Visual Acuity Tests
Norm of Visual Acuity Tests
Distance Vision |
|
Snellen Chart |
|
| Adults | 20/20 (near vision, 14/14) |
| Children | |
| Infants | 3/60 or better |
| 1–4 years | 20/40 or better |
| 4–7 years | 20/30 or better |
| >7 years | 20/20 |
Allen Cards |
|
| 3 years | 15/30 |
| 4 years | 20/30 |
| Infant testing for optokinetic nystagmus | Present at 2 months of age |
| Strabismus | Absent |
| Stereopsis | Present |
| Color vision | Present bilaterally |
| Peripheral vision | Intact bilaterally |
Usage of Visual Acuity Tests
Part of routine ophthalmologic examination; community health screening for vision testing.
Description of Visual Acuity Tests
Visual acuity testing involves testing a client's ability to read a standard Snellen chart of symbols (usually letters) at a specified distance to test distance vision, and a Jaeger card to test near vision. A Snellen chart consists of numbered rows of letters that progressively decrease in size from top to bottom. A Jaeger card contains text in progressively decreasing size. For young children and infants, substitute testing in place of the Snellen chart is performed as described below. Children are tested for distance vision, nystagmus, strabismus, stereopsis, color vision, and peripheral vision. The tests may be performed with and without current corrective lenses. For unsatisfactory tests, they will be repeated with new combinations of corrective lenses, until the best possible vision correction is obtained.
Professional Considerations of Visual Acuity Tests
Consent form NOT required.
Preparation
- Obtain charts, a handheld eye-occluder wand, an eye patch for children, and glasses for testing for stereopsis.
Procedure
- The client is positioned sitting 20 feet away from the Snellen chart.
- Each eye is tested separately as follows:
- a. The eye not being tested is occluded.
- b. The client is instructed to read the line closest to the bottom of the chart that he or she can read and then to attempt to read one line lower.
- c. The fractionated visual acuity is recorded as follows: The distance in feet the client is positioned away from the chart is the numerator (that is, 20), and the number of the lowest line read correctly is the denominator. If the client can read one symbol of a line farther down, the results are recorded as:
For example, “20/100 + 1” means the client, at a distance of 20 feet, read the line at which a normal eye could read at 100 feet, plus 1 symbol on the line below. A passing score for a line requires that the client read the entire line with no more than one error.
- Near-vision testing:
- a. The client is instructed to read a Jaeger card at normal reading distance. The numerator score is the distance at which the card was read, and the denominator score is the line number of the smallest-sized letters read correctly.
- Testing in young children:
- a. For young children, the “E” chart is substituted for the Snellen chart. The child must indicate which direction the letter E is pointing. Pictures of familiar objects may be placed above, below, left, and right of the chart for the child to use in identification of direction. The test is performed for each eye separately.
- b. Other substitutes for young children are Allen cards, which contain pictures of objects familiar to children. The numerator score is the distance at which three of the objects can be recognized by the child, and the denominator is 30. The eyes are tested separately.
- c. Strabismus testing: A light is shined into the child's eyes from 16 inches away, and the bilateral reflection of the light in the eyes is observed. Strabismus causes an off-center reflection in one eye. A second test involves occluding one eye at a time, as the child gazes at an object 1 foot away, and observing for inward or outward movement of the uncovered eye, which indicates strabismus.
- d. Stereopsis testing: Wearing stereoscopic glasses, the child is shown a stereo picture and asked if the object is on the page or in front of the page. With intact stereopsis, the child should be able to see a three-dimensional object that appears to be in front of the page. Without stereopsis, the object appears flat on the page.
- e. Color-vision testing: The child is asked to identify objects made of specifically patterned colored dots fused into gray dots.
- f. Peripheral-vision testing: As the child gazes ahead, he or she is asked to indicate on which side of the visual field an object is appearing.
- Testing of infants:
- a. Infants can be tested for optokinetic nystagmus by passing a bright object back and forth in front of the eyes and observing whether nystagmus occurs.
- b. The infant is also assessed for the ability to follow a lighted object moved in front of the visual field.
- c. Peripheral-vision testing: As the child is distracted, a bright object is moved into the peripheral visual field, and the child's response to the object is noted.
Postprocedure Care
- None.
Client and Family Teaching
- Young children may cooperate best if testing is practiced at home before this test.
- Eye exercises may be prescribed for very young children with strabismus. Simple exercises that can be performed at home involve using small pictures pasted on Popsicle sticks to strengthen the muscle. The child holds the stick at arm's length in front of the visual field. While focusing on the picture, he or she slowly and steadily moves the stick in toward the eyes and attempts to maintain single vision. When double vision occurs, the child restarts the exercise.
Factors That Affect Results
- The client must be able to follow directions.
Other Data
- Client questionnaires are also used to assess the client's subjective functional impact of visual impairments. Three that are often used include the Activities of Daily Vision Scale, the National Eye Institute Visual Function Questionnaire, and the Visual Function Index.
