Which method would be appropriate to assess a patients visual acuity using a Snellen chart?

Visual acuity is the most commonly used and intuitive measure of the performance of the visual system. Tests of visual acuity are used to determine whether people can safely drive or work in many occupations, and the expression 20/20 is widely used colloquially as a description of good vision.

This article reviews methods of measuring and recording detection and resolution acuity. Optical and neural limits on visual acuity are discussed, as well as variation of visual acuity across the retina and over life. The concepts of hyperacuity and dynamic visual acuity are introduced.

Functional Visual Acuity

The FVA system continuously measures changes in visual acuity over time (Figure 3). The test begins with the best corrected Landolt visual acuity, which is the baseline of FVA. Landolt optotypes (consists of a ring that has a gap, thus looking similar to the letter C) are presented on the frame every second within a defined time (1–5  s), which increases in size when the answer is incorrect or when there is no response within the set display times, and decreases in size when the answer is correct. Each response is recorded on a table and as graphs, which are composed of points joining the correct answers only. FVA-test parameters include the mean FVA and visual maintenance ratio (VMR). VMR measures the percentage of the time a subject is able to maintain his or her best visual acuity over the present-time testing. The mean logMAR FVA and VMR scores have been reported to decrease in dry-eye patients compared to healthy control subjects. Both parameters improve with punctum-plug occlusion (Figure 4) but may deteriorate with epiphora (watery eyes).

Visual acuity

Visual acuity is the most commonly used and universally understood measure of visual function. It is important to measure visual acuity in most circumstances because it provides a simultaneous measurement of central corneal clarity, central lens clarity, central macular function and optic nerve conduction. If it is normal, it provides a quick assessment of this central ocular pathway. If it is abnormal, it does not distinguish between the many causes of an abnormality.

Visual acuity should be measured as best corrected distance visual acuity. A recent refraction is required to obtain the best corrected visual acuity. Although the traditional distance used to measure visual acuity was 20 feet or 6 m, the distance in this case refers to a distance of at least 4 m. The use of a 4-m distance during refraction has the advantage of being one-quarter of a diopter in lens power from testing at a theoretical infinite distance. Each eye should be tested separately. The test should be conducted using a high contrast chart with an equal number of letters per line and equal spacing between lines. The stroke width of the letters should be smaller on each succeeding line such that the visual angle needed to identify the letters is reduced by two-thirds per line.

The result of a visual acuity test should be reported as a logMAR value (log of the minimum angle of resolution). Normal visual acuity for most adults is approximately –0.1 on this scale, which is equivalent to 20/16 on a Snellen visual acuity chart. A two-line or greater change from one visit to the next in a single patient should suggest additional investigation. A three-line or greater change in a single individual is usually considered clinically significant. In the evaluation of a group of subjects, changes in the mean logMAR score and in shift tables created by categorizing subjects by gains and losses in zero, one, two, three or more lines of visual acuity are often helpful in recognizing changes in visual acuity.

Additional measures of visual acuity such as best corrected near visual acuity, uncorrected distance visual acuity and uncorrected near visual acuity are rarely necessary unless it is not possible to perform a best corrected distance visual acuity. Although abnormalities may occur which alter near visual acuity without affecting best corrected distance visual acuity, these abnormalities are better characterized by measuring the accommodative amplitude together with any observed changes in refractive power in association with the best corrected distance visual acuity. Refractive power can be measured by either a manifest refraction or a cycloplegic refraction. When evaluating the effect of a drug product on refractive power, it is usually best not to perform a cycloplegic refraction as the pharmacologic action of the cycloplegic agent may alter the results.

3.5.5 Interpretation

The near VA obtained with the super pinhole VA gives an indication of the possible near VA after uncomplicated cataract surgery. The test cannot bypass dense cataracts so that in such cases the super pinhole result is likely to be worse than the postoperative VA (Vianya-Estopa et al. 2006) and just represents the minimum VA that is likely to be obtained after surgery. Other results and assessments should be taken into account when considering the likely visual outcome after cataract surgery and these include the patient's age, indications from the case history and results from a dilated fundus examination and swinging flashlight test. Visual acuity in the pseudophakic eye is another useful indicator for patients undergoing second eye surgery.

Clinical presentation

Visual acuity varies widely and correlates primarily with the integrity of the papillomacular bundle instead of the size of the optic disc. Hence, visual acuity may range from normal to no light perception (Brodsky, 1994) but usually remains stable unless amblyopia develops. Segmental optic nerve hypoplasia may occur in certain conditions. For instance, maternal diabetes mellitus has been found to be associated with a superior segmental optic nerve hypoplasia, which results in an inferior visual field defect and preserved visual acuity (Nelson et al., 1986). Optic disc coloboma is an example of an inferior segmental optic nerve hypoplasia with the remaining neuroretinal tissue confined to the superior rim.

