Bma Illustrated Medical Dictionary

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 testsThe part of an eye examina-

tionthat determines whether there is

any reduction in the ability to see. Most

vision tests (for example the Snellen

chart) are tests of visual acuity.

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 fieldmay also be

performed to assess disorders of the

eye and the nervous system. Refraction

tests can detect hypermetropia, myopia,

or astigmatism; the effect of lenses on

movements of light reflected from the

eye is observed to calculate the correc-

tive glassesor contact lensesneeded. If

presbyopiais suspected, close-reading

tests are used to assessaccommodation.

visual acuitySharpness of central vision.

Refractive errors, such as myopia, hyper-

metropiaand astigmatism, are the most

common causes of poor visual acuity.

Poor visual acuity for near objects occurs

in presbyopia.

visual fieldThe total area in which visual

perception is possible while a person is

looking straight ahead. The visual fields

VISION TESTS

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 glaucomaor stroke.

vital signAn 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 pupilof the eye when it is

exposed to a bright light.

vitaminAny 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

supplementsmay 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

VISION TESTS VITAMIN

V

Adjustable arm Phoropter

Test lens