Download Test for visual Acuity, Color vision , and Visual Field

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Visual Acuity and Visual Field
Visual Acuity
Def: Def:
•It is the ability of the eye to see the fine details of the object
Visual Acuity (cont.)
◊ Principle of measurement of VA:
-The eye can discriminates between 2 points when the 2
points stimulate 2 cones separated by unstimulated one.
- In this condition the 2 points form a visual angle of about 1
min
Visual Acuity (cont.)
Clinical Method for Stating Visual Acuity:
There are many charts that are used in testing VA
a. Snellen s letter charts → formed of english letters b. Landolts C charts → formed of broken circle or C c. Emarah arabic chart → formed of arabic letters
Visual Acuity (cont.)
◊ How to test:
-Patient sit at 6 meters (20 feet) ? -Good illumination of the chart
-Testing is done without aid or glass
-Start testing from the big rows then downward until he cannot see
the chart.
◊ Clinical expression of VA:
Visual acuity is a mathematical fraction that expresses the ratio of
two distances, or the ratio of one’s visual acuity to that of a person
with normal visual acuity.
Distance at which the patient sees the chart
VA=
Distance at which the normal person sees the chart
Visual Acuity (cont.)
◊ Clinical expression of VA:
If a person has a visual acuity of 6/12,
that person is said to see detail from
6 meter away the same as a person
with normal eyesight would see it
from 12 feet away.
It is possible to have vision superior
to 6/6: the maximum acuity of the
human eye without visual aids (such
as binoculars) is generally thought to
be around 6/3.
Visual Acuity (cont.)
◊ Other tests:
-if the patient cannot see the chart , approach him 1
meter up to 1 meter distance if he cannot see the chart ,
shift him to other tests
Name
Abbreviation
Definition
Counting Fingers
CF
Ability to count fingers at
a given distance.
Hand Motion
HM
Ability to distinguish a
hand if it is moving or not
in front of the patient's
face.
Light Perception
LP
Ability to distinguish if the
eye can perceive any light.
No Light Perception
NLP
Inability to see any light.
Total blindness.
Visual Acuity (cont.)
◊ Factors affecting VA:
1. Degree of illumination of the chart → bad illumination impair VA 2. Age: VA decreases in old age. 3. Spherical and chromatic aberrations caused by dilated pupil → impair VA . 4. Fovea centralis: it is the most sensitive point in the retina having the maximal visual acuity. 5. Errors of refractions e.g. myopia , hypermetropia and astigmatism → decrease VA
Colour Vision Vision
Def It is the ability of the eye to see the different types and characters of colors Types and characters of colors Types: — 1ry → red green and blue — Complementary colors → 2 color when mixed together they form white color Characters of colors: Hue (wave length) e.g. red color wave length is 750 nm Brightness of color is the amount of light in color Saturation of color is the purity of color
Spectrum of Visible Colors
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Mechanism of Color Vision Vision
Trichromatic Theory (Young Helmholtz theory) There are 3 types of cones; each is containing one of three pigments according to their sensitivity to a certain light wavelength: S
S ­­ cones most sensitive to short wavelength (peak cones most sensitive absorption at 420nm→perception of blue color M­cones most sensitive to medium wavelength → perception of green L
L ­­ cones most sensitive to long wavelength (560nm→ cones most sensitive perception of red — Equal stimulation of these cones →white color — Unequal stimulation→ another color
Color Blindness
— Sex­linked recessive disorder — Abnormal colour gene → inability to see one or more colours — Common in males (9%) than females (0.4%) — Protanopia (red blindness) — Deuteranopia (green blindness) — Tritanopia (blue blindness)
Test For Color Vision
Ishihara Test Chart
Normal person
reads “74,” but the redgreen color-blind
person reads “21.
Normal person reads “42.”
Red-blind person (protanope)
reads 2
green-blind person
(deuteranope) reads “4.”
Visual Field
Def.,
It is the part of external world
that can be seen by non
moving eye.
Boundaries:
Theoretically, it should be
circular, but actually it is cut off
medially by the nose and
superiorly by the roof of the
orbit .
- Upward → 50 degrees
- Downward → 80 degrees
-Nasally → 60 degrees
Visual Field
Mapping and Measurement of Visual Field
1) Confrontation test:
-Rough clinical test
-Give rough idea about the boundaries of visual field
-Visual field of patient is compared with that of the physician
Visual Field
Mapping and Measurement of Visual Field
2) Perimetery:
-Accurate test that map the field of vision.
Perimeter
Visual Field
Lesions of Visual Field:
1) Scotoma:
-loss of part of visual field
due to lesion in retina
2) Anopia:
Complete loss of visual field
3) Hemianopia:
Loss of half of the visual
field
Visual Field
Lesions of Visual pathway:
A) Ipsilateral anopia:
-loss of visual field on the same side of
lesion due to optic nerve lesion
B) Bitemporal heminopia:
-loss of visual fields on the temporal
sides of both eyes due to optic chisama
central lesion
C) Contralateral homonymous
heminopia:
-loss of visual fields on the
corresponding sides of both eyes due
to optic radiation lesion
D) Contralateral homonymous
heminopia with macular sparing: sparing:
due to lesion in cerebral cortex
S
K
N
A
H
T
Abdel Aziz Hussein, Mansoura University, Mansoura, Egypt