Worth 4-Light Test

Key points at a glance

Key points of this test

• The Worth Four Light Test (W4LT) is a clinical test that uses red-green glasses to evaluate binocular vision.
• It is used to assess sensory fusion, suppression, anomalous retinal correspondence (ARC), diplopia, and ocular dominance.
• It can be performed from around age 3 and is widely used to evaluate strabismus and amblyopia in children.
• The test uses one red, two green, and one white light at near (33 cm) and distance (6 m).
• The number of lights seen (2 to 5) and their color arrangement determine fusion, suppression, or diplopia.
• If suppression is present at distance but normal at near, it suggests monofixation syndrome.
• Changing the test distance can estimate the size of the suppression scotoma.

1. What is the Worth Four Light Test?

The Worth Four Light Test (Worth Four Dot Test; W4LT) is a clinical test for evaluating binocular vision. It was published by Worth in 1908 in his book Squint: Its causes, pathology and treatment.

This test separates the two eyes with red-green glasses and evaluates the following:

• Sensory fusion: whether the images from both eyes can be superimposed into a single perception¹⁾
• Suppression: whether the image from one eye is ignored by the brain
• Anomalous retinal correspondence: whether a non-foveal retinal area in the deviating eye functions as the corresponding point
• Diplopia: whether two separate images are perceived
• Dominant eye: The color through which the white test light is perceived

The Worth 4-dot test uses red-green filters to create binocular dissociation that differs from everyday vision, and can be measured regardless of eye position. It is the simplest binocular vision test and can be performed from around 2.5 to 3 years of age. For children, there is also the Berens 3-dot test (using a red doll, green elephant, and white circle as targets).

Q At what age can the Worth 4-dot test be taken?

A

It can be performed from around 3 years of age. The child must be able to verbally report how the targets appear. For children, the Berens 3-dot test with pictures is also used.

2. Findings that can be evaluated with the test

Patient report (subjective findings)

During the test, based on the number and color arrangement of lights the patient sees, the state of binocular vision is determined as follows.

Normal fusion

4 lights: In patients with orthophoria (normal eye alignment), this indicates normal binocular fusion. The white light is perceived in the color of the dominant eye’s filter (red or green), or appears as a mixture of red and green.

Suppression

2 red lights: Left eye suppression. The image from the green filter side is suppressed in the brain.

3 green lights: Right eye suppression. The image from the red filter side is suppressed.

Alternating suppression: A state where 3 green and 2 red lights are seen alternately. It is necessary to confirm whether 5 lights are seen simultaneously or alternately.

Diplopia

5 lights (red on the right, green on the left): Uncrossed diplopia. This is the expected result in esotropia. If this pattern appears in a patient with exotropia, it suggests anomalous retinal correspondence.

5 lights (green on the right, red on the left): Crossed diplopia. This is the expected result in exotropia. If this pattern appears in a patient with esotropia, it suggests anomalous retinal correspondence.

Anomalous retinal correspondence

4 lights with manifest strabismus: Indicates harmonious anomalous retinal correspondence. Fusion is present despite misalignment.

Diplopia pattern inconsistent with the direction of strabismus: If crossed diplopia occurs in esotropia or uncrossed diplopia in exotropia, it also suggests anomalous retinal correspondence.

Clinical interpretation (physician’s assessment)

Interpretation of test results requires knowledge of eye alignment.

• Orthophoria with 4 lights → Normal correspondence, fusion present
• Manifest strabismus with 4 lights → Harmonious anomalous correspondence
• 2 or 3 lights → Suppression in one eye
• 5 lights → No suppression, diplopia present. The amount of prism needed to reduce to 4 lights can be measured. If it matches the objective angle of strabismus, it indicates normal correspondence; if different, anomalous correspondence.
• Suppression at distance, normal at near → Monofixation syndrome

Q If 5 lights are seen, what does it mean?

A

It indicates the presence of diplopia. Based on the position of red and green lights, determine whether it is uncrossed diplopia (esotropic type) or crossed diplopia (exotropic type). If it does not match the direction of strabismus, it suggests anomalous retinal correspondence. For details, see “Principle of the test” section.

3. Indications and candidates for the test

The main indications for the Worth 4-dot test are as follows.

• Strabismus: Evaluation of binocular vision (fusion, suppression, retinal correspondence) in manifest strabismus
• Amblyopia: Used to evaluate sensory fusion ¹⁾
• Postoperative evaluation: Confirmation of binocular vision recovery after strabismus surgery
• Detection of monofixation syndrome: Estimation of suppression scotoma from discrepancy between near and far results

Note on examination order

Sensory function tests are recommended to be performed before tests involving dissociation, such as cover tests or monocular visual acuity measurement ¹⁾. This is because occlusion of one eye alters the binocular state.

The target age is approximately 3 years and older. The test is performed in a bright room, evaluating peripheral fusion at 1 m and central fusion at 5 m.

