Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

Glossary

Cycles per degree – The number of complete phases of a grating (e.g., the distance between the center of a white bar and the center of the next bright bar in a square-wave grating; or the distance between two adjacent areas of maximum brightness on a sine-wave grating) contained in 1 of visual angle.

Minimum angle of resolution – The size of the angle subtended at the eye of the smallest feature which can be reliably identified on an optotype. Minute of arc – One-sixtieth of a degree. Optotype – A letter, symbol, or other figure presented at a controlled size to measure vision. Visual angle – The angle, which a viewed object subtends at the eye.

Detection and Resolution Acuity

Visual acuity can be defined in two broad ways. Detection acuity is measured by determining the size of the smallest object which can be reliably seen (is there a circle on the first or second screen?). Detection can be elicited reliably with targets, which subtend an angle at the eye as small as 1 s of arc (1/3600 ). Even a small point of light will stimulate several photoreceptors due to the point-spread function of the eye: that is, the way in which light is diffracted through the eye’s optics (Figure 1(a)).

Tests that require the identification of a target are a measurement of resolution acuity. These tests frequently involve identifying a letter or reporting an object’s orientation (what direction is this letter C facing?). Acuity for these tests depends on the separation of the target features: if they are too close, the point-spread function from each element will overlap and they will not be identified (Figure 1(b)). The smallest separation of the elements required for identification of the target (Figure 1(c)) is known as the minimum angle of resolution (MAR). For an adult observer with good vision, a typical MAR for a centrally presented, high-contrast target can be as good as 30 s of arc (1/120 ). Figure 2 shows the feature critical for the MAR for some commonly used tests of visual acuity.

Measurement of Visual Acuity

Visual acuity tests have been used for millennia: the ancient Egyptians are reported to have used discrimination of the twin stars of Mizar and Alcor as a measurement of vision. The most familiar clinical test of visual acuity, the Snellen chart, was introduced in 1862, and is still widely used today.

Detection acuity is often measured psychophysically by means of a temporal two-alternative forced-choice experiment (did the light appear in the first or the second interval?). Detection acuity is rarely measured clinically.

In psychophysical experiments of the visual system, resolution acuity is commonly measured by asking observers to report the orientation of a grating with variable separation between each dark and light bar (Figure 2(b)). In clinical practice, gratings are rarely used, with the exception of forced-choice preferential looking tests in preverbal children. These tests consist of a uniform gray field with an isoluminant grating toward one side of the chart (Figure 3(a)). In a featureless room, the test is presented to the child and the clinician observes whether the child looks toward the grating. The finest grating toward which the child repeatedly looks is recorded as the visual acuity.

For cooperative patients, optotypes are more often used to measure clinical resolution acuity. The Landolt C (Figure 2(c)) is the standard to which letter visual acuity tests are compared. This target consists of a ring of fixed width with a gap, of height equal to the stroke width, at the top, left, right, or bottom of the circle. The observer is asked to report the position of this gap. The smallest gap whose position can be reliably reported is equivalent to the MAR.

The National Academy of Sciences standard for visual acuity measurement advocates the presentation of 10 optotypes, of equivalent difficulty to the Landolt C, at each acuity size. The horizontal spacing between each optotype should be at least one character width, and vertical spacing between lines should be 1–2 times the height of the larger optotypes. It suggests that the number of characters on each line should be equal, and that the size difference between consecutive lines is 0.1 log units: in other words, for each target size, the next line should be approximately 1.26 times smaller.

The Snellen chart (Figure 3(b)) does not meet these recommendations: the number of letters per line and step

Target

2-D PSF

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Figure 1 Schematic illustration of the point-spread function of three visual targets: (a) a point target; (b) two adjacent lines, too close to be resolved; and (c) two adjacent lines, with sufficient separation to be resolved. Middle row: two-dimensional representation of the target point-spread function; bottom row: one-dimensional representation of the point-spread function; and red line indicates the sum of energy incident on the retina. PSF, point-spread function.

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Figure 2 Examples of the limiting feature for four commonly used resolution tasks: (a) two-point discrimination task; (b) grating; (c) Landolt C; and (d) Sloan letter E (note that white gap size is equal in width to black bar elements).

size between the lines are variable, as is the horizontal and vertical spacing on the chart. There is also a marked difference in the legibility of different letters on the Snellen chart: a W, for example, has far less separation

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Figure 3 (a) A forced-choice preferential looking test consisting of a grating against an isoluminant background. Note the peephole in the center for the clinician to observe the child’s visual behavior; (b) the Snellen chart; and (c) The ETDRS chart. ETDRS, Early treatment of diabetic retinopathy study.

between the elements of the letter and is more difficult to identify than a letter L. In the 1950s, Sloan suggested the use of 10 letters with a selection of vertical, horizontal, oblique, and round strokes which are each about as legible as a Landolt C. These Sloan letters are C, D, H, K, N, O, R, S, V, and Z. Each of the Sloan letters has a stroke width of the MAR and has a total height and width of five times the MAR.

