VISION TESTS
VITAMIN
connections between the eyes and brain,
or to the visual area of the brain, can
cause loss of peripheral vision and may
be a result of
embolism
,
ischaemia
,
tumour, inflammation, or injury.
vision tests
The part of an
eye examina-
tion
that determines whether there is
any reduction in the ability to see. Most
vision tests (for example the
Snellen
chart
)
are tests of
visual acuity
.
VISION TESTS
Adjustable arm
Test lens
V
In this type of test, a device called a
phoropter is used to hold different lenses
in front of each eye. The lenses in the
phoropter are changed until the letters
near the bottom of the Snellen chart can
be read. Tests of
visual field
may also be
performed to assess disorders of the
eye and the nervous system. Refraction
tests can detect
fyspermetropia, myopia,
or
astigmatism;
the effect of lenses on
movements of light reflected from the
eye is observed to calculate the correc-
tive
glasses
or
contact lenses
needed. If
presbyopia
is suspected, close-reading
tests are used to assess
accommodation
.
visual acuity
Sharpness of central
vision.
Refractive errors, such as
myopia
,
hyper-
metropia
and
astigmatism
, are the most
common causes of poor visual acuity.
Poor visual acuity for near objects occurs
in
presbyopia.
visual field
The total area in which visual
perception is possible while a person is
looking straight ahead. The visual fields
normally extend outwards over an angle
of about 90 degrees on either side of
the midline of the face, but are more re-
stricted above and below, especially if
the eyes are deep-set or the eyebrows
are prominent. The visual fields of the 2
eyes overlap to a large extent, giving
binocular vision. Partial loss of the visual
field may occur in
glaucoma
or
stroke
.
vital sign
An indication that a person is
still alive. Vital signs include chest
movements caused by breathing, the
presence of a pulse, and the constric-
tion of the
pupil
of the eye when it is
exposed to a bright light.
vitamin
Any of a group of complex org-
anic substances that are essential in
small amounts for the normal function-
ing of the body. There are 13 vitamins:
A, C, D, E, K, B
12
, and 7 grouped under
the
vitamin B complex
. Apart from the
B vitamin niacin and vitamin D, which
the body can synthesize itself, vitamins
must be obtained from the diet. A var-
ied diet is likely to contain adequate
amounts of all the vitamins, but
vitamin
supplements
may be helpful for some
people, such as young children, women
who are pregnant or breast-feeding, or
those taking drugs that interfere with
vitamin function.
Vitamins can be categorized as fat-
soluble or water-soluble.
The fat-soluble vitamins (A, D, E, and K)
are absorbed with fats from the intestine
into the bloodstream and then stored
in fatty tissue (mainly in the liver). Body
reserves of some of these vitamins last
for several years, so a daily intake is not
usually necessary. Deficiency of a fat-
soluble vitamin is usually due to a
disorder in which intestinal absorption
of fats is impaired (see
malabsorption
)
or to a prolonged poor diet.
Vitamin C, B
12
, and those of the B
complex are water-soluble. Vitamin C
and B complex vitamins can be stored
in the body in only limited amounts and
are excreted in the urine if taken in great-
er amounts than needed. A regular intake
is therefore essential to prevent defi-
ciency. However, vitamin B
12
is stored in
the liver; these stores may last for years.
The role of all the vitamins in the body
is not fully understood. Most vitamins
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