Clinical and brain functional correlates of instrumental rigidity measurement in Parkinson’s disease

Rigidity, a cardinal symptom of Parkinson's disease (PD), remains challenging to assess objectively. A torque-angle instrument was developed to quantify muscle tone, providing two parameters: bias difference and elastic coefficient. This study aimed to investigate the association of the instrument-measured rigidity with clinical assessments and brain function. In 30 patients with PD, the muscle tone in both arms was evaluated. Ten with wearing-off phenomenon were assessed twice, off and on condition. Twentynine patients underwent brain perfusion single-photon emission computed tomography (SPECT), and expression of PD-related covariance pattern (PDRP) was computed. Bias difference and elastic coefficient showed positive correlations with physician-rated rigidity (P < 0.002). Bias difference decreased after dopaminergic medication (P = 0.022) and was associated with lower body mass index (P = 0.012). Elastic coefficient positively correlated with the Unified PD Rating Scale Part III and PDRP scores (P < 0.044). Furthermore, the higher bias difference correlated with decreased sensory-motor cortex and increased substantia nigra perfusion (P < 0.001). The Torque-angle instrument is a viable tool for quantifying rigidity in PD. The bias difference reflects treatment responsiveness and is associated with the function in the sensory-motor cortex and substantia nigra. The elastic coefficient is indicative of overall Parkinsonism severity.