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

46 Chapter 4: Visual Acuity Examination

•V'-itfi!:-:t'"-: Other lests oi Sensory Visual Function 47

when a patient is 8 to 10 years of age, recent evidence indicates that some improvement in vision can be achieved until even 12 or 13 years of age. Therefore, a trial of amblyopia therapy should be attempted in an older child after a thorough discussion with the parents and the

child of the risks and benefits of such treatment. - • • • , • ; • > •

Other Tests of Sensory Visual Function

Sensory visual functions include the ability to use both eyes together, referred to as fusion or single binocular vision. Single binocular vision y» ^ ;v':*i-1 results from simultaneous stimulation of corresponding retinal elements that have the same visual direction. For example, an object to a person's left may stimulate a spot on the temporal retina of the right eye and a corresponding spot on the nasal retina of the left eye. The brain then perceives the object being viewed as a single image in space.

Tests for binocular vision necessarily rely on both eyes being stimulated at the same time. These tests are discussed in detail in most orthoptic or ocular motility textbooks.

Stereopsis, considered a type of fusion, is the perception of depth or three-dimensionality when noncorresponding retinal elements are stimulated at the same time. To achieve stereopsis, both eyes must be used simultaneously. The tests used for stereopsis measurements consist of either polarized images or random-dot stereograms that may be nonpolarized or polarized. Figure 4.5 illustrates the Titmus (fly) test, which is probably the most common test for near stereopsis. The procedure for the Titmus stereoacuity test is summarized in Clinical Protocol 4.7. Random-dot stereogram tests also are used to test near

Figure 4.5 The Titmus stereopsis test. (A) Light-polarizing eyeglasses and targets. (B) The patient views the target through polarizing filters and reports perception of depth.

Contrast Sensitivity, Glare, and Color Vision Tests

Contrast sensitivity refers to the ability to discern relative darkness and brightness and the ability to see details, edges, or borders of images. Contrast sensitivity can be impaired even in the presence of excellent Snellen acuity. Alterations in contrast sensitivity imply abnormalities in the anterior visual receptive systems, from the tear film to the optic nerve. Specific patterns of contrast sensitivity function alterations are discussed in more advanced textbooks.

In the simplest contrast sensitivity tests, patients are shown printed charts with contrasting lines, referred to as gratings, which are presented in varying orientations. The difference in the intensity between the background of the chart and the printed lines is gradually decreased, and the patient is asked to identify the direction of the lines. The endpoint is reached when the patient can no longer correctly identify either the presence of any lines or the direction of their orientation. More technical testing methods involve presenting grating patterns or letters on an oscilloscope screen. The endpoints and reporting methods are similar in the two techniques.

Glare occurs when light from a single bright source scatters across the visual field, dazzling sight and reducing the quality of the visual image. The perception of troublesome glare, which causes distorted vision and, in some cases, mild pain, is also a symptom of potential cataract. As with contrast sensitivity testing, glare testing (by exposing the patient to bright light under controlled circumstances) can reveal the presence of cataract or other opacity

The most commonly recognized color vision abnormalities are the sex-linked congenital red-green deficiencies or confusions, but, many other color vision anomalies exist. Optic nerve or retinal disease may result in color vision defects that may be acquired or asymmetric. Most

•' patients with inherited color vision defects see red as less bright than

50 Chapter 4: Visual Acuity Examination

Suggested Resources

Amblyopia [Preferred Practice Pattern]. San Francisco: American Academy of Ophthalmology; 1992.

Frisen L. Clinical Tests of Vision. New York: Raven Press; 1990.

Optics, Refraction, and Contact Lenses. Basic and Clinical Science

Course, Section 3. San Francisco: American Academy of

Ophthalmology; updated annually.

Pediatric Ophthalmology and Strabismus. Basic and Clinical Science Course, Section 6. San Francisco: American Academy of - Ophthalmology; updated annually.

Rehabilitation: The Management of Adult Patients With Low Vision

, [Preferred Practice Pattern]. San Francisco: American Academy of Ophthalmology; 1994.

Testing Distance Visual Acuity

1. Ask the patient to stand or sit at a designated testing distance (20 feet

: adjusted to be equivalent to the corresponding Snellen acuity for the distance used. Most clinics are set up to test vision with projected charts at predetermined distances.

2.Occlude the left eye. Ask the patient or check for yourself to be sure that the occluder is not touching or pressing against the eye. Observe the patient during the test to make sure there is no conscious or inadvertent peeking.

