What is the difference between spasticity and rigidity?

From the Guidelines

Spasticity and rigidity are distinct types of muscle tone abnormalities with different characteristics and underlying causes, and understanding these differences is crucial for proper diagnosis and management of patients with movement disorders. The key differences between spasticity and rigidity are:

• Spasticity is velocity-dependent resistance to passive movement, meaning the faster a limb is moved, the more resistance is felt, often with a "clasp-knife" phenomenon where resistance suddenly gives way 1.
• Spasticity typically affects specific muscle groups, particularly flexors in the upper limbs and extensors in the lower limbs, and is associated with upper motor neuron lesions such as stroke, multiple sclerosis, or spinal cord injury 1.
• Hyperreflexia and clonus often accompany spasticity.
• In contrast, rigidity is constant resistance throughout the range of motion regardless of speed, described as "lead-pipe" rigidity or "cogwheel" rigidity when combined with tremor.
• Rigidity affects agonist and antagonist muscles equally and stems from basal ganglia disorders like Parkinson's disease.
• Rigidity is not associated with hyperreflexia and typically involves the neck, trunk, and proximal limbs. Some key points to consider when managing spasticity include:
• Early treatment is key to preventing disabling complications 1.
• Spasticity is typically treated in a stepwise approach, beginning with noninvasive and progressing to more invasive modalities.
• Positioning, passive stretching, and range-of-motion exercise may provide relief and should be done several times daily in persons with spasticity 1.
• Oral drug agents such as tizanidine, dantrolene, and oral baclofen can be used to treat spasticity that results in pain, poor skin hygiene, or decreased function 1.
• Targeted injection of botulinum toxin into localized upper limb muscles is recommended to reduce spasticity, to improve passive or active range of motion, and to improve dressing, hygiene, and limb positioning 1. The most recent and highest quality study 1 provides the most relevant guidance on the management of spasticity, and the use of targeted injection of botulinum toxin into localized upper limb muscles is recommended to reduce spasticity, to improve passive or active range of motion, and to improve dressing, hygiene, and limb positioning.

From the FDA Drug Label

Indications and Usage Baclofen tablets are useful for the alleviation of signs and symptoms of spasticity resulting from multiple sclerosis, particularly for the relief of flexor spasms and concomitant pain, clonus, and muscular rigidity.

The difference between spasticity and rigidity is not explicitly defined in the provided drug label. However, it can be inferred that they are distinct concepts, as the label mentions both spasticity and muscular rigidity as separate entities.

• Spasticity is the primary condition being treated, characterized by signs and symptoms such as flexor spasms and concomitant pain, clonus.
• Rigidity is mentioned as a concomitant symptom, but its distinction from spasticity is not clearly defined in the label 2.

From the Research

Definition and Characteristics of Spasticity and Rigidity

• Spasticity is a symptom occurring in many neurological conditions, including stroke, multiple sclerosis, and traumatic brain injury, characterized by involuntary muscle hyperactivity in the presence of central paresis 3.
• Spasticity is distinguished from rigidity by its dependence upon the speed of the muscle stretch and by the presence of other positive upper motor neuron signs 4.
• Rigidity is a form of involuntary muscle hyperactivity triggered by slow passive joint movements, whereas spasticity is triggered by rapid passive joint movements 3.

Key Differences between Spasticity and Rigidity

• Spasticity is a velocity-dependent increase in muscle tone, whereas rigidity is not velocity-dependent 5, 4.
• Spasticity is often associated with other upper motor neuron symptoms, such as tendon hyper-reflexia, clonus, and flexor and extensor spasms, whereas rigidity is not 4.
• Spasticity can be described by a documentation system grouped along clinical picture, aetiology, localisation, and additional central nervous system deficits, whereas rigidity is not typically described in this way 3.

Pathophysiology of Spasticity and Rigidity

• Spasticity is due to hyperexcitable tonic stretch reflexes, which are mediated by hyperactive spinal reflexes 4.
• Rigidity is also due to abnormal muscle tone, but its pathophysiology is not as well understood as that of spasticity 4.
• Both spasticity and rigidity can result from an upper motor neuron lesion, which disturbs the balance of supraspinal inhibitory and excitatory inputs, producing a state of net disinhibition of the spinal reflexes 4.