Measurement of visual acuity is the most important part of the ocular assessment performed by the doctor and yet it is surprising how often the nonspecialist omits it in examination. It has already been shown that the differential diagnosis of the red eye can be simplified by noting the vision in the affected eye. After injuries of the eye, it is just as important to note the vision in the uninjured eye as in the injured eye. Simple measurement of visual acuity is of limited value without a knowledge of the spectacle correction or whether the patient is wearing the appropriate spectacles. The best corrected visual acuity (i.e., with lenses in place) therefore needs to be recorded for each eye. This corrected visual acuity can also be estimated with a pinhole held in front of the eye. The effect of the pinhole is to eliminate the effect of refraction by the cornea and the lens on the extremely thin beam of light produced by the pinhole.
Measuring the visual acuity means measuring the function of the macula, which is of course only a small part of the whole retina. A patient might have grossly impaired visual acuity and yet have a normal visual field, enabling him to walk about and lead a normal life apart from being unable to read. This state of affairs is seen in patients with age-related macular degeneration and can be compared with the situation in which a patient has grossly constricted visual fields but normal macular function, as is sometimes seen in retinitis pigmentosa or advanced primary open-angle glaucoma. Here, the patient appears to be blind, being unable to find his way about, but he might surprise the ophthalmologist by reading the visual acuity chart from top to bottom once he has found it.
The simplest way to measure visual acuity might be to determine the ability to distinguish two points when placed close together (resolution). Such a method was supposed to have been used by the Arabs when choosing their horsemen. They chose only those who were able to resolve the two stars that form the second "star" in the tail of the Great Bear constellation. A point source of light such as a star, although it is infinitely small, forms an image with a diameter of about 11 mm on the retina. This is because the optical media are not perfect and allow some scattering of the light. In practice, it is possible for a person with normal vision to distinguish two points if they are separated by 1 mm when placed 10 m away. Two such points would be separated by 2 mm on the retina. This might be surprising considering that a spot of light casts a minimum size of image of 11 mm because of scatter, but such an image is not uniform, being brighter in the centre than at the periphery. In fact, the resolving power of the eye is limited by the size of the cones, which have a diameter of 1.5 mm.
In the clinic, the distance visual acuity is measured by asking the patient to read a standard set of letters, the Snellen chart. This is placed at a distance of 6 m from the eye. The single large letter at the top of this chart is designed to be just discernible to a normal-
The newsprint these days isn't what it used to be. . . .
The newsprint these days isn't what it used to be. . . .
Figure 17.1. Reading glasses in presbyopia.!
sighted person at a range of 60 m. If the patient's vision is so poor that only this and no smaller letter can be seen at 6 m, the vision is recorded as the fraction "6/60". The normal-sighted person who can read the chart down to the smaller letters designed to be discerned at 6 m is recorded as having a visual acuity of 6/6. The normal range of vision extends between 6/4 and 6/9, depending on the patient's age. In some European countries, the visual acuity is expressed as a decimal instead of a fraction. Therefore, 6/60 would be expressed as 0.1. In the USA, metres are replaced by feet, so 6/6 becomes 20/20. This is where the term "twenty twenty" vision originates from, meaning clear or near-perfect vision. Recently, a new type of visual acuity chart has entered use in the clinic and in research studies. It is called the LogMAR chart and differs from the conventional Snellen chart (Figure 3.1) by having five letters on each line rather than the "pyramid" shape of the Snellen chart. There are also smaller differences in type size between lines. Some of the advantages of using this new chart are that the measurement of poor visual acuity is more accurate as more larger letters are included and small changes in acuity are easier to detect (easier to detect disease progression or treatment success).
The near visual acuity is also measured using a standard range of reading types in the style of newsprint and, here, care must be taken to ensure that the correct spectacles for near work are used if the patient is over the age of 45 years (Figure 17.1). Normally,the results of testing the near visual acuity are in agreement with those for measuring distance vision providing the correct spectacles are worn if needed.
The visual acuity of each eye must always be measured by placing a card carefully over one eye and then transferring this to the other eye when the first eye has been tested. The visual acuity of both eyes together is usually the same or fractionally better than the vision of the better of the two eyes tested individually. In certain special circumstances, the binocular vision can be worse than the vision of each eye tested separately (e.g., in cases of macular disease causing distortion).
A number of other tests have been developed to measure visual acuity in the nonliterate patient. Infants below the reading age can be measured with surprising accuracy using the Stycar test. Here, letters of differing size are shown to the child, who is asked to point to the same letter on the card, which is given to him. Up to the age of 18 months or two years, the optokinetic drum might be used. This makes use of the phenomenon of optokinetic nystagmus produced by moving a set of vertically arranged stripes across the line of sight. When the stripes are sufficiently narrow, they are no longer visible and fail to produce any nystagmus. The eyes are examined using a graded series of stripes. This kind of test can be used to measure visual acuity in animals other than man. The "E" test is a way of measuring the visual acuity of illiterate patients. This is based on the Snellen type but the patient is presented with a series of letter "E"s of different sizes and orientations and is given a wooden letter "E" to hold in the hands. He is then instructed to turn the wooden letter to correspond with the letter indicated on the chart.
The Snellen type has the great advantage of being widely used and well standardised, but it must be realised that it is a measure of something more complex than simply the function of the macula area of the retina. It involves a degree of literacy and also speech, and testing shy children or elderly patients can sometimes be misleading.
Other ways of measuring visual acuity have been developed. One is to assess the patient's ability to resolve a grating. Here, the word "grating" refers to a row of black-and-white stripes where the black merges gradually into the white. Such a grating can be varied by altering either the contrast of black and white or the width of the stripes (the "frequency"). Thus, for a given individual, the threshold for contrast and frequency (contrast sensitivity) can be measured. This type of test has certain theoretical advantages over standard methods but it is not widely used clinically as a routine. Finally, the electrical potentials generated by the retina and optic nerve can be measured to give an estimate of visual acuity when the eye is presented with targets of varying size and contrast. This method is useful in infants and in the assessment of adults with nonorganic visual loss.
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