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 --- contd THRESHOLD • Luminance of stimuli that is seen 50% of times it is presented • Logarithmic unit d. B ( d. B prop. 1 / brightness ) • Bracketing strategy ( 4 - 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 (4 d. B).
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 d. B 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 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 <3 d. B.
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%.
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 d. B loss • 2 or more cont. non edge points with >=10 d. B loss • Diff. of 10 d. B 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 d. B increase in depth of existing defect >= 9 d. B depression adj. to abnormal point >= 11 d. B 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. ( 440 nm ) - 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
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