Automated perimetry

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