Spasticity is a motor disorder defined by a velocity-dependent increase in muscle tone, or resistance to passive movement, resulting from an upper motor neuron lesion. This condition arises when damage to the brain or spinal cord disrupts the balance of signals controlling muscle activity, leading to an exaggerated stretch reflex response. The resistance to movement becomes stronger as the speed of the movement increases, often manifesting as involuntary muscle overactivity. Testing for this hypertonia is necessary for diagnosis, allowing clinicians to quantify its severity and plan an effective management strategy.
Defining the Purpose of Spasticity Assessment
Assessment serves several distinct clinical purposes beyond simple diagnosis. The primary goal is to establish a quantitative baseline severity score for affected muscle groups, providing a measurable starting point before intervention begins. This initial scoring helps determine the extent to which involuntary muscle contractions interfere with movement and daily function.
Assessment results guide the selection of the most appropriate treatment strategy. A mild score might indicate a patient would benefit from physical therapy, while a high score might suggest the need for oral medications or targeted interventions like botulinum toxin injections. Clinicians also use the assessment to differentiate between true spasticity and other causes of restricted movement, such as fixed joint contractures or muscle stiffness.
Repeated assessments monitor the effectiveness of chosen interventions over time. A reduction in the severity score following treatment provides objective evidence that the therapy is working as intended. Without this measurable data, adjusting dosages or switching treatment plans to achieve the best functional outcome would be difficult.
Standardized Clinical Rating Scales
The most common method for assessing spasticity involves quick physical maneuvers followed by scoring using standardized, subjective scales that do not require specialized equipment. These scales rely on the clinician’s trained perception of resistance during a quick stretch, which provokes the velocity-dependent reflex. The patient is instructed to relax completely while the examiner moves the limb through its range of motion within approximately one second.
The Modified Ashworth Scale (MAS) is a widely utilized clinical tool for grading resistance, using a six-point ordinal scale ranging from 0 to 4. A score of 0 indicates no increase in muscle tone. A score of 1 represents a slight increase, manifesting as a catch and release or minimal resistance at the very end of the range of motion. The 1+ score, added in the modification, signifies a slight increase followed by minimal resistance through less than half of the remaining range.
Higher scores reflect more pronounced resistance throughout the movement. A score of 2 indicates a marked increase in tone through most of the range, though the limb is still easily moved. A score of 3 means there is considerable resistance, making passive movement difficult. A score of 4 denotes that the affected joint is rigid in flexion or extension. The MAS is valued for its simplicity and speed, making it practical for routine use in clinical settings.
The Modified Tardieu Scale (MTS) is another common assessment tool that provides a more detailed picture by measuring muscle reaction at different velocities. This scale measures two specific angles during passive movement to distinguish between the neural component (spasticity) and the biomechanical component (fixed contracture). The first measurement, R2, is the full passive range of motion (ROM) of the joint, measured at a very slow speed (V1) to avoid triggering the stretch reflex.
The second measurement, R1, is the angle at which a “catch” or resistance is first felt when the joint is moved at a fast speed (V3). The difference between these two angles (R2 minus R1) is known as the dynamic tone component, or the “spasticity angle.” A large difference suggests the resistance is predominantly due to dynamic spasticity, which may respond well to interventions targeting the nervous system.
A small difference between R2 and R1 suggests that the restricted movement is mainly due to a fixed contracture, indicating a poorer capacity for change with spasticity-focused treatments. Clonus testing is also often performed, involving a quick stretch to a muscle (such as the ankle plantarflexors) to see if it elicits sustained, rhythmic, involuntary contractions. The presence and duration of clonus confirm the hyperexcitability of the stretch reflex.
Instrumental and Objective Measurements
Beyond subjective clinical scales, several technological methods provide objective, quantifiable data on spasticity, often used in research or for complex cases. Electromyography (EMG) is a neurophysiological technique that measures the electrical activity produced by skeletal muscles. Surface EMG monitors the involuntary activation of muscles during passive joint movement or at rest.
The EMG data provides a direct measure of motor unit discharge, allowing clinicians to quantify the degree of muscle overactivity contributing to perceived resistance. Another neurophysiological test is the H-reflex, which assesses the excitability of the alpha motor neuron pool in the spinal cord. By electrically stimulating a sensory nerve, the H-reflex measures the sensitivity of the spinal reflex arc, providing a direct measure of the nervous system’s contribution to spasticity.
Biomechanical methods offer a non-subjective way to quantify the mechanical resistance of a limb. The Pendulum Test, also known as the Wartenberg Pendulum Test, is an example of this approach. In this test, the limb is dropped from an extended position, and its subsequent swinging motion is recorded using sensors like electrogoniometers.
In a healthy limb, the leg swings freely like a pendulum with several oscillations before coming to rest. In a spastic limb, however, internal resistance causes the motion to damp out much faster. Key parameters, such as the excursion of the first backward swing, are measured to quantify the resistance, offering an objective score that correlates with the severity of the hypertonia. This instrumental approach provides data on limb acceleration and deceleration to precisely document the physical manifestations of spasticity.