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Overview
Color vision deficiency results from a malfunction
or absence of cone cells in the retina. The degree
of deficiency ranges from a slight difficulty
distinguishing among different shades of one color
to the inability to see any color. There are
millions of photoreceptor cells in the human eye’s
retina that contain photosensitive pigments. There
are two types of these specialized cells: cones
and rods. Each retina contains approximately 6
million cones and over 100 million rods.
Incidence and Prevalence
Congenital color vision deficiency overwhelmingly
affects more men than women. About 10 million men
in the United States (7% of the male population)
have a color vision deficiency compared to 0.4% of
women. Caucasian men experience the highest
prevalence of this disorder.
Perceiving Color
Objects we see reflect different wavelengths of
light and give rise to the perception of color.
Seeing color is a function of cone cells, which
are stimulated by and responsive to wavelengths
within the visible spectrum. There are three
populations of cone cells, each having its own
sensitivity range: blue, green, and red. When they
are stimulated to different degrees, a match can
be made to any color in the visible spectrum. The
human eye is capable of matching over 7 million
colors. In normal color perception, all three
populations of cones are present (trichromacy) and
function normally.
Cone cells
are responsible for color vision and function only
in moderate illumination and daylight or bright
light. Each cone contains photopigments that make
it sensitive to red, green, and blue wavelengths
of light, but is most sensitive to one color of
light. People with color vision disorders usually
have a deficiency or absence of cone cells
sensitive to red or green wavelengths. The
inability to see blue light is rare. People who
perceive colors normally are known as trichromats,
because they are using three populations of cones,
one sensitive to red, one to blue, and one to
green, to match any color in the environment. Cone
cells also provide sharp visual perception and
high resolution. Cone cells are concentrated in
the fovea (see eye anatomy) and become more sparse
in the area immediately surrounding the fovea.
Rod cells
function in the dark or in very dim light. When
they are completely dark adapted, which takes
about 30 minutes for humans, rods are actually
more sensitive to light than cones. They contain
only one photopigment and primarily detect shades
of gray, thus color is perceived as black, white,
and gray in dim lighting. Rods are located in the
peripheral retina, not in the fovea. They cannot
produce sharp visual acuity.
Causes and Risk Factors
Color deficiency is usually a hereditary
condition. The trait is passed on the X
chromosome, and because males only have one X
chromosome, it is easier for them to inherit color
vision deficiency. A mother who carries one normal
X chromosome and one X chromosome with a mutation
of red and/or green pigments is not affected, but
her son has a 50% chance of having a color vision
deficit. Fathers cannot pass it to their sons,
because they supply only a Y chromosome to the
genetic mix, but they can pass the gene to their
daughters who carry the gene but do not manifest a
color deficit.
For a woman to inherit a color vision deficiency,
she must have a mother who is a carrier and a
father who is color deficient. The odds of this
occurring are very slim.
Color vision deficiency can be acquired not only
as a result of diseases or conditions of the
retina, optic nerve, or more posterior visual
pathways in the brain, but also toxins and certain
drugs. Macular degeneration, optic neuritis, and
strokes that affect certain areas of the occipital
lobe, for example, can affect color perception.
Head injuries, systemic diseases that damage
nerves (e.g., multiple sclerosis), heavy metal
poisoning, and certain medications (e.g.,
antimalarials) also can affect color vision
adversely. Unlike congenital color vision defects,
acquired defects often affect visual acuity, are
asymmetric from eye to eye, and may change as the
disease changes.
Signs and Symptoms
Children with achromatopsia demonstrate poor
vision and an inability to tolerate bright light
at a very young age. A child who is having trouble
in school should be evaluated for vision problems,
including color vision deficiency. Milder forms of
color vision deficiency are more subtle, and many
people never realize that they have a problem
seeing colors. Routine color vision testing of all
children will identify those having a color vision
problem at an early age. Anyone who notices a
change in color perception should see an eye care
specialist immediately.
Diagnosis
Inherited color vision deficiency is usually
diagnosed in early childhood using simple
screening tests. The Hardy-Rand-Rittler (H-R-R)
and
Ishihara Color Plates
are used to evaluate the type and degree of color
deficiency. In these tests, the person is asked to
identify the colored shapes or numbers that lie
within a jumble of dots and vary in color and
intensity. The physician detects and categorizes
the deficiency based on the person’s responses.
The D-15 and the Farnsworth-Munsell 100-hue
disk-matching test evaluate the ability to
identify gradations of color by placing discs in
order.
Treatment
Congenital (inherited) color vision deficiency
cannot be treated or cured. Measures can be taken,
however, to compensate for it. Some people develop
their own system of recognizing colors by their
brightness or location, such as the positions of
red, yellow, and green in a traffic light.
Specialized glasses and tinted lenses that
“normalize” colors are also available. Complete
achromats can use strong magnifiers to read and
perform near tasks and use sunglasses to reduce
light sensitivity.
Acquired color vision deficiency requires
treatment of the underlying cause. In many cases,
normal color perception returns when the
underlying condition has been resolved.
Prevention
Inherited color vision deficiency cannot be
prevented. |