3.Ask the patient to say aloud each letter or number, or name the picture

object on the lines of successively smaller optotypes, from left to right or, alternatively, as you point to each character in any order, until the patient correctly identifies only half the optotypes on a line. •;,•.

4.Note the corresponding acuity measurement shown at that line of the chart. Record the acuity value for each eye separately, with correction and without correction, as illustrated below. If the patient misses half or

fewer than half the letters on the smallest readable line, record how i& many letters were missed; for example, 20/40"2. If acuity is less than

20/20, recheck with a 2.4 mm pinhole (see Clinical Protocol 4.2).

5.Repeat steps 1 through 4 for the left eye, with die right eye covered.

6.Retest acuity with the patient using both eyes simultaneously and record acuity OU (see example below).

7. Record the power of die corrective lenses worn for the distance acuity determination (see Clinical Protocol 5.1, "Performing Manual Lensometry").

Clinical Protocol 4.2

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Clinical Protocol 4.3

Testing Near Visual Acuity

1.With the patient wearing the habitual corrective lens for near and the near card evenly illuminated, instruct the patient to hold the test card at the distance specified on the card.

4. Record the acuity value for each eye separately in the patient's chart

vaccording to the notation method accepted (see example below).

5.Repeat the procedure with the right eye occluded and the left eye viewing the test chart.

6.Repeat the procedure with both eyes viewing the test card.

7.Record the binocular acuity achieved (see example below).

Clinical Protocol 4.4

Testing for NPA and NPC

Testing for Near Point of Accommodation (NPA)

1.With the patient wearing full distance correction and with the left eve occluded, place the near testing card at a distance of 16 inches (40 cm) from the patient and ask the patient to read the 20/40 line with the unoccluded eye.

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4.Move the test card slowly away from the patient as you ask the patient to state when the letters become clear again.

5.Record this distance in centimeters or inches.

6. Repeat steps 1-5 for the other eye and record as described above.

7.The NPA is that point (in centimeters or inches) where the patient can no longer bring the image into clear focus.

Testing for Near Point of Convergence (NPC)

1.With the patient wearing appropriate correction and with neither eye occluded, hold a target, such as a pencil tip, at a distance of approximately 16 inches (40 cm) from the patient and ask the patient to fixate on it.

2.Move the object slowly forward and ask the patient to tell you when the object doubles.

3.Observe whether both eyes are converging.

4.Record the distance (in centimeters or inches) at which the image doubles or one eye deviates. The NPC is that point (in centimeters or inches) where a single image can no longer be maintained.

Clinical Protocol 4.5

Testing Acuity for Patients With

Low Vision

1.If the patient is unable to resolve the largest optotype on the distance acuity chart from the standard testing distance, ask the patient to stand or sit 10 feet from the well-illuminated test chart. A projected chart is less desirable to use in this situation than a printed wall chart. A lowvision test chart, if available, is preferable for these patients.

2.Occlude the eye not being tested.

3.Repeatedly halve the testing distance (up to 2.5 feet) and retest the distance visual acuity at each stage until the patient successfully identifies half the optotypes on a line.

continued

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*1. Seat the infant on an adult's lap, so that the infant is comfortable.

2.Select a small toy or other attention-attracting object that stimulates sight only; do not use a sound-producing object. Hold the object about

1to 2 feet from the infant's face and move it horizontally to either side.

3.Watch die infant's eyes for fixation and following movements. Infants will respond with better following movements for objects moved from temporal to nasal field; this preference decreases after an infant is approximately 6 months old.

4. If no visual response is noted, repeat the test in a darkened room using a small, bright penlight and watch the infant's fixation and following movements to the penlight. If the light is too bright, the infant will

. ••" avoid it and look away.

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• • • •

Clinical Protocol 4.7

2.Place the polarizing glasses on the patient. If glasses are usually worn, place the polarizing glasses over them.

•

J.Hold the fly image facing the patient approximately 16 inches (40 cm) away, with the surface of the page parallel to the surface of the glasses.

4.Ask the patient to touch the wings or to pick up the flyReassure young children who may respond witli surprise or fright if the image appears too real to them. If the image is perceived as having height, the patient will attempt to touch the wings as if they were above the page surface when instructed by the examiner.

5.If the fly test is positive, show the patient the three rows of animal figures. Ask which figure in each row is coming forward, is above the page, or can be picked up. You can vary these directions as you become more familiar with the test and with judging patients' responses.

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