Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996

36 Chapter 4: Visual Acuuy Examination

corrective lenses, the abbreviation sc~ is used in the measurement notation. On subsequent tests, a patient who habitually wears eyeglasses or contact lenses should wear them for the test, and the record should indicate so. Distance correction should be used to test distance vision. To avoid confusion in recording information, establish a routine for testing; by convention, the right eye is tested first. Clinical Protocol 4.1 presents instructions for performing a distance acuity test.

A variety of occluders, held by either the patient or the examiner, can be used to cover the eye that is not being tested. These include a tissue, a paddle, or an eye patch. The palm of the patient's or the examiner's hand can also be used to occlude the eye not being tested. Any occluder that is used for more than one patient should be cleaned before reuse.

Before the test begins, ask if the patient is familiar with the optotypes being used. This is particularly important for children. If a patient is comfortable with letters, use that chart, if it is available. II the patient prefers numbers, use that chart. Because people tend to memorize the sequence of images that they have seen numerous times, present different charts or optotype sequences whenever possible. If only one type of chart is available, a patient can quickly memorize the order of the optotypes, whether intentionally or not. The type of chart and the method of presentation used should be noted in the patient's record; for example, "isolated numbers," "line letters," or "pictures," whichever applies. . •»

Pinhole Acuity Test

Near Acuity Test

Figure 4.2 For near vision testing, the patient can hold the small near vision card at a normal reading distance.

ter 4: Visual Acuity Examination

Teller acuity cards, if available, can be used to estimate acuity (Figure 4.3). These cards are large photographic plates (approximately 3 feet x 1 foot) with line gratings printed at one end. T h e examiner looks through a central pinhole to determine the baby's direction of gaze. T h e baby will look preferentially toward the side of the card that has a discernable image. Once the resolving ability of the eye has been surpassed, the baby's gaze will be random. These cards are reliable

until a child is approximately 1 year of age. T h e examiner must be

experienced to obtain reliable acuity measurements, lighting must be good, and the cards must be kept meticulously clean. Detailed testing and interpretation instructions are included with the test kits.

As an alternative testing method, optokinetic nystagmus (OKN) can be elicited using any regularly striped object. This can be as simple as lines drawn on paper or a standard tape measure, or as formal as

examiner observes the movement of the baby's eves. Fine oscillatory movements, with the slow phase going in the direction that the stripes

rotate, indicate that the baby has a potential for discriminating detail

of ai least the width of the stripe. Neurologic implications in the inter-

tesi nonverbal or preverbal patients.

Toddlers and preliterate children as well as illiterate or nonverbal adult patients can be tested using a picture chart, the Landolt ring test, the tumbling E chart, or the H O T V chart and cards. When the IIOTV chart and cards are used, the patient may be given a card with the four letters marked in large print. The patient is asked to point to each letter in turn as the examiner presents them on a separate screen or chart from a specified distance. The appropriate equivalent Snellen notation is then made, with an addendum that the optotypes were II, O, T, and V.

Ask whether the patient knows letters of the alphabet or numbers, because many people can identify specific letters or numbers even though they cannot read per se. Some of the pictures are more readily recognizable than others, and only a limited number of pictures are used. Therefore, the use of picture charts generally leads to an overestimation of the visual ability of the child.

Young children often become bored with testing very quickly. Some children do better with numbers, whereas others prefer letters. If a child seems bored or hesitant with one kind of chart, trv another. If you are using the tumbling E chart, position yourself on one side of the chart and the parent on the other and ask the child, "Which direction are the legs pointing—to the ceiling, to the floor, to me, or to Mommy?" Alternatively, you can ask the child to point a hand or finger in the same direction that the legs of the letterform point, but this involves coordination abilities that are not related to vision. When

42 Chapter 4: Visual Acuity Examination

using the Landolt ring you can position yourself and the parent as with the tumbling E chart and ask the child to indicate, for example, which side of the "cookie" has a bite in it.

Variables in Acuity Measurements

Falsely high or low acuity measurements can be obtained under a variety of circumstances. In general, the near and distance acuities should be comparable. Decreased distance acuity in the presence of good near acuity indicates that the patient is probably myopic. Possible causes of near acuity being poorer than distance acuity include the following:

•Presbyopia/premature presbyopia

•Undercorrected or high hyperopia

•Overcorrected myopia

•Small, centrally located cataracts

•Drugs with anticholinergic effect

•Measurements can vary if the lighting level is not constant during testing. For acuity tests to be comparable, lighting conditions must remain equivalent.

