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A visual field test is generally done in order to assess the extent of the individual peripheral and central visual field. There are several important parameters that must be considered while screening in an optometric practice they are thresholds. A threshold the intensity at which the subject can just detect the stimulus is called absolute threshold and differential threshold is when the subject can detect a difference between the two stimuli presented eg. When a stimulus is superimposed on a background. When the subject is able to detect the size of the letter it can be termed as acuity threshold.
Reliability is an important factor in any visual field examination. Parameters to assess reliability of a test are false positives, false negatives and fixation losses.
FALSE POSITIVE:- False positive are noted when the patient responds to a stimulus when no stimulus is presented never reaches zero.
FALSE NEGATIVE:- False negative response is noted in case the subject does not respond to a stimulus. It never reaches 100%.
FIXATION LOSSES:- Fixation losses are noted when the patient does not fixates properly and keeps moving his eyes.
GLOBAL INDICES :-
MD represents is an indication of the overall visual field sensitivity and is a negative or positive value depending on an individuals contrast sensitivity more than or less than average for that same group.
PATTERN STANDARD DEVIATION:-
Indicates how each test location on average deviates from the age adjusted values of the normal population after adjustments like general depression or suersensitivity . PSD = 0 shows that the visual field has no deviation from height adjusted visual field of a normal human of the same age group. The more the PSD the more will be the amount of irregularities with steeper slopes in the visual field.
SHORT TERM FLUCTUATION:-
It can be assessed when 10 test points are tested twice , so that the difference can be noted , and the subjects response can be noted.
CORRRECTED PATTERN STANDARD DEVIATION (CPSD):-
It helps us correlate to what extent the patients hill of vision has been has been shifted from the from the shape of the normal hill of vision according to age.the statistical package (STATPAC) tries to determine the irregularities in the hill of vision by omitting the short term fluctuation results , that could cover a relative scotoma.
The standard visual field test can be classified as conventional and unconventional tests. The conventional tests can be further divided in to kinetic perimetry and static perimetry. Below is the description of few test that were performed.
Full threshold algorithm:-
The test was invented by bebie and spahr for use in the octopus perimeter.the up and down staircase technique was used. The step size was 4 dB decreasing to 2 dB after the first reversal and it terminated after the second reversal. Threshold was recorded when the subject last responded to the stimulus.
Dynamic threshold algorithm:-
There is reduced test time with reduced precision of the test. The dynamic can be joined with two or more reversal techniques.variability µ 1/sensitivity. Step size is a factor that relies on sensitivity. Therefore when sensitivity is reduces step size increases.
Tendency oriented algorithm:-
The main of this test is faster testing each test point is tested once. There are 4 submatrixes.
Binary search algorithm:-
presented in centre of the range. Half the step size after each presentation.problem with unrecoverable error
The Henson pro 6000 perimeter was brought forward in the year 2000. its size is comparatively smaller than Henson pro 3500/5000 and includes a built in touch screen along with a camera that projects the image of the subjects eye. The CCD camera field of view increases so that the while performing binocular tests so that the examiner can view both the eyes and still look at the test. Reducing the size of the instrument has been made possible by reducing the size of the bowl. The radius of the bowl is 25cm.
SITA algorithm :-
Swedish interactive threshold algorithm. shorter testing time and equivalent precision to full threshold strategy.modified ZEST algorithm. ZEST uses probability density functions that helps decide the test level.SITA starts at 4 seed locations.there is random peresntation of stimuli.
Frequency doubling technology was invented by Welch allyn .Now its made by zeiss.it is developed by the chris Johnson group.it is based on the concept of an illusion..it is portable relatively independent of room illumination and helps fast screening of individuals less than 1 min/eye and thresholds approx 4-5.5 min/eye.
The spatial frequency grating is low and is less than 1 cycle / degree whereas the counterphase flicker is greater than 15 Hz. It test 16 locations and each is 10 ' 10 degree squares.0.25 cpd sinusoidal gratings. 720 msec presentation. 25hz counterphase flicker . FDT makes use of four reversal staircase threshold strategy known as modified binary search algorithm ( MOBS).
Sita fast stops at a higher standard deviation.
Sita fast will be faster less presentations but less accurate in the final value
FDT perimetry showed large visual field defects whereas SWAP showed small defects.