Data-driven square-wave jerk definition shows limited diagnostic discrimination in PSP

Frequent square-wave jerks are included in Movement Disorder Society criteria as a suggestive oculomotor feature of progressive supranuclear palsy (PSP), but a standardised operational definition has been lacking. Researchers prospectively recorded eye movements during visual fixation in 25 people with PSP, 32 with Parkinson’s disease (PD), 35 with Alzheimer’s disease (AD) and 88 controls. They compared 32 candidate SWJ definitions using nested cross-validation, then tested the selected definition in an independent cohort of 25 people with PSP, 41 with PD and 25 controls.

The selected definition required saccade amplitudes of 1°–8°, intervals of 20–600 ms, opposing directions within ±45°, and amplitude similarity of 0.25 or less. SWJ rate, rather than the measured morphological features, distinguished PSP. Out-of-fold area under the curve was 0.81 for PSP versus controls, 0.71 versus AD, 0.63 versus PD, and 0.76 versus all non-PSP groups. Results in the external cohort were described as relatively comparable. A rate above 24 per minute had high specificity but low sensitivity for PSP versus PD.

The findings offer a reproducible operational definition for research, but the diagnostic performance does not support SWJ rate as a stand-alone marker. The study does not establish that using this threshold improves clinical diagnosis.