COLOUR VISION DEFICIENCY
C
directly.
Colour vision
is,
therefore,
poor at the periphery of vision.
W hen light hits a cone, it causes a
structural change in the pigment w ith-
in the cone, w hich in turn causes the
cone to emit an electrical signal. This
signal passes to the brain via the
optic
nerve
.
Colour
perception
requires
a
minim um level of light; below this
level, everything is perceived only by
the rods and is seen as various shades
of grey (See also
colour vision deficiency
;
eye
;
perception
;
vision
. )
colour vision deficiency
Any abnormality in
colour vision
that
causes a person to have difficulty in
distinguishing between certain colours.
TYPES
The
most
common
type
of colour
vision deficiency is the reduced ability
to discriminate between red and green.
Most cases of red-green colour vision
deficiency are the result of defects in
the light-sensitive cells in the
retina
.
These defects are usually inherited, and
tend to be sex-linked (see
genetic disor-
ders
) ;
the
majority
of sufferers
are
male, w hile females are unaffected, but
they can pass on the disorder to their
children. Occasionally defects may be
acquired as a result of diseases of the
retina or the optic nerve, or they are
caused by injury. There are two forms
of red-green deficiency. A person with
a severe green deficiency has difficulty
in
distinguishing
between
oranges,
greens, browns, and pale reds. A severe
red deficiency causes all shades of red
to appear dull.
A much rarer colour vision deficiency
exists in w hich blue cannot be distin-
guished. This condition may be inher-
ited, or it may be due to degeneration
of the retina or the optic nerve. M ono-
chromatism
(the
total
absence
of
colour vision) also exists, but this defi-
ciency is very rare.
Colpermin
A brand name for peppermint oil. This
preparation is used to relieve the pain
that results
from
muscle spasms in
irritable bowel syndrome
and
diverticular
disease
.
colposcopy
Viewing of the
cervix
(the neck of the
uterus) and
vagina
using a magnifying
instrument called a colposcope. Col-
poscopy is carried out to detect areas of
precancerous tissue (see
dysplasia
)
or
of early cervical cancer (see
cervix, can-
cer of
) .
Removal of tissue samples (see
biopsy
)
or treatment to remove any
abnormal areas can be performed dur-
ing colposcopy, and this is usually done
under local anaesthetic.
coma
A state of unconsciousness and unres-
ponsiveness to
external stimuli
(for
example, pinching) or internal stimuli
(such as a full bladder).
CAUSES
Coma results from disturbance or dam-
age to areas of the
brain
involved in
conscious activity or maintenance of
consciousness - in particular, parts of
the
cerebrum
,
upper parts of the
brain-
stem
,
and central regions of the brain,
especially the
limbic system
.
Conditions that can produce coma
include
severe
head injury
;
disorders
such as
stroke
or
cardiac arrest
,
in w hich
part or all of the brain tissue is deprived
of blood; or infectious disorders that
affect the brain, such as
meningitis
and
encephalitis
.
In
addition,
excessively
high or low blood levels of certain sub-
stances
may
result
in
coma;
for
example, a person with
diabetes mellitus
may become comatose if his or her
blood level of glucose (sugar) rises or
falls to an abnormal degree.
SYMPTOMS
There are varying depths of coma. In
less severe forms, the affected person
may make small movements and res-
pond to
certain stimuli.
In a deep
coma, the person does not make any
movements or respond to any stimu-
lus.
However,
even people
in
deep
comas
may
show
some
automatic
responses,
for
example
breathing
unaided and blinking.
If the
lower
brainstem is damaged, however, vital
functions are impaired, and artificial
ventilation and maintenance of the cir-
culation are required.
OUTLOOK
If brain damage is m inor and rever-
sible,
the
person
may
make
a full
recovery, but deep coma due to severe
trauma may result in long-term neuro-
logical
problems
such
as
muscle
weakness or changes in behaviour. A
person in a deep coma (a
persistent veg-
etative state
)
may be kept alive for years
provided the brainstem is still func-
tioning. Complete and irreversible loss
of brainstem function leads to
brain
death
(the permanent cessation of all
brain functions).
combination drug
A preparation containing more than
one active substance.
comedo
Another name for a
blackhead.
commensal
A
bacterium
or other organism that nor-
mally lives in or on the body without
either harming or benefiting its host.
comminuted fracture
A type of
fracture
in w hich the bone
shatters into more than two pieces. A
severe blow, such as an impact occur-
ring in a car accident, may result in a
comminuted fracture.
commode
A portable chair that contains a remov-
able toilet bowl in its seat.
common cold
See
cold, common.
communicable disease
Any disease due to a microorganism or
parasite that can be transmitted from
one person to another. (See also
conta-
gious; infectious disease.)
compartment syndrome
A painful
cramp
due to compression of
a group of muscles w ithin a confined
space. It may occur when muscles are
enlarged due to intensive sports train-
ing or to an injury such as
shin splints.
Cramps induced by
exercise usually
disappear when the exercise is stopped.
Severe cases may require
fasciotomy
to
improve blood flow and prevent devel-
opment of a permanent
contracture.
compensation
The adjustment made by an organ to
make up for changes in body function
or structure. An example of compensa-
tion is the increased size of one kidney
when the other has been removed.
complement
A collection of
proteins
in the
plasma
(the fluid part of
blood
) that helps to
destroy foreign cells and is an im port-
ant part of the
immune system
.
complementary medicine
A group of therapies, often described as
“alternative” , that are now increasingly
used to complement or to act as an
alternative to conventional medicine.
188
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