Rigidity in Parkinson's disease: evidence from biomechanical and neurophysiological measures

Area of Science:

• Neuroscience
• Biomedical Engineering
• Movement Disorders

Background:

• Rigidity is a key motor symptom in Parkinson's disease (PD).
• Objective measurement and understanding of its pathophysiological basis are lacking.
• Current understanding needs to differentiate neural and visco-elastic components of muscle tone.

Purpose of the Study:

• To objectively measure parkinsonian rigidity using a robotic device.
• To discriminate biomechanical sources of muscle tone (neural vs. visco-elastic).
• To clarify the contribution of neurophysiological responses, like long-latency reflexes, to objective rigidity.

Main Methods:

• Twenty PD patients and 25 controls underwent robot-assisted wrist extensions at varying angular velocities.
• Biomechanical (elastic, viscous, neural components) and neurophysiological (short/long-latency reflexes, shortening reaction) measures were synchronously assessed.
• Measures were correlated with clinical rigidity scores (Unified Parkinson's Disease Rating Scale).

Main Results:

• Objective rigidity in PD patients increased with angular velocity.
• PD patients showed elevated long-latency reflexes, but not short-latency reflexes or shortening reaction, compared to controls.
• Long-latency reflexes also increased with angular velocity specifically in PD patients.
• Specific biomechanical and neurophysiological abnormalities correlated with clinical rigidity scores.

Conclusions:

• Objective rigidity in PD is linked to velocity-dependent abnormal neuronal activity.
• Elevated long-latency reflexes are a key neurophysiological contributor to PD rigidity.
• Findings suggest a putative subcortical network underlying objective rigidity in PD.