Maddox rod and scale

This is not a rod but a series of cylindrical grooves ground usually into a coloured piece of glass and mounted in a rim. (Originally it consisted of a single cylindrical rod.) It is used to measure heterophoria by placing it in front of one eye of a subject viewing a spot of light binocularly. The Maddox rod and eye together form a long streak of light perpendicular to the axis of the grooves and this retinal image is so unlike the image formed in the other eye that the fusion reflex is not stimulated. The eyes will then stay in the passive position. If there is a phoria the streak of light will not intersect the spot of light. For horizontal phorias the rod axis is placed horizontally and for vertical phorias, vertically. The amount and type of the phoria can be quantified by placing a prism of appropriate power and direction in front of either eye such that the streak appears superimposed on the spot of light. Alternatively, the angle of the phoria could be determined using a Maddox cross and placing a rod in front of one eye; the phoria can be read directly by the patient who indicates where the streak of light appears to cross the scale.

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