automated perimetry dr.jyoti shetty medical director bangalore west lions eye hospital, bangalore
DESCRIPTION
BASIC CONCEPTS --- contd THRESHOLD Luminance of stimuli that is seen 50% of times it is presented Logarithmic unit dB ( dB prop. 1 / brightness ) Bracketing strategy ( algorithm ) Supra threshold - 95 % chance a stimulus is seen. Infra threshold - 5% chance a stimulus is seen.TRANSCRIPT
AUTOMATED PERIMETRY
DR.JYOTI SHETTYMEDICAL 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 --- contd
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• CC TV monitor• Heijl - Krakau Blind spot method• Gaze tracker
– 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 XXFALSE 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.• 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