Perimetry Algorythms

  • ·         Perimetry Algorythms:
    • Ø  First described by Lynn in 1968.
  1. 1st Generation-Threshold determination simply by altering stimulus intensities from infra-or supra luminal until threshold was crossed
  2. 2nd Generation-Staircase methods (Standard Full Threshold Testing Algorithm)
  3. 3rd Generation-Swedish Interactive Threshold Algorithm (SITA)


  1. 30-2-Central 30 deg with tested points 6 deg apart( offset 3 deg from both horizontal and vertical axes)-total 76 locations
  2. 24-2-Central 24 deg with grid spacing same like 30-2 i.e. 6 deg total 54 locations
  3. 10-2-68 locations spaced on a 2 deg within 10 deg of fixation

This tests a square grid of 16 points 2 degrees apart centred around the point of fixation .

It is used to monitor the central field in advanced glaucoma



l  Standard full threshold

l  Fastpac

l  SITA :Swedish Interactive Threshold Algorithm

      -SITA Standard

      -SITA Fast



1.Prior probability model of thresholds based on age corrected values in normal and glaucomatous popln. 1. No such model present
2.Use of information index to determine threshold endpoints i.e. it is faster. 2. Thresholds are determined in each points by up-down staircase by 4-2 steps.
3.Test pacing is more comfortable.  
4.Post test computation –greater sensitivity and reproducibility and less intertest variability 4.Not present.
5.Fewer catch trials-reduces testing time (time almost of half of SFT)  





1.At first same process to determine thresholds at 4 primary points-4-2dB step staircase  
2.If errors are larger ,a single reversal staircase of 4dB is used.(2nd staircase used if value deviates >12dB than expected value) 2.Full staircase 4-2dB is used with two reversals
3.Stimulus sequences interrupted earlier by increasing ERF cut off value  
4.Lower accuracy  



1. Longer time 1. Half the time of fastpac with same reproducibility
2.Glaucoma hemifield test is not available, can not detect early changes. 2.GHT present.


  • ·         The Threshold sensitivity is  defined as the stimulus at which the patient responds 50%of the time

- compiled & published by Dr Dhaval Patel MD AIIMS

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