automated perimetry
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AUTOMATED PERIMETRY
dr samarth mishra
VISUAL FIELD The 3 dimensional area of a subjects
surrounding that can be seen at any one time around an object of fixation.
Traquair defined visual field as “ island or hill of vision surrounded by the sea of darkness”
The shape of hill of vision correlates to density of photoreceptors and their receptive field sizes.
PERIMETRY Perimetry is the systematic measurement
of visual field function. It is the measurement of Hill of Vision in
terms of establishing the patient’s differential light sensitivity across the visual field.
TYPES OF PERIMETRYKINETIC PERIMETRY STATIC PERIMETRY Confrontation Tangent screen Goldmann
Humphrey Octopus
Humphrey automated perimetry
THE MACHINE HAS 2 PARTS- -perimetric unit- bowl type screen
- control unit- computer Use static stimuli Viewing distance of 33 cms. Background luminance- 31.5 asb Stimulus size-( Goldmann stimulus size 1-
V ) Stimulus duration-0.2 sec
Stimulus Size and Intensity Stimulus- size- diameter-mm area-
mm2 0 0.28 1/16 I 0.56 ¼ II 1.13 1 III 2.26 4 IV 4.51 16 V 9.03 64 Most commonly Goldmann size III is used In SWAP Goldmann size V is the standard
stimulus
Fixation Monitoring Heijl-Krakau method of fixation
monitoring- It provides index of quality of patient fixation during examination by periodically exposing stimuli in blind spot
Eye Monitoring by perimetrist Infra red Gaze Monitors
Threshold Strategies SUPRATHRESHOLD TESTING THRESHOLD TESTING
1. Full threshold2. FASTPAC3. SITA standard4. SITAFAST
SUPRATHRESHOLD TESTING
Field is mapped with a stimulus that is 4 to 6 dB above the threshold
Very rapid evaluation Screening purpose
THRESHOLD TESTING In this technique light sensitivity
( threshold) is determined at each testing location in the field.
Provides more accurate hill of vision Capable of detecting early and shallow
focal loss
TEST POINT PATTERNS AVAILABLE IN THRESHOLD TESTING
Central 30-2 – 76 points are tested. Each point 6 deg apart
Central 24-2 – 56 points are tested. Each point 6 deg apart
Central 10-2 – 68 points are tested . Each point 2 deg apart
Macular threshold test – square grid of 16 points each 2 deg apart
Nasal step- 14 points peripheral field testing
60-4- 60 points .
Macular programme Useful to determine macular split Used when only central 5 deg of field
remains If a defect impinges on fixation , but
other points are preserved macular pogramme is used in addition to other programmes.
Determination of thresholdBracketing or staircase procedure ( the 4-2
algorithm)
FASTPAC LESS TIME CONSUMING IT CHANGES THE STIMULUS INTENSITY IN
3 dB steps either increasing it or decreasing it depending on patients initial response
High intratest variability i.e. Short-term Fluctuation
SITA( Swedish Interactive Threshold Algorithm)
More efficient estimation of threshold 2 strategies- SITA Standard SITA FAST short test time without compromising on the
sensitivity SITA creates prior probability models for normal
and glaucomatous populations The pace of the test is dependant on patient’s
response It uses informative index derived from visual field
model for each point that determines when to stop.
TESTING METHOD Ambient light- dim Distance- 30cm Full refractive correction Pt chin on chin holder One eye occluded Pt hands on button ,explained about the test Presents 4 points one in each quadrant and
thresholds them 9d from horizontal and vertical meridians – 25dB
Size- III4e Pupil- 3-4mm
ANALYSIS OF VISUAL FIELD DATA(single field printout)
Zone 1 : Reproduciblity Zone 2: Reliability indices and foveal
threshold Zone 3: Gray scale Zone 4:Total deviation plot Zone 5: Pattern deviation plot Zone 6: Global Indices Zone 7: Glaucoma hemifield test Zone 8: Raw data
False positive error- number of times pt responds when no stimulus shown,Trigger happy,<33%, white scotomas
False negative error-brighter stimulus presented which was previosly sensitive,if pt not respond- inattentive/fatigue,<33%,clover leaf pattern
Fixation losses- <20%
GRAY SCALE A rough indicator of the extent of field
damage, but can be misleading. Each point on the gray scale is represented by a symbol of varying darkness which corresponds to the threshold level at that point. These are not indicative of disease.
