A New Standard in Objective Measurement of Spasticity

Abstract

Abnormalities of muscle tone are an integral component of many chronic motor disorders affecting the central nervous system (CNS) in children and adults. Excessive, disabling muscle tone is called ‘spasticity’. Spasticity can interfere with movement and can lead to stiff, painful joints [1]. Over half a million people in the United States and 12 million worldwide are affected by spasticity. Spasticity results from dysgenesis or injury to sensorimotor pathways in the cortex, basal ganglia, thalamus, cerebellum, brainstem, central white matter, or spinal cord. Injury occurring in children is known as cerebral palsy (CP) [2]. In adults, CNS injury or disease associated with spasticity includes stroke, spinal cord injury, multiple sclerosis and traumatic brain injury. Although the presence or absence of spasticity can be identified using current clinical scales, the accuracy of determination of spasticity severity and the relationship between severity level and deficits of voluntary movements remain elusive (see [3,4]). These questions are of interest to both researchers and clinicians, because the precise quantification of spasticity is important to establish medical and physical therapeutic effectiveness. We designed a new measure of spasticity, called the Montreal Stretch Reflex Threshold (MSRT) measure, based on a) Lance’s definition [5] of spasticity as "a motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex arc" and b) the threshold control theory of motor control [6]. This concept of measurement based on the velocitydependent stretch reflex threshold is a departure from the more common approach that characterizes the resistance to stretch of the passive muscle (e.g., [7]) and is likely to be more related to motor control deficits than resistance measures. If so, this may lead to a better ability to diagnose the motor impairment in spasticity and may lead to improved patient care. Thus, instead of determining the level of the resistance to stretch of the muscle as is usually done to assess spasticity, the main measure in the new device is the threshold joint angle at which the tonic stretch reflex (TSR) starts to produce resistance, which is clinically identified as spasticity.