Transcript Visual Fields in Glaucoma

‫مركزالبحرللعيون‬
The field of vision is defined as the area that is
perceived simultaneously by a fixating eye
• Traquair, “an island of vision in the sea of darkness”
• Depicts the visual field as a three-dimensional spatial model
• The shoreline of the island represents the peripheral limits of the visual
field (least sensitive), and the peak correspond to the fovea (greatest
sensitivity)
NORMAL VISUAL FIELD LIMITS
60
60
60
100
X
X
Fixation
75
75
X= Physiological Blind spot
Diameter of Optic Disc
Relation of Disc to Fovea
Horizontal: 1.1mm, 5.5
Nasal: 3.0mm, 15.0
Vertical: 1.5mm, 7.5
Inferior: 0.3mm, 1.5
100
Papillomacular
bundle
Horizontal
raphe
• The contour of the island of vision relates to both the anatomy of
the visual system and the level of retinal adaptation
• The highest concentration of cones is in the fovea, which project
to their own ganglion cell. This one-to-one ratio between foveal
cone and ganglion cell results in maximal resolution in the fovea
Normal Visual Field
 Visual field measurement
can be affected by:
– Patient's age
– Size and position of the nose
– Orbital structures
– Location of eye within the orbit
– Color of stimuli
– Refractive error
– Fixation
– Eye movement
– Patient cooperation
– Ease of operation of the
instrument
History of Perimetry
• In 1856 Dr. Von Graefe is the first to draw the field using white piece of
paper
• In 1889 Dr. Bjerrum was using a tangent screen on the back of his
clinic door and he described the arcuate scotoma
• In 1909 Dr. Ronne developed kinetic isopter perimetry and described
the nasal step in glaucoma
• In 1945 Dr. Goldmann designed the first cupola perimeter for manual
kinetic perimetry
• In 1973 the era of automation began with Dr. Fankhuser and his
coworkers in Bern, Switzerland
• The first standard automated perimeter
OCTOPUS 201 - In 1976
OCTOPUS 2000 - In 1980
OCTOPUS 500 - In 1983
OCTOPUS 1-2-3 - In 1989
OCTOPUS 101 - In 1993
Knee of Wilbrand`s
Meyer`s loop
KINETIC PERIMETRY
• In kinetic perimetry, a stimulus is
moved from a non seeing area of
the visual field to a seeing area
• Procedure is repeated with use of the
same stimulus along a set of
meridians, usually spaced every 15°
• Aim is to find points in the visual field
of equal retinal sensitivity. By joining
these points an isopter is defined
• Then luminance and the size of the
target is changed to plot other isopters
KINETIC PERIMETRY
• In kinetic perimetry, the island of vision is approached horizontally.
Isopters can be considered as the outline of horizontal slices of the
island of vision
• Disadvantages of this technique include the subjectivity, highly
dependant on the operator efficiency, time consuming, difficulties
with randomising targets and patient cooperation
STATIC PERIMETRY
• In static perimetry, the size and location of the test target remain constant
• Retinal sensitivity at a specific location is determined by varying only
the brightness of the test target
• The shape of the island is then defined by repeating the threshold
measurement at various locations in the field of vision
• This strategy allows for a quantitative measure of the relative density
of a defect, more easily than in kinetic perimetry
Terminology Related to Perimetry
• Isopter is a line within the visual field which connects points of equal
sensitivity or threshold.
• Apostilb (asb) is the unit of measurement of luminance (brightness).
One apostilb equals to 0.3183 candela/m², or 0.1 mililambert.
– A healthy patient can perceive a stimulus of 1 abs in the macular area.
• Decibels (dB) is the unit of measurement of neutral density filters.
Each decibel equals 1/10 log unit. Thus 10 dB equals 1 log unit or 10
fold change in intensity.
Abostilbs
Humphrey
Octopus
– The decibel scale is not standardized because the
maximal luminance varies between instruments.
– Data needs to be captured on the same instrument
for comparisons.
Decibels
Decibels
0.1
50
40
1
40
30
10
30
20
100
20
10
1000
10
0
10,000
0
SENSITIVITY VERSUS THRESHOLD
• As one ascends the hill of vision toward the
fovea, the sensitivity of the retina increases,
dimmer targets will become visible.
Least Sensitive
DECIBELS
0
APOSTILBS
10,000
GOLDMANN COMPARISON
• Therefore, as retinal sensitivity increases,
the differential light threshold measured in
apostilbs decreases.
• In automated perimetry, however, threshold
is recorded in the inverted decibel scale, and
dimmer targets have higher decibel values.
• Therefore, threshold in decibels is directly
proportional to retinal sensitivity.
One Log Unit 10
1000
Two Log Unit 20
100
Three Log Unit 30
10
Four Log Unit 40
1
Five Log Unit 50
0.1
Most Sensitive
 III 4e
 III 3e
 III 2e
III 1e
III 1a
MANUAL PERIMETRY
• Goldmann perimeter is the most widely used
instrument for manual perimetry.
• It is a calibrated bowl projection instrument
with a background intensity of 31.5 apostilbs.
• Size and intensity of targets can be varied to
plot different isopters kinetically and
determine local static thresholds.
GOLDMANN VISUAL FIELD
 The stimuli used to plot an isopter are
Size of Goldmann Targets
identified by Roman numeral, number, and a Target
Area (mm²)
0
1/16
letter
 Roman numeral represents the size of the
target, from Goldmann size 0 to Goldmann size V
I
¼
II
1
III
4
IV
16
V
64
– Each size increment equals a fourfold increase in area
 Number and letter represent the intensity of
the stimulus
– change of one number represents 5-dB change in intensity
– change each letter represents 1-dB change in intensity
Intensity of Goldmann Targets
Filter
1e
Intensity (asb)
31.5
2e
100
3e
315
4e
1000
GOLDMANN VISUAL FIELD

