Parks Three Step
Discussion
If the right eye is the hyperdeviated eye we know that the muscle that is failing is either responsible for pulling the right eye down or for pulling the left eye up. Our suspects in the right eye are the RSO or RIR. In the left eye it’s the LSR or LIO.
We can eliminate all others as possibilities.
If the left eye is the hyperdeviated eye we know that the muscle that is failing is either responsible for pulling the left eye down or for pulling the right eye up. Our suspects in the right eye are the RSR or RIO. In the left eye it’s the LSO or LIR.
We can eliminate all others as possibilities.
When we look in right gaze the muscles most responsible for the eyes' vertical position are the right rectus muscles and the left oblique muscles. Therefore our suspects are RSR, RIR, LSO, LIO.
The other four can be eliminated.
When we look in left gaze the muscles most responsible for the eyes' vertical position are the left rectus muscles and the right oblique muscles. Therefore our suspects are LSR, LIR, RSO, RIO.
The other four can be eliminated.
When the head is tilted to the right, the right eye needs to turn inward (incyclotorsion) and the left eye needs to turn outward (excyclotorsion). The muscles responsible for these movements are the RSR, RSO and the LIO, LIR. One of these could be malfunctioning.
The other four can be eliminated.
When the head is tilted to the left, the right eye needs to turn outward (excyclotorsion) and the left eye needs to turn inward (incyclotorsion). The muscles responsible for these movements are the RIO, RIR and the LSO, LSR. One of these could be malfunctioning.
The other four can be eliminated.
The Park's Three-Step Test is used to isolate the paretic muscle in acquired vertical diplopia.
The Park's Three-Step Test is not intended to be used for assessing horizontal diplopia, nor will it provide useful information in a restrictive strabismus or neuromyopathic conditions. This test is most appropriate for neuroparalytic disorders. Furthermore, be aware that patients who have had extraocular muscle surgery may not have predictable results.
The Park's Three-Step Test works by observing the vertical deviation in primary gaze, left and right gaze, and right and left head tilt. Where the hyperdeviations are of the greatest magnitude indicate where the paretic muscle should be working maximally. By comparing these deviations with each muscle's known primary field of action we can systematically eliminate them until there is only one muscle remaining.