The red glass test is useful for identifying the presence diplopia & its type of diplopia, suppression, normal & anomalous retinal correspondence Before we start, a very brief orientation. When a person fixates with both eyes on a light, it is seen singly when there is bifoveal fixation. Now if a red glass is placed in front of one eye, the eye with the red glass sees the light as red, whereas the other eye sees it as white, the fusion response produces either a pinkish light or a washed out red color. For purposes of convenience of understanding, I am indicating a fusion response with a pink color If there is a deviation, the image falls on an extrafoveal point of the deviated eye, thus producing diplopia In the same patient, a red glass in front of the fixating eye, shows the diplopia in this manner Now for the test — In a strabismic patient a red glass or filter is placed before the fixating eye The patient views a distant white light to stimulate the fovea of the fixating eye and an extrafoveal area of the fellow eye. In all of the following examples the left eye is the fixing eye & so the red glass will be placed in front of the left eye For every scenario, we have to answer these questions. So in this example, this is what the patient sees — So for these questions Diplopia — present Crossed / uncrossed — uncrossed Suppression? — No ARC / NRC ? — NRC So this is a right esotropia with NRC & absence of suppression …………………………. Scenario 2 → Diplopia — present Crossed / uncrossed — crossed Suppression? — No ARC / NRC ? — NRC So this is a right exotropia with NRC & absence of suppression ……………………………. Scenario 3 → Diplopia — — Crossed / uncrossed — — Suppression? — This is not a fusion response. Only a bright red light is seen, so yes, suppression is present So this is a case of tropia with suppresson. Suppression is a cortical response. The type of deviation cannot be inferred from this particular pattern, but it would already have been determined by the cover test ARC / NRC ? In the presence of a suppression, in order to determine whether the retinal correspondence is normal or anomalous, a vertical prism is placed over the deviated eye, to move the image out of the suppression scotoma. One of two patterns will be seen either there will be a vertical separation in addition to a horizontal separation, which indicates an NRC (the vertical separation being induced by the prism & the horizontal because of the deviation) or just a pure vertical deviation — this indicates an ARC; here the vertical deviation is due to the prism, however the pseudofovea of the deviated eye now has the same visual direction as the true fovea of the fixing eye, so there is no lateral displacement Scenario 4 → Diplopia — — Crossed / uncrossed — — Suppression? — This is not a fusion response. Only a bright red light is seen, so yes, suppression is present So this is a case of tropia with suppresson. The type of deviation cannot be inferred from this particular pattern, but it would already have been determined by the cover test ARC / NRC ? In the presence of a suppression, in order to determine whether the retinal correspondence is normal or anomalous, a vertical prism is placed over the deviated eye, to move the image out of the suppression scotoma. One of two patterns will be seen either there will be a vertical separation in addition to a horizontal separation, which indicates an NRC (the vertical separation being induced by the prism & the horizontal because of the deviation) or just a pure vertical deviation — this indicates an ARC; here the vertical deviation is due to the prism, however the pseudofovea of the deviated eye now has the same visual direction as the true fovea of the fixing eye, so there is no lateral displacement Scenario 5 → This patient has an esotropia. With the red glass test this is what he sees Diplopia — — Crossed / uncrossed — — Suppression? — is absent because this is a fusion response. ARC — yes — a fusion response in the presence of a tropia indicates the presence of an ARC. Scenario 6 → this is another patient of esotropia. A prism is placed in front of the tropic eye to neutralize the deviation. This is what the patient sees? Diplopia — — Crossed / uncrossed — — Suppression? — No, because this is a fusion response. NRC/ARC — NRC This is a right esotropia with NRC & absence of suppression The image now falls on the fovea, this is how a neutralizing prism prevents diplopia in the absence of suppression Scenario 7 → if in the previous scenario, instead of a fusion response the patient sees this, then what? Let’s answer our basic questions & it becomes easy. Diplopia — present Crossed / uncrossed — Crossed Suppression? — No, in the presence of an esotropia, instead of an uncrossed diplopia, we are seeing a crossed diplopia — this is called paradoxical diplopia, & indicates the presence of ARC. The explanation for this is — in ARC, this is the pseudofovea, which has the straight ahead orientation. So even though the image falls on the true fovea, the visual cortex interprets this is being temporal to the straight ahead position because of the ARC & so the image is perceived nasally. This occurs either if surgical correction or prism correction is done without correcting the ARC Similarly, an exotropia with prism correction will show an uncrossed diplopia if there is ARC Clinically, it is most useful to use the red glass test in adults who complain of diplopia but who seem orthophoric on cover testing. It is also a good way to fit adults with prisms. Testing children with the red filter can be difficult.