Isopters in which the sum of the Roman
numeral (size) and number (intensity) are
equal can be considered equivalent

The equivalent isopter combination with the
smallest target size usually is preferred
because detection of isopter edges is more
accurate with smaller targets

One usually starts by plotting small targets
with dim intensity (I1e) and then increasing
the intensity of the target until it is maximal
before increasing the size of the target
NO
0
I
II
III
IV
V
4
3
2
1
The usual progression
GOLDMANN VISUAL FIELD

Once an isopter is plotted, the stimulus used to plot the isopter
is used to statically test within the isopter to look for localized
defects. In this way, it acts as a suprathreshold stimulus.
AUTOMATED PERIMETRY

The introduction of computers and automation
heralded a new era in perimetric testing.

Static testing can be performed in an objective and
standardized fashion with minimal perimetrist bias.

A quantitative representation of the visual field can
be obtained more rapidly than with manual testing.

The computer presents the stimuli in a random
fashion. Patients do not know where the next
stimulus will appear, so fixation is improved. Also
increase the speed of the test by bypassing the
problem of local retinal adaptation.
AUTOMATED PERIMETRY
• Humphrey Field Analyzer uses a constant target size
equal to a Goldmann "III" (4 mm²) and varies the
target brightness only. unless otherwise instructed.
• The stimulus intensity can reach up to 10,000 asb in
Humphrey and 1000asb in Octopus.
• The background luminance in Humphery is 31.5 asb
and the testing distance 33 cm. while Octopus model
uses 4 asb and the testing distance 42.5 cm
Comparison of static and kinetic
perimetry to detect shallow scotomas
A.
Kinetic evaluation can clearly outline the normal visual field
B.
Kinetic perimetry may miss shallow scotomas and poorly define the flat
slope seen nasally
C. The edge of steeply sloped scotomas may be identified easily with
kinetic perimetry, but the steepness of the slope may not be appreciated
D. & E. Static perimetry readily detects shallow scotomas and can define the
slope of both shallow and steep scotomas
In a study of patients with open angle glaucoma,
Dr. Ourgaud reported that a defect was found in
one third of cases with static perimetry that was
missed by kinetic perimetry
J Fr Ophthalmol,1982
GLAUCOMATOUS VISUAL FIELD
DEFECTS
Any clinically or statistically significant deviation from
the normal shape of the hill of vision can be considered
a visual field defect. In glaucoma, these defects are
either diffuse depressions of the visual field or localized
defects that conform to nerve fiber bundle patterns.
DIFFUSE DEPRESSION
• Diffuse depression of the visual field results from widespread diffuse
loss of nerve fibers of the retina.
• It is common in glaucoma but it is non specific sign that can be caused
by many etiologies.
• By far the most common reason for a diffuse depression is lens opacity.
• Other factors include other media opacities, miosis, improper refraction,
patient fatigue, inattentiveness or inexperience with the examination,
ocular anomalies, and age.
DIFFUSE DEPRESSION
• In manual perimetry, is manifested by
contraction of the isopters. The isopters
retain their normal contour. The most
central isopters may disappear entirely
as the peak of the island of vision sinks.
• In automated perimetry, diffuse
depression results in relative defects
across the entire visual field.
LOCALIZED NERVE FIBER
BUNDLE DEFECTS
• Localized visual field defects in glaucoma
result from damage to the retinal nerve
fiber bundles.
• Because of the unique anatomy of the
retinal nerve fiber layer, axonal damage
causes characteristic patterns of visual
field changes.
