Good Outcome on Snellen Chart Does Not Mean Good Vision

Editorial

The Table, developed by the Dutch ophthalmologist Herman Snellen, in 1862, by its easy application, it is still the main method of measuring vision in ophthalmologists’ offices, despite their limitations:

• Does not assess contrast perception.
• Some letters are harder to identify than others.
• Manufacturers can use different scales.
• The lighting condition of the room can change the result.

Thus, the visual result measured with the Snellen chart is useful as a reference and as a parameter for measuring clinical evolution. However, it is inadequate to assess visual quality, that is, it is not reliable to measure the impact of vision on quality of life [1, 2]. The most appropriate test to assess visual quality is the Contrast Sensitivity Test (ETDRS), which has optotypes with different levels of contrast. Contrast is one of the main components of visual potential. Is it sensitivity that allows us to distinguish colors and objects, especially in low light. In the real world, it is thanks to contrast sensitivity that we see, for example, a black hole in the gray sidewalk, in the dark. This sensitivity is not measured with the Snellen Chart, which, with its letters black on white background, evaluates vision in maximum contrast [3, 4]. Thus, although Snellen chart is very practical for assessing visual acuity in office, the ophthalmologist must recognize its limitations. Even for understand, for example, the possibility of a person with nuclear cataract and vision of 20/20 complaining of not seeing at night.

The opacification of the lens nucleus is the cataract component that most compromises contrast sensitivity. As this type of opacity progresses slowly, if there is no opacification of other layers of the lens, it can take decades for contrast sensitivity to impact visual acuity measured with Snellen chart. However, in the real world, it is likely that, long before that, the reduction of contrast sensitivity will become clinically significant, impairing the activities of daily and professional life of the person, especially in low- light environments. As well as patients with mild posterior sub capsular cataract can see well in the doctors’ office and suffer, in everyday life, visual impairment in brightly environments. Thus, it is not enough for the ophthalmologist to be trained to measure the acuity visual, it is necessary that he also understands:

• The meaning of this assessment.
• The limitations of measurement methods.
• The clinical significance of the results.
• The context of the patients’ complaints.