•Charts with higher contrast will be seen more easily than those with lower contrast.

•If a chart is not kept clean, smaller letters will become more difficult to identify. When a projector chart is used, the cleanliness of the projector bulb and lens and the condition of the projecting

screen will afreet the contrast of the letters viewed by the patient.

•The distance between the projector and the chart will affect the size of the letters. The sharpness of the focus of a projected chart and the incidental glare on the screen can also influence the patients ability to read the optotypes.

•Charts that have the letters crowded together may be more difficult to read.

•Patient fatigue or boredom are difficult variables to assess but will also affect acuity measurements and may be noted in the chart at the examiner's discretion.

Optical considerations also influence the ability of the patient to discern detail. They include the following:

•If a patient is wearing eyeglasses, be sure the lenses are clean. Dirty lenses of any kind, whether trial lenses, refractor lenses, eyeglass lenses, or contact lenses, will decrease acuity, and the measurements obtained will be falsely low.

•Effects of tear film abnormalities, such as dry eye syndromes, can

be minimized by the generous use of artificial tear preparations.

•Corneal surface abnormalities may produce distortions and must be addressed medically.

•Corneal or lenticular astigmatism may necessitate the use of special spectacle or contact lenses. Discussions regarding the prescribing of these lenses can be found in specialized clinical ophthalmology and optometry texts.

•Other interferences from media opacities may have to be addressed either medically or surgically.

Patients with neurologic impairments may have motility problems or central nervous system abnormalities that can influence the measurement of acuity, as described in the list below:

•A visual or an expressive agnosia will usually be identified while

obtaining the history by the way the questions are, or are not,

•Motility defects such as nystagmus (the presence of spontaneous oscillatory movements of the eyes) or any other movement disorder that interferes with the ability to align the fovea will lower the acuity measurement.

•Nystagmus may be difficult to determine when the amplitude of the nystagmus is small. In latent nystagmus, a condition that occurs only when one eye is occluded, the unoccluded eye develops nystagmus and the measured visual acuity will be lower than expected and significantly lower than the binocular acuity.

Other neurologic considerations include the following:

•Visual field defects

•Optic nerve lesions

•Pupil abnormalities

•Impairment by drugs, legal or illicit

If latent nystagmus is suspected or diagnosed, it is best to conduct visual acuity examinations using polarized images, with a different polarized image presented to each eye. The vectographic projection slides are the best available for this purpose at present. As an alternative, the fellow eye may be blurred using a +6 or +8 lens so that the unblurred eye can be considered unoccluded when viewing the chart.

When nystagmus is present, the patient may have a null position. In this situation the patient maintains a head position to decrease the amplitude of the nystagmus. Visual acuity will improve when the head is held in that position. If the patient assumes a head position when looking at objects, determine if this improves acuity by allowing the patient to maintain that head position while you measure acuity for distance and near. Frequently patients with congenital nystagmus will have significantly better visual acuity for near than for distance. This occurs if the nystagmus dampens with convergence.

Psychologic factors, whether conscious or unconscious, affect visual acuity measurement results. In an attempt to please the examiner or "score" better on the test, children may try to peek around the occluder. Familiarity with the test may also lead to inadvertent memorization of the lines by the patient. This is especially true for children. E'xternal variables such as patient distraction, fatigue, and age should be considered when an unexplained poor acuity7 measurement is obtained.

Uncorrectable Visual Acuity

The visual acuity determination can be considered a level of performance for which the examiner may make estimates regarding a patient's potential disability. The acuity level is considered when determining reading aids and reading distances. These factors are summarized in Table 4.2. Severe visual impairment in both eyes is required for inclusion in the category "legal blindness" and is the criterion usually used to determine eligibility for disability benefits.

The disabling effect, if any, of a visual impairment depends upon the individual and may or may not be perceived by the patient as a handicap. Table 4.2 also summarizes levels of visual impairment and visual disability, which are important for evaluating legal or physical limitations for a patient. Definitions of legal blindness differ from state to state, especially regarding eligibility for a driving license. In most states, the minimum visual acuity must be correctable to 20/40 for a nonrestricted license. The Low Vision Rehabilitation Committee of the American Academy of Ophthalmology proposed this level of acuity in a 1995 Policy Statement regarding vision requirements for driving. The guidelines for issuing noncommercial driving licenses also include the recommendation that an uninterrupted visual field oi 140° horizontal diameter be present for individuals with 20/40 or better visual acuity.

Amblyopia