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Module 30 |
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Module 30: |
More EOM Related Skills: |
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Section 1: |
The Maddox Rod Test and the Von Graefe Technique | ||
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Module 30 |
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Module 30: |
More EOM Related Skills: |
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Section 1: |
The Maddox Rod Test and the Von Graefe Technique | ||
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The Maddox Rod
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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. (Review "tropia" and "phoria").
The Maddox rod can be found in a trail lens set, as part of the auxiliary lens wheel on a phoropter, and on the end of some occluders. It is a group of cylinders (usually red) lined up side by side that makes a point light source appear as a line to the patient. The line runs perpendicular to the orientation of the rods. Below are pictured the auxiliary lens wheels of an AO type phoroptor. On the right eye wheel is a setting for a red horizontal maddox rod (RMH) and a red vertical maddox rod (RMV).
On the left eye wheel is a setting for a white horizontal maddox rod (WMH) and a white vertical maddox rod (WMV).
Pictured below is the appearance of the red vertical maddox rod in the right eye port, and the white horizontal maddox rod in the left eye port.
If you want to look for or measure a horizontal deviation, place the MR (white or red) in the horizontal orientation before one eye. The fellow eye does not have a MR in place. Use the distance or near refractive correction for each eye. Hold a penlight, muscle light, or the beam from a direct ophthalmoscope at distance or at near. Do this in a dark or dimly illuminated room. The light source must be bright. The patient will see the point source of light (white) with one eye and a vertical line (red or white) created by the MR in front of the other eye. Let us assume that we are using a phoroptor and the horizontal red maddox rod (RMH on the wheel) is in place in front of the right eye. Ask the patient if he sees a white dot and a red line. If the patient cannot see one or the other, then he may be suppressing the image and the Maddox rod test will not work on him. It may be that the point light source is not bright enough and/or the room illumination is not dim enough. Give the patient enough time to "find" both images. If the patient sees both the point light source and the line, then ask if the line goes through the light, or if it appears to be to the left or to the right of the light. If the red line appears to go through the white dot (as pictured below), then the patient has no horizontal deviation.
If the MR is held over the right eye and the patient indicates the red line is to the right of the white dot, then the diplopia is uncrossed and indicates either an esophoria or an esotropia.
If the MR is held over the right eye and the patient indicates the red line is to the left of the white dot then the diplopia is crossed and it is an exo deviation.
Try the following memory technique to make the evaluation easier: If the MR is before the left eye and the red line appears to the right of the white dot then the image has crossed over. Associate crossed with the letter X. The letter X is part of the abbreviation for exotropia, which is XT. So a crossed diplopia is an exotropia (or exophoria). An uncrossed diplopia is an esotropia (or esophoria). How do you know if it is a phoria or a tropia? The Maddox rod test will not tell you this. You may know whether or not there is a tropia present from another test, such as a cover test or from the patient’s complaint of double vision . The MR is useful as a measuring test used with prisms to quantify the deviation (tell you how many prism diopters the deviation is). The MR is also useful for detecting deviations that are not obvious with the cover tests. If you want to measure a vertical deviation, place the Maddox rod in a vertical orientation (RMV) and the patient will see a horizontal line. Ask the patient if the red line goes through the white dot or if it is above or below the white dot.
If the MR is over the right eye (RMV) and the line appears to be below the white dot, then the patient has a right hyperphoria (upward deviation) or a right hypertropia. If the line appears to be above the white dot, then the patient has a left hyper deviation (or right hypo; a vertical deviation is usually identified by whichever eye is hyper). As mentioned earlier, the Maddox rod test can be used to quantify (measure) the deviation. The Risley prisms on the phoroptor can be used for this purpose. If a trial frame is used to hold the MR, then a prism bar can be used to measure the deviation.
The Risley prism on the phoroptor is pictured below (RP). It is on the same arm that holds the cross-cylinder (CC).
The picture above shows the R-prism rotated into place in front of the left eye port. The R-prism is an ingenious device consisting of two rotating 10 diopter prisms mounted in a metal case. The prisms are rotated by means of a small wheel on the case (lower arrow in picture below). When the prisms are aligned base-to-base, the total prism strength is 20 diopters. When the prisms are aligned apex-to-base, they cancel each other and the total prism strength is zero. Any prism power from zero to 20 diopters can be dialed in. A small arrow on the face tells you how much power you have dialed in (upper arrow in picture below).