• The most common location of visual field
defects occurs within an arcuate area
(Bjerrum’s area) extending from blind spot
nasally 10-20 around fixation and
terminate at the median raphe.
Nerve Fiber Bundle Defects
 The superior and inferior poles of the optic nerve head
are most vulnerable to glaucomatous damage.
 It has been postulated that these areas may be
watershed areas at the junction of the vascular supply
from adjacent ciliary vessels.
 Ultrastructural examination of the lamina cribrosa
shows that the pores in the superotemporal and
inferotemporal areas are larger. The large pores may
make these regions more vulnerable to compression.
PARACENTRAL DEFECTS
• Circumscribed paracentral defects are an early sign of localized
glaucomatous damage.
• The defects may be relative or absolute and frequently found in
Bjerrum’s area along the course of the nerve fiber bundle.
• With progression paracentral scotomas become deeper and longer and
may gradually coalesce forming an arcuate or Bjerrum’s scotoma.
ARCUATE SCOTOMAS
• More advanced loss of nerve fiber bundle leads to a scotoma that starts
at or near the blind spot, arches around fixation, and terminates
abruptly at the nasal horizontal meridian .
• In the temporal portion of the field, it is narrow because all of the nerve
fiber bundles converge onto the optic nerve.
• The scotoma spreads out on the nasal side and may be very wide
along the horizontal meridian.
Differential Diagnosis of
Arcuate Scotomas
Glaucoma
Branch Vein Occlusion
Branch Artery Occlusion
Optic Neuritis
Ischemic Optic Neuropathy
Optic Nerve Drusen
Optic Nerve Pit
Optic Nerve Cloboma
Myelinated NFL
NASAL STEP DEFECTS
• A steplike defect along the horizontal meridian results from asymmetric
loss of nerve fiber bundles in the superior and inferior hemifields.
• Nasal steps frequently occur in association with arcuate or paracentral
scotomas, but a nasal step also may occur in isolation.
• Nasal step defects may be evident in some isopters but not in others,
depending on which nerve fiber bundles are damaged.
• Approximately 7% of initial visual field defects are peripheral nasal step
defects.
TEMPORAL WEDGE DEFECTS
• Damage to nerve fibers on the nasal side of the optic disc may result
in temporal wedge-shaped defects.
• These defects are much less common than defects in the arcuate
distribution.
• Occasionally, they are seen as the sole visual field defect.
• Temporal wedge defects do not respect the horizontal meridian.
EARLY VISUAL FIELD DEFECTS
Werner and Drance found in 35 eyes with previously
normal visual fields that the earliest defects were
paracentral scotomas with a nasal step (51%), isolated
paracentral defects (26%), isolated nasal steps (20%),
and sector defects (3%).
Hart and Becker found the following initial visual field
defects in 98 eyes: nasal steps (54%), paracentral or
arcuate scotomas (41%), arcuate blind spot enlargement
(30%), isolated arcuate scotomas separated from the
blind spot (20%), and temporal defects (3%).
BLIND SPOT CHANGES
1. Enlargement or vertical elongation of the blind spot may occur with
early arcuate defect that connects with the blind spot or peripapillary
atrophy, which frequently accompanies glaucomatous damage.
2. Baring of the blind spot may be physiologic or pathologic. Physiologic
baring of the blind spot is an artifact of kinetic perimetry usually is
confined to a single central isopter in the superior visual field. Because
inferior retina is less sensitive than the superior retina.
End-stage defects
• Only a small central island and a
temporal island of vision remain.
• The temporal island is more
resistant than the central island
Visual field changes in NormalTension Glaucoma
1. Greater depth
2. Steeper slops
3. Closer to fixation
Important Points

