The Parks Three Step was first introduced by Bielschowsky in 1935 to isolate paralytic eye muscles, and is used primarily to localize the responsible muscle in oblique or cyclovertical deviations to pinpoint cranial nerve dysfunction or to plan eye muscle surgery for non-comitant strabismus. By definition a hypertropia exists on the cover test in primary gaze, and changes as the patient gazes to the left or right, and then tilts left or right. The first step of the test involves no eye or head movement; the second step involves eye movement; and the third step involves head and neck movement with the eyes counter-rolling in doll’s eye fashion. It is well-known that the Parks Three Step becomes less accurate in identifying the paretic muscle over time due to the spread of comitance. Sensory correlates of the Parks Three Step are usually limited to something like the Hess-Lancaster Screen, most recently computerized by Spectrum Software.
The Hess Test principle is also inherent in the plot of motor fields used in Home Therapy Systems’ Motor Field Test. The limitation of the Hess Test of course is that eye movements to the nine cardinal positions of gaze are involved but head and neck movements are purposely limited. It struck me one day that I’d like to combine the motor elements of the Parks Test with the sensory elements of the Hess Test and voila: Parks + Hess = Press (okay, a little bit of poetic license there) as depicted in Part 1.
The reason I’m placing emphasis on all three planes of neck rotation is that it activates the COR, or Cervico-Optical Reflex. The COR is part of the reflex arc providing positional awareness through proprioceptive feedback to the EOM nuclei.
Though the VOR (Vestibulo-Ocular Reflex) tends to get all the headlines (no pun intended), the gain of the COR and the gain of the VOR are mutually interactive in balancing eye, head, and neck movement to maintain fusion through postural interaction with the extraocular muscles. The two clinical conditions in which patients typically report cervical or neck discomfort are torticollis and whiplash, but there are subclinical version of this where patients have more subtle discomfort, headaches, dizziness, or vertigo associated abnormalities of the COR.
I am therefore proposing that the Press Three Step serves as clinical biomarker for a lag in the plasticity of the gain of the COR as the patient actively rotates his neck through the three principal planes – horizontal, vertical, and oblique. In practical terms, we’re looking for diplopia or difficulty fusing in a particular plane in contrast to its polar coordinate. In other words, if as the patient maintains fixation on the target rotating L with eyes to the R the target doubles, then rotating R with eyes L should result in more comfortable fusion. The patient’s habitual head posture in these cases is a vector representing the coordinates for best fusion.
Why bother? On to the applications of therapy and prism in Part 3.