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Visual Field Testing
During a routine eye exam,
some eye doctors may want to
determine through visual
field testing the full
horizontal and vertical
range of what you are able
to see peripherally. This is
commonly referred to as
"side vision."
Visual field tests assess
the potential presence of
blind spots (scotomas),
which could indicate eye
diseases. A blind spot in
the field of vision can be
linked to a variety of
specific eye diseases,
depending on the size and
shape of the scotoma.
Many eye and brain disorders
can cause visual field
abnormalities. For example,
optic nerve damage caused by
glaucoma creates a very
specific visual field
defect. Other vision
problems associated with
blind spots developing
within the visual field
include optic nerve damage
(optic neuropathy) from
disease or toxic exposure or
damage to the
light-sensitive inner lining
of the eye (retina).
Brain abnormalities such as
those caused by strokes or
tumors can affect the visual
field. In fact, the location
of the stroke or tumor in
the brain can frequently be
determined by the size,
shape, and site of the
visual field defect.
Types of Visual Field Tests
Confrontation visual
field testing typically
is used as a screening
visual field test. One eye
is covered while the other
eye fixates on a target
object, such as the doctor's
open eye, while the doctor
stands or sits directly in
front of you. You then are
asked to describe what is
seen on the far edges or
periphery of your field of
view. As an example, your
eye doctor may hold up
different numbers of fingers
within your peripheral field
of view and ask how many can
be seen while you continue
to fixate on the doctor's
eye.
If an eye disease is
suspected, you may need to
undergo more comprehensive,
formal types of visual field
testing to evaluate how well
you see across a wide range
of vision. Numerous tests
for measuring visual field
loss exist, and can include:
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Automated Perimetry:
Various forms of
automated perimetry
tests measure your
responses to the
presence of objects in
different areas of your
field of view. While
your head is held still,
usually with a chin rest
inside a large bowl-like
instrument, you stare
(fixate) on a source of
light straight ahead. A
series of random lights
of different intensities
are flashed in your
peripheral field of
vision. You then press a
button or use other
means to indicate your
response when you
perceive the
computer-generated light
suddenly appearing in
your field of view. If
you are unable to see
objects in an
appropriate portion of
your field of view, then
you may have a blind
spot indicating vision
loss.
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Frequency Doubling
Perimetry: Frequency
doubling is based on an
optical illusion that
uses vertical bars of
contrasting colors such
as black and white
appearing on a screen.
These bars appear to
double in number when
they alternately flicker
at higher frequencies, a
phenomenon thought to be
due to the unique
response of specific
light-sensitive cells
(photoreceptors) within
the eye's inner back
lining (retina).
Inability to see
vertical bars at certain
frequencies could
indicate optic nerve or
other types of eye
damage with accompanying
loss of vision in
certain areas of the
visual field.
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Electroretinogram:
This test automatically
measures the way cells
within the eye's
light-sensitive inner
layer (retina) respond
to different flashing
light stimuli, resulting
in creation of a pattern
of responses or "map"
used for evaluating
vision loss. This test
is conducted by placing
a device that detects
electrical impulses
(electrode) on your
eye's clear outer
surface (cornea), which
is treated with a
topical anesthetic to
ease any potential
discomfort. The
electrode then
automatically detects
responses from your
eye's retina. The
results can be directly
related to the part of a
visual field that might
be defective. This is
based on the anatomical
relationship of the
retinal images and the
visual field.
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