Both central visual acuity and field of vision may improve if the
IOP is reduced in early stages of the disease

In most patients with glaucoma, clinically recognizable disc
changes precede detectable field loss

With standard manual perimetric techniques as many as 35% of
fibers may gone in an eye with normal field

20% loss of cells, especially large gangelion cells in the central
30 of the retina, correlates with a 5-dB sensitivity loss
Examination
procedure
=
Test Program
Test Strategy
Perimetry
Technique
Program 30-2
Full Threshold
White/White
Macula 10-2
+ Supra threshold + Blue/Yellow
Nasal Step
SITA or TOP
Etc.
Etc.
Etc.
The following table indicates the threshold tests and the points
tested
Threshold Test
Extent of Visual Field/Number of Points
10-2
10 degrees/68 point grid
24-2
24 degrees/54 point grid
30-2
30 degrees/76 point grid
60-4
30 to 60 degrees/60 points
Nasal Step
50 degrees/14 points
The following table indicates screening tests and the points
tested
Screening Test
Extent of Visual Field/Number of Points
Central 40
30 degrees/40 points
Central 76
30 degrees/76 points
Central Armaly
30 degrees/84 points
Peripheral 60
30 to 60 degrees/60 points
Nasal step
50 degrees/14 points
Armaly full field
50 degrees/98 points
Full Field 81
55 degrees/81 points
Full Field 120
55 degrees/120 points
Commonly used programs
for glaucoma.

The Octopus program 32 and the Humphrey program 30-2
are tests of the central 30° with 6° of separation between
locations.

The Humphrey program 24-2 eliminates the most
peripheral ring of test locations from program 30-2
because it provides the least reliable data, except in
the nasal step region, so testing time can be shortened.
DIFFERENTIAL LIGHT THRESHOLD
• Static computerized perimetry measures retinal sensitivity at
predetermined locations in the visual field.
• These perimeters measure the ability of the eye to detect a difference
in contrast between a test target and the background luminance.
• Threshold is defined as the dimmest target perceived by the patient at
a given discrete point; psychophysicists define the term as the ability to
perceive a stimulus 50% of the time.
frequency-of-seeing curve
THRESHOLD PROGRAMS
3. Strategy

Full threshold
A staircase, or bracketing, strategy is used to estimate
threshold at each test point. Most commonly, a 4-2
algorithm is employed.
•
Testing starts with a suprathreshold stimulus. The intensity
of the stimulus is decreased in 4-db steps until the stimulus
is no longer seen ( threshold is crossed ). Threshold is
crossed a second time by increasing the stimulus intensity
in 2-db steps until it is seen again.
The 4-2 bracketing strategy
0.1
asb
1
40
dB
30
10
20
100
10
1000
0
Phase
1
2
Threshold
• Octopus perimeter estimates threshold as the average of
the last seen and unseen stimulus intensities.
• Humphrey perimeter uses the intensity of the last seen
stimulus as threshold.
• Full threshold is rarely indicated, since newer thresholding
algorithms are equally as valid and much faster.
How can test time be minimized?
• The closer the initial stimulus is to the actual threshold, the faster the test
will be. Humphrey and Octopus use a "region growing" technique to
determine the starting level for each point.
• The test begins with measuring the threshold at one spot in each
quadrant of the central field. This then determines their reference hill of
vision after correcting for age and general responsiveness of the patient.
Adjacent locations are tested with appropriate starting thresholds.
OTHER THRESHOLD PROGRAMS

FASTPAC
– Was most commonly used strategy
– Use 3dB step and only cross threshold once
– Save 25% of test time (Humphreys).
– Measurement are statistically identical to the
standard strategy
– Trade off ST fluctuation over estimated