Total deviation plot (Zone-4) is created by subtracting the actual raw data from the expected value for age matched controls, at each point.
The Pattern deviation plot (Zone-5) based on further calculations, is derived from the total deviation data and the overall depression of the visual field. It highlights focal changes which are concealed within diffuse changes
GLOBAL INDICES Mean deviation (MD): It is the weighted score ofall the points on the total deviation plot. It takes
into account both the severity of loss and amount of field affected
Pattern standard deviation (PSD): It measures the extent to which the damaged points vary from the expected hill of vision (localized loss)
Short term fluctuation (SF): Though listed under global indices it is a good indicator of intra test reliability. It measures the variation at each point on repeated thresholding in the same test.
Corrected pattern standard deviation (CPSD): It is calculated with the help of SF to adjust the
PSD.
Glaucoma Hemifield test (Zone-8) is a sophisticated analysis of 5 geometric point clusters in the superior and the inferior arcuate regions whose probability maps are compared with one another. It is very sensitive and specific at detecting asymmetry between these regions as well as symmetric deviations from normal data.
Possible test outcomes Outside Normal Limits Borderline General reduction of sensitivity Abnormally high sensitivity Within normal limits
Glaucoma Hemifield Test
Anderson Criteria 3 or more congrous ‘non edge points’ in
typical arcuate area on 30-2 programme depressed @ p< 5% with at least one point @ p<1%
PSD/CPSD@ P<5% GHT-outside normal limitsMust be demonstrated on 2 field tests
INDICATIONS OF PERIMETRY
Detection of glaucoma, progression Chorioretinal lesions Optic disc and optic nerve lesions Neuro-ophthalmological diseases
Enlargement of blind spot
Arcuate scotoma
Advanced glaucomatous field defect
ALTITUDINAL FIELD DEFECT
HOMONYMOUS HEMIANOPIA
SERIAL FIELD ANALYSIS Establishing a base line Overview printout Change Analysis Printout Glaucoma Change Probability Analysis Glaucoma Progression Analysis
Overview printouts
CHANGE ANALYSIS PRINTOUT
Glaucoma change probability
Glaucoma Progression Analysis
AETEFACTS OBSTRUCTION.Rim Artefact.Ptosis.Media Opacities.Angioscotoma MIOSIS REFRACTIVE ARTEFACTS
OCTOPUS PERIMETRY Projection system perimeters which can
perform full threshold programme Stimulus is Goldmann target size III
Reproducibility (Zone-1) Reliability factors (Zone-2) Gray scale (Zone-3) Comparison (Zone-4) Corrected comparison (Zone-5)
Numeric data/raw data(Zone-6) Visual field indices (Zone-7):
Bebie.’s curve (Zone-8)
ADVANCED TECHNIQUES FOR VISUAL FIELD EXAMINATION
SWAP[Short Wave Automated Perimetry] FDP[Frequency Doubling Perimetry] HPRP[High Pass Resolution Perimetry] Flicker Perimetry Multifocal Electroretinography ACCUMAP[Multifocal Visual Evoked
Potential] Motion Perimetry
SWAP Helps to diagnose damage due to glaucoma at
an early stage Yellow background is used to highlight isolated
blue cone response HFA and Octopus perimeters have incorporated
the SWAP SWAP uses Goldmann target V stimulus to
enhance spatial summation It is influenced by nuclear sclerosis as lens acts
a blue filter
FDP FDP is based on detecting damage to M
‘y’cells which are a subset of Magnocellular cells
Test stimulus- series of white and black bands flickering at 25 Hz
Uses target of 10 deg square FDP full threshold strategy has 2
programmes 1)C-20- 17 points are tested 2)N-30-19 points are tested
HPRP HPRP is ring perimetry Series of ring of different sizes and
having a light centre and dark annular surround are presented
These targets are spatially high pass filtered vanishing targets for determination of resolution of central 30 deg
HPRP tests the parvocellular system selectively
Based on resolution method rather than differential light sensitivity
HPRP
FLICKER PERIMETRY It detects light and dark stimulus
alternations(flicker) at various locations in the field
It targets magnocellular pathway( responsible for flicker perception)
Not influenced by media opacities and refractive error
Two types-1)Critical Fusion Frequency2) Temporal Modulation Perimetry
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