Transcript 20-Automated-perimetry-(2)

AUTOMATED PERIMETRY
DR.JYOTI SHETTY
MEDICAL DIRECTOR
BANGALORE WEST LIONS EYE
HOSPITAL, BANGALORE
BASIC CONCEPTS
• Traquair's has defined the visual field as been a
hill island of vision in a sea of darkness testing
along X-Y axes of this 3 dimensional area
determines the location in the visual field and
along the Z axis identifies the visibility threshold.
• X - Y axis - kinetic perimetry
• Z axis - static perimetry
• Automated Perimetry - " Differential light
threshold –Ability to differentiate an illuminated
target against an illuminated background."
• Threshold Perimetry - Modality of choice
BASIC CONCEPTS
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THRESHOLD
• Luminance of stimuli that is seen 50% of times it
is presented
• Logarithmic unit dB ( dB prop. 1 / brightness )
• Bracketing strategy ( 4 - 2 - 2 algorithm )
• Supra threshold - 95 % chance a stimulus is seen.
• Infra threshold - 5% chance a stimulus is seen.
BASIC MACHINE DESIGN
• Illuminated hemispherical bowl 33 cm away with
target of fixation
• Stimuli - spot of light - LED / Projection system /
Comp. Video monitor
• HFA - II ( 700 Series ) Aspherical bowl 30 cms
away ,smaller ,more ergonomic stimuli in
periphery more closer,
programmed to decrease stimuli brightness
(4dB).
FIXATION CONTROL
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CC TV monitor
Heijl - Krakau Blind spot method
Gaze tracker
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Full time two variable Gaze monitor
Image analysis
Errors - upward / downward
Fixation checked 100 % of stim. Time
No testing time for fixation check
Basic software design
• Strategies for threshold detection –
• Intensity of the stimulus presented at a
given point is related to the normal
threshold at that stimulus site.
• Bracketing strategies to define threshold at
any point.
4-2-2 algorithm
SITA
INFORMATION DISPLAY
• Numeric data display
actual dB value at each
point
• Gray scale – range of
decibels and their
corresponding luminance
• Difference / Depth defect
– actual value is
arithmetically subtracted
from a presumed
expected field.
Parameters recommended for
testing
• Foveal fixation target – small and large diamond with
yellow lights.
• Goldmann size III target for stimuli & blind spot
check. If excessive fixation loss it can be decreased to
II or I or if vision less than 6/36 than it can be
increased to V.
• White stimulus colour
• Normal testing speed. can be slow down if patient is
slow to response.
• Foveal threshold - ON / OFF
Threshold tests
• Central 30-2 – 76 points are tested . Each point 6
deg apart. Straddling the horizontal and vertical
axis so that the 2 inner most test points are 3 deg
from fixation point.
• Central 24-2 – 56 points are tested . Avoids rim
artifacts.
• Central 10-2 – 68 points space 2 deg apart.
Useful in advance disease with spilt fixation.
• Macular threshold test – square grid of 16 points
each 2 deg apart , with each point thresholded 3
times.
30 – 2
24 – 2
Macular threshold
INTERPRETATION
Factors for consistency in testing
• Best Refractive correction used. Contact lens to
avoid rim artifacts.
• Pupil Diameter – at least 3.5 mm in size.
• Visual Acuity
• Date & Time of testing
• Age-For comparison with normative data
• Short term fluctuation-Fluctuation occurring
within the test. Should be <3dB.
INTERPRETATION ----contd.
Reliability of patient
• Fatigue, anxiety and learning effect
• Fixation loss – should be less than 20%
• False positive and negative response should
be less than 33%.
FIXATION LOSSES: 10/12 XX
FALSE POS ERRORS : 0/7
FALSE NEG ERRORS :0/6
TEST DURATION : 16:20
Statistical global indices
• MD – mean deviation – sensitive to total
loss
• PSD – pattern standard deviation –
sensitive to localized loss.
• CPSD – corrected pattern standard
deviation – PSD corrected for short term
fluctuation. Very sensitive index.
Glaucoma defect with automated
perimetry- Anderson's Criteria
• 3 or more cont.non edge
points with >= 5 dB loss
• 2 or more cont. non edge
points with >=10 dB loss
• Diff. of 10 dB across nasal
hor. meridian at 2 or more
adj. points ( nasal step.)
• GHT - ONL
• PSD plot - >= 3 pts , p< 5%
of which one < 1%
• CPSD ( p <5% ) GHT ONL
INTERPRETATION ----contd.
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Progression of defect
Test parameters comparable
Defect - increased in size / depth
>= 7 dB increase in depth of existing defect
>= 9 dB depression adj. to abnormal point
>= 11 dB depression of a normal point ( New
Defect )
• Box plot change analysis
• Overview
• Glaucoma change probability analysis
SWAP
• Tests subset of Ganglions affected earlier & selectively -Blue / Yellow
• Reduces the redundancy of responsiveness to stimuli
• Intense yellow background - bleaches green / red cones
• Blue stim. ( 440nm ) - isolates blue cones
• Adaptation - 3 mts. Room illumination - minimal
• Stimulus size & BS check size V
• Mean threshold values lower than SAP - Gray scale darker
• Stat Pac probability plots more reliable
SWAP -- contd
• Field defect precedes SAP by >= 3 yrs
• Once abnormal - remain abnormal ( no recovery of damaged
blue cones )
• No role in advanced POAG / advanced lenticular changes /
colour vision abnormalities
• Most useful in younger Glaucoma suspects, OHT , POAG
with mild to mod.damage
• Time consuming - SITA optimised for SWAP / Fast Pac can
be used