الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 153 |  |  |
| | | from | 153 | | |
Abstract
Automated perimetry is a ready means to obtain standardized,
quantitative measurements of the visual field, has prompted valuable
clinical research, and has improved the care of patients with glaucoma.
Static threshold measurements are sensitive to shallow depressions
of the visual field. This increases the detection rate of early glaucomatous
defects compared to routine manual techniques. Although static threshold
measurements are not new to practitioners, the computerized format and
the generation of a large amount of numeric data present new challenges.
The increased sensitivity of the method has increased the ”noise” of the
measurements and demands careful attention to the many variables that
may affect the measured threshold. Visual field measurements should
never be interpreted in the absence of other clinical information. It is
necessary to integrate intraocular pressure, appearance of the optic nerve,
and other ocular and systemic factors when making decisions about
diagnosis and therapy.
Patient reliability IS an important initial consideration when inter
preting visual fields. This may be assessed by the rates of false - positive
and false - negative answers, the operator’s assessment of reliability, the
fluctuation of repeated threshold measurements, the number of fixation
120
losses, and the number of stimulus presentations required to complete a
visual field examination. The operator must adequately explain the testing
procedure and encourage patient cooperation and alertness during the
examination if reliable results are to be obtained.
Abnormalities of the visual field can be recognized in two ways:
by point-by-point comparison with data from normal individuals of
similar age, and by comparison with thresholds in adjacent areas of the
visual field, in the opposite hemifield of the same eye, or in the visual
field of the contralateral eye.
The classic glaucomatous visual field defects are often preceded by
widespread, shallow depressions throughout the visual field, though the
concept of uniform diffuse depression in early glaucoma is debated.
Diffuse depressions is not specific for glaucoma and is frequently
caused by media opacity, inaccurate refraction, miosis, retinal disease, or
poor test performance. Computer-assisted comparisons with normal data
may help identify early glaucomatous defects. The visual field indices
mean defect (or mean deviation), corrected loss variance (or corrected
pattern standard deviation), and short-term fluctuation, seem useful to
evaluate sequential examinations but may not be suited to detect the
earliest glaucomatous defects.
Careful comparison of sequential visual field often appropriately
directs glaucoma treatment. Knowledge of the conditions under which
121
Careful comparison of sequential visual field often appropriately
directs glaucoma treatment. Knowledge of the conditions under which
valid comparisons can be made is essential .It is best to avoid changes in
treatment, whenever possible, based on the comparison of only two visual
fields. A series of examinations may show the true course of the disease
when only two examinations would be misleading. There are no generally
accepted criteria for progression of defects, but some guidelines can be
offered. A new, confirmed defect that meets reasonable criteria for
abnormality can be considered progression. In areas of a previous defect, a
further decrease in the threshold of greater than or equal to 5dB would be
suggestive, where as a decrease of greater than or equal to IOdB can
usually be considered a significant change.
A number of confounding variables make the interpretation of visual
fields difficult, particularly for progression. The differentiation of longterm
fluctuation (test-retest variation) from true progression of visual field
loss remains one of the important challenges in modem perimetry. longterm
fluctuation in glaucomatous eyes increases with the depth of the
defect and with distance from fixation. The comparisons of sequential
visual fields is enhanced ifthese factors are considered. Computer-assisted
analyses have been developed to evaluate sequential visual field
examinations for evidence of progression beyond that which might be
122
explained, at a given level of probability, by long-term fluctuation. The
practitioner must be cautioned, however, that statistical significance should
not be equated with clinical significance.
Changes in pupil area can be significantly affect the visual field, and
may be exaggerated in the presence of even minor media opacities. Pupil
size should be measured and recorded at every visual field examination;
when it is less than 3.0Imn,patients’pupils should be dilated before testing
whenever possible. When tills is not possible, examinations should be
repeated at the same pupil size.
Cataract can have a profound effect on measured visual thresholds,
especially in the presence of a small pupil. A subjective evaluation of the
media is useful, but such evaluations are rarely sensitive enough to detect
small changes that can significantly affect visual field measurements. In
difficult situations in which changes in media clarity have occurred, it
helps to correlate apparent deterioration of the visual field with change in
optic nerve appearance.
Refractive errors of only one diopter can cause a significant
decrease of measured central visual thresholds. Aphakic eyes are best
tested after placement of contact lenses, since aphakic spactacle correction
induces significant peripheral artifact .
t23
The results of psychophysical tests generally improve as subjects
gam experience. This learning effect is small in most patients who have
previous experience with manual perimetry, but some patients still show a
dramatic improvement on their second automated test compared to their
first. The variability of test results decreases significantly with experience.
Whenever possible, the patient’s second visual field test should be used as
a baseline for subsequent comparisons.
The weight given to perimetric finding in decision making depends
on the reliability of the patient, the visibility and appearance of the optic
nerve, the level of intraocular pressure, and ocular and systemic factors.
Computerized static threshold perimetry has increased the
validity of routinely performed visual field examinations. It has made
compansons of multiple examinations more meaningful, but also more
problematic. Automated perimetry has properly shifted the emphasis away
from intraocular pressure measurements and toward the visual field in the
diagnosis and treatment of glaucoma. Perimetric results, when they are
reliable, should play prominently in the minds of practitioners when
making diagnostic and theraputic decisions.
Future work to enhance the value of automated perimetry will
undoubtedly include the development of summary indices with improved
sensitivity and specificity for early glaucomatous defects; better strategies
to differentiate true progression from long-term fluctuation; and timesavmg
testing algoritlnns with acceptable detection rates for early defects.
Automated perimetry has, perhaps, provided more questions than answers
about the early functional abnormalities in glaucoma. The sensitivity of the
technique has made us more aware of the ’noise’ inherent in
psychophysical testing; improved techniques to separate the ”signal” from
the ”noise” must be developed.
Conclusion
• The automated perimetry has been shown to be superior (0 manual
perimetry in detecting early glaucomatous visual field defect.
• The best strategy for detection and follow up is full-threshold strategy
and it must be done twice at least.
• The only accurate judge of the success of the treatment is the frequent
visual field test.