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A sensor-based, objective measure of upper limb rigidity in Parkinson's Disease based on the pendulum test
Lower Lobby
A sensor-based, objective measure of upper limb rigidity in Parkinson's Disease based on the pendulum test
University of British Columbia
BackgroundRigidity is one of the cardinal symptoms of Parkinson’s disease (PD) along with other motor symptoms such as tremor and bradykinesia. While these symptoms can be easily quantified with modern... Read more

Description

Background
Rigidity is one of the cardinal symptoms of Parkinson’s disease (PD) along with other motor symptoms such as tremor and bradykinesia. While these symptoms can be easily quantified with modern position sensors, assessment of rigidity, which requires determination of resistance to passive motion, remains elusive without external hardware capable of producing motion, such as specialized robotic devices. Currently, rigidity is assessed by the Unified Parkinson’s Disease Rating Scale (UPDRS) where stiffness of a manipulated joint is ranked on a subjective clinical scale of 0-4, and it is susceptible to inter-rater variability. The aim of this study was to utilize modified pendulum test to quantify rigidity in the arm by using lightweight motion sensors, and decreased arm swing as the clinical observation of the earliest manifestations of PD due to increased rigidity around the shoulder girdle.

Methods
The movement of arm swing of nine PD subjects (ON and OFF L-dopa medication) and controls was assessed. Similar to the “pendulum test” to assess spasticity in the lower limb, the upper limb was raised to a horizontal position against gravity and subsequently released. One accelerometer was attached to the wrist and one to the shoulder girdle in order to obtain linear acceleration and angular velocity recordings. Surface electromyography recordings of the anterior deltoid and triceps monitored the myoelectrical features of rigidity. UPDRS was utilized to assess clinical rigidity in both arms of PD subjects. The analysis was greatly extended by using quantitative accelerometers and the physical properties of a subject’s arm to fit dynamic models during the whole period of the swing. Second-order, linear time-invariant (LTI) models were built and model parameters including decay rate and damping ratio were compared.

Results
The damping ratio, which significantly correlated with clinical rigidity scores off medication, was greater in PD subjects (both ON and OFF medication) compared to controls (p < 0.0035); also, it was significantly decreased after PD subjects took L-dopa medication. The averaged effective decay rate was significant across groups: the highest decay rate was seen in PD OFF subjects, followed by PD ON subjects and controls (ANOVA, p < 0.0158). Clinical rigidity scores in both arms were significantly correlated with mean effective decay rate in PD OFF subjects (p = 0.0012).

Conclusion
The modified pendulum test is an objective quantification of rigidity that is simple, i.e. not requiring large external biomechanical devices, and accurate, as it assesses the whole duration of swing as opposed to first swing excursion. We have shown that motion in the arm in response to gravitational force can result in estimates for rigidity that correlate closely with clinical assessments by clinicians. The distinctive differences in arm swing between PD patients and controls may be helpful for early diagnosis, and for follow up of the pharmacological, physical therapy and surgical treatments. Thus, the framework presented may provide a critical role in the comprehensive automatic assessment of PD.

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