Researchers found that there were several differences between PSP patients and normal controls in terms of eye movements:
* Square-wave jerks are “more frequent, larger, and more markedly horizontal in PSP.”
Here’s a definition from the abstract: “Square-wave jerks (SWJs), consists of saccade pairs that appear purely horizontal on clinical inspection: the first saccade moves the eye away from the fixation target, and after a short interval, the second saccade brings it back toward the target.”
SWJs occur in healthy controls.
* In PSP, there is a “loss of a vertical component in fixational saccades.”
Here’s my layperson definition of a saccade: It’s a constant eye movement that changes the position of gaze. When you stare at something, your eyes are not still. They are constantly moving back and forth as well as up and down.
In PSP, the eyes are no longer able to make these small adjustments up and down (vertically). This is part of the vertical supranuclear gaze palsy that is a trademark symptom of PSP. The authors suggest that “small saccades lose their vertical component in PSP, and this property may help clinicians differentiate PSP from other parkinsonian or cerebellar disorders.”
* In PSP, “normal fixational saccades (microsaccades) are rare.”
The saccades in PSP are not small as in healthy controls. The saccades are larger movements that meet the definition of “square-wave jerks.” As we learned in the first item above, the SWJs in PSP are more frequent and larger than are seen in healthy controls.
I’ve copied the abstract below.
This is the abstract:
Journal of Neuroscience. 2011 Mar 23;31(12):4379-87.
Distinctive features of saccadic intrusions and microsaccades in progressive supranuclear palsy.
Otero-Millan J, Serra A, Leigh RJ, Troncoso XG, Macknik SL, Martinez-Conde S.
Barrow Neurological Institute, Phoenix, Arizona, Veterans Affairs Medical Center, Case Western Reserve University, Cleveland, Ohio, Institute of Clinical Neurology, University of Sassari, Sassari, Italy, California Institute of Technology, Pasadena, California, and University of Vigo, Vigo, Spain.
The eyes do not stay perfectly still during attempted fixation; fixational eye movements and saccadic intrusions (SIs) continuously change the position of gaze.
The most common type of SI, square-wave jerks (SWJs), consists of saccade pairs that appear purely horizontal on clinical inspection: the first saccade moves the eye away from the fixation target, and after a short interval, the second saccade brings it back toward the target.
SWJs are prevalent in certain neurological disorders, including progressive supranuclear palsy (PSP).
Here, we developed an objective method to identify SWJs. We found that SWJs are more frequent, larger, and more markedly horizontal in PSP patients than in healthy human subjects.
Furthermore, the loss of a vertical component in fixational saccades and SWJs was the eye movement feature that best distinguished PSP patients from controls. We moreover determined that, in PSP patients and controls, the larger the saccade the more likely it was part of a SWJ. Furthermore, saccades produced by PSP patients had equivalent properties whether they were part of a SWJ or not, suggesting that normal fixational saccades (microsaccades) are rare in PSP.
We propose that fixational saccades and SIs are generated by the same neural circuit and that, both in PSP patients and in controls, SWJs result from a coupling mechanism that generates a second corrective saccade shortly after a large fixation saccade. Because of brainstem and/or cerebellum impairment, fixational saccades in PSP are abnormally large and thus more likely to trigger a corrective saccade, giving rise to SWJs.
PubMed ID#: 21430139 (see pubmed.gov for this abstract only)