Volume 8, Number 1
Winter, 1999
Understanding Visual
Field Testing
By Jeffrey Henderer, MD
| Most
glaucoma patients have had a visual field test, and no doubt
many have a reasonably correct idea of how and why it is
done. However, questions asked by the audience at a recent
meeting of the Glaucoma Patient Support Group in which clinical
fellow Dr. Jeff Henderer spoke about "visual fields" demonstrated
that many patients want more information about this test.
- Editor |
Click here for larger view. |
The Optic Nerve and Visual
Field in Glaucoma
The retina, the light-sensitive
tissue at the back of the eye, is composed of receptors, "photoreceptors,"
that change light energy into electrical energy.
The central area of the retina
is the most sensitive to light, and we see best what is directly
in front of us. The more peripheral areas of the retina are less
sensitive to light, but they allow us to see, though less clearly,
objects off to the side or above or below straight-ahead vision.
Just how much we can see of the world around us is known as our
visual field. Each part of the retina "sees" a particular
part of the visual field. Each photoreceptor’s signal is then
picked up by a special nerve cell called a ganglion cell.
The ganglion cells then transmit the signal via the optic nerve
to the brain, allowing us to see. Thus, each ganglion cells is
responsible for "connecting" a portion of the retina to the brain.
Any disruption in the function of these cells will block the signal’s
transmission and render that portion of the retina and the accompanying
visual field less sensitive to light.
The ganglion cells that make
up the optic nerve are living entities that require energy and
nourishment. A glaucoma is a process by which the optic nerve
cells can become damaged and die, at least partially due to the
pressure within the eye. There are millions of these cells, and
one can lose perhaps as many as 40% of them before being aware
of any visual loss. In fact a small number of cells die every
year "naturally."
If doctors relied only on
reduced visual field to detect glaucoma, they would miss most
people with early glaucoma. That is why the field is only one
part of a glaucoma evaluation. But the visual field is important:
to diagnose and categorize glaucoma, to help create treatment
plans, and to establish a baseline for future comparison. If doctors
suspect a person may have glaucoma (if, for example, intraocular
pressure is not within the normal range and/or the optic nerve
looks unusual), a visual field test may help confirm or rule out
glaucoma as the cause. Especially in the later stages of glaucoma
the visual field provides essential information about whether
the glaucoma is stable or is getting worse. In these later stages
changes in the optic nerve become hard to detect. Thus, the history
and the visual field provide the essential information needed
to understand what is happening.
Visual Field Testing (Perimetry)
Visual field tests are designed
to map a person’s visual field, to document the level of peripheral
vision. As most glaucoma patients know, the test consists basically
of responding every time a flash of light is perceived, all the
while looking straight ahead. Understanding the various parts
of the print-out of the results, such as shown here, is one way
of understanding more about visual field testing.
A. Test Type 
The ideal visual field test
would be easy to take, easy to administer, and 100% reliable.
We have no such test, but fortunately for everyone involved,
recent years have seen substantial improvements in all of these
areas. Especially welcome to the glaucoma patient are tests
that are faster and less tedious.
The Glaucoma Service of
Wills Eye Hospital now relies almost completely on Humphrey
automated perimetry, often using a new testing program known
as SITA. This test can be completed anywhere from about 3 to
8 minutes depending on whether the SITA Fast or SITA Standard
program is used. This new program is not useful for all patients
and some are still tested using older, but still excellent,
strategies.
B. Patient information
The patient’s visual acuity
and age are important factors in obtaining reliable results.
The visual field test must be taken with the appropriate correction
needed for close vision. Also, since the retina of the normal
eye becomes less sensitive with age, it is important that the
age of the person being tested be taken into account. When reviewing
a visual field test print-out it’s worthwhile for the patient
to check to see if these figures are correct.
C. Reliability Parameters
The print-out provides three
kinds of information to help the doctor assess just how reliably
a visual field test reflects the patient’s actual visual field:
(1) Fixation losses.
It is very important that the patient keep the eye being tested
focused straight ahead while taking a visual field test. The
doctor wants to know what the peripheral vision is
like, that is, vision off to the sides -- up and down, right
and left. In practice it is difficult to maintain this eye
position for very long, since the natural tendency is to look
to the side, towards the flashing light. But because many
such movements may make the test unreliable, the machine records
how many times the patient moves his eye off center.
