Encyclopedia of Health and Medicine

V–X
visual acuity The ability to see objects clearly
and sharply. Visual acuity assesses central vision
and represents the function of the CORNEA, iris
(pupil size), LENS, and RETINA. The SNELLEN CHART,
which presents lines of letters of diminishing size,
is the standard measure of visual acuity. Environmental
factors that influence visual acuity include
lighting and contrast. The most common disturbances
of visual acuity are REFRACTIVE ERRORS such
as MYOPIA (nearsightedness), HYPEROPIA (farsightedness),
and ASTIGMATISM (blurred vision).
PRESBYOPIA, age-related changes in the cornea’s
FLEXIBILITY, affects near-vision visual acuity.
See also NIGHT BLINDNESS; VISION IMPAIRMENT.
visual field The total area or scope of vision. EYE
care specialists map the visual field by measuring
the boundaries of peripheral vision in degrees from
the point of central vision. A normal field of vision
is 135 degrees vertically (60 degrees up and 75
degrees down) and 160 degrees horizontally (100
degrees outward and 60 degrees inward). Everyone
has a blind spot of about 10 degrees in the direct
center of vision, the point at which the OPTIC NERVE
enters the RETINA (the optic disk). The optic disk
contains no rods or cones. Binocular vision (the
ability to see with both eyes) compensates for each
eye’s blind spot with overlapping visual fields for
each eye. People who have monocular vision (the
ability to see only through one eye) learn to accommodate
for the blind spot by frequently moving the
eye to scan the field of vision.
There are several methods for measuring visual
field. The simplest though least precise is for the
eye care specialist to sit across from the person
and, with the person looking at a fixed point the
eye care specialist slowly moves a hand or an
object such as a pen. The person tells the point at
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which he or she can see the object. The eye care
specialist may repeat this procedure several times
for each eye, measuring peripheral vision from
each side, above, and below. Other methods may
use computerized flashing lights with the person
looking at a fixed point (target) within a contained
dome. The person presses a button for each
light he or she sees, and the eye care specialist creates
a map of each eye’s visual field that allows
calculation of visual field percentages.
CONDITIONS THAT CAN AFFECT THE VISUAL FIELD
AGE-RELATED MACULAR
DEGENERATION (ARMD)
GLAUCOMA
HYPERTENSION
RETINAL DETACHMENT
RETINOPATHY
STROKE
DIABETES
GRAVES’S OPHTHALMOPATHY
MULTIPLE SCLEROSIS
RETINITIS PIGMENTOSA
SCLERITIS
TRAUMA TO THE EYE
tumors of the eye or BRAIN
See also AMSLER GRID; REFRACTION TEST; SCOTOMA;
SNELLEN CHART; VISUAL ACUITY.
vision health Personal care for the eyes to protect
the eyes and preserve vision. The two most
important elements of vision health are EYE protection
and regular ophthalmic examinations.
Protective Eyewear
Most injuries to the eyes are preventable by wearing
appropriate protective eyewear, which ranges
from sunglasses to protect the eyes from sunburn
to specialized eyewear for specific needs. Such
needs might include
• ultraviolet exposure (sunlight, welding)
• sports (protection from contact; protection
when swimming or diving)