Encyclopedia of Health and Medicine

80 The Eyes
color deficiency A VISION IMPAIRMENT in which
the ability to distinguish certain, and rarely all,
colors is impaired. Color deficiency represents a
shortage of normal cones, the specialized cells on
the RETINA that detect color. Cones contain photosensitive
chemicals that react to red, green, or
blue. The most common presentation of color
deficiency, accounting for about 98 percent of
color deficiency, is red/green deficiency, in which
the person cannot distinguish red and green. A
small percentage of people cannot distinguish blue
and yellow. Rarely, a person sees only in shades of
gray.
Color perceptions occur when lightwaves of
certain frequencies (lengths) activate the photochemicals
in cones that are sensitive to the frequency.
The BRAIN interprets the varying
intensities and blends of the photochemical
responses. Color deficiency occurs when the cones
that perceive one of the three primary colors (red,
green, blue) do not function properly.
The most common test for color vision and color
deficiency is a series of disks that contain dots of
color in random patterns with a structured pattern
of differing color within the field. The structured
pattern may be a number (most commonly) or an
object. There is no treatment to compensate for
color deficiency. People who are color-deficient
learn to accommodate the deficiency through
mechanisms such as memorizing the locations of
colored objects (such as the sequence of lights in a
traffic signal) and by making adaptations in their
personal environments. A person may have friends
or family members sort clothing by color, for example,
and label the color groups. Some people who
have mild color deficiency experience benefit from
devices such as colored glasses and colored contact
lenses that filter the lightwaves that enter the EYE. A
yellow tint may improve blue-deficient color
vision, for example.
Most color deficiency is an X-linked genetic
MUTATION, affecting about 8 percent of men and 1 ⁄2
percent of women. Color deficiency may also
develop with AGE-RELATED MACULAR DEGENERATION
(ARMD), RETINOPATHY, neurologic disorders such as
MULTIPLE SCLEROSIS, and HEAVY-METAL POISONING such
as lead or mercury. Antimalarial drugs can cause
permanent changes in the RETINA that affect color
vision; the ERECTILE DYSFUNCTION medication sildenafil
(Viagra) can temporarily intensify the perception
of blue.
See also VISION HEALTH; VISUAL ACUITY.
conjunctivitis An INFLAMMATION of the conjunctiva,
or mucous tissue that lines the inside of the
eyelids. There are many causes for conjunctivitis,
commonly called pink EYE, including INFECTION
(bacterial, viral, or fungal) and contact contamination
such as due to pollen or substances in the air
or on the fingers that irritate the tissues. Infectious
conjunctivitis is highly contagious and very common,
especially in children. Symptoms include
• red, swollen conjunctiva and sclera (inner eyelids
and the “white” of the eye)
• itchy or scratchy sensation
• thick, yellowish discharge that crusts
• PHOTOPHOBIA (sensitivity to light)
The doctor can usually diagnose conjunctivitis
from its appearance. Typical treatment is application
of an antibiotic medication in ophthalmic
preparation (drops or ointment). Most conjunctivitis
dramatically improves with 48 hours of initiating
treatment, though symptoms may resolve
gradually over 10 to 14 days, and does not require
further medical attention. The doctor may culture
the discharge when there is reason to suspect
CHLAMYDIA or GONNORHEA is the cause, or when
symptoms do not improve with treatment. Warm,
moist compresses help relieve discomfort and clear
away the discharge. Frequent HAND WASHING helps
prevent spreading the infection. Untreated conjunctivitis,
particularly when chlamydia or gonorrhea
is the infectious agent, can cause permanent
damage to the CORNEA, which results in VISUAL
IMPAIRMENT or blindness.
See also ALLERGIC CONJUNCTIVITIS; ANTIBIOTIC MED-
ICATIONS; BACTERIA; FUNGUS; SEXUALLY TRANSMITTED
DISEASE (STD) PREVENTION; VIRUS.
cornea The transparent portion of the sclera, the
EYE’s outer layer. The cornea functions as a window
to allow light to enter the eye and is the first
point of refraction (bending lightwaves to focus
them on the RETINA). Irregularities in the surface of
the cornea can distort refraction, resulting in