(2) False Positives
Errors. Sometimes the patient will push the button indicating
he has seen a flash when in fact no flash has been shown.
This misinformation obviously seriously detracts from the
test’s ability to determine what the patient is actually seeing.
One reason why the patient may indicate he has seen something
even though nothing has been shown is that, like all of us,
he wishes to do well on tests. The machine is designed to
test for this tendency by making the normal beep or whirr
but presenting no light, tempting the patient to click the
button inappropriately. Even two false positives may make
a test unreliable.
(3) False Negative Errors.
To further gauge reliability, the test repeats flashes at
the same spot at the same and at different levels of intensity.
If one time the patient reports seeing a flash at a certain
spot, but does not report seeing the same intensity flash
at the same spot the second time it is shown, the reliability
of the test is reduced. People who have glaucoma may have
normal fluctuations at the edge of their visual field loss,
so not all of these kinds of errors are truly a problem.
D. Retinal sensitivity
is not an all-or-nothing affair. Sometimes a relatively weak
flash at a particular spot that could not be seen becomes visible
if the intensity of the light is increased. By flashing lights
of varying intensity, the machine can ascertain the level of
retinal sensitivity at each representative point in the visual
field. The numbers on the print-out diagram indicate the level
of intensity required to enable the patient to see the flash.
The higher the number, the dimmer the light that could be seen.
Click here for larger view
E. A nice picture
of a patient’s visual field is obtained by assigning a lighter
shade of gray to spots on the visual field in which a patient
could see relatively weak flashes (the higher numbers in "D"),
and a darker shade of gray to spots in which a patient could
see only relatively strong flashes. Here it is appropriate to
point out that all eyes have a blind spot (scotoma) where the
optic nerve connects with the retina. It is "blind" because
there are no light receptors at this point. The blind spot in
the eye shown is indicated by the dark area in the lower left
half in this print-out.
Click here for larger view
F. As pointed out
in B., retinal sensitivity diminishes with age. The dark boxes
in this diagram indicate areas in which the person saw less
well than most people his age.
G. Many conditions
other than glaucoma can cause poor vision, for example, cataract
or corneal edema. So, if the doctor wants to know how much of
a patient’s relative insensitivity to light is due to glaucoma
rather than to something else, it is important to "subtract
out" these other factors. This can be done because these others
conditions tend to produce a similar pattern of diffuse visual
field loss, while glaucoma tends to produce localized areas
of visual field loss.
H. These numbers
indicate the extent to which the visual field is outside normal
limits. They can be followed over time to see the extent to
which it is worsening.
There are many reasons other than glaucoma
for an abnormal visual field result: the test was poorly given,
the instrument was defective, the patient did not understand how
to take the test, the patient was tired, the defect was real but
does not indicate pathology, the defect is accounted for by some
pathology other than glaucoma, eg, brain tumor, multiple sclerosis,
a vascular problem, a congenital defect, an infection, or retinal
disease such as macular degeneration, retinal detachment, or inflammation.
Or the defect could be a false defect, that is, really not present
at all!
Despite all of the shortcomings
of visual field testing it is the only way to document actual
visual loss and whether such loss is progressing or remaining
stable. As such it plays an indispensable role in helping glaucoma
patients retain their sight.
Area Screenings Get
Under Way
Drs. George Spaeth (left) and Richard
Wilson (right) of the Glaucoma Service explain the glaucoma screening
process at the kick-off event held at Wills Eye Hospital Saturday,
January 8th for a screening program to be conducted in area African-American
churches. Dr. E. Sivalingam and Dr. Walter Harris also participated.
Sponsored conjointly by the Foundation, the Penn Towne Links (a
national African-American service organization), and Merck Pharmaceuticals,
the program hopes to find individuals among the high-risk African-American
population whose glaucoma is at an early stage and can be treated
to avoid further damage. The Foundation is also conducting screenings
at area senior citizen centers under the auspices of the Philadelphia
Corporation for Aging.