Introduction

Visual acuity is the most widely used measure of visual function. In fact, visual function is often equated with visual acuity, thereby ignoring other important dimensions of visual stimuli, such as color and contrast. Visual acuity tests have proven to be useful for assessment of refractive error, screening for ocular health, following the course of eye disease, evaluating the effectiveness of medical and surgical treatment, prescribing aids for the visually impaired, and setting vision standards for employment and driving.

Given the variety of its applications, it is not surprising that many different types of visual acuity tests have evolved. Generally, these tests were developed with little concern for standardization. Since the 1980s, several attempts have been made to formulate standards for test design and administration. The Committee on Vision of the National Academy of Sciences-National Research Council (NAS-NRC)1 has published standards for clinical testing of visual acuity that are widely adopted in the USA, and the British Standards Institution2 has published similar standards for the UK. The NAS-NRC standards are used as the basis of this chapter.

Dynamic visual acuity testing

Measuring the DVA allows a functional aVOR assessment by comparing the visual acuity under static and dynamic conditions, i.e., during fast head rotations (100–150°/second) within the plane of a specific canal (Demer et al., 1994; Vital et al., 2010). With retinal slip velocity exceeding 2–4°/second, visual acuity will be reduced (Demer et al., 1994). Optotypes (Landolt rings, Snellen E) of standardized size are used to assess visual acuity. With a properly working aVOR, visual acuity decreases by fewer than three lines under dynamic conditions (Vital et al., 2010). While the DVA offers the ability to estimate real-life implications of a deficient aVOR, it heavily depends on the patient's cooperation.

Visual Acuity

Visual acuity is a measurement of the individual's capacity for visual discrimination of fine details of high contrast.17 Best corrected visual acuity should be tested for each eye.1 Distance vision is assessed with a standard Snellen chart and near vision with a handheld card. If the patient does not bring corrective lenses for the examination, a pinhole can correct most refractive errors.

Acuity is most often recorded as, for example, 20/40, in which the numerator refers to the distance (in feet) from which the patient sees the letters and the denominator the distance from which a patient with normal vision sees the same letters. Visual acuity with the near card is often recorded using the standard Jaeger notation (J1, J3, etc.). If a patient is unable to read the largest Snellen letters (20/200 or 20/400), the acuity should be characterized by the ability to count fingers (CF) (and at what distance), detect hand motions (HM), or have light perception (LP). An eye that is blind has no light perception (NLP). Contrast sensitivity testing with sine‐wave gratings is a useful adjunct in the evaluation of visual acuity. In younger preverbal patients, assessment of fixing and following in most instances is sufficient. When more accurate visual acuities are required, preferential looking tests (Teller's acuities) may be used.18 These tests are based on the principle that a child would rather look at objects with a pattern stimulus (alternating black and white lines of specific widths) than at a homogeneous field. The smallest pattern that the child seems to prefer is an indicator of best visual acuity.

Visual acuity can be altered by macular, optic nerve, or chiasmal lesions. Disturbances that are posterior to the chiasm (retrochiasmal, i.e., tract, optic radiations, and occipital lobe) can affect visual acuity only if they are bilateral.

Visual Acuity

Visual acuity usually is measured as a Snellen fraction (e.g., 20/30) for distance vision. During bedside or office examinations, however, one may use a near-vision acuity card, several of which are commercially available. Of course, patients should wear their glasses when visual acuity is being checked. Because loss of color perception can be an early sign of optic neuropathy, color vision evaluation is an important diagnostic test.63 One simple method for detecting possible early optic neuropathy is to check whether the patient perceives a difference between the two eyes in the color intensity of a red object. More advanced color vision testing should be performed by an ophthalmologist.

Measurement of Visual Acuity

Assessing visual acuity is critical to quantify function. Distance visual acuity should be measured for each eye individually with a Snellen chart (or similar) at a distance of 6 m (Figure 67.1). If distance spectacles are worn, visual acuity should be tested with these on. If visual acuity is reduced, it should be repeated using a pin-hole. Pin-holes act as universal glasses and an improvement in vision in one or both eyes using them indicates a likely refractive error – spectacles should improve vision. If the patient cannot see the largest letter then re-test at 3 m and 1 m; test ability to see fingers (count fingers, CF), hand movements (HM), a torch light (perception of light, PL) or no light perception (NPL).