnosis and then repeat examinations annually. Patients diagnosed with type 2 diabetes, on the other hand, should have an annual examination from the time of diagnosis. In addition, women with diabetes should have a comprehensive eye examination, including a visual acuity test, during the first trimester of pregnancy.
The aim of these screening examinations is to enable early intervention to prevent long-term morbidity and improve quality of life.
Bibliography:
Dr. Carmelo Chines
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