The Maddox rod is an eye test used to detect and measure heterotropias (tropias) and heterophorias (phorias). It is a dissociating test or an evaluation where the patient is presented with different objects for each eye to look at such that the images can't be merged. The test uses the principle of diplopic projection.
The Maddox rod is a portable device made of a white or red horizontal plano-convex cylindrical lens. It refracts light so that a light source appears as a line of light that is vertical to the central line of the cylinder. Fusion is disrupted when a Maddox rod is put before one of the patient's eyes. The patient will see two very distinct images instead of a single three-dimensional (3D) image.
The dissociation of the deviation created by placing a red Maddox rod that produces a line image to one eye while presenting the other eye with a white light that creates an impression of a round light is called diplopic projection. Prisms are employed to superimpose the round light and the streak image to assess the angle of deviation. The prisms power is gradually increased until the line image bisects the round light passing through the middle of the prism. The power of the lens shows the degree of deviation.
However, the Maddox rod test can't be used in patients who have sensory abnormalities such as suppression.
The commonly used tests are:
- Maddox rod test
- Double Maddox rod test
The MR test can be done at the eye clinic or doctor's office by:
- An optometrist
- An ophthalmologist
The test is a subjective evaluation that measures tropias and phorias at near and far fixation distances. Tropias are manifest deviations while phorias are latent deviations. The MR test on its own can't distinguish between a tropia and a phoria. It may be combined with the cover test.
If the unilateral cover test shows that the patient has a motor alignment, then the single MR test is a measure of phoria. Often, the test is only used to measure phorias. If the unilateral cover test detects manifest strabismus or strabismus, the single Maddox rod test gives a measure of the subjective angle of the strabismus.
The main advantage of using the test in the diagnosis of tropia is that it can measure vertical, horizontal and torsional deviations. The doctor can use a single Maddox rod to measure vertical and lateral deviations and two Maddox rods to measure torsional deviations.
Preparation & Expectation
The MR test doesn't require any preparation. There is no pain or discomfort.
The single Maddox rod test is a simple test that requires a white or red Maddox rod, prism bar and a penlight. The patient will wear corrected vision aids, such as glasses. The lighting in the examination room will be dim. The doctor will place a red Maddox rod in front of the patient's right eye while holding a penlight at 40 centimeters for near or at 6 meters for distance measurement.
S/he will orient the Maddox rod vertically for vertical deviation measurement or horizontally for lateral deviation assessment. The patient will see a red line impression for the right eye and round white light for the left one. For horizontal deviation assessment, the doctor will ask the patient to show where the round light is relative to the red perpendicular line.
The double MR test can help the doctor to assess the torsional deviation that occurs in cyclotropias and cyclophorias. The test is conducted when the patient reports a distorted vision with objects appearing tilted. The examination room will have dim lighting. The doctor will place a Maddox rod in a phoropter or trial frame in front of each of the patient's eyes. S/he will place cylinders into the trial frames vertically, which makes the two red lines horizontal. S/he may also add a vertical prism into the trial frames to separate the red lines.
The test is done at near 40 centimeters and distance 6 meters. The doctor will instruct the patient to fixate on a source of light with both eyes open. S/he may ask the patient to rotate the cylinders with the central handle on the trial frame until the two red lines are parallel. If the patient demonstrates underactive or overactive oblique extraocular muscles, the doctor may repeat the double Maddox rod test for the secondary and tertiary positions of gaze.
For a single MR test, abnormal results may include:
- Crossed diplopia (Exo deviation) - The patient will report seeing the red line to the left of the white round light
- Uncrossed diplopia (Eso deviation) - The patient will see the red line to the right of the round light
- Orthophoria - The patient will see the streak passing through the white round light
- Right hypo deviation - The patient will see the streak passing below the white round light
- Right hyper deviation - The patient will see the streak passing above the white round light
For double MR test, abnormal results might include:
- Torsion - One of the patient's eyes will see the line as horizontal, and the other eye, a line that is obliquely slanted. Sometimes each of the patient's eyes sees a diagonally slanting line
- Deviating eye - When one of the patient's eye perceives an obliquely slanting line while the other sees a horizontal line
- Excyclotorsion - The patient considers the top of the line as slanted outward, i.e., towards the ground
- Incyclotorsion - The patient perceives the top of the line as tilted ‘inward’, i.e., towards the ceiling
Risks & Complications
The Maddox rod test is a non-contact and non-invasive eye exam without any risks or complications.