|
Visual Acuity (VA):
Minimum Legible
PURPOSE To measure the clarity of vision or the ability of the visual system to resolve detail. A patient’s visual acuity depends on the accuracy of the retinal focus, the integrity of the eye's neural elements, and the interpretive faculty of the brain.
INDICATIONS
Visual acuity should be done on all patients as the first procedure following the case history.
Note: It is important to understand that there are a number of ways by which a patient’s visual function may be measured. The procedure described below, while the most common clinically, may not be applicable in certain circumstances (eg, infants, low vision patients, and illiterate patients).
EQUIPMENT
·Projector with visual acuity slide or wall mounted acuity chart.
·Nearpoint visual acuity card.
·Occluder.
·Lamp.
SET-UP
·The patient wears his habitual correction for the distance being tested.
When the examiner wants to measure the patient’s VA both with (cc) and without (sc) correction, the acuity should be measured without correction first.
·The patient holds the occluder.
·An acuity chart, with lines from 20/50 to 20/15 exposed, is shown.
STEP-BY-STEP PROCEDURE
1. Always observe the patient, not the chart. The chart should be memorized by the examiner.
2. Tell the patient to cover his left eye and not to squint.
3. Instruct the patient to read the smallest line of letters he can. If using an acuity slide and the patient is unable to read 20/50, reposition the chart so the 20/60 line becomes the lowest line.
4. Encourage the patient to read the letters on the next smaller line, even if he has to guess. Stop the patient when more than half the letters on a line have been missed.
5. Have the patient cover the right eye and repeat steps 2, 3, and 4.
6. Sometimes the patient will be unable to read even the largest letter on the chart. In this event, have the patient walk toward the chart until he can just make out the largest letter (usually a big E). Note the distance at which this occurs.
7. If the patient cannot see the letters at any distance, initiate the following testing sequence, stopping at the level at which the patient can accurately respond.
a. Counting fingers (CF): At a distance of approximately one foot, expose a selected number of fingers. Ask the patient to tell you how many fingers you are holding . Increase the distance from the patient until his responses are no longer accurate. Move back toward the patient until he can reliably report the number of fingers presented.
b. Hand motion (HM): Using a moving hand as the target, ask the patient if he can see the hand moving. Begin at approximately one foot and increase the distance until the patient reports he no longer detects the motion. Then move back toward the patient until he detects the motion once again.
c. Light projection (LProj): Holding a penlight or transilluminator at a distance of approximately 20 inches from the patient, position the light in different areas of the patient’s visual field. Each time ask the patient to point at the light and note the areas of the field in which the patient has vision.
d. Light perception (LP): Direct a penlight or transilluminator at the patient and ask if he can see the light.
8. Now test near visual acuity. Repeat steps 1 through 5 at near using the following set :
a. Provide high illumination on the near point card. The light source should be either above or slightly behind the patient. Care should be taken that the light is not directed at the patient’s eyes.
b. Instruct the patient to hold the card at the appropriate distance.
·16 in (40 cm) for a reduced Snellen Acuity Card
·14 in for a Jaeger Acuity Card
RECORDING
·Write Vcc, or VAcc: cc, means “with correction.” If the VA is taken without correction, use sc, instead of cc. If the patient’s acuity is taken through contact lenses use CL.
·Record each eye separately. Use the abbreviation of OD for the right eye, OS for the left eye, and OU for the two eyes together.
·Record the patient’s distance acuity first, followed by the near acuity.
·For each eye, record the Snellen fraction or print size for smallest (lowest) line in which more than half the letters were correctly identified. (See the additional techniques described in this section if the patient could not see any of the letters at the 20-foot testing distance.)
·If the patient read additional letters on the next line, follow the fraction or print size with a + (plus) sign and the number of letters read.
·if letters were missed, follow the fraction or print size with a - (minus) sign and the number of letters missed.
·When recording, + and - signs may be used simultaneously.
·Record the quality of the patient’s response if it was abnormal, eg, slow.
·If the patient had to walk toward the chart to discern the largest letter. Record the distance at which he could first read the letter as the numerator and the letter size (usually 400) as the denominator.
·If the patient’s distance vision is so poor that a Snellen acuity could not be obtained, measure using the sequence of techniques listed below and record the acuity that applies:
a. Counting fingers (CF) @ (distance)
b. Hand motion (HM) @ (distance)
c. Light projection (LProj). Record the areas of the visual field for which this was true.
d. Light perception (LP)
e. No light perception (NLP)
EXAMPLES
·VAcc OD 20/40+1， 20/30@16″
OS 20/25-2 20/30-2/+2@16″
·VAcc OD 20/25-2， 20/25@16″
OS 20/30+2， 20/40+2@1 6″(read very slowly)
·Vsc OD 81400， J-16@14″
OS 20/200， J-3-1@14″
·Vcc OD FC @ 4 ft
0S LProi all quadrants
EXPECTED FINDINGS
·A visual acuity of 20/20 or better is considered normal．
·The difference between the two eyes should be no greater than one line．
·Any abnormality in visual acuity must be addressed in the course of the examination and explained in the problem and plan list． |
