Perimetry Algorythms
- · Perimetry Algorythms:
- Ø First described by Lynn in 1968.
- 1st Generation-Threshold determination simply by altering stimulus intensities from infra-or supra luminal until threshold was crossed
- 2nd Generation-Staircase methods (Standard Full Threshold Testing Algorithm)
- 3rd Generation-Swedish Interactive Threshold Algorithm (SITA)
- 30-2-Central 30 deg with tested points 6 deg apart( offset 3 deg from both horizontal and vertical axes)-total 76 locations
- 24-2-Central 24 deg with grid spacing same like 30-2 i.e. 6 deg total 54 locations
- 10-2-68 locations spaced on a 2 deg within 10 deg of fixation
- MACULA TEST:
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
THRESHOLD STRATEGIES
l Standard full threshold
l Fastpac
l SITA :Swedish Interactive Threshold Algorithm
-SITA Standard
-SITA Fast
| SITA STANDARD | STANDARD FULL THRESHOLD |
| 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) |
| SITA FAST | SITA STANDARD |
| 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 |
| FASTPAC | SITA FAST |
| 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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