Swedish Interactive Thresholding Algorithm (SITA)
– SITA utilises an alternative strategy to the bracketing method.
– Fast with similar accuracy and reproduciblity
– It is available as either SITA standard or SITA fast.
– Use computer intelligence by calculating expected thresholds and
begin testing close to the actual threshold value.
– Two likelihood functions are calculated for each test location, one
based on the assumption that the test location is glaucomatous and
the other based on the assumption that the location is normal. The
likelihood functions are updated as the examination progresses. The
updating is informed by a combination of patient responses and
internal models of normality and glaucoma.
SCREENING PROGRAMS
• First the four primary points in each quadrant are thresholded to calculate
the theoretical hill of vision.
• Targets are then presented 6dB brighter than the theoretical hill of vision.
Failure to detect the stimulus after it is presented for the second time will
result in different strategies as the following:
Two Zone Points are presented the 6dB above the theoretical hill of
vision level. If the point is not seen it is tested for a second time Printouts
display circles for seen stimuli and solid squares for unseen stimuli.
Three-zone Points that are not detected after being presented twice 6dB
above theoretical hill of vision level, are retested at the brightest level
which is 10 000 asb. If target is seen a circle is displayed, "x‘ on the
printout for a relative defect or a solid block if the target is not seen.
Quantify defects The points missed twice at the 6dB brighter than the
theoretical hill of vision level, are thresholded to quantify the depth of the
defect at that location. Printouts display circles for seen stimuli, and
numbers for defects.
READING FIELD PRINTOUT
1.
Name, ID and Age
Ensure that this data is accurate. The correct age is essential as the
patient is compared to age matched normals.
2.
Type of Test
This indicates whether the test was a threshold or screening test.
•
Screening tests are a fast effective method to detect suspect areas
in the visual field and indicate the need for further evaluation
•
Threshold tests determine the sensitivity at various points in the
visual field and detect early changes in retinal sensitivity.
4.
Pupil Diameter
–
–
While large pupils do not affect the results significantly, miotic pupils can
induce a defect. The pupil should be at least 3mm to avoid false defects.
Automated pupil size measurement (Humphry)
5.
Glasses Used
–
The proper near add refraction, as determined by the patient's age
and the diameter of the perimeter's cupola, must be used.
–
This lens must be positioned properly to prevent artifactual defects
caused by the rim of the lens.
–
Use Trial lenses only for central tests (within 30), or the central
part of a full field test. For Peripheral test > 30 degrees, remove
the lenses.
–
Uncorrected refractive errors cause defocusing of the test target
and apparent depression of retinal sensitivity. Each diopter of
uncorrected refraction causes a 1.26-db depression of retinal
sensitivity
ASSESSING RELIABILITY
(Reliability Indices)
 Fixation losses
• Fixation is central to the validity of a visual field.
• The following strategies are employed to ensure adequate
fixation:
– Video monitoring of the eye or Gaze tracking (Octopus)
– The manual method which requires constant supervision of the
patient during the test (Goldmann)
– Heijl-Krakau Technique in which fixation during the examination is
periodically monitored by presenting stimuli in blind spot (Humphry)
• If fixation losses exceed 20% indicative of poor fixation or that the
blind spot was not correctly mapped out then XX will be printed
next to the numbers
ASSESSING RELIABILITY
(Reliability Indices)
 False positives Catch Trials
• By withholding of a stimulus projection (only sound is presented) and
patient is still responding. The patient responds to the sound clue alone
• The scores are flagged with XX if errors exceed 33% of the trials
• High score suggests a ‘trigger happy’ patient
 False negatives Catch Trials
• Failure to respond to a stimulus 9 dB brighter than previously seen at
same location
• The scores are flagged with XX if errors exceed 33% of the trials
• High score indicates inattention, or advanced field loss
 The numeric data
Expresses the patient's test responses in decibels. The STATPAC
software analyses this info and gives it age adjusted significance and
it is then that this information is really relevant and worth drawing
conclusions from.
 The Grayscale
The grayscale is a colour scheme of the visual loss. It is useful to
provide an overview of the visual field loss but cannot be relied on by
the clinician to make a definitive diagnosis of the extent of the visual
field loss. It is useful for the patients to understand the extent of the
visual field loss and the risks that they face.
Deviations
 Humphery Field Analyzer's statistical package (STATPAC)
uses a model based on test results of patients with normal
fields, retinal sensitivity, and pupil size for each different age
group. It compares the patient's test results against this model
to determine how their threshold results, for each tested point,
compares or falls outside the normal population model.
1. Total deviation (Comparisons in Octopus)
– Upper numerical display shows difference (dB) between patient’s
results and age-matched normal
– These negative values become diagnostic when they reach (-5) or
greater and more so if there are several grouped together.
– Lower graphic display shows these differences as grey scale ie.
the defect depth
Deviations
2.
3.
Pattern deviation (Corrected Comparisons in Octopus)
–
Similar to total deviation except the STATPAC correct total it for
diffuse effects eg. cataract, miotic pupils or incorrect testing lens
–
Display any superimposed pattern of localized loss (eg. subtle
glaucoma changes) that is hidden under a generalized depression
Probability plot
– Indicate the degree of abnormality
– The darker the symbol in the probability
plot the more significant the deviation from
normal
– P<1% means that this deviation happens
in less than 1% of the normal population