The Bailey–Lovie chart, introduced before the recommendations of the National Academy of Sciences, conforms to most of these requirements, although it only has five letters per line. Further, the letters on the Bailey–Lovie chart are taller than they are wide: their height-to-width ratio is 5:4 and they are selected from the British Standards set of letters (D, E, F, H, N, P, U, V, R, and Z). The ETDRS chart (Figure 3(c)), developed for the early treatment of diabetic retinopathy study (ETDRS), is similar in design but does use the recommended 5 5 Sloan letters.

A criterion of 7/10 letters being read correctly for a line to be marked as seen was suggested by the National Academy of Sciences. This threshold reduces the chance of the line being scored correctly by chance (by a blind observer) to around 1 in 9 000 000. On a chart with five letters per line, recording a visual acuity where four of the five letters are read correctly equates to a chance success rate of 1 in 46 000. There is a theoretical advantage if the observer knows there are only 10 letters which can be presented on the chart: if an observer guesses from all 26 letters rather than the ten Sloan letters, the probability of the observer getting four out of five letters correct reduces to about 1 in 100 000.

Test–retest variability of the Snellen chart is around0.3 logMAR, while the ETDRS chart has far better repeatability (test–retest variability 0.1–0.2 logMAR). Despite the many limitations of the Snellen chart, it is still widely used in clinical practice. While this is likely to be largely due to clinicians’ familiarity with the Snellen chart, there is also a perception that Snellen acuity measurement is quicker than that on the Bailey–Lovie or ETDRS charts.

Various modified versions of the ETDRS chart exist: for example, a version with an altered letter set (A, B, E, H, N, O, P, T, X, and Y) has been developed for use by readers of most European languages, including those based on Cyrillic or Hellenic alphabets.

For observers unable to report letters on a sight chart, other frequently used optotypes include the tumbling E chart (formerly and less politically correctly known as the illiterate E chart), where a letter E is shown in each of four rotations; the HOTV chart, where only these four letters are used; symbols such as the Lea or Kay pictures; and simple shapes, such as the Cardiff card.

Reporting Visual Acuity

Clinicians have traditionally used Snellen fractions to record visual acuity, where the numerator is the test distance and the denominator the target size. The target size is expressed, counterintuitively, as the distance from which the target has an MAR of 1 min of arc. Therefore, a visual acuity of 6/6 indicates that from 6 m, letters with MAR 1-min arc are correctly identified, while a visual acuity of 3/36 indicates that from 3 m, the targets identified have a MAR of 1 min of arc when viewed from 36 m. The reciprocal of the Snellen fraction gives the visual acuity in MAR: so a visual acuity of 3/36 indicates a MAR of 12 min of arc.

In much of Europe, the Snellen fraction is reduced into a decimal fraction.

A further confusion with the Snellen system is that in countries not using the metric system, distances are expressed in feet rather than meters, with 20/20 being exactly equivalent to 6/6 but with a test distance of 20 ft rather than 6 m. Although Snellen recommended adoption of the metric system in 1875 and, in 1980, the US National Academy of Sciences favored adoption of a standard defined in meters, given the imminent adoption of the metric system, the feet system is still widely used in the USA, and among lay people in the UK.

The accepted standard for expressing visual acuity in clinical research, and increasingly in clinical practice, is to use the base 10 logarithm of the MAR (logMAR), such that 0.0 logMAR is equivalent to 6/6 or 20/20, and 1.0 logMAR is the same as 6/60 or 20/200. Table 1 gives approximately equivalent values in MAR, cycles per degree, Snellen fractions in meters and feet, decimal acuity, and logMAR for a range of visual acuities.

Optical and Neural Limits on Visual Acuity

Visual acuity is limited by many factors: the optics and refraction of the eye; the clarity of the optical media; the spacing and function of the retinal photoreceptors; the ratio of retinal ganglion cells to photoreceptors; and the resolution of the primary visual cortex and higher areas of visual processing.

Each diopter of myopia reduces visual acuity: a –1.00DS myope will typically have uncorrected visual acuity of around 0.5 logMAR (6/18; 20/60) and a two-diopter myope will have vision of around 0.8 logMAR on a distance test. Hypermetropia can often be relieved by accommodation in young people, but each diopter of hypermetropia