4. Examination procedure and interpretation of results

Equipment

• Red-green glasses: Red filter over the right eye, green filter over the left eye
• Worth 4-dot test: Flashlight type or wall-mounted display. Four lights (1 red, 2 green, 1 white) arranged in a diamond pattern

The arrangement of the lights is as follows:

• 12 o’clock: Red
• 3 and 9 o’clock: Green
• 6 o’clock: White

Examination Procedure

1. Check eye position under refractive correction
2. Place red-green glasses over the patient’s usual glasses or contact lenses
3. Present the target at both near (33 cm) and far (6 m) distances
4. Ask the patient to report the number, position, and color of the lights seen
5. Also check whether all lights are seen simultaneously or appear and disappear alternately

Room lights should be turned on if they promote fusion, or turned off if they enhance dissociation during the test.

Test Distance and Stimulus Angle

The angle at which the target is projected onto the retina changes depending on the test distance. This property can be used to estimate the size of the suppression scotoma.

Test Distance	Stimulus Angle
1/6 m	12 degrees
1/3 m	6 degrees
1/2 m	4 degrees
1 m	2 degrees

Detection of Monofixation Syndrome

In monofixation syndrome, a central suppression scotoma of 1 to 4 degrees is usually present. At distances of 3 m or more, the light of the target projects onto the central retina within 1 degree, so it falls within the suppression scotoma and is not perceived. At near distances, the light projects outside the scotoma and can be perceived binocularly.

For example, in a right esotropia of 8 prism diopters (4 degrees), the following occurs:

• Near (1/3 m): The four lights project onto the retina at a distance of 6 degrees. Even with a 4-degree suppression scotoma, they project outside the scotoma, so all four lights are visible.
• Distance (6 m): The four lights project onto the retina at a distance of 1.25 degrees. They fall within the 4-degree suppression scotoma, so the right eye’s lights are not visible, and only the three lights of the left eye are seen.

Q Why do test results differ between near and far distances?

A

Because the retinal projection angle of the target changes with test distance. At near distances, the projection angle is larger and more likely to fall outside the suppression scotoma, while at far distances, the projection angle is smaller and more likely to fall within the scotoma. This difference can be used to detect monofixation syndrome.

6. Principle of the Test (Optical Mechanism)

Binocular Separation with Red-Green Filters

Red and green are complementary colors. Through a red filter, red targets are visible but green targets are blocked. The green filter does the opposite. White targets are visible through either filter.

Based on this principle, the right eye (red filter) perceives two lights (red and white), and the left eye (green filter) perceives three lights (two green and one white). The white light is a common target for both eyes; if fusion occurs, it is perceived as a mixture of red and green, or as the filter color of the dominant eye.

Position in binocular vision function

Binocular vision function consists of three functions: simultaneous perception, fusion, and stereopsis. Fusion is further divided into sensory fusion and motor fusion.

• Sensory fusion: The function of superimposing images from corresponding points on both retinas to achieve single vision.
• Motor fusion: The function that prevents the manifestation of eye misalignment and maintains sensory fusion. Normal values are approximately 25 degrees for convergence, 5 degrees for divergence, 1–2 degrees for vertical, and 8 degrees for cyclorotational.

The Worth 4-dot test primarily evaluates sensory fusion and suppression. For quantitative assessment of motor fusion, prism bars or rotary prisms are used ¹⁾, and for stereopsis evaluation, the Randot stereo test or similar is used.

Comparison with other binocular vision tests

The Worth 4-dot test separates the two eyes with red-green glasses, creating a test environment far removed from everyday vision. The degree of this binocular dissociation affects the interpretation of results.

Test method	Degree of binocular dissociation	Characteristics
Bagolini striated lens test	Small	Close to everyday vision
Worth 4-dot test	Large	Abnormal correspondence is more likely to appear
Major amblyoscope	Large	Testable even with manifest strabismus

Tests with greater binocular dissociation tend to detect anomalous correspondence more easily. However, it should be noted that anomalous retinal correspondence detected in tests closer to everyday vision does not necessarily normalize more easily with treatment. It is recommended to perform tests in order from those farthest from everyday vision to comprehensively assess the degree of retinal correspondence abnormality.

Q What is the difference between the Worth 4-dot test and the Bagolini striated lens test?

A

The Bagolini striated lens test has low binocular dissociation and examines sensory fusion, suppression, and retinal correspondence in a state close to everyday vision. The Worth 4-dot test uses red-green glasses to dissociate the eyes, creating an environment far from everyday vision, making anomalous correspondence more likely to be detected. Combining both tests allows assessment of the degree of retinal correspondence abnormality.

8. References

1. American Academy of Ophthalmology Pediatric Ophthalmology/Strabismus Preferred Practice Pattern Panel. Amblyopia Preferred Practice Pattern. 2024.
2. Roper-Hall G. The “worth” of the worth four dot test. Am Orthopt J. 2004;54:112-9. PMID: 21149094.
3. Etezad Razavi M, Najaran M, Moravvej R, Ansari Astaneh MR, Azimi A. Correlation between Worth Four Dot Test Results and Fusional Control in Intermittent Exotropia. J Ophthalmic Vis Res. 2012;7(2):134-8. PMID: 23275822.