Glaucoma Hemifield Test
– GHT is based on the fact that glaucoma usually
causes asymmetric field loss and not a
generalised global depression.
– GHT evaluates five zones in the superior field
and compares these zones to their mirror image
zones in the inferior field. Then prints one of
three messages below the graytone format:
» GHT
within normal limits
Outside Normal limits
Borderline
– The test is not available with tests using Fastpac

Defect (Bebie) Curve in Octopus
GLOBAL INDICES
1. Mean deviation (Mean defect in Octopus)
–
Reflects deviation of patient’s overall field from normal
•
•
•
•
It is simply the average (Octopus) or the weighted average (HFA)
of the deviation values for all locations tested.
p values are < 5%, < 2%, < 1% and < 0.5%
The lower the p value the greater the significance
The mean deviation is most sensitive to diffuse changes and is
less sensitive to small localized scotomas.
2. Pattern standard deviation (Loss variance in Octopus)
–
measurement of the degree to which the shape of the patient's
field departs from the age-matched normals reference field.
•
•
•
Represent the local non-uniformity of the visual field
low PSD indicates a smooth hill of vision or if the damage is more or
less even
high PSD indicates an irregular hill or presence of scotoma
GLOBAL INDICES
3.
4.
Short-term fluctuation
–
Represent intra test variability and measure the consistency of responses
–
The threshold is measured twice at 10 pre-selected points. A fluctuation
value is then determined by using the difference between the first and the
second readings.
–
2 dB or less indicates reliable field
–
> 3 dB indicates either poor patient compliance or a sign of
glaucomatous field loss and flagged with p values, eg. P< 0.01
Corrected pattern standard deviation CPSD
(Corrected Lloss variance in Octopus)
–
Measurement of how much the total shape of the patient's hill of vision
deviates from the shape of the "NORMAL" hill of vision for the patient's age,
after being corrected for intra-test variability (short-term fluctuation)
–
It is increased when localized defects are present
40 dB
40 dB
PSD – SF = CPSD
40 dB
MD = 1.9
CPSD = .7
40 dB
MD = 11.9
CATARACT
NORMAL
40 dB
MD = 1.9
CPSD = .7
CPSD = 2.6
GLAUCOMA
40 dB
MD = 11.9
CPSD = 2.6
GLAUCOMA + CATARACT
INTEREYE COMPARISONS
• The difference in the mean sensitivity between a patient's
two eyes is less than 1 db 95% of the time and less than
1.4 db 99% of the time.
• Intereye differences greater than these values are
suspicious if they are unexplained by non glaucomatous
factors, such as unilateral cataract or miosis.
Clover leafe field
Glaucoma (1)
Humphrey Central 24-2 Threshold Test
Glaucoma (2)
Glaucoma (3)
Glaucoma
Incomplete Left Superior Quadrantonopia (Temporal lobe Syx)
Neuro (1)
Neuro (2)
Neuro (3)
Bilateral Optic Neuropathy
Pseudotumour cerebri
Retinal Toxicity
secondary to
Plaquenil
Post portum CVA
2nd VF largely
resolved
Conclusion
• Visual field measurement is a critical component in the
armament against potentially blinding diseases.
• Visual field measurement has undergone an evolution from the
mechanical to the automated measurement process, resulting
in greater accuracy, ease of use and greater depth of analysis.
•
• Other psychophysical methods for testing the visual field for
damage are now being explored. These methods include
contrast sensitivity, acuity perimetry, and color perimetry.
References
•
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•
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•
Haley MR. (ed). The Field Analyser Primer. San Leandro, California: Allergan Humphrey,
1987.
•
Townsend JC, Selvin GJ, Griffin JR, Comer GW. Visual Fields. Clinical Case Presentations.
Boston: Butterworth-Heinemann, 1991 pp 3-37.
•
Lalle P. A. Visual Fields. In Fingeret M & Lewis TL, eds. Primary Care of the Glaucomas.
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•
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•
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•
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•
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•
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•
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