 
Research Committee Charts
Course
Optic nerve imaging, neuroprotection, telemedicine,
surgery, and genetics were identified as Foundation research
priorities at a meeting of the Research Committee on January
17th. Participating were Glaucoma Service Director Dr. George
Spaeth, Assistant Research Director Dr. Jonathan Myers, staff
physicians Drs Richard Wilson, Jay Katz, Marlene Moster, Courtland
Schmidt, and Anne Chan, Research Coordinator Kelly Flartey,
and Foundation Board Member Mary Chatterton, JD, MBA.
Optic Nerve Imaging
Several instruments are currently available to
provide important information about the characteristics of the
optic nerve, useful for studying the differences between the
appearance of the optic nerve in normal eyes and in eyes with
various types of glaucoma.Dr. Spaeth spoke in detail about the
strengths and weaknesses of these various instruments, noting
that none has sufficient sensitivity and specificity in their
current forms to replace clinical judgment. Nevertheless, he
recommended continuing efforts to evaluate these methods for
optic nerve analysis.
Neuroprotection
Dr. Katz summarized the status of neuroprotection
research in glaucoma. Since visual loss in glaucoma is the result
of damaged optic nerve cells, there is much current interest
in finding ways of "protecting" these cells to keep them healthy.
Both brimonidine (Alphagan) and betaxolol (Betoptic), drugs
designed to lower intraocular pressure, along with the systemic
drug memantine have been shown in some models to be neuroprotective.
The Service currently has two protocols on brimonidine and neuroprotection:
brimonidine vs. timolol (Timoptic) in low-pressure glaucoma
(a multicenter trial), and brimonidine vs. timolol to prevent
post-vitrectomy visual field defects (a two-center trial). In
the spring a trial of memantine will also be initiated.
Telemedicine
Dr. Wilson presented an update on his efforts
to bring telemedicine capabilities to the Hospital. The technology
is available to enable doctors to be electronically present
to guide in the evaluation and management of patients who are
unable actually to come to the doctor. Dr. Wilson emphasized
that telemedicine will undoubtedly become increasingly important,
and institutions with early experience may be at a great advantage.
Glaucoma Surgery
Dr. Moster addressed the newest techniques and
ideas in glaucoma surgery. The most promising of these are the
non-penetrating trabeculectomies, which have been described
in a variety of styles. Dr. Moster has obtained the instruments
for performing one of these, viscocanulostomy, and suggested
that the entire Service participate in a prospective trial.
Genetics
Dr. Myers discussed the current state of genetic
research in glaucoma. Seven genes have been localized for open
angle and congenital glaucomas. These account for only a small
percentage of patients with glaucoma. Dr. Mansoor Sarfarazi
of the University of Connecticut has isolated four of these
genes. Thanks to the initiative of Dr. Ordan Lehman, a visiting
fellow on the Glaucoma Service in 1998 from Moorfields Eye Hospital
in London, the Service is now working with Dr. Sarfarazi on
recruiting families with three or more relatives diagnosed with
glaucoma. The Glaucoma Service has an ample patient base
and is poised to make a significant contribution in this field.
Dr. Myers has recommended continued and increased involvement
in this research.
Current limitations in moving forward in these
areas center on funding and space. Dr. Spaeth and Ms. Chatterton
suggested ways in which funds could be obtained to create new
positions and entities, for example seeking donors who would
be interested in having a Glaucoma Center for Surgical
Research or a Glaucoma Center for Genetic Research named
for them or someone of their choosing. Ideas for space include:
a reconfigured Glaucoma Service Diagnostic Laboratory, Dr. Augsburger’s
office, and the Glaucoma Foundation office on the seventh floor.
1998 Annual Fund Makes A Strong Advance
Thanks to the generosity of our rapidly increasing
number of donors, the Foundation’s 1998 Annual Fund was a resounding
success. In only its second year, the amount raised by the Fund
from individuals, foundations, and corporations for unrestricted
operating expenses more than doubled – from $70,223.52 to $192,776.39
– and the total number of gifts received increased 43% from 968
to 1386. To each and every one of you, THANK YOU!
Note: For a complete listing of all 1998 contribuators
or to make a donation please contact Ken Parker at parker@willsglaucoma.org.
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