The R-prism an be oriented horizontally by rotating the zero position on the scale to a click stop at the 90 degree position, as picture in the top photo above. Notice in the top photo that the small arrow on the R-prism is pointed toward the "3" on the right side of the "0". This indicates that the prism power is 3 diopters, and the fact that the arrow is to the right of the zero indicates that the base direction of the prism is in that direction. Since the R-prism is in front of the left eye, the base direction is "out". If the arrow was pointing toward the "3" on the left (your left) side of the "0", then the prism would be 3 diopters base-in.
The picture above shows the R-prism in the vertical prism orientation, with 3 diopters of base-up prism dialed in. The R-prism is used to superimpose the dot and the line; to bring them together. The amount of prism diopters and the direction of the prism base are measurements of the degree of deviation.
MR Procedures
To measure a horizontal deviation: The RMH is placed before the right eye and the R-prism is placed before the left eye with the "0" at the 90 degree click stop (12 o'clock). The R-prism power should be zero initially. The patient should see a vertical red streak with the right eye and a bright white dot with the left eye. If the streak appears to the patient to go through the dot, then the patient is "ortho" (no deviation) and no further horizontal measurement is necessary. If the streak appears to the right of the dot, then an uncrossed (eso) deviation is present. Eso deviations are measured with base-out prism. Remember that the apex of the prism points toward the direction of the deviation. Dial in prism power on the R-prism, moving the pointer toward the patient's temple for base-out prism. Stop when the patient reports that the line and the dot have come together. Record the prism amount and direction (BO) for the horizontal measurement. If the streak appears to the left of the dot, then a crossed (exo) deviation is present. Exo deviations are measured with base-in prism. Dial in prism power on the R-prism, moving the pointer toward the patient's nose for base-in prism. Stop when the patient reports that the line and the dot have come together. Record the prism amount and direction (BI) for the horizontal measurement.
To measure a vertical deviation: The RMV is placed before the right eye and the R-prism is placed before the left eye with the "0" at the 180 degree click stop (9 o'clock). Turn the R-prism wheel so that the pointer is on "0" initially. The patient should see a red horizontal line with the right eye and a white dot with the left eye. If the streak appears to the patient to go through the dot, then the patient is "ortho" (no deviation) and no further vertical measurement is necessary. If the streak appears above the dot, then the right eye is pointing downward compared to the left eye (a right hypo-deviation), or you can think of it as the left eye pointing upward compared to the right eye (a left hyper-deviation). If you have trouble with this concept, remember how retinal projection works. Move the pointer on the R-prism upward to dial in base-up prism until the patient reports that the line and the dot are together. Record the prism amount and direction (BU) for the vertical measurement. If the streak appears below the dot, then the right eye is pointing upward compared to the left eye (a right hyper-deviation). Move the pointer on the R-prism slowly downward until the patient reports that the line and the dot have converged. Record the prism amount and direction (BD) for the vertical measurement. If a MR is used with a trial-frame, or if a MR is simply held in front of the patient's glasses lens, then a prism bar can be used to measure the deviation. A prism bar is particularly useful for this purpose. The prism bar is held up in front of the Maddox rod with the apex of the prisms pointing in the direction of the deviation. For example, if the MR is before the left eye and the diplopia is crossed, then we are dealing with an exo deviation and the apex of the prisms would point temporally or toward the patient’s left. The prism bar is moved from the lowest power toward the highest as the patient is asked to indicate when the line and the dot are superimposed. This is the endpoint. The prism power that does the job is equivalent to the amount of deviation in diopters.
The Von Graefe technique is a "quick and dirty'' technique for measuring phorias. It may not be as precise as Maddox rod measurements, but it is more "user friendly", and it is usually quicker. It is performed with the Risley prisms on the phoroptor.
Testing Horizontal deviations:
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. Another short cut: Some phoropters have the lenses needed available on the auxiliary lens wheel. Pictured below is a left aux. lens wheel with a 10 prism diopter base-in lens setting. The right side aux. wheel has a 6 prism diopter base-up lens.
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