Visual Acuity

There are references in antiquity suggesting that the ‘sharpness of vision’ could be measured by the ability to resolve double stars, and from the 17^th century by Daça de Valdes, who described a method based on the ability to resolve mustard seeds, but the first convincing arguments for the standardisation of vision tests were not made until 1843 by Kuechler, who developed 3 vision charts to avoid memorisation.

Unfortunately these did not gain popularity. In 1854 Jaeger published near vision reading plates and in 1861 Donders proposed a measurement of the ‘sharpness in vision’ while undertaking his studies on refraction and accommodation. A year later his co-worker Herman Snellen published his standardised acuity charts that soon became adopted worldwide.

Visual acuity is a measure of the ability to discriminate two stimuli separated in space. It is measured at high levels of illumination and at high contrast, and although it reflects only a small part of an individuals’ visual performance, it is traditionally the most widely used measure to describe the functional capacity of the eye.

Snellen notation is the most frequently used when measuring acuity. It consists of black letters (‘optotypes’) on a white, brightly lit background, each of which subtends an angle of 5 minutes of arc and the space between each optotype subtends 1 minute of arc. Conventional test charts contain about 10 lines of letters in a progression of sizes, each designated by the distance at which the overall height of the letters on that line continues to subtend 5 minutes of arc, with the width of each ‘limb’ of the letter subtending 1 minute of arc. The acuity is recorded as a ‘fraction’ in which the numerator is the recording distance (in meters or feet), and the denominator is the distance at which the letter subtends the standard visual angle of 5 minutes of arc.

Thus, on the 6/6 line (20/20 in feet), the letters subtend an angle of 5 minutes of arc when viewed at 6 m, and on the 6/12 line (20/40), the letters subtend an angle of 10 minutes of arc when viewed at 6 m, or 5 minutes of arc when viewed at 12 m.

Snellen charts, however, posed a number of problems including the observation that the optotypes are not related to each other by geometric progressions in size, and that there are different numbers of letters per line. Thus, other charts have been developed, such as the Bailey-Lovie (or ETDRS) chart [51]. In this chart, LogMAR notation is used, which provides a measure of visual acuity known as the Logarithm of the Minimal angle of Resolution (LogMAR). The sizes of the letters progress systematically in geometric progression, each letter carries a value of 0.02 LogMAR

and there are 5 letters per line (Figure 6). Thus, each line represents a change in acuity by 0.1 LogMAR and acuity doubles every third line. 0.00 LogMAR is equivalent to 6/6 Snellen acuity and 1.0 LogMAR is equivalent to 6/60 Snellen acuity. The Snellen fraction can also be converted to the minimum angle of resolution, and the base-10 logarithm of this (LogMAR) determined [52]. This has allowed the standardisation of measurements such as counting fingers and hand movements to be denoted as 2.0 LogMAR and 3.0 LogMAR respectively. Perception of light (PL) and Nil Perception of Light (NPL) vision are arbitrarily denoted as 5.0 LogMAR and 6.0 LogMAR respectively [53]. Both the Snellen and Bailey-Lovie charts rely on the literacy of the subject being tested, and require the detection, resolution and recognition of the letters, and then the relaying of this information to the examiner.

Figure 6 – ETDRS visual acuity chart. LogMAR acuity scale is on the right and equivalent Snellen acuity on the left.

1.3.1.1 Visual acuity testing in children

In young children and infants, the above methods are not possible. Letter matching tests in which the child can point to letters on a key card once they recognise the letter they are shown, most closely relate to the Bailey-Lovie test. Examples of these include the Sheridan Gardiner test, the Sonksen-Silver acuity system and Cambridge crowding tests, among others. Some tests are ‘pictorial’, where the child is asked to name or match the pictures shown to them, such as the Kay Picture Test, although this test tends to underestimate the visual acuity. ‘Preferential looking’ may be used to estimate the visual acuity in infants and non-verbal children who cannot complete letter/picture matching. It is based on the observation that infants would rather look at a pattern than a blank stimulus. Such tests include the Teller and the Keeler acuity cards, which consist of a series of large grey cards with a central peephole that have a black and white grating of specific frequencies on one side. The cards are shown to the child and their preference recorded. However, although these tests attempt to provide a measure of visual acuity, they do not equate to Snellen vision. The Cardiff Acuity test, another example of a preferential looking test, uses ‘vanishing optotypes’

which consist of pictures with increasingly fine outlines that are correspondingly difficult to see. In the youngest infants (from age 3 months), the objective is to determine whether they have the ability to steadily fixate and follow an object, and at the same time assess if they have any nystagmus or a manifest strabismus. Another crude method of assessing acuity is to measure the optokinetic response in an infant, which should be present before age 3 months. This can be performed using an OKN drum or tape.