Worth's Four Dot Test - WFDT

 Worth’s four dot test (WFDT)

WFDT provides information on all aspects of binocular vision including state of retinal correspondence.

Principle:
The test is based on complementary colours. The white light provides fusional stimulus. Red and green light provide dissociation.

Apparatus:
Red-green google
Worth’s - 4 light - two green, one red and one white presented on black or grey background.

Three sizes, to be viewed at 6meter, 0.5meter and 33cm for testing both central and peripheral BSV. The overall diameter of the four lights subtends an angle of 1.25 degree at 6 meter and 0.5 meter.

Precautions:
Patient must be able to count and differentiate red from green.
Keep red-green glasses before switching on the test.
If light is reflected by the coloured glass covering the apertures or if background is grey rather than black, non-illuminated circles may be visible. The examiner should ensure that the patient describes only those dots that are lighted up.

Methods:
Patient sits at a desired distance, generally 6 meters away from the test object. Put red glass in front of right eye and green in front of the left eye. Switch on the test. Ask to the patient, how many coloured dots patient sees with both eyes open.

Modification:
Green light gives a stronger stimulus, if it is placed in front of the suppressing eye it may the patient aware of the image.

Result:
Possible responses are:
1.    Four lights: one red, two green, one fluctuating mixture of red and green indicates;
i)    Normal fusion (no manifest deviation).
ii)    Abnormal fusion (ARC) – (with manifest deviation).
2.    Three green – right eye suppression.
3.    Two red – left eye suppression.
4.    Red and green light alternate – alternating suppression.
5.    Five lights: two red and three green – diplopic response
i)    Red on right, green on left – uncrossed diplopia – esotropia
ii)    Red on left, green on right – crossed diplopia – exotropia use prism to differentiate normal and abnormal projections.
a.    Light superimposed with a prism equal to angle of deviation – normal retinal correspondence.
b.    Change of crossed to uncrossed or reverse of dots with prism equal to amount of deviation – abnormal retinal correcspondance.

*patient with alternate suppression occasionally may give diplopia response – hence look for behaviour of eyes behind glasses.
Value: simple test to detect retinal correspondence and state of binocular single vision