Visual acuity (VA) is the most widely used parameter to assess visual performance to date, is measured using Snellen tables. Although this test is widely used in clinical practice, there is no standard version: it can include characters of different sizes, shapes and spacing, all of which could influence the final result. In an attempt to create a standard, some of Snellen's graphs have been drawn using a logarithm, called the logarithm of the minimum angle of resolution (logMAR), with characters of constantly increasing size (0.1 log). For example, the graph for the Early Treatment Diabetic Retinopathy Study (ETDRS) uses logMAR and Sloan's series, i.e. 10 graphical representations of letters (octotypes) drawn to provide uniform readability. In addition, AV measurement is inappropriate for assessing 'real-world' visual performancein which the visual system is stimulated by a wide range of light intensities and contrast thresholds. To this end, the test for contrast sensitivity (SC) is more appropriatein that it assesses the visual system's ability to discriminate the contours of an object that are affected by even small variations in brightness. Although a patient with an excellent SC is likely to have an excellent AV, the reverse is not necessarily true: it is therefore necessary to measure the SC independently of the AV. An effective way to measure SC is to determine the contrast sensitivity threshold (CST) against a light background. In this test, the patient reads characters of a defined size with a contrast that gradually becomes lighter and lighter until it is no longer possible to observe any image by lowering the brightness of the background. An example of this is the Pelli-Robson chart: 8 lines with 6 Sloan characters each, with decreasing contrast with each triplet of letters. Patients have to read as far as they can and the CST is defined when two of the three letters are correctly identified. However, even the latter has limitations, such as the small number of contrast gradations that can be expected and the interference given by the brightness in the examination room. There are also electronic systems with liquid crystal monitors that are commonly used to measure AV and can also be used to determine CST and SC more easily.
How can visual performance be improved?
A crucial role in improving visual performance is played by the macular carotenoids: lutein (L), zeaxanthin (Z) and meso-zeaxanthin (MZ). These elements, introduced through the intake of leafy vegetables and coloured fruit and vegetables, are deposited in the foveal region of the retina where make up the yellow-orange component of macular pigment (PM). Thanks to its colouring, the PM protects the retina in two ways:
-acting as an antioxidant against the formation of free radicals, which could be formed by radiant energy from sunlight;
-absorbing high-energy, low-frequency light (blue light), which is potentially harmful.
Studies have shown that PM is associated with a decreased risk of developing age-related macular degeneration.
Although a recommended daily dose for macular carotenoids has not yet been established, many studies suggest an intake of 6-24 mg/day and show benefits from doses of 12 mg/day or higher. Furthermore, in a recent study (CREST Normal Trial) it was shown that a year's administration of 10 mg/day of carotenoids induced a significant improvement in SC that was also matched by an increase in macular pigment optical density (MPOD macular pigment optical density).
From a physiological point of view, it seems that the improvement in SC is due to the enhancement of the circuitry of the visual system with increased sensitivity to lateral inhibition, a natural mechanism whereby a visual message reaches the brain undisturbed, i.e. without 'interference' caused by other visual information.
Since SC decreases with the normal ageing process and can be improved with dietary supplementation, ophthalmologists should adequately inform patients about the beneficial effects of carotenoid supplementation on vision.
Furthermore, in a recent article published in Molecular Nutrition & Food Research, a new model to understand the visual impact of SC in real conditions: Roark's model. This model compares changes in SC to equivalent changes in AV. For example: an improvement of CST from 3.2% to 2.0% at a character size of 20/100 would seem to have a comparable visual impact to the improvement of AV from 20/40 to 20/25.
Since SC refers to the ability of the visual system to discriminate contours and delineate the boundaries of an object in real-life situations, it would be more appropriate for the scientific community to consider adopting the measurement of SC as a standard examination or use a combination of AV and CS. In fact, these tests can provide physicians with a more accurate picture of a patient's visual function and would allow for better clinical management.
Source:
Roark, M. W. and Stringham, J. M. (2019), Visual Performance in the 'Real World': Contrast Sensitivity, Visual Acuity, and Effects of Macular Carotenoids. Mol. Nutr. Food Res.. Accepted Author Manuscript. doi:10.1002/mnfr.201801053
Dr. Carmelo Chines
Direttore responsabile