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