diplopia

Diplopia is the simultaneous perception of two images of the same object in different positions. In strabismus the cause is an image of one object falling on noncorresponding retinal points of the two eyes.¹

In esotropia the image falls on the nasal retina of the deviated eye and is projected into the temporal field, producing homonymous, or uncrossed, diplopia.^1,2 With a red filter before the right eye of an esotropic patient, the red light appears to the right of the white light.³ In exotropia the image falls on the temporal retina and is projected into the nasal field, producing heteronymous, or crossed, diplopia. With the red filter before the right eye, the red light then appears to the left of the white light.^1,2,3

Fusional vergence can control diplopia, and occluding one eye abolishes binocular diplopia.² Some patients mean blur or a partial overlap of contours when they report double vision, so a prism before one eye quickly shows whether true diplopia is present.⁴ Double vision with no visible deviation should raise the possibility of monocular diplopia, and covering each eye in turn shows whether the diplopia is monocular or binocular.⁴