The Maddox rod is a dissociating test that will reveal and measure a phoria or a tropia. A dissociating test is a test that presents dissimilar objects for each eye to view, so that the images cannot be fused. The MR test is most commonly used only to measure phorias.
The problem with the Moddox rod test is that it can be confusing to tester, not to mention the testee. You might consider performing the Von Graefe technique. This test gives the same results, it is easier to remember how to perform, and it is usually faster to perform.
The Von Graefe Setup
The patient looks through the photoptor with the distance or near Rx in place.
The target is an isolated (small box) letter on the Snellen chart one line above the BVA of the worst seeing eye. 12 diopters base-in prism is dialed into the Risley prism in front of the right eye. Six diopters base-up prism is dialed into the Risley prism in front of the left eye. The setup should look like this:
The patient should see something like this:
Confirm that the patient sees double. If not, the patient may be suppressing, or not understand. Either way, the test cannot be completed without the perception of two images.
Testing Horizontal deviations:
Always test horizontal deviations first. This is controlled by the Risley prism in front of the right eye. Move the Risley prism wheel slowly in one direction and ask the patient if the images are getting closer or farther apart horizontally. If they are going farther apart, move in the other direction. Ask the patient to tell you when the images are lined up directly one above the other. Record the prism power and base direction on the Risley prism at that point. This is the measure of the horizontal deviation. The end point should look like this to the patient:
A more precise measurement can be made by moving past the endpoint to horizontal separation again and coming back from the other direction to the endpoint once more. Record this second reading and average the two.
A base-in measurement indicates an exo deviation and a base-out measurement indicates an eso deviation.
Testing Vertical deviations:
Move the Risley prism in front of the right eye back to the 12 diopter base-in position. Confirm that the patient is seeing the two separate images. Vertical measurements are controlled by the R-prism in front of the left eye. Move the Risley prism wheel slowly in one direction and ask the patient if the images are getting closer or farther apart vertically. If they are getting farther apart, reverse the direction. Ask the patient to tell you when the images are directly across from one another on a horizontal line. This is the endpoint and it should look something like this to the patient:
Record the base direction and the amount of prism diopters on the scale.
Again, for a more precise measurement, you can dial in more prism power until there is once again vertical separation of the images. The prism power is then reversed until the the images are once again aligned on a horizontal plane. The two measurements are averaged.
Notes: There is nothing magical about the starting diopter values for this test. You can use other values, and sometimes you may have to. For instance, if the patient has a 12 D exophoria, then images will be vertically aligned at the outset. The point is that you want to start the measurement with some degree of vertical and horizontal separation of the images.