In the Worth 4-dot test, a red glass is worn in front of 1 eye and a green glass in front of the other (Fig 4-10). The eye behind the red glass can see red light but not green light because the red glass blocks this wavelength. Similarly, the eye behind the green glass can see green light but not red light. A polarized Worth 4-dot test is available; it is administered and interpreted much like the traditional test except that polarized glasses are worn rather than red and green ones. As with the red-glass test, the Worth 4-dot test can produce a diplopic response in nonsuppression heterotropic NRC and either a diplopic or a fusion response in ARC, depending on the depth of the ARC adaptation. As mentioned earlier, this test must be performed in conjunction with cover testing.
When testing a patient for monofixation syndrome (see the section Monofixation Syndrome later in this chapter), the Worth 4-dot test can be used to demonstrate both the presence of peripheral fusion and the absence of bifixation. The standard Worth 4-dot flashlight projects onto a central retinal area of 1° or less when viewed at 10 ft, well within the 1°–4° scotoma characteristic of monofixation syndrome. Therefore, patients with monofixation syndrome will report 2 or 3 lights when viewing at 10 ft, depending on their ocular fixation preference. As the Worth 4-dot flashlight is brought closer to the patient, the dots begin to project onto peripheral retina outside the central monofixation scotoma until a fusion response (4 lights) is obtained. This usually occurs between 2 and 3 ft.